New Applications of Lasers to Chemistry - American Chemical Society

9 Laser Applications in Photoelectrochemistry 1

1

S. P. PERONE , J. H. RICHARDSON, B. S. SHEPARD , J. ROSENTHAL , J. E. HARRAR, and S. M. GEORGE 1

Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0085.ch009

General Chemistry Division, Lawrence Livermore, Laboratory, Livermore, CA 94550

Photoemission S t u d i e s . The U V - v i s i b l e i r r a d i a t i o n of an e l e c t r o d e / s o l u t i o n i n t e r f a c e can stimulate any of s e v e r a l i n t e r e s t i n g e l e c t r o d e processes. I f the s o l u t i o n does not absorb and the e l e c t r o d e i s a m e t a l l i c conductor, photoemission of e l e c t r o n s may occur, with the formation of solvated e l e c t r o n s and initiation of r e a c t i o n s with a v a i l a b l e scavengers. I f the s o l u t i o n absorbs the r a d i a t i o n and p h o t o l y s i s occurs, e l e c t r o a c t i v e p h o t o l y t i c intermediates and products may be detected at an i n d i c a t o r e l e c t r o d e by t h e i r e l e c t r o l y s i s currents under pot e n t i o s t a t i c c o n d i t i o n s . I f the e l e c t r o d e i s a semiconductor and absorbs r a d i a t i o n s u f f i c i e n t to promote e l e c t r o n s through the band gap, o x i d a t i o n or r e d u c t i o n process may be induced which would not occur i n the absence of r a d i a t i o n . I f a dye absorbed on a semiconductor e l e c t r o d e i s e x c i t e d by the i r r a d i a t i o n , the e x c i t e d state may undergo an o x i d a t i v e or r e d u c t i v e e l e c t r o n t r a n s f e r step which would not occur with the ground state s p e c i e s . These are examples of some of the more i n t e r e s t i n g r a d i a tion-induced e l e c t r o d e processes which have been studied. Much of the i n t e r e s t has been stimulated r e c e n t l y by the prospects f o r d i r e c t s o l a r - t o - e l e c t r i c a l energy or s o l a r - t o - c h e m i c a l energy conversion which might be p o s s i b l e with photoelectrochemical ce l i s . ( 1 - 3 ) The work reported here was designed to demonstrate the u t i l i t y of l a s e r sources for these photoelectrochemical s t u d i e s ; we have focused our a t t e n t i o n on those phenomena u n i quely r e l a t e d to the c h a r a c t e r i s t i c s of l a s e r i r r a d i a t i o n . In p a r t i c u l a r , we report here the r e s u l t s of our studies of l a s e r - i n d u c e d photoemission processes and l a s e r - induced p h o t o l y s i s . The former study was undertaken to i l l u s t r a t e the e f f o r t s of wavelength, source power, and i n t e n s i t y on photoem i s s i o n - r e l a t e d process; whereas the l a t t e r study was designed to i l l u s t r a t e the c a p a b i l i t i e s f o r photoelectrochemical quantum y i e l d measurements on t r a n s i e n t p h o t o l y t i c species made p o s s i b l e with a l a s e r source. 1Current address: Department of Chemistry, Purdue University, West Lafayette, IN 47907 This chapter not subject to U.S. copyright. Published 1978 American Chemical Society

In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.


9.

PÉRONÉ ET AL.

Laser Applications in Photoelectrochemistry

127

Photoemission of e l e c t r o n s from mercury e l e c t r o d e s i n t o e l e c t r o l y t e s o l u t i o n s has been studied e x t e n s i v e l y i n recent years.(4-10) O c c a s i o n a l l y other metals have been used as a source o f photoemitted electrons,(11) but the dropping mercury e l e c t r o d e (DME) i s g e n e r a l l y recognized to have d e s i r a b l e chara c t e r i s t i c s . (12.) I n t e r e s t i n photo-related currents has not only been i n c h a r a c t e r i z i n g the emission process but a l s o i n studying the r e a c t i o n s o f the r e s u l t i n g hydrated e l e c t r o n s with various s u i t able scavengers.(13-16) T h e o r e t i c a l studies have a l s o been i n i t i a t e d both with respect to the k i n e t i c s o f scavenging i t s e l f (17_-18) and t r a n s i e n t e f f e c t s i n the e l e c t r o c h e m i c a l detect i o n o f the photorelated phenomena.(21) Many previous experimental studies have been c a r r i e d out with continuous or chopped r a d i a i o n sources, monitoring the steady state (DC)(22) or modulated(4«9.15,23) photocurrents under p o t e n t i o s t a t i c c o n d i t i o n s . Most o f the more recent work has used pulsed xenon flashlamps. ( 5 .6 .10 .13 .14.24.25 ) Detect i o n o f photo-related phenomena was by p o t e n t i o s t a t i c ( 1 0 , 2 4 ) or coulostatic(13,14,17) monitoring of the e l e c t r o d e process. Very l i t t l e work has involved the use o f l a s e r s as the r a d i a t o n source. The few laser-induced photoelectrochemistry studies which have appeared mostly used s o l i d - s t a t e l a s e r s (ruby or neodymium) with one or two l i n e s and a slow r e p e t i t i o n rate (sometimes s i n g l e - s h o t only) . ( 13 ,26-28) A recent report(J29) used the n i t r o g e n l a s e r at 337.1 nm. The work reported here had the general o b j e c t i v e o f studying the e f f e c t s o f very intense l a s e r sources on e l e c t r o d e photoemission processes. Thus, the s p e c i f i c goals of t h i s work were t h r e e f o l d : 1) to develop appropriate i l l u m i n a t i o n and measurement instrumentation f o r both pulsed and cw l a s e r sources i n conjunction with a conventional DME assembly; 2) to determine the e f f e c t o f source c h a r a c t e r i s t i c s on e l e c t r o d e photoemission processes i n the presence o f s u i t a b l e hydrated e l e c t r o n scavengers; 3) to evaluate the general u t i l i t y o f l a s e r sources f o r photoelectrochemical s t u d i e s . In order to achieve the above goals we have used both a pulsed n i t r o g e n pumped dye l a s e r and a cw argon i o n l a s e r . The dye l a s e r i s continuously tunable from 258 to 750 nm, the output c o n s i s t i n g o f 10 nsec pulses at a r e p e t i t i o n rate o f 1 to 50 Hz and peak powers o f s e v e r a l k i l o w a t t s . The argon ion l a s e r was a cw source, u s u a l l y chopped around 1 kHz. I t was operated on one o f four f i x e d wavelengths, with output powers up to 6 W. A conventional DME polarographic assembly was i l l u m i n a t e d under c o n t r o l l e d p o t e n t i a l conditions with c a p a b i l i t i e s f o r both polarographic steady state and t r a n s i e n t (boxcar i n t e g r a tor and o s c i l l o s c o p e ) current measurement c a p a b i l i t i e s . Scavengers used i n these studies included N2O, NO3", s e v e r a l d i v a l e n t


NEW

128

2 +

2 +

2+

APPLICATIONS OF LASERS TO CHEMISTRY

2 +

2 +

2 +

2 +

cations (e.g., C o , F e , Mn , N i , C u , C d and P b ) , and Co(NH ) . The photo-related current was studied as a f u n c t i o n of wavelength, e l e c t r o d e p o t e n t i a l , and i n t e n s i t y f o r a l l the s c a vengers, using both sources and a l l three d e t e c t i o n c a p a b i l i ties. D e f i n i t e scavenging of e l e c t r o n s was not observed with the metal c a t i o n s , i n contrast to NO3" and N2O. A h i g h l y non l i n e a r photo-effect was observed with the high peak-powered, pulsed l a s e r system. This e f f e c t was p a r t i c u l a r l y n o t i c e a b l e with the metal c a t i o n s . To our knowledge such a pronounced e f f e c t has not been observed before. Our work i l l u s t r a t e s some of the l i m i t a t i o n s as w e l l as advantages of using l a s e r sources with e l e c t r o c h e m i c a l d e t e c t i o n of photoemission c u r r e n t s . 3 +


3

6

Experimental Laser P h o t o l y s i s - Quantum Y i e l d Studies. E l e c t r o a c t i v e species can be q u a l i t a t i v e l y and q u a n t i t a t i v e l y c h a r a c t e r i z e d by chronoamperometric measurements at a microelectrode i n the p h o t o l y s i s cell.(30-33) I f the f a r a d a i c current f o r p h o t o l y t i c species i s d i f f u s i o n - c o n t r o l l e d and uncomplicated by k i n e t i c e f f e c t s , the C o t t r e l l equation a p p l i e s :

i

= nFAD

1 / 2

C°/(ut)

1 / 2

(1)

where i is the f a r a d a i c current at time t, η i s the number od e l e c t r o n s t r a n s f e r r e d , F i s the Faraday, A i s the e f f e c t i v e e l e c t r o d e area, and D i s the d i f f u s i o n c o e f f i c i e n t of the e l e c t r o a c t i v e s p e c i e s . Thus, a p l o t of i versus 1/ t should be l i n e a r with a slope p r o p o r t i o n a l to C°, the bulk concentra t i o n of e l e c t r o a c t i v e species i n s o l u t i o n . A t h e o r e t i c a l expression has been derived(30) d e s c r i b i n g the i n i t i a l concentration of intermediate, R, produced when a pulse of l i g h t i s passed through a s o l u t i o n containing a photor e a c t i v e s p e c i e s , 0,

Cj(b) = 4Q [aC°]exp[-abC°]

(2)

o

where C R i s the concentration of intermediate as a f u n c t i o n of pathlength (b), the quantum e f f i c i e n c y (Φ), the instantaneous i n i t i a l quanta of monochromatic l i g h t per u n i t area ( Q ) , the absorption c o e f f i c i e n t of the photoreactive species (a), and the concentration of the reactant before the f l a s h (Cq) . This r e l a t i o n s h i p assumes i d e a l conditons where C Q does not change Q



9.

