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Ultrashort vibrational population lifetime of large polyatomic molecules in the vapor phase
CtlEMICAL PHYSICS LETTERS
Volume 46, number 3
ULTRASHORT
VIBRATIONAL
15 March 1977
POPULATION LIFETIME
OF LARGE POLYATOMIC MOLECULES IN THE VAPOR PHASE J.P. MAIER, A. SEILMEIER, A. LAUBEREAU and W. KAISER Rzysik-Department der Technisrhen Unirersr&. Munchen. Munich, Germany Received 17 December 1976
A vibrationa mode of a polyatornic n~~lcculcin the vapor phase is f’kt excited by an ultrashort infrared puke. Fhedesce of excitation is monItored by a delayed probe pulse which promotes the excited molecules to the fluorescent first singlet state. The lifctlmc of an overtone at 5950 cm” of coumarin 6 is found to be 4 +- 1 ps in the vapor phase at 1 torr.
1. Introduction
In recent years, methods have been developed to measure the population lifetimes of weII-defined vibrational modes of polyatomic molecules in liquids [l-4]. Very short time constants of the order of IO-I2 s were observed and inter- as well as intramolecular interaction processes were established [2,3]. In the condensed phases the molecular packing is very dense and the interactions of a vibrationally excited molecule with its surrounding are important. Very recently, we have introduced a double resonance technique where the population lifetime of vibrational modes in the electronic ground state can be investigated in highly diluted systems [4]. The vibrational mode of interest is first excited by an ultrafast resonant infrared pulse and a propedy delayed interrogating pulse of higher frequency promotes the molecule close to the bottom of the fluorescent first singlet state. The observed fluorescence signal is a direct measure of the instantaneous popuIation of the vibrational state. In this paper we apply this method to coumarin 6 in the vapor phase. WC present for the first time data of population lifetimes in the electronic ground state. We have previously studied the same molecule in a diluted Iiquid system.
2. Experimental In fig. 1, out experimental
set-up is depicted sche-
L
Sample Celt
PO Q 50bPV I ig. 1. Schcmdaic of the experimental syttcm to measure population lifetimes m the vapor phase. A sin& picosecond puk generates ultrashort infrared pulses in a singfe pass through two nonlinear L1hSb@3 crystals. Tuning of the infrared frcquencies is achiticd by crystal rotation. The infkar& Frcqucncy and the bandwidth are monitored with spectrometer SP 1. The probe pulse is converted to the second harmonic frequency in a KDP crystal and optically dcldyed before intcrcogating the vlbrationally excited volume. Spectrometer, SP 3, measures the produced fluorcscencc signal.
matically. A mode-locked Nd-glass laser generates in conjunction with a subsequent clectro-optic switch - a single, nearly bandwidth limited pulse of 6 ps duration. This laser pulse at a frequency 0f7~ = 9455 cm-l traverses two properly oriented LiNbO, crystals generating two new light pulses at the signal and idler frequencies [ 51. The parametric three-photon process in LiNbO, allows a tuning range between approximately 527
VoIumc 46, number 3
CHEICIICAL l’lIYSfCS
cm- t to 2500 cm-l. ~j~ the help of an infrared spectrometer, SP 1, the frequency and bandwidth of the JR-pulses arc dctcnnincd. In the present experiment we work with a frequency of jiiR = 5950 cm-’ and a pulse duration of 3 X JO-*2 s. Jn this frequency range we arc not far from the subharmonic frequency and as a result the infrared bandwidth is datively large, ATrR = GOcm--l. The infrared beam is focused into the cell containing the santpte. A second laser pulse of smalfer intensity Js generated by a beam splitter at t&e left side of the picture. This pulse is sharpcncd at its Jcading edge by a nonJinear absorber, DC @I3 and CORvertcd to the second harmonic frequency p&_ = f 89 10 cm-l in a KDP-crystaf. The green pulse serves to interrogatc the vibrationally excited moIecules. Both light beams cross in a small ccl1of silica glass placed in the center of a small tdmpcrature-controlled heating oven. The Sower part of the oven was held at a sorncwhat higher tcmperaturc (typically 20’) to avoid condensation of the material at the cell windows. Temnperature values quoted in this paper refer to the upper part (the reservoir) of our system. The celi was filfcd with I to S mg of solid sample, evacuated and seafed off. The fiuoresccnce emitted by the vapor was measured with a spectrometer, SP 2, and a photomultiplier.
