Picosecond Spectroscopy

The generation

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GENERATION of ultrashort pulses of coherent light has pro­ vided a means of studying directly molecular processes which occur on a previously inaccessible time scale. Before describing the progress made to date in areas such as vibrational relaxation in large molecules, intersystem crossing, and energy transfer an outline of the operation of pulsed solid-state lasers would be useful. A typical solid-state laser (sketched in Figure 1) consists of an active oscillator medium—e.g., a Nd 3 +-doped glass or ruby (Cr s + doped Al 2 0 3 )rod—situated between parallel mirrors. One mirror is usually of high reflectance at the laser line (R « 1.0), while the other is partially transmitting (R = 0.Ι­ Ο.5). The arrangement of mirrors

THE

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chemists are

Picosecond Spectroscopy

si r .i-'

v„. =

where L is the separation of the mirrors, c is the speed of light, and η is an integer (~10 6 for ruby or Nd 8 +:glass). Adjacent modes are separated from each other by a fre­ quency, Av, given by Av

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C, capacitor; L, flash

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and rod described constitute one form of Fabry-Perot interferometer, the theory of which is treated in ele­ mentary texts (J). Standing light waves may bo set up within the cavity at a series of discrete optical frequencies. These frequencies, known as Fabry-Perot modes, are those for which the following rela­ tion holds :

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Figure l b . A laser cavity for mode-locked operation; the laser rod is cut at the Brewster's angle to eliminate unwanted reflections from the faces of the rod

20 A · ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

REPORT FOR ANALYTICAL. CHEMISTS

P. M . R E N T Z E P i S

C. J. M I T S C H E L E

Bell Telephone Laboratories, Inc., Murray H i l l , N . J . 0 7 9 7 4

-LASER SPECTRAL ENVELOPE

^-CAVITY ί RESONANCE k MODES

FREQUENCY

Figure 2. The relationship between the spectral band of the laser transition and the Fabry-Perot frequencies f o r the laser cavity

7

PUMPING

For an actual laser interferometer, the allowed modes have a finite spectral bandwidth, Sv, which is de­ pendent on the reflectivity and flat­ ness of the mirrors, and the effective aperture of the laser rod. The rela­ tionships are illustrated in Figure 2. If the laser rod is optically pumped by a high intensity, broad­ band flashlamp, a population inver­ sion between the lasing levels of the active ion (Nd 3 + or Cr 3 +) will re­ sult (Figure 3). In this situation, spontaneous emission from the ex­ cited ions will gain in intensity by stimulating emission as it passes through the laser material. If the resulting light is collinear with the optic axis and of a frequency fall­ ing within the bandwidth of one of the Fabry-Perot modes, it will bo

LEVELS

reflected back and forth through the rod and amplified by stimulating further emission each time it passes through the pumped laser rod. Since stimulated emission is co­ herent and propagates in the same direction as the stimulating light, ultimately the predominant output of the laser through the partially reflecting mirror is intense, coherent radiation of low divergence. This output consists of a series of random spikes with variable duration (mi­ croseconds or less), commencing a short time after initiation of optical pumping. Typically, a total of ~ 1 0 1 0 photons (~10 J) of red or infrared light will be emitted. A few thousand joules are discharged through the flashlamp ; thus the con­ version efficiency is of the order of

PUMPING

LEVELS

RADIATIONLESS TRANSITION TO LASING LEVEL

PUMPING LIGHT

LASER TRANSITION

GROUND T H R E E - L E V E L SYSTEM (RUBY)

PUMPING LIGHT

LASER TRANSITION

RADIATIONLESS TRANSITIONS

Iz

STATE

FOUR-LEVEL SYSTEM (NEODYMIUM)

Figure 3. The processes involved in three- and four-level laser systems ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

·

21 A

Report

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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 can p a r t i a l l y overcome this difficulty by Q-switching. T h e 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 shutter. If, at t h e p e a k of this build-up of excited states, t h e shutter is suddenly opened, t h e energy stored in t h e laser rod will be emitted v e r y r a p i d l y 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 initial 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 t o t h e 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 offsets 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, t h e 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 energy 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 outp u t from Q-switched lasers h a v e

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22 A .

ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

m a d e it possible t o study a v a r i e t y of n e w p h e n o m e n a . T h e high intensity h a s led, for example, to t h e observation of stimulated R a m a n scattering (3), h a r monic generation ( 4 ) , a n d 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 by 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 gant 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 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 a n d 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 t o chemists. These a r e t h e processes occurring in a molecule v e r y soon (within 10~10 to 10~13 sec) after absorption of a photon. A technique, t e r m e d mode-locking, h a s 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 -

Report for

ous regenerative oscillation in any or all of the allowed Fabry-Perot modes; thus the output will nor­ mally contain light of all such fre­ quencies. Normally, there is no unique phase relationship between two different modes or even between two identical modes oscillating in different regions of the laser cavity. If this is the case, sharply localized pulses cannot arise through con­ structive interference, for this re­ quires closely matched phases be­ tween each contributing mode. With the appropriate passive dye Q-switch however, it is possible to lock in-phase each of the resonant modes. This technique produces pulses whose widths approach the limit set by the bandwidth—i.e., At = l/Δν where At is the pulsewidth and Δν is the laser bandwidth. In the usual dye Q-switched laser, as the "giant" pulse grows in in­ tensity the dye is bleached to some degree by each pass of the pulse through it. If the recovery time of the dye is comparable to or longer than the round-trip transit time for a pulse within the laser cavity, then the dye remains bleached until most of the energy stored in the rod is depleted by the giant pulse. The output will be the normal Q-switched pulse, nanoseconds in duration. If, however, the relaxa­ tion time is very short compared to the round-trip time, the fact that only intense pulses can bleach the dye coupled with rapid recovery of dye absorbance strongly favors amplification of a single intense spike propagating between the mir"***&

Analytical

B E L - A R T

Chemists

rors of the laser cavity. The Q-switch also acts to shorten this pulse in time, since it is absorption of the leading edge which causes saturation of the Q-switch; for a period corresponding to the dye re­ laxation time the pulse is trans­ mitted without attenuation. As the single pulse within the laser cavity propagates back and forth between the mirrors, a fraction is trans­ mitted each time the pulse is re­ flected at the output mirror. Thus, the output is a series of pulses sep­ arated by a time corresponding to 2-L/c, the time for a pulse roundtrip in the laser cavity (Figure 4). On the basis of the relationship be­ tween pulse duration and band­ width mentioned above, ruby, with a bandwidth of ~ 1 0 c m - 1 , can pro­ duce pulses with a lower limit of ~ 3 >< 10~ 12 sec, while ÏSÎd3+: glass, with a bandwidth of ~100 c m - 1 , can produce pulses with a lower limit of ~ 3 Χ 10" 1 3 sec. M e a s u r e m e n t of P i c o s e c o n d Pulses

The measurement of the pulsewidth of the laser output in modelocked operation presents an experi­ mental obstacle. The fastest avail­ able oscilloscope-photodiode detec­ tion system has a resolution time on the order of tenths of nanoseconds, while estimates of pulsewidth de­ rived from the relationship between the spectral bandwidth of the pulse and its duration give only a lower limit. To obtain an accurate mea­ sure of the duration of mode-locked pulses other, nonelectronic, pro­ cesses must be employed.

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A N A L Y T I C A L C H E M I S T R Y , VOL. 4 2 , NO. 14, DECEMBER 1 9 7 0