PÉRONÉ ET AL.

129


during the f l a s h , the l i g h t pulse i s c o l l i m a t e d and monochromat i c , and no other absorbing species are produced during the flash. Replacement i n t h i s work of the p r e v i o u s l y used xenon flashlamp with a pulsed dye l a s e r e x c i t a t i o n source extends the v a l i d i t y and, consequently, the usefulness o f t h i s expression. The pulsed l a s e r s u p p l i e s c o l l i m a t e d , monochromatic l i g h t p u l ses o f high i n t e n s i t y and very short d u r a t i o n , and, t h e r e f o r e , allows measurements to be made at s e l e c t e d wavelengths and on shorter time s c a l e s . The present study involved l a s e r f l a s h p h o t o l y s i s e x p e r i ments using the F e ( l I I ) oxalate system. Both the photochemist r y (34-J37.) and the photoelectrochemistry(38 ,39) o f thé F e ( I I I ) oxalate system have been studied. These studies have shown by s p e c t r o s c o p i c and e l e c t r o c h e m i c a l techniques that F e ( l I I ) can be photoreduced by both v i s i b l e (blue) and uv l i g h t . Although a number of c o n f l i c t i n g r e a c t i o n mechanisms have been proposed f o r t h i s system, i t i s known that the f i n a l product i s F e ( l l ) . Quantum y i e l d s f o r the production o f F e ( I I ) have been reported f o r wavelengths l e s s than 600 nm.(40-42) In a d d i t i o n , f l a s h photoelectrochemical experiments have shown that the production of F e ( l l ) can be monitored at a p o t e n t i a l where the F e ( l l ) i s o x i d i z e d to F e ( l I I ) . Furthermore, when these o x i d a t i o n currents are d i f f u s i o n - c o n t r o l l e d and f o l l o w C o t t r e l l behavior, q u a n t i t a t i v e determinations o f the F e ( l l ) produced by the f l a s h can be made. (43) In the studies reported here the concentration of F e ( I I ) produced from laser-induced p h o t o l y s i s was determined from C o t t r e l l p l o t s . The l a s e r p h o t o l y s i s source used was a Xerox flash-lamp-pumped dye l a s e r with ~ 0.5 psec pulse width and - 1 j o u l e output energy i n the v i s i b l e r e g i o n . Because the l a s e r pulse i s monochromatic and c o l l i m a t e d , the r e l a t i o n s h i p between the concentration and the pathlength p r e d i c t e d by Equat i o n 2 was observed. In a d d i t i o n , working at a known pathlength and measuring both the photon f l u x and the F e ( I I ) i n i t i a l l y produced, quantum y i e l d s were determined at 442 and 457 nm. Reagents. A l l i n o r g a n i c s a l t s used i n t h i s work were reagent grade and were used without f u r t h e r p u r i f i c a t i o n : NaOH, N i C l , M n C l , C o C l , F e C l , F e N H ^ S O ^ · 12H 0, K C 0 4 , and P b C l (Baker), C u C l , C d C l and KC1 ( M a l l i n c k r o d t ) , Ο ο ( Ν Η ) 0 1 (Kodak) and NaNC>3 (MCB). Nitrous oxide (N 0) was obtained from Matheson and used without f u r t h e r p u r i f i c a t i o n . A l l s o l u t i o n s were made up i n l a b o r a t o r y deionized water which had been f u r t h e r p u r i f i e d by a Corning demineralizer and water s t i l l . F e r r i c oxalate s o l u t i o n s were made by d i s o l v i n g the appro p r i a t e amount o f FeNH4(S04) ·12H 0 i n an aqueous s o l u t i o n o f K C 04'H 0. The pH of the s o l u t i o n was adjusted by the a d d i t i o n of d i l u t e H S04 or KOH. A l l data reported here are for 0.853 χ 10~ M f e r r o i x a l a t e s o l u t i o n s at pH 6.0 and 0.25 M oxalate. The 2

2

2

2

2

2

2

2

2

2

3

6

2

2

2

2

2

2

2

2


3


130

NEW APPLICATIONS OF LASERS TO CHEMISTRY

F e ( l I I ) c o n c e n t r a t i o n was determined s p e c t r o - p h o t o m e t r i c a l l y with thiocyanate reagent.(44) Instrumentation f o r Photoemission Studies. Two l a s e r sys tems were used: 1) a n i t r o g e n pumped dye l a s e r , 2) an argon i o n l a s e r . A Molectron UV1000 n i t r o g e n l a s e r (1 MW peak power, 10 nanosecond pulse with 1-50 Hz r e p e t i t i o n rate) was used to t r a n s v e r s e l y pump a Molectron dye l a s e r operated i n the DL200 c o n f i g u r a t i o n . Doubling the fundamentals was accomplished by using angle-tuned KDP c r y s t a l s f o r second harmonic generation, with a Corning 7-54 f i l t e r to block the fundamental. The l a s e r was u s u a l l y operated at 30 Hz. T y p i c a l peak powers of funda mentals d e l i v e r e d to the e l e c t r o c h e m i c a l c e l l were 1-4 KW, c o r responding to a maximum power d e n s i t y o f ca. 0.5 MW/cm^. (The actual peak power d e l i v e r e d out of the l a s e r i s more than an order o f magnitude greater than t h i s , but there are c o n s i d e r able losses i n s t e e r i n g and shaping the beam before i t gets to the DME.) Peak powers o f doubled wavelengths were approximately 5% that o f the fundamental. Average powers at the c e l l were measured with a Scientech 362 power meter, and the r e l a t i v e power monitored p e r i o d i c a l l y with a Molectron J3 p y r o e l e c t r i c joulemeter. A Spectra-Physics 170-09 argon i o n l a s e r was used as the cw source. Four d i s c r e t e wavelengths were used: 514.5, 488.0, 457.9 and 351/364 nm. The cw output was modulated, u s u a l l y at 1 kHz, with a 50% duty cycle using an Ithaco 383A v a r i a b l e speed chopper. For the v i s i b l e l i n e s the average power d e l i vered to the c e l l was approximately 25% of the cw output power. For the uv l i n e s t h i s f i g u r e dropped to 8%. The l a s e r output was d i r e c t e d i n t o a sample chamber con t a i n i n g the DME. Figure 1 i s a schematic of the experimental apparatus. Figure 2 i s a p i c t u r e o f the e l e c t r o c h e m i c a l c e l l mounted i n the sample chamber. The c e l l i t s e l f i s transparent, the lower p o r t i o n c o n s i s t i n g o f a 1 χ 2 χ 4 cm S u p r a s i l curvette ( f l u r o r e s c e n c e type, P r e c i s i o n C e l l s ) and the upper p a r t Pyrex. The t o t a l volume i s approximately 40 ml. The c e l l s i t s i n a n e a r l y l i g h t t i g h t sample chamber s u i t a b l e f o r s p e c t r o s c o p i c s t u d i e s . A more d e t a i l e d d e s c r i p t i o n o f the s p e c t r o s c o p i c c h a r a c t e r i s t i c s o f the sample chamber has been published pre v i o u s l y . (45) The e l e c t r o c h e m i c a l c e l l c o n s i s t s of a three e l e c t r o d e system: a dropping mercury working e l e c t r o d e , a platinum wire counter e l e c t r o d e which can be p o s i t i o n e d very close to the DME, and a saturated calomel reference e l e c t r o d e (SCE) which measures the p o t e n t i a l o f the s o l u t i o n near the DME v i a a T e f l o n tube. The c e l l i s sealed with a T e f l o n cover. P r o v i s i o n i s made f o r bubbling v a r i o u s gases through the s o l u t i o n , flowing gases over the top o f the s o l u t i o n (to exclude oxygen) and emptying, r i n sing and f i l l i n g the c e l l without d i s t u r b i n g the o p t i c a l a l i g n ment. The e n t i r e e l e c t r o c h e m i c a l c e l l can be f i n e l y p o s i t i o n e d


PERONE ET AL.


XY


recorder

Signal

Polarograph

Ar laser power supply +

Plasma

Electrode system DME Sample chamber Beam dump

Dye laser

2 laser

1P28

, N

Baffles and iris Lens Filter

Beam splitter

Trigger and reference High voltage supply

(Oscilloscope

Monochromator

8850 Boxcar integrator

t

Signal

Output

XY recorder

High voltage supply

Uigure 1. Schematic of the experimental apparatus




Figure 2. Electrochemical cell and polarograph interface mounted on sample chamber



9.

PERONE ET

AL.

