Report
02ΡΡΜ Mhos ^, ^Λ Ohms
«-^
Langley ^ Dissolved Oxygen T,, etgs/cm2-sec Vapor 'g Pressure s D
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1 % . Since this light is coherent and nearly parallel, it m a y be focused to an area of dimensions approaching the wavelength of light. Although use is m a d e of this p r o p e r t y of t h e laser o u t p u t in applications such as welding, t h e erratic behavior of this pulsed laser gives it limited use in problems of interest t o chemists. One c a n p a r t i a l l y overcome this dif ficulty by Q-switching. The Q-Switched Laser
If a s h u t t e r is placed inside t h e laser cavity a n d k e p t closed during the initial portion of t h e p u m p i n g pulse, t h e excited-state population in t h e laser rod will n o t be depleted by t h e lasing process which would n o r m a l l y occur, a n d hence will build u p t o a level greatly in excess of t h a t a t t a i n e d without such a shut ter. If, at t h e p e a k of this build-up of excited states, t h e shutter is sud denly opened, t h e energy stored in t h e laser rod will be emitted v e r y rapidly in a single, " g i a n t " pulse (2). A cell containing a solution of a photo-bleachable dye is often used as t h e shutter, or Q-switch. T o function in this capacity t h e d y e m u s t h a v e an absorption b a n d which overlaps t h e laser emission band. I n this case, during t h e ini tial stages of p u m p i n g all emission from t h e laser rod will pass t h r o u g h the Q-switch cell a n d will be a t t e n u a t e d owing to the absorption of t h e dye. T h e d y e concentration a n d t h e p u m p i n g power are adjusted so t h a t t h e initial loss in t h e Q-switch off sets t h e gain in t h e rod ; a t this point the " s h u t t e r " is closed. However, as t h e excited-state population grows with continued pumping, t h e gain increases exponentially a n d eventually a condition of net a m p l i fication is reached. A t this point, the power of t h e light pulse s t a r t s t o grow r a p i d l y a n d becomes intense enough t o bleach t h e Q-switch dye transition, opening t h e " s h u t t e r . " T h e energy stored in t h e rod is t h e n emitted over a period of typically 20—30 nsec duration. T h e t o t a l en ergy o u t p u t is reduced somewhat from t h a t obtained in n o n - Q switched operation, b u t since t h e pulse d u r a t i o n is shortened, t h e power is increased. T h e high power (tens t o h u n d r e d s of m e g a w a t t s ) and short d u r a t i o n of t h e light out p u t from Q-switched lasers h a v e
Circle No. 179 on Readers' Service Card 22 A
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ANALYTICAL CHEMISTRY, VOL. 4 2 , N O . 14, DECEMBER 1 9 7 0
m a d e it possible t o study a v a r i e t y of n e w phenomena. T h e high intensity h a s led, for example, to t h e observation of stim ulated R a m a n scattering ( 3 ) , h a r monic generation ( 4 ) , and t w o - p h o ton absorption (δ). T h e latter ef fect h a s been detected by t h e a p p e a r a n c e of fluorescence—e.g., in a n t h r a c e n e (6)—or b y chemical r e action of molecules t r a n s p a r e n t a t t h e laser wavelength [polymeriza tion of styrene ( 7 ) , photodissocia tion of chlorine (8) ]. T h e light flux available from Q-switched lasers can produce sufficient chemi cal reaction in molecular beams t o allow detection of photodecomposition products and use h a s been m a d e of laser p u m p i n g in an ele g a n t series of molecular beam ex periments (9). Upon irradiation by Q-switched lasers, certain crystals generate o p tical harmonics from t h e laser fun d a m e n t a l frequency (4) ; with such " n o n l i n e a r " crystals one can obtain a 1 0 % energy conversion of t h e laser frequency t o its second h a r monic. F o r r u b y (14,400 cm-"1) t h e second harmonic is at 28,800 c m ' 1 ; for N d 3 + : glass (9431 cm- 1 ) t h e second harmonic is a t 18,863 cm" 1 , and t h e fourth harmonic (which can be obtained b y frequency 7 doubling the second harmonic) is at 37,726 cm- 1 . These energies are sufficient ly high to enable one t o excite di rectly by one-photon absorption most of the larger organic molecules. T h e absorption spectra of excited singlets and other short-lived inter mediates of a n u m b e r of molecules h a v e been observed with t h e use of Q-switched pulses a n d second h a r monic generation (10, 11). T h e Mode-Locked Laser
W h i l e t h e use of Q-switched la sers as an intense light source of nanosecond d u r a t i o n h a s led t o much interesting research, this t i m e scale is too long for detection a n d m e a s u r e m e n t of t h e most funda m e n t a l molecular processes of inter est to chemists. These are t h e processes occurring in a molecule v e r y soon (within 10- 10 to ΙΟ - 1 3 sec) after absorption of a photon. A technique, termed mode-locking, has m a d e it possible t o study these very rapid processes directly. T h e laser will support s i m u l t a n e -