ETTERS
15 March1977
7000
3. Results and discussions We J~avcmade a series of mcasurcmcnfs of the absorption, the f’hrcsccncc and, most important, ofthc population lifetime of pofyatomic moJccuJcs in tJra vapor phase, The results discussed here are obtained on couma~n 6 (see fig. 3 top), a rno~~cu~ewe have reccntJy studied in Jiquid solutions [4f. At a temperature of 305”C, where time-dependent data were taken, the coumarin G molecule is quite stable over many hours. At temperatures exceeding 38O”C, decomposition of the sample was observed within approximately one hour. In fig. 2b, the absorption spectrum of caumarin 6 at a temperature of 30S°C is presented. We used an evacuated and sealed cuvette. 1 cm in length. The broad abssrption band is typical for dye vapors. A comparison with the absorption of coum~rin 6 in CC14 solutioa
frequency
5
ted’3
I& 2. Abr;orptionand fkorescencc of coumarin 6 molcculcs: (a) bquid solution of ftY5 ~moljlitcrf in CC14and (b) vapor phaseat 305°C(1 torr). The frequency zz of the interrogating pulse and the frequency Fl+ 72 (infrared mdjcated Pt the abscisa
plus
green pulse)arc
absorption cross-section at the peak of the absorption band at 25 500 cm-l is found to be G = 2..5X I O-16 cm2 f%, = 4 X lo4 QmoFf cm-‘) which is very similar co the value of o = 1.6 X IQ--‘6 cm2 obtained in liquid soJutions af Ccl,. At the abscissa of figs- 2a and 2b, we marked the frequency F2 of the interrogating pulse. It is readily seen from the figure that the absorption at the probing frequency js very small and as a result, the fluorescence produced by the prubing pulse alone is small. Quite different is the situation after popuhrtion of the Librational mode at frequency ‘;r by the infrared puisc. As indicated at the abscissa of figs_2a and b, with two pulses of frequencies ;;i and “2 we reach the strongly absorbing first singlet state of the moJec~c. In the liquid sofution (fig. 2a) we populated previously a Cl-l,-stretching mode at FI = 2970 CZZI-~;in the present work WCexcited an overtone at rl = 5950 cm_1
Volume 46, number
3
CHEMICAL
15 BfarcR 1977
PHYSICS LETTERS
in the vapor phase and in the liquid state. There arc three reasons which made us choose the overtone at 5950 cm-l for direct infrared excitation. (i) Infrared measurements of coumarin 6 in Ccl4 17] revealed distinct absorption around 5950 cm-l ; this absorption corresponds to u = 2 overtones of the CH aromatic vibrations [8 ] . (ii) Tuning our ‘;; frequency through this frequency range gave a peak in the fluorescence signal at rI = 5950 cm-l indicating a good Franck-Condon factor for the transition to the S,state. (iii) The thermal population of this high lying overtone vibration is very small (approximately 10B6) at the temperature of our vapor at 30S°C. As a result, the fluorescence signal without IR-excitation is very low. We recall the small absorption at the Z$frequency (see fig. 2). The fluorescence of coumarin 6 vapor shows a good quantum efficiency somewhat smaller than the value in liquid solutions (V = 0.7). The fluorescence was cxcited close to the peak of the absorption band with the filtered 405 nm (24700 cm-l) line of a small Hg-lamp. We measured the fluorescence spectrum of the vapor at 305’C with a spectrograph and an optical multichannel analyscr. In thrs way, 350 data points were obtained between 16000 and 26000 cm-l, while illuminating the sample for 30 s only. The measured fluorescence spectrum (corrected for equipment parameters) is depicted in fig. 2b left; it indicates a larger Stokes shift than found in solution (fig. 2a). The O-O transition shifts to 23400 cm-l, i.e. approximately 2000 cm-l to higher frequencies in the vapor phase. Similar shifts were recently observed on several structurally quite different, large polyatomic molecules 191. Next we present out time resolved data on the population lifetime of the overtone-level at 5950 cm-l. This energy state was populated by a first resonant infrared pulse and the fluorescence signal generated by a second probe pulse was observed. In fig. 3, the measured fluorescence is plotted as a function of delay time between the infrared pump and the green probe pulse. The frequency width of the observed fluorescence is indicated in fig. 2 by the cross-hatched regions. Fig. 3 clearly shows the rise of the fluorescence signal to a maximum vaIue during the population of the vibrational state. (Note, at time zero the peaks of the two pulses overlap.) After the excitation has terminated, the population of the excited state decreases exponentially with a time constant of r = 4 +, 1 ps, i.e. the ini-
I
-5
I 0
I :
10 Oeiay Tune IDE 5 3
I 15 E IC
I-rg. 3. Vtbratronal relaxation time of coum~rm G in the vapor at 305°C. An ovcrtonc at 5950 cm-’ IS prlmardy cxcitcd by an infwcd pulse. The fluorcsccncc, produced by rhc probe pulse, is plotted versus delay time between the exciting infrarcd and probing green pulse. The exponenth: decay of the sgnal gives J population lifetime of4 *_ 1 ps. Tap: The coumdrin 6 molcculc.
