It is shown that sub-Doppler spectroscopy enables one to resolve

individual rotational states in the S^ manifold of polyatomic molecules.

This i s an essential to the understanding of the primary

photophysics within the molecule. Spectra of benzene are found to undergo

substantial changes as the vibrational energy i s raised within S^.

Due to the increased density of vibrational states, Coriolis coupling,

which is already seen at low energies, can lead to effective IVR above

3000 cm""1 excess energy. This onset of IVR may be responsible for the

onset of "Channel Three" in benzene and probably produces gross changes

in the photophysical behavior of any polyatomic molecule

Neusser, H. J.

Riedle, Eberhard

Schlag, E. W.

English

Universität München: Elektronischen Publikationen

628 
HIGH RESOLUTION SUB-DOPPLER EXPERIMENTS ON BENZENE 
E. W. Schlag, Η. J. Neusser, Ε. Riedle 
Ins t i t u t für Physikalische und Theoretische Chemie 
Technische Universität München, D-8046 Garching, West-Germany 
ABSTRACT 
I t is shown that sub-Doppler spectroscopy enables one to resolve 
individual rotational states in the Ŝ  manifold of polyatomic molecules. 
This i s an essential to the understanding of the primary 
photophysics within the molecule. Spectra of benzene are found to under-
go substantial changes as the vibrational energy i s raised within Ŝ . 
Due to the increased density of vibrational states, Coriolis coupling, 
which is already seen at low energies, can lead to effective IVR above 
3000 cm""1 excess energy. This onset of IVR may be responsible for the 
onset of "Channel Three" in benzene and probably produces gross changes 
in the photophysical behavior of any polyatomic molecule. 
INTRODUCTION 
Contrary to small molecules, which can only decay by radiative 
processes or at most dissociation, excited large polyatomic molecules 
can also undergo intramolecular nonradiative decay. One of the prime 
examples that showed the existence of such processes is the benzene mo-
lecule. This was concluded from the distinct deviation of the fluores-
cence quantum yield from unity after excitation of the molecule into 
the S-j electronic state The reason for this deviation is the rela-
xation of the molecule into the t r i p l e t manifold, I.e. Inter-System-
Crossing (ISC). Due to the high density of t r i p l e t states at the energy 
of the S-j state, ISC is believed to be in the s t a t i s t i c a l l i m i t and 
leads to the irreversible decay observed. A f a i r l y smooth increase In 
the decay rate with the vibrational excess energy in Sj would be expec-
ted from this model. However, a sudden nearly complete disappearance 
of fluorescence was observed starting at about 3000 cm"1 excess energy 
/ 2 / . This drop in quantum yield could not be explained with an ISC 
process alone. In addition, from experiments aimed at estimating l i n e -
widths of states above 3000 cm"1 in Sj an even more drastic increase 
of the nonradiative rate at around this energy was concluded /3/ . Recent 
decay time measurements confirmed the strong increase of the decay rate, 
but the increase was not found to be as large A/. Since none of the 
known relaxation processes of large molecules could explain these obser-
vations, a mystic "Channel Three" was invoked /3/ . The nature of this 
"Channel Three" could not be determined from low resolution experiments 
with vibronic state resolution, and especially the discrepancy between 
decay rate measurements and "linewidth" measurements of /3/ remains a 
puzzle. To cl a r i f y the situation, decay time measurements and measu-
rements of the homogeneous linewidth of single defined quantum states 
at rotational resolution are needed. Due to the Doppler broadening of 
about 1.7 GHz such experiments have to be performed with sub-Doppler 
resolution. To gain insight into the exact nature of the nonradiative 
process, states at the onset of "Channel Three" have to be observed. 
© 1988 American Institute of Physics 
629 
EXPERIMENTAL TECHNIQUES 