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23 A

Report for

Analytical

Chemists

The first technique developed for measuring ultrashort light pulses is based on two-photon absorption (IB). Excitation of a molecule with a strong absorption band can take place by the usual absorption of a single photon of appropriate fre­ quency V], or, under intense irradia­ tion by absorption of two photons of frequency y2 = V2 vi. Such twophoton absorption takes place via a virtual state of essentially zero life­ time and can occur only when the two photons are absorbed simul­ taneously. If the resulting excited state fluoresces, one can monitor the extent of two-photon absorption by following the omission intensity. The quadratic dependence of two-photon absorption on the in­ stantaneous light intensity can be used to measure the duration of light pulses as follows. If a modelocked train of pulses is directed through a cell containing a solution of a dye which can undergo only two-photon excited fluorescence, the resultant trail of two-photon excited emission can be photographed and will appear as a bright streak. If the train of pulses is reflected back on itself by a 100% reflecting mir­ ror, spots of greater brightness will appear at the mirror and at regular intervals corresponding to one-half the separation between pulses in the train. These bright spots are due to the greater instantaneous inten­

sity which occurs whenever a pulse overlaps either itself (at the mirror) or one of the trailing pulses, in the train. Thus, the length of the bright spot is a measure of the length of the pulse. A photograph of such a fluorescence trace and the photodensitometer reading for the trace is shown in Figure 5. Since the contrast ratio between the peaks and the background in a photograph such as that described is only 3:1, elimination of the back­ ground intensity would improve the resolution significantly. This can be accomplished by selecting as the two-photon absorber a fluorescent dye which is transparent at twice the laser fundamental frequency, 2v\, but which absorbs strongly at three times the laser frequency, 3vi. In this case the intensity of the laser fundamental, vi, may be as high as is convenient, yet no two-photon fluorescence will result. If, how­ ever, a train of mode-locked pulses of the second harmonic (va = 2vi) (generation of the laser second har­ monic is discussed in the next sec­ tion) of the laser frequency is made to overlap in the dye cell with the mode-locked train of laser funda­ mental v\, two-photon absorption (^ + η = 3vi) can occur (IS). The experimental arrangement used is shown in Figure 6. The dispers­ ing cell separates the pulses of the fundamental (vi) and second har­

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ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

Expanding the Laser Spectrum

Mode-locked, solid-state lasers supply only a small set of frequen­ cies, in practice those of ruby, 14,400 cm-1, and Nd 3 +:glass, 9431 cm -1 . This limitation can be over­ come in large part by three tech­ niques which produce picosecond pulses covering a broad spectral range. Second harmonic genera­ tion yields light at twice the funda­ mental frequency. Stimulated Ra­ man scattering yields coherent, collimated pulses which are Stokesshifted by amounts characteristic of the scattering molecule. There are a large number of Raman lines available which are red-shifted by as little as 216 cmr1 («-sulfur Ra­ man shift) to as much as 4155 cm - 1 (hydrogen Raman shift). Finally, dye lasers optically pumped by mode-locked ruby or Nd 3 +:glass lasers produce pulses picoseconds in duration; by using a small number of dyes and varying the experi­ mental conditions one can span the entire visible spectrum (14). Applications of Pulsed Lasers

1

1» l·

Figure 5. A photograph of a two-photon fluorescence trace and the corresponding microdensitometer trace 24 A

monic (y2), the longer wavelength pulses traveling faster in the dis­ persing medium (bromobenzeno). The set of two trains of pulses enters the dye cell; the longer wavelength pulse reaches the 100% mirror first and is reflected back to meet the trailing pulse of second harmonic. At the points of overlap of the two different harmonics two-photon flu­ orescence occurs, and there is no background. This arrangement can be modified to enhance the degree of resolution by combining the dis­ persing medium and two-photon fluorescent dye in the same cell. The two laser harmonics enter the cell simultaneously but the pulse of the laser fundamental travels faster than does the pulse of the second harmonic. In this manner the distance of overlap is extended from millimeters to centimeters, al­ lowing more accurate measurement. This technique has shown that subpicosecond structure ( ~ 4 X 10~13 sec half-width) is present in normal picosecond pulses from a modelocked Nd s +:glass laser (13).

