The generation o f second pulses o f light
is outlined.
A
number o f
T
applications o f such pulses to problems o f interest to chemists are described.
Figure l a . A typical laser cavity. lamp; R, laser rod; M , mirror
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HE GESERATIOS of ultrashort pulses of coherent light has provided 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 niolecules, intersystem crossing, and energy transfer an outline of the operation of pulsed solid-state lasers ~ o u l dbe useful. h typical solid-state laser (sketched in Figure 1) consists of an active oscillator medium-e,g., a IYd3--doped glass or ruby (Cr3Ldoped h1203’, rod-situated between parallel mirrors. One mirror is usually of high reflectance at the laser line ( R N 1.0),while the other is partially transmitting ( R = 0.10.5). The arrangement of mirrors
C, capacitor;
L, flash
and rod described constitute one form of Fabry-Perot interferometer, the theory of \yhich is treated in elementary texts ( I ) . Standing light Jvaves may be set up within the cavity a t a series of discrete optical frequencies. These frequencies, known as Fabry-Perot modes, are those for which the following relation holds: vn, = nc/2L
where L is the separation of the mirrors. c is the speed of light, and n is an integer (-lo6 for ruby or S d 3f : glass) , 4 d j acent modes are separated from each other by a frequency. Av, giyen by Av
= vn -
-
1
= c/2L
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
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
REPORT FOR ANALYTICAL CHI
ITS
P. M. RENTZEPIS C. J. MlTSCHELE Bell Telephone Laboratories, lnc., Murray Hill, N.J. 07974
For an actual laser interferometer, the allowed inodes have a finite spectral bandwidth, S V , which is dependent on the reflectirity and flatness of the mirrors. and the effective aperture of the laser rod. The relationships are illustrated in Figure 2 . If the laser rod is optically Fumped by a high intensity, broadband flashlamp. a population inversion between the lasing lerels of the active ion ( S d 3 + or Cr3+) will result (Figure 31. I n this situation, spontaneous emission from the excited 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 falling \Tithin the bandwidth of one of t5e Fabry-Perot modes, it will be
Figure 2. The relationship between the spectral band of the laser transition and the Fabry-Perot frequencies for the laser cavity
PUMPING LEVELS
PUMPING LEVELS
D l AT I ON LESS RANSITION T O LASING LEVEL
c,
RAD1AT IONLESS TRANS I T IONS
PU L
TRANSITION
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 coherent and propagates in the same direction as the stimulating light, ultimately the predoniinant output of the laser through the partially reflecting mirror is intense, coherent radiation of lo^ divergence. This output consists of a series of random spikes XTith variable duration (microseconds or less), commencing a short t h e after initiation of optical pumping. Typically, a total of -lO1g photons (-10 A of red or infrared light will be emitted. A few thousand joules are discharged through the flashlamp; thus the conversion efficiency is of the order of
PUMPING LIGHT
I I
LASER TRANSIT I ON
c'
GROUND S T A T E THREE- L E V E L SYSTEM (RUBY)
F O U R - L E V E L SYSTEM (NEODY M l U M )
Figure 3. The processes involved in three- and four-level laser systems A N A L Y T I C A L CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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1%. Since this light is coherent aiid nearly parallel, it may be focused to an area of dimensions approaching the Lyavelength of light. Although use is made of this property of the laser output in applications such as Lyelding, the erratic behavior of this pulsed laser gives it limited use in problems of interest to chemists. One can partially overcome this difficulty by &-switching. The Q-Switched Laser
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If a shutter is placed inside the laser cavity and kept closed during the initial portion of the pumping pulse, the excited-state population in the laser rod will not be depleted by the lasing process which would normally occur, and hence will build up to a level greatly in excess of that attained without such a shutter. If, a t the peak of this build-up of excited states, the shutter is suddenly opened, the energy stored in the laser rod will be emitted yery rapidly in a single, “giant” pulse ( 2 ) . A cell containing a solution of a photo-bleachable dye is often used as the shutter, or &-switch. T o function in this capacity the dye must have an absorption band which overlaps the laser emission band. I n this case, during the initial stages of pumping all emission from the laser rod will pass through the Q-switch cell and will be attenuated owing to the absorption of the dye. The dye concentration and the pumping power are adjusted so that the initial loss in the Q-switch offsets the gain in the rod : a t this point the “shutter” is closed. However. as the excited-state population grows with continued pumping, the gain increases exponentially and eventually a condition of net aniplification is reached. At this point, the power of the light pulse starts to grow rapidly and becomes intense enough to bleach the Q-switch dye transition, opening the “shutter.” The energy stored in the rod is then emitted over a period of typically 20-30 nsec duration. The total energy output is reduced somewhat from that obtained in non-Qswitched operation, but since the pulse duration is shortened, the pom-er is increased. The high power (tens to hundreds of megawatts) and short duration of the light output from &-switched lasers have