The experimental set up used for the recording of Doppler-free two-
photon spectra and the investigation of the decay of individual levels 
has been described In detail previously /5,6,J/. Doppler-free excita-
tion takes place i f a molecule simultaneously absorbs two photons of 
equal energy and opposite direction of propagation from a standing wave 
light f i e l d . The light source in our experiments Is a cw frequency 
stabilized single mode ring dye laser. For extremely high spectral reso-
lution and very high sensitivity a cw Doppler-free experiment is per-
formed within a concentric external cavity whose length i s variable and 
locked to the laser frequency /5/ . The UV-fluorescenoe following the 
two-photon-excitation of the molecules i s monitored. The use of the 
external cavity increases the sensitivity by two orders of magnitude 
as compared to the simpler arrangement of a backreflected laser beam. 
A resolution of better than 10 MHz can be obtained with this set up /5/ . 
For the investigation of the decay of individual levels pulsed 
excitation of the molecules has to be used. Light pulses of 500 KW peak 
power and nearly Fourier transform-limited bandwidth are produced by 
pulsed amplification of the cw light /6/. with an additional parasitic 
cavity around the second amplifier the pulse length can be varied bet-
ween 2.5 ns and 10 ns /It, The decay time measurements are not per-
formed within an external resonator lik e in the cw set up. Rather after 
passing through the sample c e l l the laser beam is reflected back into 
i t s e l f to allow the Doppler-free absorption. The resulting UV-fluores-
cenoe signal i s either integrated for the recording of spectra or i t s 
time behavior i s recorded with a transient digiti z e r . In the Doppler-
free two-photon absorption process a l l molecules regardless of their 
velocity contribute to the observed signal. Molecules-are only excited 
to one single level i f the laser frequency i s set to a resolved rota-
tional l i n e in the spectrum regardless of the number of lines within 
the Doppler width. This allows the observation ,of the decay of an indi-
vidual level /6 f7/. 
RESULTS AT LOW EXCESS ENERGY 
To understand the rotational structure of vibronic bands* that i s 
observable at sub-Doppler resolution, vibronic bands with very low ex-
cess energy i n S-j have to be analyzed at f i r s t . The deviations obser-
ved i n the "Channel Three" regime can then be attributed to the unknown 
nonradiative process. 
The lowest S-| state reached by a strong two-photon transition is 
the 141 state at 1570 cm*"1 vibrational excess energy. Both with the ew 
set up and the pulsed one the observed sub-Doppler spectrum of the 
141o band is a line spectrum, with most of the lines corresponding to 
single rovlbronic transition /8,9/. Part of the ow recording of the 
blue part of the Q-branch of this band is shown in figure 1. Each ro-
tational li n e shown has been assigned within the model of a semirigid 
symmetric top. For the whole band more than 5000 lines were identified 
and a f i t to the observed line positions yields extremely accurate 
values for the rotational constants and the quartio centrifugal distor-
tion constants /10/· 
In contrast to the situation described so far, in some places of 
630 
Figure 1. Part of the Doppler-free two-photon spectrum of the Q-branch 
of the 14 1 0 band of C6H6 /10/. 
the spectrum, a line predicted by the symmetric top model i s not found 
in the experimental spectrum. Instead two smaller lines are observed, 
one each to both sides of the expected position. These two lines can 
be explained as transitions to the two quasi-elgenstates, that originate 
from the coupling of the " l i g h t " zero-order rotational state of the 
1U1 manifold to a "dark" zero-order state Due to the isolated 
nature of the coupling i t can be concluded that the dark state belongs 
also to the S<| manifold A detailed analysis shows that for fixed 
Κ values a l l lines within a range of J values show this kind of pertur­