The electronic energy of excited molecules is dissipated either by emission, or by nonradiative mo-

Report for Analytical

Chemists

lecular processes which compete with emission. Most often compe­ tition comes either from intersystem crossing; for example, the radiationless transition from the excited singlet state, &>., to a lower lying triplet, Tx ; or from internal con­ version when there is strong cou­ pling between electronic levels of the same spin. In many organic molecules these radiationless transi­ tions are the main paths of energy relaxation. When light emission is absent and relaxation rates are fast (>10° sccr1) one can sometimes ob­ tain an indirect estimate of the kinetics of these relaxation pro­ cesses. Using picosecond pulse techniques we have been able to measure these rates directly. Mea­ surements were carried out on azulene, benzophenone, and a few other large organic systems. Azulene. Azulene exhibits anom­ alous fluorescence in that the emission orginates not from the lowest excited singlet (Sx -=• S0) as is usually the case, but from the second excited singlet (S2 -> Sx). Furthermore, the first excited-state singlet lies substantially lower in energy at ~ 14,400 c u r 1 (almost exactly one-half the energy of the second excited singlet, //////////////. ψπν, \ Ι φ £ϋ ιο""6

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cited state as it propogates along the return path of the νχ pulse. The fluorescence produced as a function of the delay between the arrivals of the two pulses provides a direct measure of the lifetime of the vibra­ tional level. Photodcnsitometer traces in Figure 8 show the length of the pulse to be ~ 2 χ 10~ 12 sec, while relaxation time of the vibra­ tional level is 7.5 Χ ΙΟ"12 sec (15). This procedure allows direct ob­ servation of lifetimes of the order of picoseconds. However, it cannot provide an answer to the question of whether the predominant de­ populating process for the Sx state of azulene is vibrational relaxation in the same manifold, internal con­ version to the ground state, or inter­ system crossing to a triplet level. To identify the actual path, a vari­ ation of the above method was used that enables us to measure the inter­ system crossing rate not only of azulene, but of nearly any other molecule. We have observed fluorescence from the Sx level of azulene under strong optical pumping by laser light. The quantum efficiency of (Continued

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on page 29 A)

ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 14, DECEMBER 1970

Figure 7. A schematic representation of the energy levels in azulene. The ex­ periment to determine the rate of vibra­ tional relaxation in the first excited singlet level is outlined.

—»- /

(a)

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1-·— 2 X i o ~ 1 2 sec

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Figure 8a. A microdensitometer read­ ing of the picosecond pulsewidth as measured by the two-photon fluores­ cence method

Figure 8b. A microdensitometer read­ ing of the fluorescence from azulene excited by one photon at 18,863 c m - 1 to a vibrationally excited level of the first excited singlet state from which it can be raised to the fluorescent sec­ ond singlet by a photon at 9431 c m 1 . The increase in the length of the fluo­ rescence spot is due to the finite (7.5χ10~ 1 ζ sec) lifetime of the vibra­ tional level of the first singlet

Report for Analytical Chemists

fluorescence is ~ 1 0 " ° and only the most intense bands are observed. This emission is increased by a fac­ tor of ,~·3 in deuterated azulene {16). Obviously, radiationless pro­ cesses predominate in the depopu­ lation of the lowest excited singlet state. One of the most likely of the possibilities for this t y p e of process is energy transfer to a lower lying triplet state. T h e absorption of azulene in solution begins at —14,400 cm."1, which is conveniently resonant with the frequency of the ruby laser. A t r a i n of picosecond pulses ( ~ 4 X 10~12 sec) from a ruby laser separated by ~ 5 X 10~8 sec is passed through a cell contain­ ing an azulene solution, exciting a large fraction of the azulene mole­ cules in its p a t h to the lowest vibra­ tional level of the first singlet level. Reflection by a mirror situated after the cell causes reflected and oncorning pulses to pass through one another in the cell. I n regions where azulene remains in the Si state, molecules m a y be excited by a second pulse into the second sin­ glet electronic level. T h e resulting fluorescence (S-2 -> S0) spot is mea­ sured by the same method described previously. Since the level popu­ lated by the laser frequency in the S 0 —» S-i transition is the lowest vi­ brational state of the Si state, the depopulation time of this level is a measure of cither intersystem cross­ ing or direct relaxation to the ground state. To establish the p a t h of the relaxation process, heavy atom solvents were substituted for the alcohol. I t is generally accepted t h a t a heavy atom solvent affects the intersystem crossing rate by enhancing the spin-orbit coupling of the two states. Our experiments indicate t h a t there is only a small effect owing to heavy solvent so t h a t the ~ 8 X 10~12 sec relaxation time from the state most probably arises from direct coupling with the ground state. T h e inter­ system crossing is of less impor­ tance, and in any event can be de­ termined directly. W e excited azu­ lene by a 14,400 cixr 1 picosecond pulse but interrogate the population build up of only the triplet state by a second picosecond pulse, using a Raman-shifted frequency of the ruby fundamental as the probe. F r o m t h e r a t e of increase of triplet-