Circle No. 179 on Readers’ Service Card
22 A
a
A N A L Y T I C A L CHEMISTRY, VOL,
.2, NO. 14, DECEMBER 1970
macle it possible to study a variety of new phenomena. The high intensity has led, for example, to the observation of stimulated Raman scattering (3), harmonic generation ( 4 ), and two-photon absorption ( 5 ) . The latter effect has been detected by the appearance of fluorescence-e.g., in anthracene (6)-or by chemical reaction of molecules transparent a t the laser wayelength [polymerization of styrene ( 7 ), photodissociation of chlorine (S)]. The light flux available from Q-switched lasers can produce sufficient chemical reaction in molecular beams to allow detection of photodecomposition products and use has been made of laser pumping in an elegant series of molecular beam experiments 191, Upon irradiation by Q-switched lasers, certain crystals generate optical harmonics from the laser fundamental frequency ( 4 ) ; with such “nonlinear” crystals one can obtain a 10% energy conversion of the laser frequency to its second harnionic. For ruby (14.400 em-? the second harmonic is a t 28,800 c1~1-l; for ?Sd3-:glass (9431 em-l) the second harmonic is a t 18,863 cm-I, aiid the fourth harmonic (which can be obtained by frequency doubling the second harmonic1 is a t 37,726 cm-1. These energies are sufficiently high t o enable one t o excite directly by one-photon absorption most of the larger organic molecules. The absorption spectra of excited singlets and other short-lired intermediates of a nuniber of molecules have been observed with the use of Q-sx-itched pulses and second harmonic generation (10. 11). The Mode-Locked Laser
While the use of &-switched lasers as an intense light source of nanosecond duration has led to much interesting research, this time scale is too long for detection and measurement of the most fundamental molecular processes of interest to chemists. These are the processes occurring in a molecule very soon ixyithin 10-lO to sec) after absorption of a photon. A technique. termed mode-locking, has made it possible to study these very rapid proresses directly. The laser mill support simultane-
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Report for Analytical Chemists
ous regeneratis-e oscillation in any or all of the allowed Fabry-Perot modes; thus the output mill normally contain light of all such frequencies. 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 constructive interference, for this requires closely matched phases between 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-ie.. at = ~ / I V Tvhere I t is the pulsen-idth and IV is the laser bandwidth. I n the usual dye Q-switched laser, as the "giant" pulse grows in intensity the dye is bleached to some degree by each pass of the pulse through it. If the recovery time of the dye is coniparable to or longer than the round-trip traixit time for a pulse within the laser carity, 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 &-switched pulse, nanoseconds in duration. If, however, the relaxation time is very short compared to the round-trip time, the fact that only intense pulses can bleach the dye coupled with rapid recoyery of dye absorbance strongly favors amplification of a single intense spike propagating between the mir-
Tors of the laser cavity. Q-switch also acts to shorten The this pulse in time, since it is absorption of the leading edge Tvhich causes saturation of the Q-switch: for a period corresponding to the dye relaxation time the pulse is transmitted without attenuation. As the single pulse within the laser cas-ity propagates back and forth betveen the mirrors. a fraction is transmitted each time the pulse is refleeted a t the output mirror. Thus, the output is a series of pulses separated by a time corresponding t o f L / c , the time for a pulse roundtrip in the laser cavity (Figure 4 ) . On the basis of the relationship betn.een pulse duration and bandwidth mentioned aboye. ruby. n-ith a bandwidth of -10 cm-l, can produce pulses with a loxT-er limit of -3 >< sec, while S d 3 + :glass, v i t h a bandwidth of -100 can produce pulses v i t h a lov-er limit of -3 X sec. Measurement of Picosecond Pulses
The measurement of the pulsewidth of the laser output in modelocked operation presents an experimental obstacle. The fastest amilable oscilloscope-photodiode detectlon system has a resolution time on the order of tenths of nanoseconds, while estimates of pulsevidth derdved froin the relationship between the spectral bandwidth of the pulse and its duration give only a lower limit. To obtain a n accurate measure of the duration of mode-locked pulses other, nonelectronic, processes must be employed.