bation. The distance of the two quasi-eigenstates from the expected 
position of the l i g h t state shows the typical form of an avoided cros­
sing /11Λ Since both eigenstates belonging to one l i g h t state can be 
observed in our spectra, we are able to determine the size of the coup­
ling matrix element for each pair. The coupling element on the order 
of 1 GHz is found to depend strongly on both J and K. I t s exact depen­
dence on the rotational quantum numbers agrees very well with that 
expected for perpendicular Coriolis coupling. We therefore conclude 
that some of the rotational states of the 141 vibronic state are coupled 
to rotational states of another vibrational state within S-j by this 
mechanism. Since the two vibronic states have s l i g h t l y d i f f e r i n g rota­
tional constants, only some of the rotational states w i l l be in reso­
nance and this explains the isolated nature of the perturbation. 
With the pulsed set up the decay time of selected single quasi-
eigenstates in 141 can be measured. We observed, that a l l states found 
to be unperturbed possess the same decay time /6/. This shows that the 
nonradiative decay of benzene does indeed happen i n the s t a t i s t i c a l 
631 
l i m i t and does not depend on the rotation of the molecule. However, for 
a l l perturbed states a significantly shorter decay time was found /6/. 
This can be explained by the fact that the dark vibronic state coupled 
to the Hi 1 state dacays rauch faster. The observed decay is then depen­
dent on the degree of admixture of the dark state. Combining our know­
ledge from the spectroscopic analysis and from the decay time measure­
ments we can even determine the decay time of the pure dark state. In 
this way the perturbations observed in the spectrum allow us to inves­
tigate the decay behavior of vibronic states that cannot be directly 
excited by either one- or two-photon absorption. The fact that the 
dark states have a faster nonradiative decay is important for the under­
standing of the observations within the "Channel Three" regime. 
RESULTS AT HIGH EXCESS ENERGY 
The 141 state of benzene discussed so far has 3. vibrational excess 
energy well below the onset of "Channel Three" at around 3000 cm"1. On 
the contrary, the energy of the 1 4 1 1 2 state i s 3*112 cm"1 and just 
s l i g h t l y above this threshold. The rotational structure of the 1 4 1 Q 1 2 O 
two-photon transition leading to this state should be nearly identical 
with that of the 1 4 1 Q band. However, a strong deviation is observed 
in the experiment / 1 2Λ This i s demonstrated i n figure 2 , where the blue 
edge of the Q-branch of the 14"·Q1 2 Q is compared with the same part of 
-GO -50 -XO -30 -20 -10 0 
AV [GHz] 
Figure 2 . Comparison of identical parts of the Doppler-free two-photon 
spectrum of the 11*1 o 1 10 b a n d a n d H 10 l 20 band of CgHö /12Λ 
632 
the l^o^o b a n d / 1 2 / · ^ structure of the l a t t e r band was found to 
agree quite well with a symmetric top model /9Λ In the H 1o l 20 b a n d 
a strongly decreased number of lines i s seen. In an absorption spectrum 
a l l rotational lines would be present, but due to a stronlgy rotational 
dependent nonradiative decay most rotational states do not fluoresce 
and are therefore not observed in our fluorescence-excitation spectrum. 
From the positions of the remaining lines and their alternating inten­
sities i t was found, that in the part up to about -60 GHz only lines 
with Κ = 0 are seen /5,12/. This can be understood by a strong parallel 
Coriolis coupling to a fast decaying dark state, since parallel Corio-
l i s coupling does affect a l l rotational states but the Κ • 0 states. 
The excistence of such a state with a very fast nonradiative decay i s 
consistent with the observation made at low excess energy. 
With our pulsed set up the decay times of the Κ = 0 states (seen 
as sharp lines In the* spectrum of figure 2) were measured. Typical re­
sults are shown in figure 3 /7/. For a l l states single exponential de­