tia1Iy excited vibrational state is very rapidly dcpopuIated. We performed the same experiment at room temperature with coumarin 6 dissolved in CCI, and a concentration close to that of the vapor. PreIiminary data indicated a time constant shorter than 2 ps, a va1ue very similar to the one Found for the CK3-mode in an earlier experiment [4]. Quite obviously, the retaxation time in the vapor is comparable to that in the liquid state. In our experiments we measure the occupation density times the Franck-Condon factor of the primari.Iy excited vibrational states and of the energy states which are populated during the subsequent vibrationa reIaxation. There are two processes which effect our measured time constant. First, energy transfer to cnergetitally neighboring vibrational states and second, decay of the high vibrational state to lower vibrationaL levels. We point to the high density of states at * 6000 cm-l in our polyatomic molecule. Energy states below = 4500 cm-’ do not contribute to our probing signaL 529
Volume 46, number 3
CilEMICAL PItYSICS LETTERS
The observed time constant is equal to the population lifetime of the primarily excited vibrational mode if the subsequent vibrational states do not give a significant fluorescence signal. At the moment we cannot say which of the two processes determines the obserJed time constant. What we do know in the vapor phase is the intramoleeular nature of the relaxation process. For a kinetic cross section of ~he order of l 0-33 cm 2 one estimates a collision time of approximately 10 -7 s in the vapor, i.e. the observed time constant is not effected by intermolecular processes. The following number~ are of interest. The incident infrared pulse contains ~ I034 photons, approximately 10 -4 of which are absorbed by the molecular vapor. The probe pulse interrogates the vibrationally excited vol~Jme with lO ! 1 photons over a beam diameter of 0.3 ,'-am. The resulting light intensities aro relatively small and disturbing nonli~,~-ar optical processes (e.g. stimulated emission) and ~.;eessive heating o f the sample may be neglected. A comment should be made concerning the time resolution of our experimental system. It is important to note that the shape - more precisely, the wings of the ultrashort pulsez are essential for the ultimate time resolution. Very favorable conditions are provided here by the sharpened leading edge of the p,obe pulse and by the steeply rising wings of the infra,ed pulse resulting from the highly nonlinear parame'ric generation process. In fact, we have tested directly the time resolution using the very fast (10-15 s) frequency summation, v"1 + ~'2, of the two light pulses in a nonlinear cr~ tel [t0]. Experimentally, we observe a system resolution of O.4 +-.0.2 ps. This result convinces us that the reported lifetime of 4 ps in the vapor is a true time constant of the investigated molecular vapor. in a recent paper [I I ], perylene and POPOP molecules in the vapor phase were excited several thousand wave numbers above the 0 - 0 transition by a train picosecond pulses. The fluorescence rise time was measured with a streak camera to be less than or equal to 30 and 20 ps for perylene and POPOP, respectively. The re~olution time of the streak camera was about 15 ps. The authors of re(. [ 11 ] consider the rise time
530
~5 M~rd~ 1977
of the fluorescence to be a measure of the non-radiative relaxation time. On account of the shot1 time con. stanl presented here, the data of re(. [I || appear ~o be exceedingly large..
us to make these meamremen~ ~ t h good ~ u r a c y .