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TECHNILAB INSTRUMENTS, INC. Figure 4. An oscilloscope (Tektronix 519) trace of the output of a mode-locked laser. The time scale is 50 nsec/cm; the separation between pulses corresponds to a time equal to 2 L / C , in this case, -6 nsec
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A N A L Y T I C A L CHEMISTRY, VOL. 42, NO. 14, DECEMBER 1970
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Report: for Analytical Chemists
The first technique deyeloped for measuring ultrashort light pulses is based on two-photon absorption ( 1 2 ) . Excitation of a molecule with a strong absorption band can take place by the usual absorption of a single photon of appropriate frequency vl, or, under intense irradiation by absorption of two photons of frequency v2 = I J ~ . Such tn-ophoton absorption takes place via a virtual state of essentially zero lifetime and can occur only when the two photons are absorbed simultaneously. If the resulting excited state fluoresces, one can monitor the extent of two-photon absorption by following the emission intensity. The quadratic dependence of t1Y.o-photon absorption on the instantaneous 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 mirror, spots of greater brightness will appear at the mirror and a t regular intervals corresponding to one-half the separation betveen 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 I 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. h photograph of such a fluorescence trace and the photodensitonieter 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, eliiniriation of the background intensity would improve the resolution significantly. This can be acconiplished by selecting as the two-photon absorber a fluorescent dye which is transparent a t twice the laser fundamental frequency, 2vl, but which absorbs strongly a t three times the laser frequency, 3v1. I n this case the intensity of the laser fundamental, vl, may be as high as is convenient, yet no two-photon fluorescence will result. If, however, a train of mode-locked pulses of the second harnionic ( v 2 = 2vli (generation of the laser second harmonic is discussed in the next section) of the laser frequency is made to overlap in the dye cell with the mode-locked train of laser fundamental vl. two-photon absorption (v2 v1 = 3v1\ can occur 113). The experimental arrangement used is shown in Figure 6. The dispersing cell separates the pulses of the fundamental (vl) and second har-
+
WY Y)I
g!
iig
Figure 5. A photograph of a two-photon fluorescence trace and the corresponding microdensitometer trace
*
Expanding the Laser Spectrum
Mode-locked, solid-state lasers supply only a small set of frequencies, in practice those of ruby, 14.400 cm-l. and Nd3-:glass, 9431 c w 1 . This limitation can be overcome in large part by three techniques which produce picosecond pulses covering a broad spectral range. Second harmonic generation yields light a t twice the fundamental frequency. Stimulated Ranian scattering yields coherent, collimated pulses which are Stokesshifted by amounts characteristic of the scattering molecule. There are a large number of Ranian lines available which are red-shifted by as little as 216 cm-1 (a-sulfur R a nian shift I to as much as 4155 cin-1 (hydrogen Raman shift). Finally, dye lasers optically pumped by mode-locked ruby or Hd3&:glass lasers produce pulses picoseconds in duration: by using a small number of dyes and varying the experiiiieiital conditions one can span the entire visible spectrum (141. Applications of Pulsed Lasers
A P F ~ O X I M A T E ~ I S T A N C(MMI E
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moiiic ( v Z ) , the longer wavelength pulses traveling faster in the dispersing medium (bromobenaene) . 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 harmoiiics tn-o-photon fluorescence occurs. and there is no background. This arrangement can be modified to enhance the degree of resolution by combining the dispersing inediuin 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. I n this manner the distance of overlap is extended from millimeters to centimeters, alloving more accurate measurement. This technique has shown that subpicosecond structure (-4 X sec half-u-idth) is present in normal picosecond pulses froin a modelocked Kd3+:glass laser ( 1 3 ) .