cays were found within the experimental resolution and the decay rate 
increases monotonically with J. The values of the decay rate agree very 
well with the values found from the measurement of the homogeneous l i n e -
width of the same lines /7/. The J-dependence of the decay f i t s nicely 
the dependence expected for an additional perpendicular Coriolis coup­
ling to a broadened dark state. The magnitude of the coupling matrix 
element is identical with the one found from the analysis of the pertur­
bations in the 1 1 1 0 band. However, since the dark state is strongly 
broadened, no s p l i t t i n g of the lines results, but rather the l i g h t state 
is broadened and superimposed on the dark state. The decay behavior of 
the sharp one of the two lines, i.e. the observed increase in the non­
radiative decay rate, Is determined by the degree of mixture and the 
decay behavior of the dark state. The broad dark state i s believed to 
be a combinational state containing quanta of low frequency out-of-
plane modes, which are known to enhance the nonradiative decay /13/. 
0 
ι I ι I ι I ι I 1 
14V 
-1 
-2 -
-3 
-4 
-5 - 1 
I 
Λ · · β \ ^ JK , = 6° 
ι I ι 1 · 1 ι 1 
0 20 40 60 80 100 
TIME (ns) 
Figure 3. Decay curves for Κ - 0 states of the 141 state of C5H6 /7Λ 
633 
SUMMARY AND CONCLUSIONS 
The Doppler-free spectra of Sf benzene discussed above clearly show 
the strong dependence of the nonradiative decay on the rotation of 
the molecule. Especially the strong influence of Coriolis coupling is 
seen. Since the rotational structure of vibronic bands can not be re­
solved with Doppler-limited spectroscopy, a l l previous measurements 
could only show the averaged decay behavior of bunches of eigenstates. 
This could even result in the observation of biexponentlal decays, as 
have been observed for states in the "Channel Three" regime /4/. 
In view of the results presented, the "Channel Three" phenomenon 
is now understood as a strong increase in internal vibrational redis­
tribution (IVR) to fast decaying dark states within the same electronic 
state. The density of vibrational states in Ŝ  needed for effective IVR 
is just reached at 3000 cm"1 excess energy. IVR is at least in part 
caused by Coriolis coupling. For a l l quasi-eigenstates investigated 
the nonradiative decay rate determined from decay time measurements 
Is identical to that determined from linewidth measurements. The devia­
tion found in previous low resolution experiments Is believed to be 
mainly due to the limited experimental resolution and the different 
selection of the bunches of eigenstates in the different experiments. 
ACKNOWLEGEMENT 
The authors wish to thank U. Schubert and H. Sieber for valuable 
experimental help. Financial support of the Deutsche Forschungsgemein­
schaft i s greatfully acknowledged. 
REFERENCES 
/1/ For a review see, C. S. Parmenter, Adv. Chem. Phys. 22, 365 (1972) 
/2/ C. S. Parmenter and Μ .W. Schuyler, Transitions NonRadlat. 
Mol.f Reunion Soc. Chim. Phys., 20th, 1969, p. 92 
/3/ J. H. Calloraon, J. E. Parkin and R. Lopez-Delgado, Chem. Pys. 
Lett. 13, 125 (1972) 
/4/ L. Wunsch, Η. J. Neusser and Ε. W. Schlag, Ζ. Naturforsch. Teil 
A 36, 1340 (1981) 
Μ. Sumitani, D. V. O'Connor, Y. Takagi, Ν. Nakashima, A. Kama-
gawa, Y. Udagawa and K. Yoshihara, Chem. Phys. 93, 359 (1985) 
/5/ E. Riedle and H. J. Neusser, J. Chem. Phys. 80, 4686 (1984) 
/6/ u . Schubert, E. Riedle and H. J . Neusser, J . Chem. Phys. 84, 
5326 (1986) 
/!/ U. Schubert, E. Riedle, Η. J. Neusser and Ε. W. Schlag, 
J. Chem. Phys. 84, 6182 (1986) 
/8/ E. Riedle, Η. J. Neusser and Ε. W. Schlag, J. Chem. Phys. 75, 
4231 (1981) 
/9/ E. Riedle, R. Moder and H.J. Neusser, Opt. Commun. 43, 388 (1982) 
/10/ H. Sieber, E. Riedle and H.J. Neusser, submitted to J. Chem. Phys. 
/11/ E. Riedle, Η. Stepp and Η. J. Neusser, Chem. Phys. Lett. 110, 
452 (1984) 
/12/ E. Riedle, Η. J. Neusser and Ε. W. Schlag, J. Phys. Chem. 86, 
4847 (1982) 
/13/ H. Hornburger and J. Brand, Chem. Phys. Lett. 88, 153 (1982) 