Acknowledgement The authors wish to thank Prof~sor W. l.fittke for valuable information on the infrared spectrum of eoumatin 6. They are also grateful to Pcofe~sora S. Fischer and F.P. Sch~fer for helpful discur~ion~.
References ! ! ] A. Laubereau, D. yon der Linde and W. Kaiser, I~yi. Rev. Letters 28 (19721 1162.
12l A. Laubereau, L. Ktrschner and W. Kaiser. OpL Commun. 9 (1973) 1831 A. Laubereau, G. Kehi and W. i~i~er. Opt. Commun. t 1 13974) 74; K. Spanner, A. Laubel'eau and W. K~iser, Chem. Phy~o Letlers 44 (3976) 88. t31 R.R. Alfano and S.L. Shapiro. Phyg Rev. Letters 29 11972) t655; R.R. Monson, S. Patum|ev~piba|, IC$. K'.,ufmann and G.W. Robinson, Chem. Phys. Letters 28 ~|974~ 3| 2. [41 A. Laubereau, A. SeiimeJer and W. KaLes, Chem. ~y~. Lelters 36 (]975} 232. 15] A. Laubereau, L. Greiter and W. Ka/ser, AppL Phyg Letters 25 (1974) 87. [6] A. Penzkofer, D. yon tier LLnde, A. Lzuberezu ~r~ W. Kaiser, AppL Phys. Letters 20 H972~ 35L 17l W. Liittke, private communication. 181 H. Kempter and R. Meeke, Z. Naturforsc~. 2a ~|947~
549. [9] B. Steyer and F.P. Seh~fer, AppL Phys. 7 ~1975) ] 33. |10] A. Seilmeier, IC Spanner, A. Laubereau and W. Kaiser, to be publ~hed. [ 11 ] S.L. Shapiro, R.C. Hyer and A J . Camp~o. Phys. Re-¢. Lc~ers 33 f 1974) 513.
Volume 46, number 3
ULTRASHORT
VIBRATIONAL
15 March 1977
POPULATION LIFETIME
OF LARGE POLYATOMIC MOLECULES IN THE VAPOR PHASE J.P. MAIER, A. SEILMEIER, A. LAUBEREAU and W. KAISER Rzysik-Department der Technisrhen Unirersr&. Munchen. Munich, Germany Received 17 December 1976
A vibrationa mode of a polyatornic n~~lcculcin the vapor phase is f’kt excited by an ultrashort infrared puke. Fhedesce of excitation is monItored by a delayed probe pulse which promotes the excited molecules to the fluorescent first singlet state. The lifctlmc of an overtone at 5950 cm” of coumarin 6 is found to be 4 +- 1 ps in the vapor phase at 1 torr.
1. Introduction
In recent years, methods have been developed to measure the population lifetimes of weII-defined vibrational modes of polyatomic molecules in liquids [l-4]. Very short time constants of the order of IO-I2 s were observed and inter- as well as intramolecular interaction processes were established [2,3]. In the condensed phases the molecular packing is very dense and the interactions of a vibrationally excited molecule with its surrounding are important. Very recently, we have introduced a double resonance technique where the population lifetime of vibrational modes in the electronic ground state can be investigated in highly diluted systems [4]. The vibrational mode of interest is first excited by an ultrafast resonant infrared pulse and a propedy delayed interrogating pulse of higher frequency promotes the molecule close to the bottom of the fluorescent first singlet state. The observed fluorescence signal is a direct measure of the instantaneous popuIation of the vibrational state. In this paper we apply this method to coumarin 6 in the vapor phase. WC present for the first time data of population lifetimes in the electronic ground state. We have previously studied the same molecule in a diluted Iiquid system.