Laser Applications

in

Photoelectrochemistry

133

i n the three xyz d i r e c t i o n s , thus enabling the mercury drop to i n t e r s e c t the focused l a s e r beam i n l i n e with the r i g h t angle spectroscopic viewing assembling. A Princeton Applied Research (PAR) model 174A polarographic analyzer was used to c o n t r o l and scan the DME p o t e n t i a l , c o n t r o l the drop timer and monitor the c u r r e n t . A T e k t r o n i x 7904 o s c i l l o s c o p e with 7A15A and 7A19 p l u g - i n a m p l i f i e r s was used to observe the s i g n a l . The o s c i l l o s c o p e was p r i m a r i l y used f o r alignment, s i n g l e waveform monitoring and other d i a g n o s t i c purposes. The t r i g g e r was provided by the dye l a s e r pulse or modulated argon i o n l a s e r output. A PAR 162/163/164 boxcar i n t e g r a t o r was used f o r data ac q u i s i t i o n and averaging. The AC coupling i n the 164 gated i n t r e g r a t o r was modified to eliminate the l a r g e , slowly v a r y i n g DC component which was due to v a r i a t i o n of the mercury drop s i z e . The boxcar was used f o r s i n g l e point a n a l y s i s p r i m a r i l y , delaying the window ( t y p i c a l l y 50 ps) by an appropriate amount to c o i n c i d e with the s i g n a l maximum. Output from both the box car and the polarograph was displayed on x-y r e c o r d e r s . Procedures f o r Photoemission Studies. The c e l l was r i n s e d s e v e r a l times before the f i n a l s a l t s o l u t i o n was added. The s o l u t i o n was deoxygenated f o r a l e a s t 10 minutes, u s u a l l y with scrubbed (chromous c h l o r i d e and zinc amalgam) and water s a t u r ated argon. For the studies with N 0 the gas was allowed to bubble through f o r several minutes u n t i l a s u f f i c i e n t photoemis s i o n s i g n a l could be obtained. No attempt was made to determine the N 0 concentration. The m a j o r i t y of the other s o l u t i o n s were 3 mM i n the scavenging i o n . The supporting e l e c t r o l y t e s o l u t i o n was u s u a l l y 0.1 M KC1. To reduce polarographic maxima a d i l u t e s o l u t i o n of T r i t o n X-100 was added dropwise to the e l e c t r o c h e m i c a l c e l l u n t i l no f u r t h e r apparent r e d u c t i o n i n the maxima was noted. The polarograph was operated without any e l e c t r o n i c f i l t e r ing ( i . e . , no a d d i t i o n a l time c o n s t a n t s ) . The hangtime of the mercury drop was u s u a l l y 5 seconds; t y p i c a l s e n s i t i v i t i e s of the polarograph were 0.5 to 75 μA f u l l s c a l e . The polarograph drove the x-axis of both of the recorders, thus p e r m i t t i n g s i multaneous recording of both the DC polarograph and photoemis sion current vs p o t e n t i a l . The l a s e r beam was a l i g n e d with the DME both v i s u a l l y and i n s t r u m e n t a l l y ( i . e . , by monitoring the photo-related s i g n a l on the o s c i l l o s c o p e ) . P r e l i m i n a r y spectro s c o p i c scans were made by f i x i n g the p o t e n t i a l and scanning the monochromator, viewing the luminescence at r i g h t angles to the l a s e r beam. The luminescence was detected by an RCA 8850 photo m u l t i p l i e r tube, processed by the boxcar and recorder. Occa s i o n a l l y the emission wavelength was f i x e d and the p o t e n t i a l s canned. The n i t r o g e n l a s e r i s w e l l s h i e l d e d e l e c t r i c a l l y . In the present experiments the t r a n s i e n t current s i g n a l was monitored 2

2



134


many microseconds a f t e r the l a s e r pulse, and no e f f e c t a t t r i b u t able to the l a s e r discharge was detected. No e l e c t r i c a l i n t e r ference was n o t i c e a b l e from the cw argon ion l a s e r . Background s i g n a l s from blank s o l u t i o n s ( i . e . , e l e c t r o l y t e s o l u t i o n ) were measured f r e q u e n t l y as various conditons were changed (e.g., l a s e r i n t e n s i t y , wavelength, s e n s i t i v i t y , potential). Thus i t was s t r a i g h t f o r w a r d to compare any observed photo-related phenomena i n the presence and absence of scavenger. Instrumentation f o r Laser P h o t o l y s i s Studies. A Phase-R model 2100B fiashlamp-pumped tunable dye l a s e r (Phase-R Co., New Durham, N.H.) was used as the p h o t o l y s i s l i g h t source. When pumped with a model DL-18 c o a x i a l flashlamp, with a t r i a x adapter d i m i n i s h i n g the beam diameter to 12 mm, output pulses with energies as high as 1 - 5 joules and widths as narrow as 0.5 usee could be generated at a r e p e t i t i o n rate of 20 ppm. A commercial p y r o e l e c t r i c joulemeter (model J3-05DW, Molectron Corp., Sunnyvale, CA) was used f o r l i g h t i n t e n s i t y measurements. The p h o t o l y s i s c e l l was constructed from 1/4-inch t h i c k p o l y a c r y l i c sheet cut to s i z e and bonded together with c h l o r o form. The c e l l held a t o t a l s o l u t i o n volume of approximately 7 ml. A quartz window was located i n the bottom of the c e l l through which the p h o t o l y s i s source was d i r e c t e d . The reference and counter e l e c t r o d e s were mounted permanently i n the c e l l w a l l to e l i m i n a t e any problems a s s o c i a t e d with reproducing t h e i r pos i t i o n s . Tubing connected the c e l l with a separate s o l u t i o n r e s e r v o i r , an a s p i r a t o r f o r s o l u t i o n removal, and a scrubbed nitrogen l i n e . The t h r e e - e l e c t r o d e monitoring system c o n s i s t e d of a hanging mercury drop working e l e c t r o d e (HMDE), a Pt counter e l e c trode, and a saturated calomel reference e l e c t r o d e (SCE). The mercury drop was suspended from a micrometer dispensing assembly (Metrohm E410 Hanging Mercury Drop E l e c t r o d e , Brinkman I n s t r u ments, Inc., Westbury, NY) f o r accurate c o n t r o l of drop s i z e . The e n t i r e HMDE assembly was p o s i t i o n e d above the c e l l v e r t i c a l l y and h o r i z o n t a l l y with a p r e c i s i o n of + 0.1 mm using a V e r t i c a l - T r a n s v e r s e Motion Mount ( E a l i n g Corp., Cambridge, MA). The counter e l e c t r o d e was constructed from copper metal covered with a t h i n platinum sheet and sealed i n the back w a l l of the cell. The reference e l e c t r o d e was made of a three-compartment c e l l with glass f r i t s s e p a r a t i n g the compartments. The f i r s t compartment contained the SCE, the second contained a 1 M KC1 s o l u t i o n , and the t h i r d contained a mixture of s o l v e n t , e l e c t r o l y t e , and b u f f e r . The t h i r d compartment was connected to a Luggin c a p i l l a r y mounted on the back w a l l of the c e l l with a Teflon f i t t i n g . The p o t e n t i o s t a t , described i n Ref. 46, had a c o n t r o l u n i t y - g a i n bandwidth o f 900 kHz and a monitoring bandwidth of 100 kHz. The computerized data a c q u i s i t i o n system has been described.21


9.

PÉRONÉ ET AL.


135

Procedures f o r Laser P h o t o l y s i s Studies. The temperatures of the l a s e r dye and the water surrounding the t r i a x adapter were i n i t i a l l y e q u i l i b r a t e d to + 0.1°C. The absolute temperature v a r i e d between 18 and 20°C. These temperatures were continuously monitored throughout each s e r i e s o f experiments and adjusted i f necessary. The l a s e r was f i r e d at 17 kV, which corresponded to a charging energy o f 289 j o u l e s . The l a s e r dyes used were Coumarin 440 i n methanol ( λ = 442 nm) and Coumarin 460 i n ethanol ( λ = 457 nm), (Phase-R Co., New Durham, NH). F e ( l I I ) oxalate s o l u t i o n s were deaerated f o r at l e a s t t h i r t y minutes before s t a r t i n g each experiment. Following p h o t o l y s i s , the s o l u t i o n was a s p i r a t e d from the c e l l and new s o l u t i o n was obtained from the r e s e r v o i r . For each experiment, the c e l l containing the s o l u t i o n was o p t i c a l l y shielded from the l a s e r by means o f a removable shut ter, and a blank was run to measure any a m p l i f i e r d r i f t and/or any background current due to e l e c t r o n i c disturbances. The shutter was then removed, the Fe(ox)3~^ s o l u t i o n was photolyzed, and F e ( l l ) o x i d a t i o n currents as a f u n c t i o n of time were monitored p o t e n t i o s t a t i c a l l y at -0.05V vs SCE. The current mea sured i n the blank was then subtracted. This procedure was r e peated a minimum o f three times at each set of c o n d i t i o n s , and r e s u l t i n g current-time curves were averaged to give the net result. Each averaged current-time curve was corrected f o r f a r a daic-induced charging current before f u r t h e r a n a l y s i s . The data were c o r r e c t e d f i r s t by the " d e r i v a t i v e method", based on the f o l l o w i n g r e l a t i o n s h i p : 0


