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11 A n Apparatus for Obtaining Accurate Transient

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Absorption Spectra on the Picosecond Time Scale R. BRUCE WEISMAN and BENJAMIN I. GREENE Laboratory for Research on the Structure of Matter, Department of Chemistry, University of Pennsylvania, Philadelphia, PA 19104

Steady state absorption spectra of chemical and biological systems have traditionally yielded a wealth of information essential to many types of research. Within the last decade, as time resolved studies have pushed into the picosecond regime, a powerful probe of the ultrafast dynamical behavior of systems has correspondingly proved to be their transient absorption properties. The bulk of these picosecond transient absorption experiments have measured kinetics at a single probe wavelength following optical excitation of the sample. Yet it is increasingly clear that such single wavelength data cannot be properly interpreted without a great deal of auxiliary knowledge concerning the identities and spectra of the states of all components in the system(1).Sincesuch information is obviously rarely available, one must instead attempt to obtain the maximum amount of spectral information from the sample at each time interval — that is, a full transient absorption spectrum — in order to unravel dynamical pathways. The barriers to this approach have been technical in nature. Mode-locked Nd:glass lasers remain a common light source for picosecond spectroscopic studies, but they suffer from poor reproducibility and very low repetition rates. These features combine to make wavelength scanning techniques unsuitable with such lasers. The alternative approach is to employ multichannel optical detection and thereby obtain full spectral coverage with each laser shot. It is also necessary to eliminate the effects of shot-to-shot variations of the laser output. In t h i s p a p e r w e r e p o r t the c o n s t r u c t i o n and u s e o f a d e v i c e w h i c h m e e t s these goals. It i s a d o u b l e - b e a m t r a n s i e n t a b s o r p tion spectrometer b a s e d on a low repetition rate m o d e - l o c k e d 0-8412-0504-3/79/47-102-227$05.00/0 © 1979 American Chemical Society Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

