bk-1985-0288.ch021

Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on September 22, 2018 at 08:15:20 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

21 Laser-Induced Thermal Desorption with Fourier Transform Mass Spectrometric Detection M. G. Sherman, J. R. Kingsley, R. T. McIver, Jr., and J. C. Hemminger Department of Chemistry, University of California—Irvine, Irvine, CA 92717

Fourier transform mass spectrometry (FTMS) is utilized to detect molecules thermally desorbed by a pulsed laser from a single crystal surface. Desorbed species are ionized by electron impact and detected in the analyzer cell of the FTMS spectrometer. FTMS has many advantageous features, such as high sensitivity, ultra-high mass resolution, simultaneous detection over a large mass range, and close proximity of the detector with respect to the crystal. The characteristics of the surface temperature jumps and resulting molecular desorption which can be obtained with an excimer laser are described. Laser desorption of CO, C H , C N , CHOH, and C H has been observed from a Pt(s)[7(111) x (100)] surface. In all cases only neutral molecular species are seen to desorb. In the case of benzene, the molecular ion C H is observed even in the absence of electron bombardment ionization. It is likely that the benzene is ionized by resonant multiphoton ionization after the desorption process. 2

4

2

2

3

6

6

+

6

6

Thermal desorption spectroscopy and temperature programmed reaction experiments have provided s i g n i f i c a n t insight into the chemistry of a wide variety of reactions on well characterized surfaces. I n such experiments, characterized, adsorbate covered, surfaces are heated at rates of 10-100 K/sec and molecular species which desorb are monitored by mass spectrometry. T y p i c a l l y , several masses are monitored i n each experiment by computer multiplexing techniques. Often, i n such experiments, the species desorbed are the r e s u l t of a surface reaction during the temperature ramp. The laser desorption experiments which we describe here u t i l i z e pulsed laser r a d i a t i o n , which i s p a r t i a l l y absorbed by the metal substrate, to generate a temperature jump i n the surface region of the sample. The neutral species desorbed are ionized and detected by Fourier transform mass spectrometry (FTMS). This technique has 0097-6156/85/0288-0238$06.00/0 © 1985 American Chemical Society Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

21.

SHERMAN ET AL.

239

Laser-Induced Thermal Desorption

many varied applications. Adsorbate surface d i f f u s i o n can be monitored i f one laser pulse i s used to generate a concentration gradient and subsequent laser pulses are used to interrogate the time evolution of the concentration g r a d i e n t ^ ) . Since the pulsed laser experiments are inherently time resolved, i t i s possible to conceive of monitoring the d e t a i l s of desorption dynamics. Most importantly, f o r surface chemistry studies, laser desorption methods provide a way to observe actual species on the surface and measure reaction k i n e t i c s ( 2 ) . To carry out such experiments, several inherent problems must be addressed s u c c e s s f u l l y . The experiments are inherently pulsed (laser pulses of 10-20 nsec are commonly u t i l i z e d ) . This means that desorbed molecules w i l l s t r i k e the walls of the chamber within a few hundred microseconds a f t e r the laser pulse. Thus i f a scanning mass spectrometer, such as a quadrupole, i s used as a detector as i n references (1-3)> only a single mass can be monitored with each laser shot i f wall c o l l i s i o n s are to be avoided. In a d d i t i o n , t y p i c a l experiments w i l l only r e s u l t i n small numbers of molecules desorbed. The FTMS technique provides an extremely s e n s i t i v e detection method which i s e s p e c i a l l y well suited to pulsed experiments. In t h i s manuscript we w i l l f i r s t describe the c h a r a c t e r i s t i c s of the temperature jumps and the r e s u l t i n g molecular desorption which can be produced by a laser pulse. We then describe how we have implemented FTMS as a detection method i n these experiments and present our r e s u l t s on several adsorbate systems. Laser-Induced Temperature Jumps and Molecular Desorption The laser we use i n these experiments i s an excimer pulse width of approximately 20 nsec. In t h i s time laser heating can be treated using the d i f f e r e n t i a l heat flow with a well defined value f o r the thermal (κ) and the thermal conductivity (K) (Jl).

laser with a regime the equation f o r diffusivity

2

V T(x,y,z,t) - (1/κ) 3T(x,y,z,t)/9t = -A(x,y,z,t)/K

(1 )

where A(x,y,z,t) represents the heat source. The d e t a i l s of the laser time and s p a t i a l p r o f i l e s i n conjunction with the thermal parameters of the metal w i l l determine the c h a r a c t e r i s t i c s of the surface temperature jump. I t i s useful f o r i l l u s t r a t i v e purposes to consider a laser beam with a Gaussian s p a t i a l p r o f i l e and a square pulse time p r o f i l e . I f the laser has a Gaussian s p a t i a l beam p r o f i l e the temperature at the surface of the i r r a d i a t e d s o l i d (z=0) at a time t after the laser pulse i s started i s given by (JO: ο wo fF(t-t») exp{-r /(iJKt'+d )} dt' AT(r,t) = {d mU/W \ 5 (2) J t^'^Kt'+cr) 2

d

2

/d

T 7 3

where r = the l a t e r a l distance from the center of the laser spot, d = the Gaussian beam radius. F(t) = the adsorbed power per unit area at the center of the Gaussian spot.

Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

240

CATALYST CHARACTERIZATION SCIENCE

For the case of a laser pulse having a square time p r o f i l e with width τ and constant power F at the center of the Gaussian beam, the above i n t e g r a l can be evaluated i n closed form. For r » 0 (the center of the laser beam) the temperature increase i s given by: 0

1/2

1

AT(r-O.t) = ( F d / K i r ) t a n ~ { 2 ( K t ) 0

1/2

1

1/2

/d}

for t*x

1/2

(3)

1

1/2

«(F d/^ )[tan" {2(Kt) /d}-tan"" {2[K(t-T)] }] f o r t>x 0

For the s o l i d s of interest to us at present (metals) κ i s t y p i c a l l y 0.1cm /sec. I f we r e s t r i c t t to be less than 10 microseconds then Htct < 10- cm and we w i l l always have Mict « d . In t h i s l i m i t the temperature jump f o r nonzero values of r i s simply: 6

2

2

2

AT(r,t) « T(r«0,t) e x p ( - r / d )

(*0

Figure 1 shows the expected surface temperature r i s e under the l a s e r f o r a n i c k e l sample and 10 MW/cm of adsorbed power. The peak surface temperature under the center of the laser beam i s 990°C above the s t a r t i n g surface temperature. The detailed temporal shape of the surface temperature w i l l depend on the detailed time p r o f i l e of the laser pulse. However, two important c h a r a c t e r i s t i c s of the temperature jump which i s generated w i l l be retained: (1) i t i s l o c a l i z e d i n time, and (2) there i s very l i t t l e l a t e r a l spreading of the temperature gradient (only the material under the laser beam changes temperature s i g n i f i c a n t l y ) . The s p a t i a l l o c a l i z a t i o n of the heating allows us to probe the surface without modifying the areas a few millimeters from the l a s e r . This allows for experiments i n which the time evolution of chemistry on the surface i s followed by sequentially probing d i f f e r e n t areas on the surface. 2

Desorption Rates. Using the above model f o r the temperature jump associated with pulsed laser heating, the rate of desorption versus time and the t o t a l number of molecules desorbed from a f i n i t e surface area heated by the laser can be calculated. For the p a r t i c u l a r case of f i r s t - o r d e r desorption k i n e t i c s , the desorption rate i s : Rate(t) = -de/dt - v6exp{-E/RT(t)}

(5)

f

This leads to: 1η{θ/θ } « -v/dt exp{-E/RT(t»)}. S u b s t i t u t i n g into equation 5 f o r θ gives the following f o r the rate. 0

Rate(t) « ve exp{-v/dt'exp{-E/RT(t')}} exp{-E/RT(t)} 0

where θ - the i n i t i a l surface coverage and Ε i s the a c t i v a t i o n energy f o r desorption. Figure 2 shows a plot of the desorption rate and the integrated number of molecules versus time f o r an a c t i v a t i o n energy of 20 kcal/mole and the temperature jump shown i n Figure 1. For the case of i n i t i a l coverage of 1 0 molecule/cm about 1.5X10 molecules w i l l be desorbed from an area of 1 mm radius by a s i n g l e laser pulse. This corresponds to about h a l f of the molecules o r i g i n a l l y i n t h i s area.

(6)

0

15

13


2

SHERMAN ET AL.

241


1000

600 LU

OC

3

400

α. u 200

-L 100

80

40 60 TIME (nsec)

20

Figure 1. Calculated temperature increase under the center of the laser beam assuming the thermal parameters of bulk n i c k e l and 10MW/cm of absorbed power. 2

0

3

6

9

12

15

18

21

24

27

30

TIME (nsec)

Figure 2. Plot of the desorption r a t e , molecules/sec, ( s o l i d c i r c l e s ) and the integrated number of molecules desorbed ( s o l i d l i n e ) f o r an adsorbate with a desorption a c t i v a t i o n energy of 20Kcal/mole and a preexponential of 1 0 s e c - . The temperature jump shown i n Figure 1 was used f o r t h i s c a l c u l a t i o n . 1 3