0

d i

T

where i p i s the Faradaic c u r r e n t , i ^ i s the t o t a l current, R i s the uncompensated c e l l r e s i s t a n c e , Cnj_, i s the c a p a c i tance o f the working e l e c t r o d e double l a y e r , and ( d i ^ / d t ) i s the time d e r i v a t i v e of the t o t a l current. The c e l l time con stant ( R C J ) L ) J b determined experimentally as o u t l i n e d p r e v i o u s l y . ( 3 1 ) I f data show C o t t r e l l behavior f o l l o w i n g t h i s c o r r e c t i o n , they can be assumed to be d i f f u s i o n - c o n t r o l l e d . The d e r i v a t i v e method tends to introduce a large amount o f noise i n t o the c a l c u l a t e d Faradaic c u r r e n t . I f C o t t r e l l behav i o r i s observed, however, the raw data can be c o r r e c t e d by a tabulated t h e o r e t i c a l c o r r e c t i o n f a c t o r which does not introduce noise.(47) As with the d e r i v a t i v e c o r r e c t i o n method, knowledge of the c e l l time constant i s r e q u i r e d . I t has been pointed out(31) that the " e f f e c t i v e " RC ( R C f f ) a f t e r l i g h t i r r a d i a t i o n may d i f f e r s i g n i f i c a n t l y from the experimentally determined u

c

a

n

e

U

e


NEW

136

APPLICATIONS OF LASERS TO

CHEMISTRY

RC because only part of the s p h e r i c a l e l e c t r o d e i s exposed to photolyzed s o l u t i o n (~ 50-75%). Thus, the e f f e c t i v e c e l l time constant may be less than that measured experimentally, where the t o t a l surface i s i n v o l v e d . F o r t u n a t e l y , when Faradaic currents are d i f f u s i o n - l i m i t e d , the value of R C f f can be e s timated from the raw data, as theory p r e d i c t s that ( ^ - - ) 0.85 RC.(31) Thus, the e f f e c t i v e RC can be determined from the p o t e n t i o s t a t i c current-time curve f o l l o w i n g f l a s h i r r a d i a t i o n by observing the time at which the current goes through a maxi mum. I t was t h i s procedure that was followed i n the work r e ported here. e

t

=


ma

Results of Photoemission

K

Studies

To evaluate the e f f e c t s of l a s e r source c h a r a c t e r i s t i c s on e l e c t r o d e photoemission processes three d i f f e r e n t types of aqu eous s o l u t i o n s were used. One of these contained i n e r t e l e c t r o l y t e only (KC1 or NaOH). The second contained i n e r t e l e c t r o l y t e and a w e l l c h a r a c t e r i z e d scavenger (N 0 or NO3""). The t h i r d contained i n e r t e l e c t r o l y t e and one of s e v e r a l d i v a l e n t metal ions ( F e , N i , M n , C o , C u , P b , or C d . The f i r s t two types o f s o l u t i o n s provided f o r d i r e c t comparison with pre vious studies using more conventional i l l u m i n a t i o n sources. The t h i r d type o f s o l u t i o n provided e l e c t r o a c t i v e species for which d i s t i n c t s e n s i t i v i t y to source c h a r a c t e r i s t i c s was observed. In a l l o f the d i s c u s s i o n s below we w i l l use the general terms "photo-related" currents or "photocurrents" to describe any current s i g n a l s which are dependent on e l e c t r o d e i l l u m i n a t i o n , regardless o f whether photoemission o f e l e c t r o n s i s known to occur. The term "photoemission-related" currents w i l l be used whenever the s p e c i f i c phenomenon o f e l e c t r o n photoemission i s to be considered. The behavior o f N 0 as a scavenger i s w e l l known, and i t s photoelectrochemical c h a r a c t e r i s t i c s have been p r e v i o u s l y described.(4,9J3.14.16.27.29) I t r e a c t s very r a p i d l y with hydrated e l e c t r o n s to form molecular n i t r o g e n and hydroxyl r a d i c a l . The l a t t e r species i s r e d u c i b l e over the e n t i r e mer cury e l e c t r o d e p o t e n t i a l range. Thus, e l e c t r o d e photoemission i n the presence o f N 0 y i e l d s a net cathodic current at a l l p o t e n t i a l s negative to the photoemission t h r e s h o l d value f o r the p a r t i c u l a r wavelength of r a d i a t i o n . When Ν 0 ~ i s the scavenger the i n i t i a l product i s NO3*" , which reacts r a p i d l y with water (τχ/ < 15 με) to form N0 . The N0 i s e a s i l y reduced to n i t r i t e ion at p o t e n t i a l s negative of about -0.9 V vs SCE. Thus, cathodic photoemission-related currents are seen i n the presence o f ΝΟβ" with s u b s t a n t i a l en hancement at s u f f i c i e n t l y negative p o t e n t i a l s . The behavior o f c e r t a i n d i v a l e n t t r a n s i t i o n metal ions as hy drated e l e c t r o n scavengers has been reported previously.(22.24.25) 2

2 +

2 +

2+

2 +

2 +

2 +

2 +

2

2

2

3

2

2


2

9.

PÉRONÉ ET AL.


137

The i n i t i a l product postulated i s the s h o r t - l i v e d univalent c a tion. In the absence o f an o x i d i z a b l e species the univalent c a t i o n i s r e o x i d i z e d to the 2 state at the e l e c t r o d e or by r e a c t i o n with the solvent. A photoelectrochemical study(£4) with N i ^ as the scavenger described the e f f e c t s o f t h i s type of mechanism and observed photocurrents. The f o l l o w i n g mechanism was suggested to e x p l a i n the current pulses that were observed with pulsed i r r a d i a t i o n o f a DME i n the polarographic plateau region:(24) +


+

e"\ . + Ni (aq)

Ni

+

+ 2

-> N i

+ H 0 -* N i

+ 2

2

+

(4)

5

+ H + 0H~

H + OH 2

(7)

Thus, i t was suggested that the observed photocurrents were photoemission-related and that r e a c t i o n s (2) and (3) i n the d i f f u s i o n l a y e r would r e s u l t i n enhanced currents. Current enhancement occurs because not only are photoemitted e l e c t r o n s scavenged, but a l s o the ultimate product, H atoms, i s reducible f u r t h e r . Moreover, there i s no net d e p l e t i o n of N i ^ , as i t i s regenerated by r e a c t i o n o f N i with solvent. Although no studies with other d i v a l e n t metal ion scavengers have been r e ported, i t i s l i k e l y that the N i 2 e l e c t r o d e process provides a model system. The magnitudes of photoemission-related currents depend on several f a c t o r s . F i r s t l y , the quantum e f f i c i e n c y o f the photo emission event i t s e l f increases with negative p o t e n t i a l . Se condly, i f the scavenging r e a c t i o n r e s u l t s i n an e l e c t r o i n a c t i v e product, e l e c t r o n s are permanently removed from the e l e c t r o d e , and the net cathodic current w i l l i n c r e a s e , up to a p o i n t , with the scavenging rate constant. T h i r d l y , i f the scavenging reac t i o n r e s u l t s i n an e l e c t r o r e d u c i b l e s p e c i e s , the net cathodic current w i l l be enhanced f u r t h e r . The fundamental aspects of e l e c t r o d e photoemission and subsequent scavenging processes have been discussed e l s e where ( ^ ^ J J ^ ^ J ^ ) and w i l l not be repeated here. However, i t should be emphasized that the i n i t i a l e l e c t r o n emission event and subsequent scavenging of that e l e c t r o n must be complete i n less than about 1 a f t e r l i g h t absorption by the e l e c t r o d e . A l s o , these events do not u s u a l l y extend beyond 50-100 Â from the electrode surface. +

+

+



138

The studies reported here were conducted with the two l a s e r sources described i n the Experimental s e c t i o n . Various uv and v i s i b l e wavelengths were used, and the beam c h a r a c t e r i s t i c s were monitored and documented f o r each of the studies reported below. In each case, photocurrents were monitored i n three d i f f e r e n t ways: 1) conventional polarographic output; 2) an o s c i l l i s c o p e d i s p l a y of t r a n s i e n t or modulated s i g n a l s ; and 3) boxcar averaging of t r a n s i e n t or modulated currents synchronized with e i t h e r l a s e r source. N0 Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0085.ch009

2

and NO3"

Solutions

Results with Chopped CW Laser Source. Figure 3 i s a t y p i cal o s c i l l o s c o p e d i s p l a y of both the 1 kHz modulated l a s e r r a d i a t i o n and the AC coupled synchronous photoemission current observed at -1.6 V vs SCE i n the presence of NO3"" scavenger. In the absense of r a d i a t i o n there i s no detectable current. Furthermore, while there was some small amount of photorelated current i n the absence of scavenger, there was a tremendous enhancement of current a t t r i b u t a b l e to photoemission a f t e r a d d i t i o n of N 0 or NO3". Figure 4 i s a DC polarogram i l l u s t r a t i n g the large change i n DC current upon i r r a d i a t i o n . F i g u r e 5 i l l u s t r a t e s the AC coupled synchronously detected boxcar averaged s i g n a l . A small photorelated current (probably thermal i n o r i g i n , v i d e i n f r a ) i s seen even i n the blank i n the v i c i n i t y of -0.2 V; however, the photorelated current a t t r i b u t a b l e to photoemission and scavenging e a s i l y dominates Figure 5B. Figure 5C i l l u s t r a t e s the complete photoemission s i g n a l as a f u n c t i o n of DME p o t e n t i a l . The decrease i n photoemission current at more negative p o t e n t i a l s was c o n s i s t e n t l y observed, c o i n c i d i n g with the p o t e n t i a l at which solvent r e d u c t i o n commenced. We i n v e s t i g a t e d the dependence of photorelated current on both wavelength and l i g h t i n t e n s i t y using both N 0 and NO3" scavengers. Figure 6 i l l u s t r a t e s the f u n c t i o n a l dependence of the photoemission current (ipg) ° DME p o t e n t i a l : the theorect i c a l l y expected(7.8.16.23) l i n e a r i t y of ( i p g ) ' ^ with respect to p o t e n t i a l i s observed. The uv output i s not s t r i c t l y monochomatic (351/364 nm or 3.53/3.41 ev), which p o s s i b l y accounts for the d i f f e r e n t slope observed. A l s o , N03~ (the i n i t i a l product of N03~ scavenging) can be r e o x i d i z e d p o s i t i v e of -1.1 V (vs SCE),(.2£} which would tend to lower the apparent photoemission c u r r e n t . A l l of the data represented by Figure 6 were obtained from boxcar signal-averaged p l o t s such as Figure 5C. D i f f e r e n t absolute v e r t i c a l scales apply to each wavelength, but the r e l a t i v e dependence on wavelength and p o t e n t i a l is apparent . F i g u r e 7 i l l u s t r a t e s the dependence of the photorelated Current on l a s e r i n t e n s i t y . Once again, r e l a t i v e currents were obtained from the boxcar signal-averaged output as i n Figure 5C. In Figure 7 the photoemission current was monitored at a f i x e d 2