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Ndrglass laser. T h e two c r u c i a l f e a t u r e s of t h i s a p p a r a t u s a r e a n o p t i c a l c o n f i g u r a t i o n d e s i g n e d s p e c i f i c a l l y to o p t i m i z e the s p e c t r o m e t r i c r a n g e and a c c u r a c y , a n d a n a d v a n c e d two d i m e n sional o p t i c a l m u l t i c h a n n e l a n a l y z e r s y s t e m w h i c h a c q u i r e s and p r o c e s s e s two f u l l s p e c t r a l data t r a c k s f o r e a c h l a s e r shot. In the f o l l o w i n g s e c t i o n s we p r e s e n t d e t a i l s of the s y s t e m ' s d e s i g n f o l l o w e d b y e x a m p l e s of i t s h i g h a c c u r a c y a n d w i d e u t i l i t y i n scientific applications. DESCRIPTION OF T H E APPARATUS T h e t i m e r e s o l u t i o n of o u r a p p a r a t u s r e s u l t s f r o m the u s e of p i c o s e c o n d - s c a l e o p t i c a l p u l s e s g e n e r a t e d b y a m o d e - l o c k e d Nd:glass laser system. T h e o s c i l l a t o r stage of t h i s s y s t e m c o n s i s t s of a n O w e n s - I l l i n o i s E D - 2 B r e w s t e r a n g l e d r o d , a 7 m e t e r r a d i u s t o t a l r e f l e c t o r , a n a p e r t u r e to r e s t r i c t the t r a n s v e r s e m o d e s t r u c t u r e to T E M , a type I K D P s e c o n d h a r m o n i c g e n e r a t o r f o r s t a b i l i z a t i o n ^ a n d a flat output c o u p l e r i n contact w i t h a f l o w i n g dye c e l l f o r the E a s t m a n 9860 p a s s i v e m o d e - l o c k i n g solution. P r o p e r m o d e - l o c k i n g i s obtained only within a n a r r o w r a n g e of a l i g n m e n t a n d o p e r a t i n g p a r a m e t e r s . The oscillator t h e n p r o d u c e s a l o w a m p l i t u d e t r a i n of a p p r o x i m a t e l y 70 p u l s e s s p a c e d b y 5ns and h a v i n g 8 p s c h a r a c t e r i s t i c w i d t h , 1. 06 μπι wavelength, and fundamental G a u s s i a n t r a n s v e r s e intensity d i s t r i b u t i o n . We u s e a n e l e c t r o n i c a l l y d r i v e n P o c k e l s c e l l p l a c e d b e t w e e n c r o s s e d p o l a r i z i n g p r i s m s to e x t r a c t one of t h e s e p u l s e s f r o m e a r l y i n the t r a i n f o r u s e i n the e x p e r i m e n t s . T h e e x t r a c t e d p u l s e i s t h e n a m p l i f i e d to a n e n e r g y of 30 m J i n two d o u b l e p a s s e d a m p l i f i e r r o d s . It next p a s s e s t h r o u g h two n o n l i n e a r K D P c r y s t a l s w h i c h g e n e r a t e the s e c o n d h a r m o n i c , at 530 n m , a n d the t h i r d h a r m o n i c , at 353 n m , w i t h h i g h e f f i c i e n c y . T h e l a s e r s y s t e m i s f i r e d at 100 s i n t e r v a l s . We a r e i n t e r e s t e d i n m e a s u r i n g the t r a n s i e n t a b s o r p t i o n s p e c t r a of s a m p l e s at v a r i o u s t i m e s after t h e i r s u d d e n e x c i t a t i o n to h i g h e r e l e c t r o n i c s t a t e s . F o r t h i s p u r p o s e a c o n v e n t i o n a l two pulse excite-and-probe configuration is employed, i n which i n t e r p u l s e j i t t e r i s n e g l i g i b l e b e c a u s e the s a m e s i n g l e a m p l i f i e d l a s e r p u l s e i s the s o u r c e f o r b o t h of t h o s e u s e d i n the m e a s u r e ment. F i g u r e 1 s c h e m a t i c a l l y depicts our o p t i c a l configuration. A d i c h r o i c b e a m s p l i t t e r r e f l e c t s the u l t r a v i o l e t t h i r d h a r m o n i c p u l s e , u s e d f o r e x c i t a t i o n , w h i l e t r a n s m i t t i n g the f i r s t a n d s e c o n d h a r m o n i c s . F i l t e r s a r e p l a c e d i n the e x c i t a t i o n b e a m to e H m i n a t e v i s i b l e l i g h t l e a k a g e a n d to attenuate the u l t r a v i o l e t b e a m to a n a p p r o p r i a t e e n e r g y , w h i c h c o r r e s p o n d s t y p i c a l l y to

Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979.

Talmi; Multichannel Image Detectors ACS Symposium Series; American Chemical Society: Washington, DC, 1979. 0

Figure 1. Schematic diagram of the apparatus. The laser system which produces a 1.06 μm ultra short pulse and its 353 nm third harmonic is not shown. F denotes filter; L, lens; CC, continuum generation cell; D, diffusing plate; CL, cylindrical lens; S, spectrograph slit; G, concave grating; FP, focal pfone; DC, detector controller. 1 and 1 label the continuum beams passing through unexcited and excited sample volumes.