High Resolution Sub-Doppler Experiments on Benzene

Open Access LMU

628 HIGH RESOLUTION SUB-DOPPLER EXPERIMENTS ON BENZENE E. W. Schlag, Η. J. Neusser, Ε. Riedle Ins t i t u t für Physikalische und Theoretische Chemie Technische Universität München, D-8046 Garching, West-Germany ABSTRACT I t is shown that sub-Doppler spectroscopy enables one to resolve individual rotational states in the S^  manifold of polyatomic molecules. This i s an essential to the understanding of the primary photophysics within the molecule. Spectra of benzene are found to under-go substantial changes as the vibrational energy i s raised within S^ . Due to the increased density of vibrational states, Coriolis coupling, which is already seen at low energies, can lead to effective IVR above 3000 cm""1 excess energy. This onset of IVR may be responsible for the onset of "Channel Three" in benzene and probably produces gross changes in the photophysical behavior of any polyatomic molecule. INTRODUCTION Contrary to small molecules, which can only decay by radiative processes or at most dissociation, excited large polyatomic molecules can also undergo intramolecular nonradiative decay. One of the prime examples that showed the existence of such processes is the benzene mo-lecule. This was concluded from the distinct deviation of the fluores-cence quantum yield from unity after excitation of the molecule into the S-j electronic state The reason for this deviation is the rela-xation of the molecule into the t r i p l e t manifold, I.e. Inter-System-Crossing (ISC). Due to the high density of t r i p l e t states at the energy of the S-j state, ISC is believed to be in the s t a t i s t i c a l l i m i t and leads to the irreversible decay observed. A f a i r l y smooth increase In the decay rate with the vibrational excess energy in Sj would be expec-ted from this model. However, a sudden nearly complete disappearance of fluorescence was observed starting at about 3000 cm"1 excess energy / 2 / . This drop in quantum yield could not be explained with an ISC process alone. In addition, from experiments aimed at estimating l i n e -widths of states above 3000 cm"1 in Sj an even more drastic increase of the nonradiative rate at around this energy was concluded /3/ . Recent decay time measurements confirmed the strong increase of the decay rate, but the increase was not found to be as large A/. Since none of the known relaxation processes of large molecules could explain these obser-vations, a mystic "Channel Three" was invoked /3/ . The nature of this "Channel Three" could not be determined from low resolution experiments with vibronic state resolution, and especially the discrepancy between decay rate measurements and "linewidth" measurements of /3/ remains a puzzle. To cl a r i f y the situation, decay time measurements and measu-rements of the homogeneous linewidth of single defined quantum states at rotational resolution are needed. Due to the Doppler broadening of about 1.7 GHz such experiments have to be performed with sub-Doppler resolution. To gain insight into the exact nature of the nonradiative process, states at the onset of "Channel Three" have to be observed. © 1988 American Institute of Physics 629 EXPERIMENTAL TECHNIQUES The experimental set up used for the recording of Doppler-free two-photon spectra and the investigation of the decay of individual levels has been described In detail previously /5,6,J/. Doppler-free excita-tion takes place i f a molecule simultaneously absorbs two photons of equal energy and opposite direction of propagation from a standing wave light f i e l d . The light source in our experiments Is a cw frequency stabilized single mode ring dye laser. For extremely high spectral reso-lution and very high sensitivity a cw Doppler-free experiment is per-formed within a concentric external cavity whose length i s variable and locked to the laser frequency /5/ . The UV-fluorescenoe following the two-photon-excitation of the molecules i s monitored. The use of the external cavity increases the sensitivity by two orders of magnitude as compared to the simpler arrangement of a backreflected laser beam. A resolution of better than 10 MHz can be obtained with this set up /5/ . For the investigation of the decay of individual levels pulsed excitation of the molecules has to be used. Light pulses of 500 KW peak power and nearly Fourier transform-limited bandwidth are produced by pulsed amplification of the cw light /6/. with an additional parasitic cavity around the second amplifier the pulse length can be varied bet-ween 2.5 ns and 10 ns /It, The decay time measurements are not per-formed within an external resonator lik e in the cw set up. Rather after passing through the sample c e l l the laser beam is reflected back into i t s e l f to allow the Doppler-free absorption. The resulting UV-fluores-cenoe signal i s either integrated for the recording of spectra or i t s time behavior i s recorded with a transient digiti z e r . In the Doppler-free two-photon absorption process a l l molecules regardless of their velocity contribute to the observed