1


242


Of p a r t i c u l a r importance i n the a p p l i c a t i o n of laser desorption to the study of chemistry on surfaces i s what i s to be expected for a s i t u a t i o n i n which the adsorbate has two (or more) competing thermally activated channels (eg. desorption versus decomposition). For i l l u s t r a t i v e purposes, consider the case of two competing channels, both of which obey simple f i r s t - o r d e r k i n e t i c s with constant preexponential f a c t o r s and a c t i v a t i o n energies (see f i g u r e 3 a ) . This scheme describes a reaction i n which molecular adsorbate A can either react to form surface species Β or d i r e c t l y desorb to give gas phase A. I f the a c t i v a t i o n energy for desorption i s greater than that f o r reaction (Ε. > Ε ), the reaction channel w i l l predominate i n a conventional TPR experiment with a slow heating r a t e . In contrast, r a p i d l a s e r heating can.cause the branching r a t i o between r e a c t i o n and desorption to be determined by the preexponential f a c t o r s v and v rather than the a c t i v a t i o n energies. This occurs when the temperature increase i s so f a s t that the a c t i v a t i o n energy terms i n the rate expression approach unity before a s i g n i f i c a n t amount of the lower energy reaction has occured. Figure 3b shows the calculated branching r a t i o as a function of heating rate f o r the s p e c i f i c case of ν * 1 0 sec- ; Ε = 20 kcal/mole; v. = l O ^ s e c - ; and E, * 35 kcaï/mole. Notice that under the t y p i c a l heating rates or a thermal desorption experiment (10K/sec) the reaction proceeds predominantly v i a the lower energy channel, whereas at the more rapid heating rates the higher energy channel can dominate. Our model c a l c u l a t i o n s assume that the i n t e r n a l degrees of freedom of the adsorbate are equilibrated with the instantaneous surface temperature. Recent c a l c u l a t i o n s by T u l l y ( 5 ) indicate that i n t e r n a l degrees of freedom of the adsorbate w i l l remain r e l a t i v e l y cold during a fast surface temperature jump. This w i l l lead to an even more pronounced depression of the reaction channel for systems that require thermal a c t i v a t i o n v i a i n t e r n a l degrees of freedom ( i e . v i b r a t i o n s ) of the adsorbate. I t should be mentioned that preexponential factors f o r d i r e c t desorption can be expected to be large f o r many systems because of the increase i n entropy going from an adsorbate with r e s t r i c t e d t r a n s l a t i o n s to a gas phase molecule. I t i s well known that a larger heating rate r e s u l t s i n the desorption occuring at higher temperatures i n a thermal desorption experiment (6). I f i t i s necessary to desorb a large f r a c t i o n of a monolayer to observe a s i g n a l , the peak temperatures required quickly exceed the threshold for surface damage, as the heating rate i s increased. As a consequence, when extremely high heating rates are used, very l i t t l e of the adsorbate (A) undergoes either channel (reaction or desorption). This i s indicated i n figure 3b where i n addition to the channel branching r a t i o we have plotted the f r a c t i o n of o r i g i n a l adsorbate (A) which undergoes either of the two processes. Note that f o r the p a r t i c u l a r parameters used here only *5% of the adsorbate undergoes either process when a heating rate of 5X10 K/sec i s used. The reason behind t h i s phenomena i s that at very high heating r a t e s , the time scale of the temperature jump becomes short compared to reaction times. These considerations lead us to conclude that the high s e n s i t i v i t y of FTMS would be of c r i t i c a l importance for these experiments. p

d

10

1

1

10


21.

S H E R M A N ET

AL.