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250 \iJ at the s a m p l e . A f t e r the t h i r d h a r m o n i c h a s b e e n filtered it p a s s e s through a fixed optical delay line and a lens w h i c h f o c u s e s i t i n t o the 1 m m p a t h l e n g t h s a m p l e c e l l t h r o u g h a 3 2 0 μ ι η d i a m e t e r a p e r t u r e i n contact w i t h the c e l l ' s f r o n t w i n d o w . The probe b e a m for our w o r k i s a broadband p i c o s e c o n d d u r a t i o n c o n t i n u u m f r o m w h i c h we m e a s u r e i n d u c e d a b s o r p t i o n s . A s found b y A l f a n o and S h a p i r o J , s u c h a c o n t i n u u m m a y be g e n e r a t e d b y f o c u s i n g a p o w e r f u l l a s e r p u l s e into a s u i t a b l e l i q u i d medium. We take the 1. 06 μπι f i r s t h a r m o n i c t r a n s m i t t e d t h r o u g h the d i c h r o i c b e a m s p l i t t e r , f i l t e r i t to r e m o v e s e c o n d h a r m o n i c l i g h t , a n d f o c u s i t i n t o a 5 c m c e l l of p h o s p h o r i c a c i d . The e m e r g i n g continuum light is spatially f i l t e r e d b y t r a n s m i s s i o n t h r o u g h g r o u n d g l a s s s c a t t e r p l a t e s and s e p a r a t e d f r o m r e s i d u a l 1. 06 μπι l i g h t w i t h a n i n f r a r e d a b s o r b i n g c o l o r f i l t e r . After r e c o l l i m a t i o n it proceeds through a reflective delay line w h i c h c a n be a d j u s t e d to p r o v i d e f o r p r o b e p u l s e a r r i v a l t i m e s of a p p r o x i m a t e l y - 1 0 0 p s t o +700 p s r e l a t i v e to the e x c i t a t i o n pulse. B e c a u s e the p i c o s e c o n d c o n t i n u u m i s g e n e r a t e d t h r o u g h a h i g h l y n o n l i n e a r p r o c e s s , i t s d e t a i l e d s p a t i a l , s p e c t r a l , and i n t e n s i t y c h a r a c t e r i s t i c s v a r y f r o m shot to shot m o r e s e v e r e l y t h a n do the l a s e r p u l s e s u s e d to g e n e r a t e i t . In o r d e r to a c h i e v e a h i g h d e g r e e of r e l i a b i l i t y i n o u r s p e c t r a l m e a s u r e m e n t s , i t i s t h e r e f o r e n e c e s s a r y to o b t a i n double b e a m s p e c t r a i n w h i c h the d a t a a r e c o r r e c t e d f o r c o n t i n u u m f l u c t u a t i o n s f o r e v e r y shot. T h i s i s a c c o m p l i s h e d b y s p l i t t i n g the c o n t i n u u m into two p a r a l l e l b e a m s d i s p l a c e d f r o m one a n o t h e r b y s e v e r a l m i l l i m e t e r s . B o t h a r e f o c u s e d i n t o the s a m p l e c e l l t h r o u g h a d i c h r o i c m i r r o r u s e d to c o l l i n e a r l y r e c o m b i n e the u l t r a v i o l e t e x c i t a t i o n p u l s e w i t h the m a i n continuum beam. In the s a m p l e , t h e n , that c o n t i n u u m b e a m p a s s e s t h r o u g h the e x c i t e d v o l u m e w h i l e the r e p l i c a c o n t i n u u m b e a m t r a v e r s e s a n u n e x c i t e d r e g i o n . A c o m p a r i s o n of the two resulting spectral intensity distributions allows a determination of the d e s i r e d i n d u c e d a b s o r b a n c e s p e c t r u m . T o a c q u i r e t h i s i n f o r m a t i o n , the two d i s p l a c e d c o n t i n u u m b e a m s a r e i m a g e d w i t h a c y l i n d r i c a l and a s p h e r i c a l l e n s onto d i f f e r e n t p o s i t i o n s a l o n g the l e n g t h of the e n t r a n c e s l i t of a low d i s p e r s i o n s p e c t r o g r a p h (Instruments S A , m o d e l U F S - 2 0 0 ) . T h e two r e s u l t i n g p a r a l l e l d i s p e r s e d s p e c t r a a r e f u l l y s e p a r a t e d f r o m e a c h o t h e r at the f o c a l p l a n e , w h e r e t h e y a r e d e t e c t e d b y the m o d e l 1254 SIT d e t e c t o r h e a d of a n E G + G P r i n c e t o n A p p l i e d R e s e a r c h C o r p o r a t i o n o p t i c a l m u l t i c h a n n e l a n a l y z e r s y s t e m . In c o n j u n c t i o n w i t h a m o d e l 1216 d e t e c t o r c o n t r o l l e r and m o d e l 1215 c o n s o l e , t h i s d e t e c t o r i s p r o g r a m m e d w i t h a two d i m e n s i o n a l