signal. Molecules-are only excited to one single level i f the laser frequency i s set to a resolved rota-tional l i n e in the spectrum regardless of the number of lines within the Doppler width. This allows the observation ,of the decay of an indi-vidual level /6 f7/. RESULTS AT LOW EXCESS ENERGY To understand the rotational structure of vibronic bands* that i s observable at sub-Doppler resolution, vibronic bands with very low ex-cess energy i n S-j have to be analyzed at f i r s t . The deviations obser-ved i n the "Channel Three" regime can then be attributed to the unknown nonradiative process. The lowest S-| state reached by a strong two-photon transition is the 141 state at 1570 cm*"1 vibrational excess energy. Both with the ew set up and the pulsed one the observed sub-Doppler spectrum of the 141o band is a line spectrum, with most of the lines corresponding to single rovlbronic transition /8,9/. Part of the ow recording of the blue part of the Q-branch of this band is shown in figure 1. Each ro-tational li n e shown has been assigned within the model of a semirigid symmetric top. For the whole band more than 5000 lines were identified and a f i t to the observed line positions yields extremely accurate values for the rotational constants and the quartio centrifugal distor-tion constants /10/· In contrast to the situation described so far, in some places of 630 Figure 1. Part of the Doppler-free two-photon spectrum of the Q-branch of the 14 1 0 band of C6H6 /10/. the spectrum, a line predicted by the symmetric top model i s not found in the experimental spectrum. Instead two smaller lines are observed, one each to both sides of the expected position. These two lines can be explained as transitions to the two quasi-elgenstates, that originate from the coupling of the " l i g h t " zero-order rotational state of the 1U1 manifold to a "dark" zero-order state Due to the isolated nature of the coupling i t can be concluded that the dark state belongs also to the S<| manifold A detailed analysis shows that for fixed Κ values a l l lines within a range of J values show this kind of pertur­bation. The distance of the two quasi-eigenstates from the expected position of the l i g h t state shows the typical form of an avoided cros­sing /11Λ Since both eigenstates belonging to one l i g h t state can be observed in our spectra, we are able to determine the size of the coup­ling matrix element for each pair. The coupling element on the order of 1 GHz is found to depend strongly on both J and K. I t s exact depen­dence on the rotational quantum numbers agrees very well with that expected for perpendicular Coriolis coupling. We therefore conclude that some of the rotational states of the 141 vibronic state are coupled to rotational states of another vibrational state within S-j by this mechanism. Since the two vibronic states have s l i g h t l y d i f f e r i n g rota­tional constants, only some of the rotational states w i l l be in reso­nance and this explains the isolated nature of the perturbation. With the pulsed set up the decay time of selected single quasi-eigenstates in 141 can be measured. We observed, that a l l states found to be unperturbed possess the same decay time /6/. This shows that the nonradiative decay of benzene does indeed happen i n the s t a t i s t i c a l 631 l i m i t and does not depend on the rotation of the molecule. However, for a l l perturbed states a significantly shorter decay time was found /6/. This can be explained by the fact that the dark vibronic state coupled to the Hi 1 state dacays rauch faster. The observed decay is then depen­dent on the degree of admixture of the dark state. Combining our know­ledge from the spectroscopic analysis and from the decay time measure­ments we can even determine the decay time of the pure dark state. In this way the perturbations observed in the spectrum allow us to inves­tigate the decay behavior of vibronic states that cannot be directly excited by either one- or two-photon absorption. The fact that the dark states have a faster nonradiative decay is important for the under­standing of the observations within the "Channel Three" regime. RESULTS AT HIGH EXCESS ENERGY The 141 state of benzene discussed so far has 3. vibrational excess energy well below the onset of "Channel Three" at around 3000 cm"1. On the contrary, the energy of the 1 4 1 1 2 state i s 3*112 cm"1 and just s l i g h t l y above this threshold. The rotational structure of the 1 4 1 Q 1 2 O two-photon transition leading to this state should be nearly identical with that of the 1 4 1 Q band. However, a strong deviation is observed in the experiment / 1 2Λ This i s demonstrated i n figure 2 , where the blue edge of the Q-branch of the 14"·Q1 2 Q is compared with the same part of -GO -50 -XO -30 -20 -10 0 AV [GHz] Figure 2 . Comparison of identical parts of the Doppler-free two-photon spectrum of the 11*1 o 1 10 b a n d a n d H 10 l 20 band of CgHö /12Λ 632 the l^o^o b a n d / 1 2 / · ^ structure of the l a t t e r band was found to agree quite well with a symmetric top model /9Λ In the H 1o l 