2. Experimental In fig. 1, out experimental
set-up is depicted sche-
L
Sample Celt
PO Q 50bPV I ig. 1. Schcmdaic of the experimental syttcm to measure population lifetimes m the vapor phase. A sin& picosecond puk generates ultrashort infrared pulses in a singfe pass through two nonlinear L1hSb@3 crystals. Tuning of the infrared frcquencies is achiticd by crystal rotation. The infkar& Frcqucncy and the bandwidth are monitored with spectrometer SP 1. The probe pulse is converted to the second harmonic frequency in a KDP crystal and optically dcldyed before intcrcogating the vlbrationally excited volume. Spectrometer, SP 3, measures the produced fluorcscencc signal.
matically. A mode-locked Nd-glass laser generates in conjunction with a subsequent clectro-optic switch - a single, nearly bandwidth limited pulse of 6 ps duration. This laser pulse at a frequency 0f7~ = 9455 cm-l traverses two properly oriented LiNbO, crystals generating two new light pulses at the signal and idler frequencies [ 51. The parametric three-photon process in LiNbO, allows a tuning range between approximately 527
VoIumc 46, number 3
CHEICIICAL l’lIYSfCS
cm- t to 2500 cm-l. ~j~ the help of an infrared spectrometer, SP 1, the frequency and bandwidth of the JR-pulses arc dctcnnincd. In the present experiment we work with a frequency of jiiR = 5950 cm-’ and a pulse duration of 3 X JO-*2 s. Jn this frequency range we arc not far from the subharmonic frequency and as a result the infrared bandwidth is datively large, ATrR = GOcm--l. The infrared beam is focused into the cell containing the santpte. A second laser pulse of smalfer intensity Js generated by a beam splitter at t&e left side of the picture. This pulse is sharpcncd at its Jcading edge by a nonJinear absorber, DC @I3 and CORvertcd to the second harmonic frequency p&_ = f 89 10 cm-l in a KDP-crystaf. The green pulse serves to interrogatc the vibrationally excited moIecules. Both light beams cross in a small ccl1of silica glass placed in the center of a small tdmpcrature-controlled heating oven. The Sower part of the oven was held at a sorncwhat higher tcmperaturc (typically 20’) to avoid condensation of the material at the cell windows. Temnperature values quoted in this paper refer to the upper part (the reservoir) of our system. The celi was filfcd with I to S mg of solid sample, evacuated and seafed off. The fiuoresccnce emitted by the vapor was measured with a spectrometer, SP 2, and a photomultiplier.
ETTERS
15 March1977
7000
3. Results and discussions We J~avcmade a series of mcasurcmcnfs of the absorption, the f’hrcsccncc and, most important, ofthc population lifetime of pofyatomic moJccuJcs in tJra vapor phase, The results discussed here are obtained on couma~n 6 (see fig. 3 top), a rno~~cu~ewe have reccntJy studied in Jiquid solutions [4f. At a temperature of 305”C, where time-dependent data were taken, the coumarin G molecule is quite stable over many hours. At temperatures exceeding 38O”C, decomposition of the sample was observed within approximately one hour. In fig. 2b, the absorption spectrum of caumarin 6 at a temperature of 30S°C is presented. We used an evacuated and sealed cuvette. 1 cm in length. The broad abssrption band is typical for dye vapors. A comparison with the absorption of coum~rin 6 in CC14 solutioa
frequency
5
ted’3
I& 2. Abr;orptionand fkorescencc of coumarin 6 molcculcs: (a) bquid solution of ftY5 ~moljlitcrf in CC14and (b) vapor phaseat 305°C(1 torr). The frequency zz of the interrogating pulse and the frequency Fl+ 72 (infrared mdjcated Pt the abscisa
plus
green pulse)arc