2

n

0

2

2



9. PÉRONÉ ET AL.


139

(A)

B)

A

(C)

Figure 3. Oscilloscope display of synchronous DC-coupled photoemission current with chopped cw-Kr hser (407 nm, 150 mW). (A) Reference signal, light on (5 V/division); (B) reference signal, light off; (C) polarographic signal, light off. Solution was 3 mM NaN0 in 0.1M KCl, Ε = -1.70 V (vs. SCE). +

3


NEW


ι

(A)

APPLICATIONS OF

J

Ί

CHEMISTRY

Γ

, , ι 11 11 I I 11 "

ρττττ

LASERS TO

I

I

{ 1

u

L

Γ (Β)

Λ λ

A À

trrrrrrr_L 0

-0.2

-0.4

J

-0.6

I

-0.8

I

-1.0

I

-1.2

L

-1.4

V (vs. SCE)

Figure 4. DC polarogram for chopped cw-laser irradiation with and without NO ~ scavenger, λ — 457.9 nm, laser output power = 0.5 W. (A) 0.1M KCl, 2.5 μΑ/division; (B) 3 mM NaN0 in 0.1M KCl, 2.5 μΑ/division. s

3


PERONE ET AL.

0.0


-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4


V (vs.SCE)

0.0

-0.2

-0.4

-0.6

-0.8

-1.0

-1.2

-1.4

V (vs. S C E )

V (vs.SCE)

Figure 5. Boxcar-averaged photo-related current for chopped cw-laser irradiation with and without N0 ~ scavenger, ACcoupled, synchronously-detected, λ = 457.9 nm, laser output power = 0.5 W. (A) 0.1M KCl, amplified 7.7X; (Β) 3 mM NaN0 in 0.1M KCl, amplified 1.95X; (C) S mM NaN0 in 0.1 M KCl, .77χ. 3

3

3



NEW

APPLICATIONS OF

LASERS TO

•

CHEMISTRY

D

•

° •

»

"I

Δ Δ

A Ο •

°

03

α •

ι

03

«

cc

3

Δ

Ο

°

Δ

°

Δ

°

ο

Δ

•

• Δ

ο ο

Δ •

Ο Δ

•

ο

Δ • •

J -0.4

I

-0.6

-0.8

Δ

I -1.0

ο

Δ Ο

I -1.2 V (vs.

L -1.4

-1.6

-1.8

-2.0

SCE)

0A

Figure 6. (i ) vs. DME potential for chopped cw-hser source, supporting electrolyte was 0.1M KCl in all cases. (Φ) N 0 351/ 364 nm (3.53/3.41 eV); ({J) 3 nM NaNO , 457.9 nm (2.71 eV); (A)3mM NaN0 , 488.0 nm (2.54 eV); (O) 3 mM NaNO , 514.5 nm (2.41 eV). PE

2

s

3

s




PERONE ET AL.

ΙΟ"

10°

1

Ί0

1

Watts

Figure 7. Dependence of photo-related current on cw-hser intensity, 3 mM NaN0 in 0.1M KCl, λ = 514.5 nm, rehtive photoemission current vs. hser output. (Ο) Ε = 1.75 V; (Φ) Ε = -0.3 V. 3



144

NEW

APPLICATIONS OF

LASERS TO

CHEMISTRY

p o t e n t i a l while the l a s e r power was increased. Beyond 4 W (or ca. 2 W peak power a c t u a l l y focused onto the mercury drop) satu r a t i o n occurs, corresponding to a maximum quantum e f f i c i e n c y f o r photoemision of ca. 0.1% before s a t u r a t i o n . Figure 7 also i l l u s t r a t e s the dependence on r e l a t i v e i n t e n s i t y at two d i f f e r e n t potentials. In t h i s case n e u t r a l d e n s i t y f i l t e r s ( c a l i b r a t e d for high energy pulsed l a s e r s ) were used to decrease the r e l a t i v e i n t e n s i t y at a p o t e n t i a l where photoemission occurs (-1.75 V) and a l s o at a p o t e n t i a l where the photo-related current may be due to thermal heating and p e r t u r b a t i o n of the double l a y e r (-0.3 V). In each case a l i n e a r dependence on l a s e r i n t e n s i t y was observed (as i n d i c a t e d by u n i t y slope on a l o g - l o g p l o t ) . Results with Pulsed Dye Laser Source. Figure 8 i l l u s t r a t e s the temporal response of the photoemission current observed with the pulsed l a s e r . The decay time observed e s s e n t i a l l y r e f l e c t s the c e l l time constant, since both the l a s e r pulse width (10 ns) and scavenging time constant (< 1 με) are c o n s i d erably l e s s than the s e v e r a l hundred microseconds observed f o r the current s i g n a l . Data taken with the high peak power pulsed l a s e r were much more ambiguous than those obtained with the cw or modulated a r gon ion l a s e r . For example there was a much more s i g n i f i c a n t t r a n s i e n t photorelated current detectable at more negative po t e n t i a l s , even i n the absence of scavenger. This phenomenon i s i l l u s t r a t e d i n Figure 9A. With the a d d i t i o n of scavenger and attenuation of the l a s e r power (accomplished by adding n e u t r a l density f i l t e r s ) * , a n o t i c e a b l e increase i n t r a n s i e n t current at p o t e n t i a l s near ~ -1.6 V and the appearance of t r a n s i e n t photo-related currents at l e s s negative p o t e n t i a l s were observed (Figure 9B). However, unattenuated l a s e r r a d i a t i o n led to a tremendous enhancement i n photo-related current, extending f a r p o s i t i v e of the photoemission threshold (Figure 9C). No n o t i c e able threshold could be observed i n a DC polarogram. However, t h i s i s not s u r p r i s i n g because of the low average power of the pulsed l a s e r (ca. 0.6 mW). The phenomenon represented i n F i g u r e 9C was accompanied by v i s u a l l y observable d i s r u p t i o n (streaming) i n the v i c i n i t y of the mercury drop. This streaming would con tinue b r i e f l y a f t e r the l i g h t was blocked before once again be coming quiescent. This phenomenon could only be observed with l i g h t at 520 nm, the most intense and t i g h t l y focused wavelength p o s s i b l e . A s l i g h t attenuation i n power, e i t h e r by neutal den s i t y f i l t e r s or tuning, eliminated t h i s unusual streaming pheno menon, i n d i c a t i n g a thermal or non-linear o r i g i n . Figure 10 i l l u s t r a t e s the dependence of t r a n s i e n t photo-re l a t e d currents on pulsed l a s e r i n t e n s i t y . The r e s i d u a l back ground current shows an e s s e n t i a l l y l i n e a r dependence on l a s e r i n t e n s i t y whereas the data f o r NaN0 does not. The t o t a l photoemission current at -1.80 V (vs SCE) shows a non-linear behavior, and represents a sum of at l e a s t two components: 1) the background c o n t r i b u t i o n which may r e f l e c t thermal e f f e c t s , 3



PERONE ET AL.


Time

Figure 8. Oscilloscope disphy of AC-coupled photoemission current synchronized to laser pulse, λ = 580 nm, Ε = —1.5 V, 5 mM ΝαΝ0 in 0.1M NaOH, 200 mV/division.

3


NEW

146

APPLICATIONS OF

LASERS TO

CHEMISTRY

i m p u r i t i e s , multiphoton events, and e " ( ) - e (aq) a n n i h i l a t i o n react i o n s ; and 2) the simple scavenging of hydrated e l e c t r o n s by n i t r a t e ions. The photo-related current at -1.20 V (vs SCE) has a nearly l i n e a r but large slope on a l o g - l o g p l o t , i n d i c a t ing a very high but simple non-linear dependence on l a s e r power. Figure 11 i l l u s t r a t e s the dependence of the t r a n s i e n t photoemission current on p o t e n t i a l using N2O scavenger and v a r i ous wavelengths. Once again, data were obtained from boxcar averaged traces such as Figure 9B and a d i f f e r e n t absolute vert i c a l s c a l e was used f o r each wavelength. Several i n t e r e s t i n g points are represented i n t h i s f i g u r e . F i r s t l y , the currents observed at fundamental ( v i s i b l e ) frequencies show a nearly l i n e a r r i s e continuously over a very wide p o t e n t i a l range, as expected f o r photoemission current. Currents of the two uv wavelengths do not, but show a more d i s t i n c t l y saturated e f f e c t w i t h i n h a l f a v o l t from the apparent t h r e s h o l d . Secondly, the s h i f t i n threshold i s c o n s i s t e n t with the s h i f t i n photon energy u n t i l reaching the uv l i n e s . The thresholds f o r 260 nm and 305 nm are not at p o s i t i v e p o t e n t i a l s (vs SCE) as they should be f o r photoemission. This e f f e c t i s a l s o n o t i c e a b l e but l e s s pronounced with the argon ion l a s e r data (Figure 6). Good agreement e x i s t s between the two sets of data ( f o r cw and p u l sed l a s e r sources) when comparing the data f o r v i s i b l e wavelengths; s l i g h t d i f f e r e n c e s may be a t t r i b u t e d to i n t e n s i t y d i f f e r e n c e s and the p r e v i o u s l y mentioned e f f e c t s using the high peak power l a s e r . In general, the enhancement of current over the background observed with the pulsed dye l a s e r was only a f a c t o r of 2 to 7, whereas the enhancement observed with the argon ion l a s e r was i n excess of a f a c t o r of 100.