243

FTMS and Laser Desorption Results The usual method of detecting the desorbed molecules i n TPR and laser desorption i s with a quadrupole mass spectrometer placed a few centimeters from the surface of the c r y s t a l . The use of a quadrupole mass spectrometer l i m i t s the experiment i n several respects. Since the r e s o l u t i o n of a t y p i c a l quadrupole i s quite low, ions of the same nominal mass, such as CO and 0 Η,, , cannot be distinguished. Also, since the quadrupole i s a single-channel, scanning device, only a s i n g l e mass i s detected at a time. As a r e s u l t , l a s e r desorption experiments must be repeated many times to observe a l l the masses of i n t e r e s t . Many of these l i m i t a t i o n s can be a l l e v i a t e d by the use of Fourier transform mass spectrometry (FTMS) (7"13)« In FTMS, ions are stored i n an analyzer c e l l which i s s i t u a t e d between the pole caps of an electromagnet. The ions move i n c i r c u l a r o r b i t s perpendicular to the magnetic f i e l d with a cyclotron frequency ω - qB/m, where m/q i s the mass-to-charge r a t i o , and Β i s the magnetic f i e l d strength. When accelerated by a radio frequency (RF) pulse, the ions are detected by observing the coherent image currents induced i n a pair of receiver p l a t e s . This phenomenon i s i l l u s t r a t e d i n Figure H. I f , for instance, a packet of p o s i t i v e ions moves away from the f i r s t electrode and towards the second, the e l e c t r i c f i e l d of the ions induces electrons i n the external c i r c u i t to flow through the r e s i s t o r and accumulate on the second electrode. During the other h a l f of the cyclotron o r b i t , the electrons leave the second electrode and accumulate on the f i r s t electrode as the p o s i t i v e ions approach. This flow of electrons i n the external c i r c u i t i s c a l l e d an image current. I t i s an a l t e r n a t i n g current that has the same frequency as the cyclotron frequency of the ions that induced i t , and the amplitude of the current i s proportional to the number of ions. Thus, ions of d i f f e r e n t masses, each having a c h a r a c t e r i s t i c cyclotron frequency, create a composite s i g n a l , allowing simultaneous detection of a l l ions i n the analyzer c e l l . Fourier transform analysis of the image current s i g n a l y i e l d s the mass spectrum. During the l a s t year we have b u i l t an FTMS instrument s p e c i f i c a l l y designed f o r laser-induced thermal desorption from s i n g l e - c r y s t a l surfaces. Figure 5 i s a perspective drawing of the instrument. The chamber i s pumped by a 150 1/s ion pump and has a base pressure of 2.0 X 10t o r r . Gases are introduced through sapphire-sealed leak valves from a d i f f u s i o n pumped gas manifold. A Pt(s)C7(111) x (100)] c r y s t a l i s positioned i n front of a hole i n one of the plates of the analyzer c e l l . Ions formed by electron impact are trapped i n the analyzer c e l l and detected by FTMS. An excimer l a s e r , having a pulse width of 20 nsec, i s used to desorb molecules from the Pt c r y s t a l . Figure 6 shows the sequence of events i n a l a s e r desorption FTMS experiment. F i r s t , a focused l a s e r beam traverses the analyzer c e l l and s t r i k e s the c r y s t a l normal to the surface. Molecules desorbed by the thermal spike r a p i d l y move away from the c r y s t a l and are ionized by an electron beam which passes through the c e l l p a r a l l e l to the magnetic f i e l d and 3 cm i n front of the c r y s t a l . The ions are trapped by the combined e f f e c t s of the magnetic f i e l d 2

10



244

8

E 3 5 Kcal/mole d

*6AS

v^lO^sec"

1

*SURFACE * BsuRFACE

k

*v e

r

•E /RT r

E

' *

2

0

K

c

a

1

/

m

o

,

e

v MO 8ec-' ,0

r

r

1.0 Total y Reaction ^ Probability

0.8

\ \

0.6 0.4 Branching Ratio

0.2 0.0

102 1

'TPR

Heating

I0

4

1

HEATING RATE (Ksec" )

Figure 3 . (a) Model reaction scheme showing d i r e c t desorption of adsorbate A competing with a surface reaction of A t o form the surface species B. The k i n e t i c parameters shown are those used to generate the curves, (b) Plot of the calculated branching r a t i o f o r desorption, A(gas)/{A(gas)+B(surface)}, and the t o t a l reaction p r o b a b i l i t y , {A(gas)+ B(surface)} divided by the i n i t i a l amount of adsorbate as a function of heating r a t e . A temperature jump of 1000 Κ and s t a r t i n g temperature of 300K i s used f o r a l l heating rates.

ELECTRODE 2

ELECTRODE 1 \ ions

F i g u r e 4. Ions undergoing coherent c y c l o t r o n motion induce image c u r r e n t s i n t h e p l a t e s o f t h e FTMS a n a l y z e r c e l l . Reproduced with per m i s s i o n from Ref. 18. Copyright 1985, North-Holland P h y s i c s P u b l i s h i n g . Deviney and Gland; Catalyst Characterization Science ACS Symposium Series; American Chemical Society: Washington, DC, 1985.


F i g u r e 5. P e r s p e c t i v e drawing o f t h e l a s e r d e s o r p t i o n FTMS instrument showing the r e l a t i v e p o s i t i o n s o f the c r y s t a l , the l a s e r beam, and the FTMS analyzer c e l l . Reproduced with permission from Ref. 18. Copyright 1985, North-Holland Physics P u b l i s h i n g .


246

ELECTRON

o)TRAPPING PLATE +6.6 V O L T S ^ ^

•Î

f

6

6 V f

DESORBED /MOLECULES

•t

CRYSTAL-^

SUPPORT

\«= LASER BEAM

ff. . . . . . • « Ο VOLTS TRANSMITTER PLATE

Ο VOLTS TRANSMITTER PLATE • 6.6 VOLTS TRAPPING P L A T E

" " T ^ ^ Y ^ ^ ^ ^ ^ O N • 6.6 V

c)

466V J

^-TRAPPED Ç IONS

d)

E

I

Î

φ φ φ φ

-I

Y

ACCELERATED IONS

t

Τ

FORMED B

-20 V

^,

l·

V

RF PULSE

RF PULSE

Ι

bk-1985-0288.ch021

Recommend Documents