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s c a n p a t t e r n c o n s i s t i n g of two p a r a l l e l 5 0 0 - c h a n n e l t r a c k s w i t h p e r p e n d i c u l a r w i d t h s a n d p o s i t i o n s a d j u s t e d to i n c l u d e o n l y the l o c a t i o n s of the d i s p e r s e d c o n t i n u u m b e a m s . E a c h channel c o r r e s p o n d s to a s p e c t r a l w i d t h of 0. 6 n m . T h e s p e c t r a l r e s o l u t i o n i s a p p r o x i m a t e l y 6 n m ( d e t e r m i n e d b y the s l i t w i d t h of the s p e c t r o g r a p h ) a n d the r a n g e , 300 n m . F o r e a c h l a s e r shot, the two r a w s p e c t r a a r e d i g i t a l l y s t o r e d i n the m e m o r y of the c o n s o l e a n d p r o c e s s e d c h a n n e l - b y c h a n n e l i n the f o l l o w i n g p r o g r a m m e d s e q u e n c e . F i r s t , the c o n t r i b u t i o n of d a r k c u r r e n t a n d f l a s h l a m p b a c k g r o u n d l i g h t i s s u b t r a c t e d f r o m b o t h t r a c k s . T h e r e s u l t i n g net 1^ s p e c t r u m i s t h e n d i v i d e d b y the net I s p e c t r u m a n d the l o g a r i t h m of the r a t i o s p e c t r u m c a l c u l a t e d . T h i s r e s u l t i s h e l d f o r future u s e . O n the next l a s e r shot, the u l t r a v i o l e t e x c i t a t i o n b e a m i s b l o c k e d but the d a t a a r e a c q u i r e d and p r o c e s s e d e x a c t l y a s d e s c r i b e d a b o v e . T h e n the d i f f e r e n c e i n the two l o g a r i t h m i c s p e c t r a i s c o m p u t e d , t h e r e b y g i v i n g the f u l l d o u b l e - b e a m t r a n s i e n t a b s o r b a n c e s p e c t r u m at the t i m e d e l a y c o r r e s p o n d i n g to the s e t t i n g of the v a r i able d e l a y l i n e . T o i m p r o v e the s i g n a l - t o - n o i s e r a t i o a n d a c c u r a c y of the s p e c t r u m , t h i s d a t a c o l l e c t i o n c y c l e i s r e p e a t e d s e v e r a l t i m e s and the r e s u l t i n g s p e c t r a a r e n o r m a l i z e d to the relative excitation energies and then averaged together. The S a v i t z k y - G o l a y s m o o t h i n g r o u t i n e s u p p l i e d i n the O M A s o f t w a r e p r o v i d e s some additional noise reduction. A l t e r n a t i o n of e x c i t e d a n d u n e x c i t e d data i s i m p o r t a n t i n e l i m i n a t i n g e r r o r s c a u s e d b y slow optical alignment d r i f t s . Another key factor i n a c h i e v i n g r e l i a b l e r e s u l t s i s the c a r e f u l m o n i t o r i n g of l a s e r p u l s e t r a i n s , w i t h r e j e c t i o n of a l l d a t a f r o m shots i n w h i c h the p u l s e t r a i n s f a i l to m e e t s t r i n g e n t q u a l i t y c r i t e r i a . A n important test of a transient s p e c t r o m e t e r ' s a c c u r a c y , s t a b i l i t y , and n o i s e l e v e l i s p r o v i d e d b y a b a s e l i n e s p e c t r u m . T h i s i s a n a v e r a g e d s p e c t r u m o b t a i n e d i n e x a c t l y the s a m e w a y a s a c t u a l d a t a , e x c e p t that the u l t r a v i o l e t e x c i t a t i o n b e a m i s kept b l o c k e d when it would otherwise be open. T h e lowest t r a c e shown i n F i g u r e 2 i s a t y p i c a l 5 - c y c l e b a s e l i n e s p e c t r u m f o r o u r system. S y s t e m a t i c d e v i a t i o n f r o m z e r o i s l e s s t h a n 0. 01 a b s o r b a n c e u n i t s t h r o u g h o u t , a n d the r . m . s. n o i s e l e v e l v a r i e s f r o m 0. 03 n e a r the e d g e s to 0. 007 n e a r the c e n t e r of the s p e c trum. T h e s e n o i s e v a r i a t i o n s a r e i n v e r s e l y r e l a t e d to the d e t e c t e d s i n g l e b e a m i n t e n s i t y s p e c t r u m , w h i c h d r o p s o n the b l u e side b e c a u s e of the c o n t i n u u m d i s t r i b u t i o n a n d the t r a n s m i s s i v e p r o p e r t i e s of o u r b e a m c o m b i n e r , a n d o n the r e d s i d e b e c a u s e of the photocathode r e s p o n s e of the SIT d e t e c t o r h e a d . A h i g h d e g r e e of i n t e n s i t y l i n e a r i t y i n the O M A i s n e c e s s a r y f o r o u r