ANALYTICAL CHEMISTRY, VOL. 42, NO. 14,DECEMBER 1970
The electronic energy of excited molecules is dissipated either by emission, or by nonradiative mo-
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lecular processes which compete with emission. Most often competition comes either from intersystem crossing; for example, the radiationless transition from the excited singlet state, SI, to a lower lying triplet, T,: or from internal conversion when there is strong coupling between electronic levels of Figure 6. A schematic representation of the apparatus used for the measurement of pulses of frequencies v + 2~ the same spin. I n many organic molecules these radiationless transitions are the main paths of energy 2 8 2 9 3 cm-' relaxation. Wheii light emission is absent and relaxation rates are fast ( >loo sec-l) one can sometimes obtain an indirect estimate of the kinetics of these relaxation pro18863 cm-' cesses. Using picosecond pulse techniques me have been able to T ( " ~ ) F R O M I8863Cm-'= 7 . 5 i~d Z s e c measure these rates directly. Mea14400 cm-' surements were carried out on azu,d2sec lene, benzophenone, and a few other large organic systems. Azulene. Azulene exhibits anomalous fluorescence in that the emission orginates not from the lowest excited singlet (SI+ So) as is usually the case, but from the second excited singlet is2 + 81). Furthermore, the first excited-state singlet lies substantially lower in Figure 7. A schematic representation energy at w 14,400 cm-l (almost cited state as it propogates along of the energy levels in azulene. The exexactly one-half the energy of the the return path of the "1 pulse. The periment to determine the rate of vibrasecond excited singlet, ~ 2 8 . 0 0 0 fluorescence produced as a function tional relaxation in the first excited singlet level is outlined. crn-l~ than might have been exof the delay between the arrivals of pected on the basis of comparison the tn-o pulses provides a direct with chemically similar molecules. measure of the lifetime of the vibraThese properties were utilized in an tional level. Photodensitonieter experiment designed to measure the traces in Figure 8 s h o ~ the length rates of radiationless processes QCof the pulse to be - 2 x sec, curring in the lovest excited state while relaxation time of the vibra+-2 x sec of azuleiie 116). A schematic ensec 125). tional level is 7 . 5 x ergy diagram for azulene is shown This procedure allom direct ohin Figure 7. (0) servation of lifetimes of the order The experimental arrangement is of picoseconds. Howeyer. it cannot Figure 8a. A microdensitometer reading of the picosecond pulsewidth as similar to that used for the tT1-oproyide an ansTyer to the question measured by the two-photon fluoresphoton fluorescence measurement of of whether the predominant decence method pulse duration. For the measurepopulating process for the 8, state ment of vibrational relaxation, a o f azulene is vibrational relaxation mode-locked S d 3 :glass laser mas in the same manifold, internal conused. Light a t the second harmonic version to the ground state, or interfrequency, v;, = 18,863 cni-l, raises system crossing to a triplet level. 7.5 x I G - I ~s e c azulene to approximately the fifth T o identify the actual path, a varivibrational level of the first excited ation of the above method was used (b) singlet, SI1 v 5 ) . The subsequent that enables us to measure the interFigure 8b. A microdensitometer readarrival of a pulse of the laser fundasystem crossing rate not only of ing of the fluorescence from azulene mental. yl = 9431 cnirl, can excite azulene, but of nearly any other excited by one photon at 18,863 cm-1 the azulene molecules from SIto 8, molecule. to a vibrationally excited level of the first excited singlet state from which it provided they are still in the vibraW e have obserred fluorescence can be raised to the fluorescent sectionally excited state when the secfrom the SIlevel of azulene under ond singlet by a photon at 9431 cm'. ond pulse arrives. Therefore, the The increase in the length of the fluostrong optical pumping by laser rescence spot is due to the finite "2 pulse interrogates the population light. The quantum efficiency of ( 7 . 5 ~ 1 0 - ~sec) ? lifetime of the vibrao f azuleiie in the vibrationally extional level of the first singlet (Continued o n poge 29 A )
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ANALYTICAL C H E M I S T R Y , VOL. 42, NO. 14, D E C E M B E R 1970