a q

T r a n s i t i o n metal c a t i o n s . The f o l l o w i n g t r a n s i t i o n metal ions were i n v e s t i g a t e d , u s u a l l y with both the chopped cw argon ion l a s e r and the pulsed dye l a s e r : Mn , F e , Co*, N i , C u , C u , P b , C d and Co(NH3)5^ . The half-wave p o t e n t i a l s (E1/2) f o r the uncomplexed ions i n 0.1 M KCl were -1.45 V, -1.30V, -1.25 V, -1.10 V, -0.20 V, -0.45 V, and -0.60 V, r e s p e c t i v e l y . The two reduct i o n p o t e n t i a l s for C o ( N H ) were -0.35 V, and -1.35 V. Results with Chopped CW Laser Source. The various metal c a t i o n s may be categorized according to whether or not t h e i r reduction wave occurs w e l l before, approximately c o i n c i d e n t with or w e l l a f t e r the expected photoemission t h r e s h o l d . For example, C u , P b and C d r e d u c t i o n p o t e n t i a l s a l l f a l l more p o s i t i v e than the photoemission threshold f o r 514 nm r a d i a t i o n . A l a r g e DC component of the photo-related s i g n a l i s observed. This e f f e c t appeared to be more s i g n i f i c a n t f o r t h i s group of metal cations which were more e a s i l y reduced, since s i m i l a r r e s u l t s were 2+

2 +

2 +

2 +

2 +

2 +

2 +

+

3 +

3

2 +

2 +

6

2 +



Figure 9A. Boxcar-signal-averaged, AC-coupled, synchronously detected photo-related currents using pulsed laser as radiation source. Supporting electrolyte was 0.1 M KCl, λ = 520 nm. No scavenger, no neutral densityfilter,amplified 7.7X.


ι—'

«β:

Co"

s

es

Ci

ο S"

OK Ο

ο

s*

Co

δ δ*

"S-

M

§

NEW

APPLICATIONS

OF

LASERS TO


148


CHEMISTRY

9.

PÉRONÉ ET AL.


149

«•ι | | Downloaded by GEORGETOWN UNIV on August 25, 2015 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0085.ch009

Co 00

S

8

κ

ta

§|

lox 00 .S*

•if* 1

§ Il §

§ SΟ

g

Re lat


8

Δ

Ο

Ο

_

•

• • I

I

-1.2

-1.4

— I

I

-1.6

-1.8

-2

V (vs. SCE)

Figure 11. Dependence of transient photoemission-related cur rent on DME potential and wavelength. Supporting electrolyte was 0.1M KCl, scavenger was N 0. (Φ) 260 nm (4.77 eV); (A) 305 nm (4.06 eV); ({J) 386 nm (3.21 eV); (A) 406 nm (3.05 eV); (O ) 520 nm (2.38 eV); (U) 610 nm (2.03 eV). 2


152

NEW APPLICATIONS OF LASERS TO CHEMISTRY 2 +

2 +

obtained for C u and P b . Because copper i s s e v e r a l orders of magnitude l e s s soluble i n mercury than i s cadmium,(48) i t appears that the fate o f the r e d u c t i o n product i s i r r e l e v a n t . I t i s not c l e a r why the photo-related current s i g n a l i n Figure 12 i s not completely modulated. This may be due to slow p h y s i c a l or chemical steps involved i n the photocurrent process a c t i n g as a dampening f a c t o r . No polarographic maximum was observed f o r C d , but a maximum was observed for C u . A d i l u t e s o l u t i o n o f T r i t o n X-100 was added u n t i l no f u r t h e r reduction i n the maximum was obtained. However, except for a small s h i f t i n the apparent threshold, the a d d i t i o n o f maximum suppressor had no e f f e c t on the photo-related current observed with chopped cw l a s e r r a d i a t i o n (Figure 13). The unique p o t e n t i a l dependence o f photo-re lated current i l l u s t r a t e d i n Figure 13 was g e n e r a l l y observed f o r a l l the d i v a l e n t c a t i o n s o l u t i o n s used here. Invariably the r i s e i n photo-related current corresponded to the r i s e i n polarographic current at Έ>\/2' Figure 14 i l l u s t r a t e s a s i m i l a r phenomenon observed with C0CI2, which has a reduction p o t e n t i a l j u s t s l i g h t l y more ne gative than any expected photoemision. S i m i l a r r e s u l t s were obtained for FeCl2, which a l s o have r e d u c t i o n p o t e n t i a l s appro ximately coincident with the photoemission t h r e s h o l d . Finally, r e s u l t s s i m i l a r to C0CI2 were also obtained for M n , which has a reduction p o t e n t i a l much more negative than the photoemis sion threshold expected for 457.9 nm (ca. -0.9 V, see Figure 6 ) . Q u a l i t a t i v e l y a l l ' the photorelated"~currents observed with chopped cw l a s e r i r r a d i a t i o n for the various metal cations were s i m i l a r , commencing at the onset o f c a t i o n reduction and slowly tapering o f f as the DME p o t e n t i a l became more negative. It is important to note that the onset o f photo-related current i s i n no way c o r r e l a t e d with the photoemission threshold — sometimes commencing e a r l i e r , sometimes l a t e r . Any p o s s i b l e photoemission was obscured by t h i s other phenomenon. However, t h i s photorel a t e d current d i d not have an unusual dependence on l a s e r power i n t e n s i t y . The slope i s nearly 1.0, i n marked contrast to the r e s u l t s obtained with the pulsed l a s e r (Figures 10 and 15). Results with Pulsed Laser Source. Data taken with the pulsed dye l a s e r a l s o provided no d e f i n i t e evidence f o r a d d i t i o n a l photoemision r e l a t e d current beyond that observed with the blank. With the exception o f r a d i a t i o n at 520 nm, the only d i s t i n c t i o n between the blank and the s o l u t i o n c o n t a i n i n g the metal c a t i o n (when t r a n s i e n t photo-related currents were moni tored) corresponded to a d i s c o n t i n u i t y near the r e d u c t i o n poten t i a l wave, mostly seen when the reduction occured i n a region where photocurrents were observed. For example a d i s c o n t i n u i t y was observed with M n (E^/2 ~1·45 V) but not with a C u so l u t i o n (E1/2 0.20 V) when i r r a d i a t i n g with 406 nm. The only exception to t h i s was ΟοίΝΗβ)^" ", which had r e d u c t i o n waves at 2 +


2 +

2+

2+

=

2 +

=

1



PERONE ET AL.


(B)

(C)

Figure 12. Oscilloscope display of DC-coupled, photo related current synchronized to square-wave-modulated argon-ion laser, λ = 514.5 nm, laser output power = 0.5 W, 3 mM CdCl in 0.1M KCl, Ε = -0.7 V (vs. SCE). (A) Light on; (B) light off; (C) ground; lower trace is 1 kHz chopped argon-ion laser output. Curves (A), (B), and (C) were at the same vertical sensitivity and dc offset. 2




•

|

.

|

ι

ι

ι

ι

1

1

1

1

1

J É I If «

ι I

I

I

ι 1

I

I

ι

I

1

I

ι 1

I

I

ι

1

to

1

1

1

ι

1

1

1

1i1J I 0

1

1 -0.2

ι

I -0.4

ι

I -0.6

ι

I -0.8

ι

I -1.0

•

1I -1.2

1• 1I -1.4

V (vs. S C E )

Figure 13. Boxcar-averaged, photo-related current for chopped cw laser. AC-coupled, synchronously detected, laser output power = 2 W, 3 mM CuCl in 0.1M KCl, λ = 514.5 nm, same vertical sensitivity. (A) Without Triton X-100; (B) with Triton X-100. 2



PERONE ET AL.


_ W ,

ι

ι

'

1

'

1

' l '

1

,

ι

•

ι

,

ι

,

-1.2

.

ι

1

1

ι

•

1

ι_

-1.4

V (vs. S C E )

Figure 14. Boxcar-averaged, photo-related current for chopped cw laser, AC-coupled, synchronously detected, 3 mM CoCl in 0.1M KCl. (A) No light, amplified I.55X; (Β) λ = 5.14.5 nm, laser output power = 3W, .77χ. 2


NEW

APPLICATIONS OF

LASERS TO

CHEMISTRY


156

Figure 15A. Rehtive photo-related current (% S/S ) vs. relative pulsed laser intensity (l/l ), λ = 520 nm, 0.1 M KCl was the sup porting electrolyte. 3 mM CoCl , (Φ) Ε = —1.75 V, (Ο) Ε = -1.41 V. 0

0

2




PERONE ET AL.

l/l

0

Figure 15B. Relative photo-related current (% S/S ) vs. relative pulsed laser intensity (1/I ), λ = 520 nm, 0.1M KCl was the sup porting electrolyte. 10 mM CoCl , (%) Ε = -1.8 V, (Ο) Ε = -1.35 V, (Π) Ε « - 0 . 9 V. 0