M U L T I C H A N N E L IMAGE DETECTORS

τ

'

1

'

1

Benzophenone / EtOH

400

500

600

λ

700

(nm)

Figure 2. Transient absorbance and baseline spectra for benzophenone in ethanol solution. The lowest trace is a baseline obtained with the 353 nm excitation pulse blocked. The dotted line spectrum was taken 10 ps after sample excitation; the solid line spectrum was taken at a delay of 22 ps. These two transient spectra are drawn normalized to the same peak height to facilitate bandshape compari sons.


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a p p l i c a t i o n . T h i s i s a s s u r e d b y u s i n g m u l t i p l e r e a d o u t s c a n s to s u p p r e s s t a r g e t l a g effects a n d b y a v o i d i n g l i g h t l e v e l s l a r g e enough to c a u s e l o c a l d e t e c t o r s a t u r a t i o n . W i t h t h e s e p r e c a u t i o n s e v e n l a r g e v a r i a t i o n s i n the s i n g l e b e a m s p e c t r a a r e found to n o r m a l i z e a c c u r a t e l y . O u r t e c h n i q u e s a r e of c o u r s e s u i t a b l e f o r the study of i n d u c e d t r a n s m i s s i o n (bleaching) a s w e l l as i n duced absorption effects. F o r l a r g e t r a n s i e n t s i g n a l s i t i s n e c e s s a r y to c o n s i d e r p o s s i b l e d i s t o r t i o n s of the o b s e r v e d s p e c t r a that r e s u l t f r o m b e a m g e o m e t r i c a l effects. These spectral distortions are c a u s e d b y s p a t i a l l y n o n - u n i f o r m c o n c e n t r a t i o n s of the e x c i t e d s p e c i e s a c r o s s the p r o b e d a p e r t u r e , r e f l e c t i n g the t r a n s v e r s e i n t e n s i t y d i s t r i b u t i o n of the e x c i t a t i o n b e a m . T h e effect t e n d s to r e d u c e the a p p a r e n t r e l a t i v e s i z e of i n d u c e d a b s o r p t i o n p e a k s and to m a g n i f y i n d u c e d t r a n s m i s s i o n p e a k s . When l a s e r s with m u l timode t r a n s v e r s e structures are used i n transient absorption s p e c t r o s c o p y , i t i s i m p o s s i b l e to p r e d i c t the extent of s p e c t r o m e t r i c e r r o r s c a u s e d b y the b e a m s e x t r e m e a n d i r r e p r o d u c i b l e spatial intensity v a r i a t i o n s . B y contrast, our system employs a n e x c i t a t i o n b e a m w i t h a s m o o t h G a u s s i a n i n t e n s i t y p r o f i l e of known size and a c o l l i n e a r p r o b i n g b e a m w h i c h i s e s s e n t i a l l y u n i f o r m a c r o s s the s a m p l e a p e r t u r e . We have n u m e r i c a l l y s o l v e d the e q u a t i o n that p r e d i c t s the d e g r e e to w h i c h the r e s u l t ing r a d i a l d i s t r i b u t i o n of excited states causes s p e c t r o m e t r i c n o n l i n e a r i t i e s i n the o b s e r v e d s p e c t r a a n d have found that the d i s t o r t i o n s a r e not s i g n i f i c a n t f o r p e a k i n d u c e d a b s o r b a n c e s of c a . 0. 5 o r l e s s i n o u r e x p e r i m e n t a l c o n f i g u r a t i o n . Normally our spectral peaks fall within this l i m i t . 1