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fluorescence is -.10-6 and only the most intense bands are observed. This emission is increased by a factor of ~3 in deuterated azuleiie ( 1 6 ) . Obviously, radiationless processes predominate in the depopulation of the lovxst excited singlet state. One of the most likely of the possibilities for this type of process is energy transfer to a lower lying triplet state. The absorption of azulene in solution begins a t w 14,400 which is conveniently resonant with the frequency of the ruby laser. h train of picosecond pulses (-4 x sec) from a ruby laser separated by - 5 X sec is paesed through a cell contaiiiing an azulene solution, exciting a large fraction of the azulene molecules in its path to the lowest vibrational level of the first singlet level. Reflection by a mirror situated after the cell causes reflected and oncoming pulses to pass through one another in the cell. I n regions where azulene remains in the SI state. molecules may be excited by a second pulse into the second einglet electronic level. The resulting fluorescence is, -+ S o ) spot is nieasured b y the dame method described preriously. Since the level populated by the laser frequency in the So -+ Xltransition is the lowest vibrational state of the SI state. the depopulation time of this level is a measure of either intersyFtem crossing or direct relaxation to the ground state. T o establish the path of the relaxation process, heavy atom solvents were substituted for the alcohol. It is generally accepted that a heavy atom solvent affects the intersystem croasing rate by enhaiiciiig the spin-orbit coupling of tlie two states. Our experiments indicate that there is only a small effect owing to heavy solvent so that the -8 X see relaxation time froin the ~ ! 3 =~ (0’~ state most probably arises from direct coupling with the ground state. The intersystem crossing is of less importance. and in any event can be determined directly. We excited mulene by a 14,400 em-I 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. From the rate of increase of triplet-
triplet absorption, we rectly the rate of intersystem crossing (-60 X 10-l2 sec) since the population of the triplet state is achieved by only this path 116). Benzophenone. Benzophenone is an example of a molecule in which spin-orbit coupling b e h e e n the lowest excited singlet and the manifold of nearby triplet states is very strong. The usual explanation of this is that there is a m” triplet state lyiiig near the first excited singlet, TThich is a n na” state. Theory predicts that the spin-orbit interaction is very large betTYeeii two such states, in comparison to m-’or na””- nn“ spin-orbit interactions. Indeed. for benzophenone 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-l, to excite benzophenone to the lowest vibrational level of the first excited singlet, Sl(v= 0 ) . a weak emission (with a quantum yield E was observed. This emission is attributed to fluorescence from the first excited singlet of benzophenone ( 1 7 ) . The high intensity of the mode-locked laser as a n excitation source made detection of this weak eniission possible : ordinary light sources cannot provide suffieient light flux. Further experimenta haye shown that excitation of benzophenone by the second harmonic of ruby (28.800 em-l’i to the fourth iTibrationa1 level of the lowest singlet, S,(11 = 4 ) . results in no detectable fluorescence. If \-e assume t h a t thp Franck-Condon factors between the ground and first excited singlet state favor emission From only tlie lowest vibrational level of the S1 state. \ye can infer from this interesting result that intersystem cros+ig froin the upper y,-ibrational levels of the SIstate is more rapid than vibrational relaxation in the SIstate. Support is lent t o this interpretation by the results of measurements of the lifetime of the SI(,) = 0) level and the mean lifetime for the S1 state \Then initially excited to the v = 4 level. The second harmonic of ruby at 28,800 em-1 was used to excite ben-
Peter IM. Rentzepis i s a inember of
the Chemical Physzcs Research D e partmeiit a t Bell Laboratories, M u r r a y Hill,S.J . . and lze i s presently engaged in picosecond laser reseasch work. H e also i s Projessof- of Cheinzstry at the University o , ~P e n n s y k a n i a . Dr. Rentxepis iozned Bell Laboratories in 1963. H i s research uork has included high energy radiatzon. photochemistry of o/*ganic systems, and studies with oxygen, nitrogen, and hydroGen atoms o n polymers. A native of Calaviata, Greece, Dr. Rentacpis received the Ph.D. degree in physical chemistry f r o m Cambridge Universitv (England) in 1963. H e is a Fellou) of the LYew York A c a d e m y of Sciences. and a membcr of the Faraday Society, the Philosophical Society, the American Plzysical Society. and Sigma X i .