0

2


NEW

158

APPLICATIONS OF

LASERS TO

CHEMISTRY

-0.35 V and -1.35 V (vs SCE) but i n no way d i f f e r e d from the blank i n i t s t r a n s i e n t response to the l a s e r pulse. Of a l l the metal ions which had reduction waves i n the photocurrent region, Co(NH3)5 alone d i d not e x h i b i t a polarographic maximum. The e x c e p t i o n a l behavior observed with 520 nm r a d i a t i o n included non-linear photocurrent dependence on l a s e r i n t e n s i t y and streaming phenomena ( v i d a supra). F i g u r e 15 represents the dependence of the t r a n s i e n t photo-related current on pulsed l a s e r i n t e n s i t y at 520 nm f o r two d i f f e r e n t s o l u t i o n s . The r e s u l t s are s i m i l a r : a nearly l i n e a r dependence on i n t e n s i t y at more negative p o t e n t i a l s , i n c r e a s i n g to a much higher-order pro cess at less negative p o t e n t i a l s . In the case of C o (Figure 15B) t h i s higher order dependence i s observed even before the reduction wave. I n t e r e s t i n g l y enough t h i s e f f e c t was n e a r l y absent i n the ΟοίΝΗβ)^" " s o l u t i o n . This r e s u l t can probably be a t t r i b u t e d to absorption by the s o l u t i o n , as the e x t i n c t i o n c o e f f i c i e n t at 520 nm i s ~ 20 cnT^M""! and the e f f e c t i s very i n t e n s i t y dependent. 3+


2 +

1

D i s c u s s i o n of Photoemission Studies This study i l l u s t r a t e s the s i g n i f i c a n t e f f e c t of compres sing a given amount of energy i n t o a narrow pulse. Even under the most favorable circumstances, b a r e l y d i s c e r n i b l e photoemis s i o n currents were obtained with the cw argon l a s e r when i t s output power was reduced to the average output power of the pulsed l a s e r (-0.6 mW). At higher average powers the photoemis sion currents seen with the cw argon l a s e r f a r exceeded those seen with the pulsed l a s e r . On the other hand, even at 1.5 W the unusual photorelated currents observed with the pulsed l a s e r were not observed with the cw argon l a s e r . The e f f e c t of p o l a r i z a t i o n on photoemission currents i s s t i l l somewhat ambiguous.(8,18,28) In these studies the argon ion l a s e r was v e r t i c a l l y p o l a r i z e d , but the pulsed l a s e r was e s s e n t i a l l y unpolarized. T u n a b i l i t y i n the pulsed dye l a s e r proved to be a substan t i a l asset. Our r e s u l t s are the f i r s t s e r i e s of current vs po t e n t i a l curves obtained at s e v e r a l narrow bandwidth wavelengths. The absence of a s h i f t i n photoemission threshold p r o p o r t i o n a l to the change i n photon energy i n the uv i s noteworthy (Figure 11). This has been suggested before.(49) and can even be detec ted i n e a r l i e r work.(4,) There are s e v e r a l p o s s i b i l i t i e s which might c o n t r i b u t e to t h i s observation. Two f a c t o r s are a lower concentration of scavenger and a lower concentration of support ing e l e c t r o l y t e i n our work. The former would tend to decrease the observed photoemission current, due to an increased probabi l i t y of the hydrated e l e c t r o n r e t u r n i n g to the e l e c t r o d e . T h i s might lead to a more negative measured t h r e s h o l d . The l a t t e r would tend to increase the mean distance from the e l e c t r o d e where the e l e c t r o n s are s o l v a t e d . The l a r g e r t h i s d i s t a n c e , of


9.

PÉRONÉ ET AL.


course, the harder i t i s f o r e l e c t r o n s to r e t u r n to the e l e c trode and hence the l a r g e r the photo-related c u r r e n t . This e f f e c t would tend to s h i f t the t h r e s h o l d to l e s s negative potent i a l s . (S_ 50) A l s o , a t h i r d f a c t o r to be considered i s that the uv data arose from doubling fundamental l a s e r frequencies; with the accompanying loss i n photon f l u x there would be a decrease i n photoemission current, and hence a more negative measured threshold. Another i n t e r e s t i n g p o i n t , a l s o not completely explained, is the r e l a t i v e l y large background photo-related s i g n a l observed with the pulsed l a s e r . Q u a l i t a t i v e l y our blanks resemble those p r e v i o u s l y obtained using conventional sources ;(6,10) the n u l l point was observed at about -0.6 v o l t s (vs SCE). Some r e s i d u a l photo-related current may have r e s u l t e d from i m p u r i t i e s i n the water or s a l t s used. No p a r t i c u l a r e f f o r t , such as treatment wi th SO3" / uv i r r a d i a t i o n , ( 5 ) was made to exclude t h i s as a p o s s i b l e c o n t r i b u t i o n . However, the r e l a t i v e l y large r e s i d u a l current noted with the pulsed l a s e r , both i n comparison to other workers using conventional r a d i a t i o n sources and our own concurrent work with the argon ion l a s e r , makes i t u n l i k e l y that imp u r i t i e s were p l a y i n g a s i g n i f i c a n t r o l e . I t i s f a r more l i k e l y that e~"(aq) - e""(aq) a n n i h i l a t i o n r e a c t i o n s are s i g n i f i c a n t i n pulsed l a s e r s t u d i e s . The r e l a t i v e l y high f l u x would create a correspondingly l a r g e r l o c a l concentration o f e"(aq). The slope i n Figure 10 i s c e r t a i n l y greater than 1.0, although a slope o f 2.0 would be expected for a s t r i c t l y bimolecular e""(aq) - e~(aq) which accounted f o r a l l o f the photorelated current.(29) Thermal p e r t u r b a t i o n i s undoubtedly r e s p o n s i b l e for a s u b s t a n t i a l p o r t i o n o f the photo-related current observed here.(5,6,8,13) This i s p a r t i c u l a r l y true of the r e s u l t s with the pulsed dye l a s e r , where wavelength c o n s i d e r a t i o n s e l i m i n a t e photoemission as a cause. Furthermore, compared to other l a s e r studies done using a mercury pool electrode;(26,28) one might e a s i l y e n v i s i o n a greater thermal e f f e c t here using the much smaller DME. I t i s even conceivable that the focused pulsed l a s e r led to d i s r u p t i o n o f the mercury or c a t a l y s i s of hydrogen reduction; such a p o s s i b i l i t y has been suggested(27) and i s c o n s i s t e n t with our observations of a streaming from the mercury s u r f a c e . On the other hand, the l i m i t f o r d e s t r u c t i o n of the double l a y e r has been estimated to be i n excess o f 10 MW cm"" , (29) and our power f l u x e s were considerably l e s s than t h a t . Two-photon emission has not been c o n c l u s i v e l y shown.(18,27) Our photon i n t e n s i t y vs current data does not unequivocally suggest a two-photon process to e x p l a i n anomalous observations with 520 nm pulsed l a s e r i r r a d i a t i o n . In f a c t , the observed higher order processes are c o n s i s t e n t with what has been suggested might occur under large thermal excursions.(27) Photo-related currents observed i n the presence o f t r a n s i t i o n metal ions pose d i f f i c u l t questions. Contrary to the conclusion of other workers, (,22 . 24) the observed photo-related currents do 3i


159

2

2



160


not appear to be a s s o c i a t e d p r i m a r i l y with photoemission. A l though t r a n s i e n t photo-related currents are observed on the reduct i o n plateau of each metal i o n ( j u s t as reported e a r l i e r (24))> these s i g n a l s do not d i f f e r appreciably from those observed with blank e l e c t r o l y t e s o l u t i o n . Moreover, the onset of photo-related currents i s t i e d to the onset of polarographic reduction c u r r e n t s , but i s not s p e c i f i c a l l y r e l a t e d to the photoemission p o t e n t i a l (vide supra). Thus, although photoemission and scavenging reactions must be occuring with t a n s i t i o n metal ion s o l u t i o n s , the net e f f e c t on observed currents i s r e l a t i v e l y small compared to the primary phenomenon g i v i n g r i s e to photo-related currents. Our explanation f o r the observed photo-related currents i n duced with the CW l a s e r i n the presence of metal ion reduction i s simply that the chopped l a s e r source perturbs the Nernstian e q u i l i b r i u m which e x i s t s at the e l e c t r o d e surface i n a manner s i m i l a r to that imposed by d i f f e r e n t i a l pulse polarography. This p e r t u r b a t i o n may be thermal i n nature. In any event, the r e s u l t i s the generation of current pulses which go through a maximum near the R\/2> y i e l d i n g a p l o t of photo-related current vs pot e n t i a l which looks very s i m i l a r to a d i f f e r e n t i a l pulse p o l a r o gram (Figures 13,14). As t h i s r e s u l t was h i g h l y dependent on l a s e r power, these observations provide another example of the p o t e n t i a l of l a s e r i r r a d i a t i o n to provide more d e t a i l e d informat i o n about the e l e c t r o d e - s o l u t i o n i n t e r f a c e . In t h i s regard i t i s noteworthy that a d d i t i o n of T r i t o n X-100 d i d not appreciably change the shape or magnitude of the photo-related current, but d i d s h i f t the apparent threshold f o r C u (Figure 15). T r i t o n X-100, being a r e l a t i v e l y large organic molecule, would be expected to have c e r t a i n i n s u l a t i n g p r o p e r t i e s , both e l e c t r o c h e m i c a l l y and from a d i f f u s i o n standpoint.(8.50) However, i t s t i l l i s not easy to c o r r e l a t e t h i s e f f e c t with a molecular model. 2 +

Results and D i s c u s s i o n f o r Laser P h o t o l y s i s Studies Determination of F e ( I I ) Concentration from F l a s h Photoreduction To a c c u r a t e l y determine the F e ( I I ) concentration produced from the photoreduction of F e ( I I I ) oxalate, the measured F e ( I I ) oxidat i o n currents must be corrected f o r faradaic-induced charging current.(31.47) Because raw data corrected by the " D e r i v a t i v e method" (see Experimental) showed C o t t r e l l behavior, the currents were subsequently c o r r e c t e d according to the " c o r r e c t i o n f a c t o r " procedure o u t l i n e d above. C o t t r e l l p l o t s f o r the two currenttime curves are shown i n Figure 16. The c o r r e c t e d data c l e a r l y show more i d e a l C o t t r e l l behavior. In order to c a l c u l a t e C° from the slope of the C o t t r e l l p l o t , the e f f e c t i v e e l e c t r o d e area and the d i f f u s i o n c o e f f i c i e n t of the e l e c t r o - a c t i v e species must be known. The t o t a l area of the working e l e c t r o d e was determined by c o l l e c t i n g ten d r o p l e t s , weighing, and b a c k - c a l c u l a t i n g from the known density of mercury, assuming s p h e r i c a l drops. Since the l i g h t i s c o l l i m a t e d , the e f f e c t i v e working area of the e l e c t r o d e ,



PERONE ET AL.