4

A n o t h e r p o s s i b l e s o u r c e of s p e c t r o m e t r i c e r r o r i n s t r o n g i n d u c e d a b s o r p t i o n s i s l e a k a g e o f l i g h t f r o m one b e a m into the o t h e r o r b e t w e e n w a v e l e n g t h r e g i o n s w i t h i n one b e a m . B y b l o c k i n g e i t h e r of the c o n t i n u u m p a t h s w i t h a n opaque o b j e c t at the s a m p l e c e l l , we f i n d l e s s t h a n 1% l e a k a g e i n t o the other b e a m . S c a t t e r w i t h i n a t r a c k i s a l s o low b e c a u s e of the h o l o g r a p h i c g r a t i n g of o u r s p e c t r o g r a p h . T h u s t h e s e effects i n t r o d u c e n e g l i g i b l e e r r o r s into o u r r e s u l t s . T h e t i m e r e s o l u t i o n of the s y s t e m i s d e t e r m i n e d p r i m a r i l y b y the c o n v o l u t i o n of the e x c i t a t i o n and p r o b i n g p u l s e w i d t h s . T h i s p a r a m e t e r w a s found to be a p p r o x i m a t e l y 10 p s b y a d e t e r m i n a t i o n of the a p p a r e n t g r o w t h k i n e t i c s of a p r o m p t l y r i s i n g e x c i t e d s i n g l e t a b s o r p t i o n , a m e a s u r e m e n t w h i c h a l s o s p e c i f i e d the d e l a y l i n e setting f o r w h i c h the two p u l s e s a r e s y n c h r o n i z e d . B e c a u s e of g r o u p v e l o c i t y d i s p e r s i o n i n t r a n s m i s s i v e o p t i c s , v a r i o u s f r e q u e n c y c o m p o n e n t s o f the p r o b i n g c o n t i n u u m w i l l s u f f e r d i f f e r i n g