absorption cross-section at the peak of the absorption band at 25 500 cm-l is found to be G = 2..5X I O-16 cm2 f%, = 4 X lo4 QmoFf cm-‘) which is very similar co the value of o = 1.6 X IQ--‘6 cm2 obtained in liquid soJutions af Ccl,. At the abscissa of figs- 2a and 2b, we marked the frequency F2 of the interrogating pulse. It is readily seen from the figure that the absorption at the probing frequency js very small and as a result, the fluorescence produced by the prubing pulse alone is small. Quite different is the situation after popuhrtion of the Librational mode at frequency ‘;r by the infrared puisc. As indicated at the abscissa of figs_2a and b, with two pulses of frequencies ;;i and “2 we reach the strongly absorbing first singlet state of the moJec~c. In the liquid sofution (fig. 2a) we populated previously a Cl-l,-stretching mode at FI = 2970 CZZI-~;in the present work WCexcited an overtone at rl = 5950 cm_1
Volume 46, number
3
CHEMICAL
15 BfarcR 1977
PHYSICS LETTERS
in the vapor phase and in the liquid state. There arc three reasons which made us choose the overtone at 5950 cm-l for direct infrared excitation. (i) Infrared measurements of coumarin 6 in Ccl4 17] revealed distinct absorption around 5950 cm-l ; this absorption corresponds to u = 2 overtones of the CH aromatic vibrations [8 ] . (ii) Tuning our ‘;; frequency through this frequency range gave a peak in the fluorescence signal at rI = 5950 cm-l indicating a good Franck-Condon factor for the transition to the S,state. (iii) The thermal population of this high lying overtone vibration is very small (approximately 10B6) at the temperature of our vapor at 30S°C. As a result, the fluorescence signal without IR-excitation is very low. We recall the small absorption at the Z$frequency (see fig. 2). The fluorescence of coumarin 6 vapor shows a good quantum efficiency somewhat smaller than the value in liquid solutions (V = 0.7). The fluorescence was cxcited close to the peak of the absorption band with the filtered 405 nm (24700 cm-l) line of a small Hg-lamp. We measured the fluorescence spectrum of the vapor at 305’C with a spectrograph and an optical multichannel analyscr. In thrs way, 350 data points were obtained between 16000 and 26000 cm-l, while illuminating the sample for 30 s only. The measured fluorescence spectrum (corrected for equipment parameters) is depicted in fig. 2b left; it indicates a larger Stokes shift than found in solution (fig. 2a). The O-O transition shifts to 23400 cm-l, i.e. approximately 2000 cm-l to higher frequencies in the vapor phase. Similar shifts were recently observed on several structurally quite different, large polyatomic molecules 191. Next we present out time resolved data on the population lifetime of the overtone-level at 5950 cm-l. This energy state was populated by a first resonant infrared pulse and the fluorescence signal generated by a second probe pulse was observed. In fig. 3, the measured fluorescence is plotted as a function of delay time between the infrared pump and the green probe pulse. The frequency width of the observed fluorescence is indicated in fig. 2 by the cross-hatched regions. Fig. 3 clearly shows the rise of the fluorescence signal to a maximum vaIue during the population of the vibrational state. (Note, at time zero the peaks of the two pulses overlap.) After the excitation has terminated, the population of the excited state decreases exponentially with a time constant of r = 4 +, 1 ps, i.e. the ini-
I
-5
I 0
I :
10 Oeiay Tune IDE 5 3
I 15 E IC
I-rg. 3. Vtbratronal relaxation time of coum~rm G in the vapor at 305°C. An ovcrtonc at 5950 cm-’ IS prlmardy cxcitcd by an infwcd pulse. The fluorcsccncc, produced by rhc probe pulse, is plotted versus delay time between the exciting infrarcd and probing green pulse. The exponenth: decay of the sgnal gives J population lifetime of4 *_ 1 ps. Tap: The coumdrin 6 molcculc.