Charles J. Mitschele of Bell L a b -
oratories, X u l * r a y H111,S.J., i s engaged n erploratory research using picosecond pulsed lasers to study the rezction rates of short-lived photochemical intermediates and to inLestiytte rapid physical processes such as energy relaxation zn large molecules. D r . Ilfztschele r e c e k e d an A.A. degree from Santa A n a College i n 1960. an A B . degree from Pomona College in 1962. and earned his P h D . degree a t the Cniiersity of Ca12,fornia at RiLerside i n 1968. H e has done postdoctoral work both a t Ru gel‘s, the State Tniversity (1.967-79) and a t Bell Telepho?ze Laboratories /1969-present).
A N A L Y T I C A L CHEMISTRY, VOL. 42,
NO. 14,
DECEMBER 1970
29A
krsat.de
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Report for Analytical Chemists
Figure 9. times
The experimental arrangement used for determining excited-state life-
zophenone to the S1(v = 4) level. From this level, benzophenone can be excited to the second singlet, X,, by absorption of a photon a t 10,245 cm-l (Raman-shifted ruby fundamental), If relaxation to a lower vibrational level takes place, absorption a t 10.245 em-l cannot occur. Thus, if after excitation to X1 v = 4) by a picosecond pulse of the second harmonic, the excited state population is interrogated by a picosecond pulse a t 10,245 em-l the degree of absorption of this secoiid pulse is proportional t o the population of S1( V = 4 ) . T-ariation of the delay between the exciting and interrogation pulses allows measurement of the lifetime of the & ( v = 4) level, found t o be -20 psec. The ruby fundamental (14,400 cm-l) is absorbed by any of the vibronic levels of the 8 1 state; with the fundamental as the interrogating pulse in the above experiment the lifetime was again measured as -20 psec. The experimental arrangement is shown in Figure 9 ; a schematic representation of the energy levels of benzophenone is shown in Figure 10. This result implies t h a t the mean lifetime of the lower vibrational levels of XI is the same as or shorter than t h a t of the S,(V = 4 1 level. If benzophenone is excited to & i~= 0 ) by the Raman-shifted second harmonic of ruby a t 26,000 cm-l, followed by interrogation by the ruby fundamental, the lifetime of the & ( v = 0) is only -6 psec. We conclude that intersystem crossing from the Xli~ = 0) level is more rapid than is intersystem crossing or vibrational relaxation in the upper vibrational levels of the Xl state of benzophenone. Evidently. there is less spin-orbit coupling in the higher yibratioiial leyeia than is
present in the S,(V= 0) level. probably because the triplet TT++state participating most strongly in the spin-orbit interaction is closer to the & ( v = 0 ) level than to upper vibrational levels. Biphenylene. This molecule exhibits no luminescence, is photostable, and has no absorption between excited states detectable by flash photolysis or other methods. This is interesting, for the first allowed transition is sufficiently high in energy (-26.000 cn-l) to exclude the possibility. which exists in azulene, of normal radiationless conversion to the ground state. Iloreorer, no triplet state has been detected in emission, eren a t low temperatures. TTith laser excitation we have detected fluorescence
EXCITED SINGLET
with a small quantum yield (-let-4) and a lifetime of sec in rigid media a t low temperatures ( 1 8 ) . This evidence indicates that the fluorescence in fluid solution i;: dynamically quenched. This in itself is unusual, as fluorescence processes are usually too fast to be affected except by impurity quenchers. for example, oxygen. T h 2 experimental arrangement in Figure 9 is similar to that employed in the study of the repopulation of the €;round state of biphenylene, but a polarizer and/or quarter wave plate: were added. Thc biphenylene niolecule is excited by the first pulse of a picoseconl pulse train. The laser output is linearly polarized for this exper ment. Every successi-cre onconiirg pulse has the same frequency (28,800 cni-l), length. and polarization characteristics. The frequency of the pulse corresponds to thc biphenylene long-axis transition; thus a given molecule is excited mly if its long molecular axis has E component parallel to the electric field of the photon. 44tthis stage, a second pulse is introduced for tl-e purpose of determining the nunib3r of niolecules remaining a t the erergy level of the excited state and rr aintaining the same molecular
I
(s,) -6 ps(?C
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I
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Figure 10. A schematic energy diagram for benzophenone. The experiments with the first ( v ~ ) ,second (pi), and Raman-shifted first ( v ’ J and second (v”?) harmonics of ruby are depicted A N A L Y T I C A L CHEMISTRY, VOL. 42,
NO, 14,
DECEMBER 1970
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