20 b a n d a strongly decreased number of lines i s seen. In an absorption spectrum a l l rotational lines would be present, but due to a stronlgy rotational dependent nonradiative decay most rotational states do not fluoresce and are therefore not observed in our fluorescence-excitation spectrum. From the positions of the remaining lines and their alternating inten­sities i t was found, that in the part up to about -60 GHz only lines with Κ = 0 are seen /5,12/. This can be understood by a strong parallel Coriolis coupling to a fast decaying dark state, since parallel Corio-l i s coupling does affect a l l rotational states but the Κ • 0 states. The excistence of such a state with a very fast nonradiative decay i s consistent with the observation made at low excess energy. With our pulsed set up the decay times of the Κ = 0 states (seen as sharp lines In the* spectrum of figure 2) were measured. Typical re­sults are shown in figure 3 /7/. For a l l states single exponential de­cays were found within the experimental resolution and the decay rate increases monotonically with J. The values of the decay rate agree very well with the values found from the measurement of the homogeneous l i n e -width of the same lines /7/. The J-dependence of the decay f i t s nicely the dependence expected for an additional perpendicular Coriolis coup­ling to a broadened dark state. The magnitude of the coupling matrix element is identical with the one found from the analysis of the pertur­bations in the 1 1 1 0 band. However, since the dark state is strongly broadened, no s p l i t t i n g of the lines results, but rather the l i g h t state is broadened and superimposed on the dark state. The decay behavior of the sharp one of the two lines, i.e. the observed increase in the non­radiative decay rate, Is determined by the degree of mixture and the decay behavior of the dark state. The broad dark state i s believed to be a combinational state containing quanta of low frequency out-of-plane modes, which are known to enhance the nonradiative decay /13/. 0 ι I ι I ι I ι I 1 14V -1 -2 --3 -4 -5 - 1 I Λ · · β \ ^ JK , = 6° ι I ι 1 · 1 ι 1 0 20 40 60 80 100 TIME (ns) Figure 3. Decay curves for Κ - 0 states of the 141 state of C5H6 /7Λ 633 SUMMARY AND CONCLUSIONS The Doppler-free spectra of Sf benzene discussed above clearly show the strong dependence of the nonradiative decay on the rotation of the molecule. Especially the strong influence of Coriolis coupling is seen. Since the rotational structure of vibronic bands can not be re­solved with Doppler-limited spectroscopy, a l l previous measurements could only show the averaged decay behavior of bunches of eigenstates. This could even result in the observation of biexponentlal decays, as have been observed for states in the "Channel Three" regime /4/. In view of the results presented, the "Channel Three" phenomenon is now understood as a strong increase in internal vibrational redis­tribution (IVR) to fast decaying dark states within the same electronic state. The density of vibrational states in S^  needed for effective IVR is just reached at 3000 cm"1 excess energy. IVR is at least in part caused by Coriolis coupling. For a l l quasi-eigenstates investigated the nonradiative decay rate determined from decay time measurements Is identical to that determined from linewidth measurements. The devia­tion found in previous low resolution experiments Is believed to be mainly due to the limited experimental resolution and the different selection of the bunches of eigenstates in the different experiments. ACKNOWLEGEMENT The authors wish to thank U. Schubert and H. Sieber for valuable experimental help. Financial support of the Deutsche Forschungsgemein­schaft i s greatfully acknowledged. REFERENCES /1/ For a review see, C. S. Parmenter, Adv. Chem. Phys. 22, 365 (1972) /2/ C. S. Parmenter and Μ .W. Schuyler, Transitions NonRadlat. Mol.f Reunion Soc. Chim. Phys., 20th, 1969, p. 92 /3/ J. H. Calloraon, J. E. Parkin and R. Lopez-Delgado, Chem. Pys. Lett. 13, 125 (1972) /4/ L. Wunsch, Η. J. Neusser and Ε. W. Schlag, Ζ. Naturforsch. Teil A 36, 1340 (1981) Μ. Sumitani, D. V. O'Connor, Y. Takagi, Ν. Nakashima, A. Kama-gawa, Y. Udagawa and K. Yoshihara, Chem. Phys. 93, 359 (1985) /5/ E. Riedle and H. J. Neusser, J. Chem. Phys. 80, 4686 (1984) /6/ u . Schubert, E. Riedle and H. J . Neusser, J . Chem. Phys. 84, 5326 (1986) /!/ U. Schubert, E. Riedle, Η. J. Neusser and Ε. W. Schlag, J. Chem. Phys. 84, 6182 (1986) /8/ E. Riedle, Η. J. Neusser and Ε. W. Schlag, J. Chem. Phys. 75, 4231 (1981) /9/ E. Riedle, R. Moder and H.J. Neusser, Opt. Commun. 43, 388 (1982) /10/ H. Sieber, E. Riedle and H.J. Neusser, submitted to J. Chem. Phys. /11/ E. Riedle, Η. Stepp and Η. J. Neusser, Chem. Phys. Lett. 110, 452 (1984) /12/ E. Riedle, Η. J. Neusser and Ε. W. Schlag, J. Phys. Chem. 86, 4847 (1982) /13/ H. Hornburger and J. Brand, Chem. Phys. Lett. 88, 153 (1982) 

1/ For a review see,

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High Resolution Sub-Doppler Experiments on Benzene

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