234

M U L T I C H A N N E L I M A G E DETECTORS

d e l a y s a n d t h u s have d i f f e r e n t a r r i v a l t i m e s at the s a m p l e p o s i t i o n . T h i s effect i s k n o w n a s " c h i r p " a n d f o r o u r c o n f i g u r a t i o n d e l a y s the b l u e edge f r o m the r e d edge b y about 6 p s . T h e s p e c t r a l d i s t o r t i o n c a u s e d b y c h i r p i s g r e a t e s t w h e n the t r a n s i e n t s p e c t r u m c h a n g e s r a p i d l y , a s often h a p p e n s n e a r t = 0. A l though we have not c o r r e c t e d f o r t h i s d i s t o r t i o n , we do t r y to m i n i m i z e i t s s e v e r i t y b y a v o i d i n g v e r y e a r l y d e l a y settings f o r spectra. O f c o u r s e , the a p p a r a t u s m a y be u s e d f o r s i n g l e w a v e l e n g t h k i n e t i c s t u d i e s w i t h no s u c h r e s t r i c t i o n . APPLICATIONS T o i l l u s t r a t e the q u a l i t y of d a t a o b t a i n e d w i t h t h i s a p p a r a t u s , we p r e s e n t i n F i g u r e 2 t r a n s i e n t s p e c t r a of b e n z o p h e n o n e i n e t h a n o l s o l u t i o n m e a s u r e d at two d i f f e r e n t d e l a y t i m e s f o l l o w i n g e x c i t a t i o n at 353 n m . T h e dotted l i n e r e p r e s e n t s the s p e c t r u m o b t a i n e d at 10 p s d e l a y w h i l e the s o l i d l i n e shows the s p e c t r u m at 22 p s n o r m a l i z e d to the s a m e p e a k h e i g h t . It i s c l e a r that the benzophenone s p e c t r u m n a r r o w s s l i g h t l y a n d shifts to the b l u e o v e r t h i s t i m e i n t e r v a l , a n effect w h i c h i s h i g h l y s i g n i f i c a n t i n r e l a t i o n to the n a t u r e of e x c i t e d state r e l a xation p r o c e s s e s i n this molecular system. Y e t the s p e c t r a l c h a n g e s i n v o l v e d a r e subtle a n d w o u l d s u r e l y have r e m a i n e d u n d e t e c t e d i f not f o r the h i g h l e v e l of a c c u r a c y a f f o r d e d b y t h i s apparatus. T h e r e a r e a l s o other c a s e s f o r w h i c h t h i s q u a n t i t a t i v e i m p r o v e m e n t i n the q u a l i t y o f p i c o s e c o n d t r a n s i e n t d a t a h a s l e d to q u a l i t a t i v e l y d i f f e r e n t s c i e n t i f i c c o n c l u s i o n s . S o m e of t h e s e w i l l be p r e s e n t e d , a l o n g w i t h a d e t a i l e d i n t e r p r e t a t i o n of the above benzophenone r e s u l t s , i n a f u t u r e p u b l i c a t i o n , k In s u m m a r y , we h a v e c o m b i n e d state of the a r t o p t i c a l m u l t i c h a n n e l a n a l y z e r t e c h n i q u e s w i t h w e l l e s t a b l i s h e d low r e p e t i t i o n r a t e p i c o s e c o n d l a s e r t e c h n o l o g y to c o n s t r u c t a n i n s t r u m e n t c a p a b l e of m e a s u r i n g t r a n s i e n t s p e c t r a w i t h u n p r e c e d e n t e d r e l i a b i l i t y . It i s , i n i t s p r e s e n t f o r m , a p o w e r f u l t o o l f o r the i n v e s t i g a t i o n of u l t r a f a s t p r o c e s s e s i n b i o l o g i c a l , c h e m i c a l , a n d p h y s i c a l s y s t e m s . We f o r e s e e s t r a i g h t f o r w a r d e x t e n s i o n of the t e c h n i q u e to the u s e of f o u r t h h a r m o n i c e x c i t a t i o n (at 265 nm) and a l s o a f u t u r e c a p a b i l i t y to study g a s e o u s as w e l l a s c o n d e n sed phase s a m p l e s over a m o r e extended s p e c t r a l range.


11.

WEISMAN AND GREENE


235

ACKNOWLEDGEMENTS We w i s h t o thank P r o f . R o b i n H o c h s t r a s s e r , i n w h o s e l a b oratory this w o r k was p e r f o r m e d , for advice and support. F u n d i n g w a s p r o v i d e d b y a g r a n t b y the N a t i o n a l S c i e n c e F o u n d a t i o n ( C H E 7 6 - 8 4 4 2 8 ) a n d i n p a r t b y the M R L P r o g r a m u n d e r G r a n t N o . DMR76-80994.

LITERATURE CITED 1. 2. 3. 4.

Greene, Β. I., Hochstrasser, R. Μ . , and Weisman, R. B., in Picosecond Phenomena, Shank, C . V . , Ippen, Ε. P . , Shapiro, S. L., Eds., Springer-Verlag, Berlin, 1978. Weisman, R. B. and Rice, S. Α . , Spectroscopy Lett. (1975) 8, 329. Alfano, R. R. and Shapiro, S. L., Phys. Rev. Lett. (1970) 24, 584. Greene, B. I., Hochstrasser, R. M. and Weisman, R. Β . , J. Chem. Phys., in press.

RECEIVED

January 10, 1979.


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