triplet absorption, we obtain di­ rectly the rate of intersystem cross­ ing ( ~ 6 0 X 10~12 sec) since the population of the triplet state is achieved by only this p a t h (16). Benzophenone. Benzophenone is an example of a molecule in which spin-orbit coupling between the lowest excited singlet and the m a n i ­ fold of nearby triplet states is very strong. T h e usual explanation of this is t h a t there is a w * triplet state lying near . the first excited singlet, which is an ηπ* state. Theory predicts t h a t the spin-orbit interaction is very large between two such states, in comparison to •mi-* — ΤΓΪΓ* or n-7rif — iW* spin-orbit interactions. Indeed, for benzo­ phenone the intersystem crossing rate, determined to a large extent by the degree of spin-orbit coupling, is so large as to preclude measureable fluorescence under the usual conditions of excitation. However, using the second harmonic of ruby, Raman-shifted to 26,800 cm- 1 , to excite benzophenone to the lowest vibrational level of the first excited singlet, Sx (v = 0 ) , a weak emission (with a q u a n t u m yield Φ ~ 10~5) was observed. T h i s emission is at­ tributed to fluorescence from the first excited singlet of benzophe­ none (17). T h e high intensity of the mode-locked laser as an excita­ tion source made detection of this weak emission possible ; ordinary light sources cannot provide suffi­ cient light flux. F u r t h e r experi­ ments have shown t h a t excitation of benzophenone by the second h a r ­ monic of ruby (28,800 cm-"1) to the fourth vibrational level of the low­ est singlet. Si (v = 4 ) , results in no detectable fluorescence. If we as­ sume t h a t the F r a n c k - C o n d o n fac­ tors between the ground and first excited singlet state favor emission from only the lowest vibrational level of the S x state, we can infer from this interesting result t h a t in­ tersystem crossing from the upper vibrational levels of the Si state is more rapid t h a n vibrational relaxa­ tion in the. Sj state. Support is lent to this interpretation by the results of measurements of the lifetime of the Si(v = 0) level and the mean lifetime for the Si state when ini­ tially excited to the ν = 4 level. The second harmonic of ruby at 28,800 c m - 1 was used to excite ben-

S *

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Peter ιΜ. Rentzepis is a member of the Chemical Physics Research De­ partment at Bell Laboratories, Murray Hill, N. J:, and he is pres­ ently engaged in picosecond laser research work. He also is Pro­ fessor of Chemistry at the Univer­ sity of Pennsylvania. Dr. Rent­ zepis joined Bell Laboratories in 1963. His research work has in­ cluded high energy radiation, photo­ chemistry of organic systems, and studies with oxygen, nitrogen, and hydrogen atoms on polymers. A native of Calamata, Greece, Dr. Rentzepis received the Ph.D. degree in physical chemistry from Cam­ bridge University (England) in 1963. Tie is a Fellow of the New York Academy of Sciences, and a member of the Faraday Society, the Philosophical Society, the American Physical Society, and Sigma Xi.

Charles J. Mitschele of Bell Lab­ oratories, Murray Hill, λτ. J., is en­ gaged in exploratory research using picosecond pulsed lasers to study the reaction rates of short-lived, photochemical intermediates and to investigate rapid physical processes such as energy relaxation in large molecules. Dr. Mitschele received an A.A. degree, from, Santa Ana Col­ lege in 1960, an A.B. degree from Pomona,College in 1962, and earned his Ph.D. degree at the University of California at, Riverside in 1968. He has done postdoctoral work both at Rutgers, the State University (1967-39) and at Bell Telephone Laboratories (1.969--present).

ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

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29 A

Report for Analytical Chemists

Figure 9. times

The experimental arrangement used for determining excited-state life-

zophenone to the Sx(v = 4) level. From this level, benzophenone can be excited to the second singlet, S2, by absorption of a photon at 10,245 cm -1 (Raman-shifted ruby funda­ mental) . If relaxation to a lower vibrational level takes place, ab­ sorption at 10,245 cm -1 cannot oc­ cur. Thus, if after excitation to S± (ν = 4) by a picosecond pulse of the second harmonic, the excited state population is interrogated by a picosecond pulse at 10,245 cm -1 the degree of absorption of this sec­ ond pulse is proportional to the pop­ ulation of Si (j/ = 4). Variation of the delay between the exciting and interrogation pulses allows mea­ surement of the lifetime of the Stiv = 4) level, found to be ~ 2 0 psec. The ruby fundamental (14,400 enr 1 ) is absorbed by any of the vibronic levels of the Sx state; with the fundamental as the interro­ gating pulse in the above experi­ ment the lifetime was again mea­ sured as ~ 2 0 psec. The experi­ mental arrangement is shown in Figure 9; a schematic representa­ tion of the energy levels of benzo­ phenone is shown in Figure 10. This result implies that the mean lifetime of the lower vibrational levels of S : is the same as or shorter than that of the Si (v = 4) level. If benzophenone is excited to S x (v = 0) by the Raman-shifted second harmonic of ruby at 26,000 cm -1 , followed by interrogation by the ruby fundamental, the lifetime of the Sx(v = 0) is only ~ 6 psec. We conclude that intersystem cross­ ing from the Si {v = 0) level is more rapid than is intersystem crossing or vibrational relaxation in the upper vibrational levels of the Sx state of benzophenone. Evidently, there is less spin-orbit coupling in the higher vibrational levels than is

present in the Si (y = 0) level, prob­ ably because the triplet m-* state participating most strongly in the spin-orbit interaction is closer to the Sx(v = 0) level than to upper vibrational levels. Biphenylene. This molecule ex­ hibits no luminescence, is photostable, and has no absorption be­ tween excited states detectable by flash photolysis or other methods. This is interesting, for the first al­ lowed transition is sufficiently high in energy (~26,000 cm -1 ) to ex­ clude the possibility, which exists in azulene, of normal radiationless conversion to the ground state. Moreover, no triplet state has been detected in emission, even at low temperatures. With laser excita­ tion we have detected fluorescence

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with a small quantum yield (~10- 4 ) and a lifetime of ~10" 9 sec in rigid media at low tempera­ tures {18). This evidence indicates that the fluorescence in fluid solu­ tion is dynamically quenched. This in itself is unusual, as fluorescence processes are usually too fast to be affected except by impurity quench­ ers, for example, oxygen. . The experimental arrangement in Figure 9 is similar to that employed in the study of the repopulation of the ground state of biphenylene, but a polarizer and/or quarter wave plates were added. The biphenylene molecule is ex­ cited by the first pulse of a pico­ second pulse train. The laser out­ put is linearly polarized for this experiment. Every successive on­ coming pulse has the same fre­ quency (28,800 c m 4 ) , length, and polarization characteristics. The frequency of the pulse corresponds to the biphenylene long-axis transi­ tion; thus a given molecule is ex­ cited only if its long molecular axis has ε, component parallel to the electric field of the photon. At this stage, a second pulse is introduced for the purpose of determining the number of molecules remaining at the energy level of the excited state and maintaining the same molecular

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Figure 10. A schematic energy diagram for benzophenone. The experiments with the first ( Ι Ί ) , second («,), and Raman-shifted first (ιΛ) and second (i>"2) harmonics of ruby are depicted ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970

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LET O U R CATALOG G6 INTRODUCE Y O U T O THE W O R L D OF PCR.

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