Figure 16. Cottrell plot of current-time data for Fe(II) oxida tion after laser flash irradiaion of ferrie oxalate solution. E = -0.5 V vs. SCE; RC = 706 psec; λ = 437 nm; b = 1 mm; [Fe(Ox)t] = 0.853 χ 10 U. eff

2


NEW

162

APPLICATIONS OF

L A S E R S TO

CHEMISTRY

or the area i r r a d i a t e d , was assumed to be 50% of the t o t a l area or 1.07 χ lO-^cm^. χ d i f f u s i o n c o e f f i c i e n t of F e ( l l ) was assumed to equal that of F e ( l I I ) , which was experimentally deter mined to be 7.09 χ 10~^ cm /sec by p o t e n t i a l - s t e p chronoamperometry. The observation of d i f f u s i o n - c o n t r o l l e d currents over a time period of about 5 msec i s s i g n i f i c a n t i n view of e a r l i e r F e ( l l l ) oxalate studies conducted i n t h i s laboratory.(38,39) The e a r l i e r studies were based on chronoamperometric data obtained from timedelay p o t e n t i o s t a t i c e l e c t r o l y s i s . Time-delay a n a l y s i s ( 3 8 ) em ploys f a s t e l e c t r o a n a l y t i c a l sampling of the photolyzed s o l u t i o n at v a r i o u s times a f t e r the f l a s h . By v a r y i n g the sampling time f o r consecutive experiments, the time-dependent behavior of the e l e c t r o a c t i v e species can be followed. The F e ( l l ) o x i d a t i o n cur rent was found to change with time, passing through a minimum at about 10 msec and then i n c r e a s i n g i n magnitude at longer times. Present r e s u l t s i n d i c a t e that the amount of o x i d i z a b l e spe c i e s produced by the f l a s h does not change with time. These findings r a i s e v a l i d questions about the p r e v i o u s l y proposed mechanism of the r e a c t i o n . The present data imply that the F e ( l l ) concentration may not be changing with time, and that the f i n a l product concentration i s being measured. Even i f there i s a r e a c t i o n i n v o l v i n g conversion of one o x i d i z a b l e form to another, the net current should be p r o p o r t i o n a l to f i n a l product as long as C o t t r e l l behavior i s observed at a l l times. I d e n t i c a l r e s u l t s were obtained i n a p r e l i m i n a r y study of the F e ( l I I ) oxalate system using a xenon f l a s h lamp e x c i t a t i o n source. F e ( I I ) o x i d a t i o n currents were found to be d i f f u s i o n - c o n t r o l l e d over the e n t i r e time range (.3-250 msec). These r e s u l t s are s i g n i f i c a n t because they r e i n f o r c e the f i n d i n g s of the laser-induced p h o t o l y s i s study and e l i m i n a t e the p o s s i b i l i t y that the " d i s c r e pancy" with the e a r l i e r studies i s a wavelength-dependent pheno menon. An explanation of the c o n f l i c t i n g r e s u l t s i s not obvious. The monitoring techniques are d i f f e r e n t - continuous versus timedelay, and the e a r l i e r workers were not aware of the f a r a d a i c - i n duced charging current c o n t r i b u t i o n s to the t o t a l c u r r e n t . I t i s p o s s i b l e that the apparent changes i n F e ( I I ) c o n c e n t r a t i o n noted e a r l i e r may have been due to o p e n - c i r c u i t d e p l e t i o n of F e ( l l ) due to double-layer charging a f t e r the f l a s h . η θ


2

Concentration of F e ( l l ) vs.

Pathlength

B i r k and Perone(30) have developed the t h e o r e t i c a l r e l a t i o n ship (Equation 2) d e s c r i b i n g the pathlength dependence of the i n i t i a l concentration of intermediate produced by f l a s h photo l y s i s , based on a d e r i v a t i o n s i m i l a r to that by Hercules f o r o p t i c a l flourescence.(51) Because of the complexity of t h i s r e l a t i o n s h i p and the necessary assumptions made i n i t s d e r i v a t i o n , B i r k points out that i t i s more p r a c t i c a l to examine a


9.

PERONE ET

AL.


163

r a t i o of CJj[ measurements made at two path lengths, b and b + Ab. From Equation 2, an expression r e l a t i n g the change i n i n t e r mediate c o n c e n t r a t i o n to the change i n pathlength f o l l o w s

C

R

( b )

= exp(a'AbC°)

( 8 )


C° (b+Ab)

where a" i s the e f f e c t i v e absorption c o e f f i c i e n t . The mono chromatic, c o l l i m a t e d , pulsed l a s e r source makes the v e r i f i c a t i o n of Equation 8 f e a s i b l e for the F e ( I I I ) oxalate system. The concentrations of F e ( l l ) produced by the photoreduction of F e ( l I I ) oxalate at 442 nm and 457 nm are given i n Table I f o r three pathlengths. As the pathlength increases the F e ( l l ) con c e n t r a t i o n decreases as expected. Table I.

Dependence of F e ( l l ) Concentration on Pathlength i n P h o t o l y s i s C e l l All

Data Set

1 2 3

4

concentrations, χ

Pathlength, b(mm)

10 M

CR(b)(\44 ) 2

1 5 10

CR(b)(X45 )

1.58 1.41 1.02

7

.93 .65 .47

Table II compares the data with the t h e o r e t i c a l r e l a t i o n s h i p expressed i n Equation 8. L i t e r a t u r e values(52) of 0:442 and 0:457 were used to c a l c u l a t e the p r e d i c t e d concentration r a t i o s . These values were 0:442 = 13.8, 0:457 9.0 M ^ c m (note: α = 2.3 ε ) . As can be seen there i s considerable d e v i a t i o n from the p r e d i c tions . =

-1


NEW

164 Table I I .

\(nm)

442


CHEMISTRY

Comparison o f F e ( I I ) Concentration Dependence to Theory. C a l c u l a t i o n of

Measured concentration r a t i o s from Table I

Predicted r a t i o using

= = 1.12 1.12 = 1.38 = 1.54

1.05 1.05 1.06 1.11

C1 = 1.43 1.43 C i //CC9 = C / C = 1.38 C1/C3 = 1.98

1.03 1.03 1.04 1.07

d/C C!/C C /C C1/C3 2

2

457

APPLICATIONS OF LASERS TO

9

3

2

2

3

Average a " values:

o^Ccm^M 1)

.33 X 3. 3.33 χ 10 7,.58 X 10 5,.64 X 10

4

4

4

.52 10 10.52 7 .58 8 .93

X χ 10 X 10 X 10

4

4

4

4

5.52

χ 1 0 (\=442nm)

9.01

χ 1 0 (\=457nm)

4

The d e v i a t i o n s from Equation 8 can be explained by two e f f e c t s . F i r s t , the intense l a s e r pulse may cause some heating of the s o l u t i o n . This changes the r e f r a c t i v e index of the l i quid and the l a s e r beam i s dispersed by a "negative l e n s " e f f e c t . (53) This decreases the amount of l i g h t a v a i l a b l e for p h o t o l y s i s at the Hg drop, and, t h e r e f o r e , a smaller concentra t i o n o f F e ( I I ) i s produced than that p r e d i c t e d . This beam d i s p e r s i o n increases with increased pathlength, which explains the more pronounced d e v i a t i o n s i n F e ( I I ) c o n c e n t r a t i o n f o r l a r g e r changes i n b. The second p o s s i b l e reason for poor agreement i n Table I I i s due to a t r a n s i e n t inner f i l t e r e f f e c t ; i . e . , the amount o f a v a i l a b l e i n c i d e n t l i g h t at the mercury drop i s being reduced by a s t r o n g l y absorbing intermediate s p e c i e s . This e f f e c t would a l s o be l a r g e r over longer pathlengths. This l a t t e r e f f e c t i s probably the most s i g n i f i c a n t . From the concentration r a t i o s i n Table I I , i t i s p o s s i b l e to c a l c u l a t e an e f f e c t i v e absorption c o e f f i c i e n t a', which accounts f o r the absorption o f a l l absorbing species produced from the reactant as w e l l as the reactant i t s e l f . I t may a l s o account f o r the "negative l e n s " e f f e c t . Average values o f a' for the two wavelengths are reported i n Table I I . I t i s not s u r p r i s i n g that these absorption c o e f f i c i e n t s are l a r g e r than the experimentally measured a values for F e ( l l l ) , because previous workers have reported a p h o t o l y t i c intermediate which i s more s t r o n g l y absorbing than the s t a r t i n g m a t e r i a l i n the wavelength region o f interest.(40.41) The c a l c u l a t e d a' values w i l l be o f subsequent use i n determining Φ 4 4 2 ^ Φ457 from Equation 2. a n

New Applications of Lasers to Chemistry - American Chemical Society

Recommend Documents