tia1Iy excited vibrational state is very rapidly dcpopuIated. We performed the same experiment at room temperature with coumarin 6 dissolved in CCI, and a concentration close to that of the vapor. PreIiminary data indicated a time constant shorter than 2 ps, a va1ue very similar to the one Found for the CK3-mode in an earlier experiment [4]. Quite obviously, the retaxation time in the vapor is comparable to that in the liquid state. In our experiments we measure the occupation density times the Franck-Condon factor of the primari.Iy excited vibrational states and of the energy states which are populated during the subsequent vibrationa reIaxation. There are two processes which effect our measured time constant. First, energy transfer to cnergetitally neighboring vibrational states and second, decay of the high vibrational state to lower vibrationaL levels. We point to the high density of states at * 6000 cm-l in our polyatomic molecule. Energy states below = 4500 cm-’ do not contribute to our probing signaL 529
Volume 46, number 3
CilEMICAL PItYSICS LETTERS
The observed time constant is equal to the population lifetime of the primarily excited vibrational mode if the subsequent vibrational states do not give a significant fluorescence signal. At the moment we cannot say which of the two processes determines the obserJed time constant. What we do know in the vapor phase is the intramoleeular nature of the relaxation process. For a kinetic cross section of ~he order of l 0-33 cm 2 one estimates a collision time of approximately 10 -7 s in the vapor, i.e. the observed time constant is not effected by intermolecular processes. The following number~ are of interest. The incident infrared pulse contains ~ I034 photons, approximately 10 -4 of which are absorbed by the molecular vapor. The probe pulse interrogates the vibrationally excited vol~Jme with lO ! 1 photons over a beam diameter of 0.3 ,'-am. The resulting light intensities aro relatively small and disturbing nonli~,~-ar optical processes (e.g. stimulated emission) and ~.;eessive heating o f the sample may be neglected. A comment should be made concerning the time resolution of our experimental system. It is important to note that the shape - more precisely, the wings of the ultrashort pulsez are essential for the ultimate time resolution. Very favorable conditions are provided here by the sharpened leading edge of the p,obe pulse and by the steeply rising wings of the infra,ed pulse resulting from the highly nonlinear parame'ric generation process. In fact, we have tested directly the time resolution using the very fast (10-15 s) frequency summation, v"1 + ~'2, of the two light pulses in a nonlinear cr~ tel [t0]. Experimentally, we observe a system resolution of O.4 +-.0.2 ps. This result convinces us that the reported lifetime of 4 ps in the vapor is a true time constant of the investigated molecular vapor. in a recent paper [I I ], perylene and POPOP molecules in the vapor phase were excited several thousand wave numbers above the 0 - 0 transition by a train picosecond pulses. The fluorescence rise time was measured with a streak camera to be less than or equal to 30 and 20 ps for perylene and POPOP, respectively. The re~olution time of the streak camera was about 15 ps. The authors of re(. [ 11 ] consider the rise time
530
~5 M~rd~ 1977
of the fluorescence to be a measure of the non-radiative relaxation time. On account of the shot1 time con. stanl presented here, the data of re(. [I || appear ~o be exceedingly large..
us to make these meamremen~ ~ t h good ~ u r a c y .
Acknowledgement The authors wish to thank Prof~sor W. l.fittke for valuable information on the infrared spectrum of eoumatin 6. They are also grateful to Pcofe~sora S. Fischer and F.P. Sch~fer for helpful discur~ion~.
References ! ! ] A. Laubereau, D. yon der Linde and W. Kaiser, I~yi. Rev. Letters 28 (19721 1162.
12l A. Laubereau, L. Ktrschner and W. Kaiser. OpL Commun. 9 (1973) 1831 A. Laubereau, G. Kehi and W. i~i~er. Opt. Commun. t 1 13974) 74; K. Spanner, A. Laubel'eau and W. K~iser, Chem. Phy~o Letlers 44 (3976) 88. t31 R.R. Alfano and S.L. Shapiro. Phyg Rev. Letters 29 11972) t655; R.R. Monson, S. Patum|ev~piba|, IC$. K'.,ufmann and G.W. Robinson, Chem. Phys. Letters 28 ~|974~ 3| 2. [41 A. Laubereau, A. SeiimeJer and W. KaLes, Chem. ~y~. Lelters 36 (]975} 232. 15] A. Laubereau, L. Greiter and W. Ka/ser, AppL Phyg Letters 25 (1974) 87. [6] A. Penzkofer, D. yon tier LLnde, A. Lzuberezu ~r~ W. Kaiser, AppL Phys. Letters 20 H972~ 35L 17l W. Liittke, private communication. 181 H. Kempter and R. Meeke, Z. Naturforsc~. 2a ~|947~
549. [9] B. Steyer and F.P. Seh~fer, AppL Phys. 7 ~1975) ] 33. |10] A. Seilmeier, IC Spanner, A. Laubereau and W. Kaiser, to be publ~hed. [ 11 ] S.L. Shapiro, R.C. Hyer and A J . Camp~o. Phys. Re-¢. Lc~ers 33 f 1974) 513.