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The Dynamics of Populating The Lowest Triplet State Of Benzophenone Following Singlet Excitation
Volume 24, number 2
CHEMICAL PHYSICS LETTERS
15 January 1974
THE DYNAMICS OF POPULATING THE LOWEST TRIPLET STATE OF BENZOPHENONE FOLLOWING SINGLET EXCITATION* Robin M. HOCHSTRASSER, Hanspeter LUTZ and Gary W. SCOTT Deparrtnent of Chemistry and Laboratory for Research on the Structure of Matter, The ffniversity of Pennsylvania, Philadelphia, Pennsylvania I Pi 74. USA
Received 19 October 1973
Measurements of the build-up of triplet-triplet absorption at 5300 A have been made during and subsequent to the excitation of benzophenone by 5 psec pulses of 3545 A radiation (frequency tripled Nd*: glass fundamental). The data were fitted to a simple model that included the finite gaussian pulse width and results for the assumed proCeSSSi%Tr are: k-r = 16.5 f 3 p~ec for benzophenone in ethanol; k-’ = 30 t 5 psec for benzophenone &benzene. This new type of solvent effect on triplet build-up is discussed.
1. Introduction The mechanism mation matic
following ketones
and dynamics excitation
of triplet state for-
of singlet states
in the condensed
of aro-
phase have been
studied theoretically and experimentally from many viewpoints. With reference to benzophenone a current idea is that the very fast intersystem crossing is induced by the interaction
of the benzophenone
states
with the
environment
[I]. A number of experiments indicate that in the condensed phase the lntr* state relaxes to some other electronic state in times that are very short compared with the natural radiative rate for Inn* + IA. In addition to the extremely low quantum yield for fluorescence, there have been measurements of linewidths in low temperature high resolution absorption spectra [2], and direct measurements of short-lived transients 13-51 following excitation of J nn*, both of which suggest electronic relaxation times in the range of 10 psec for inn*. It was always assumed that the disappearance of the Inrr* state would yield triplet states of benzophenone: a question arises as to the dynamics of populating the lowest triplet state, 3nrr*, which is known to appear with virtually unit quantum yield by * lhis research was supported by a grant from the National Science Foundation (GP25334) and by the NSF-MRL at the University of Pennsylvania. ’ N.I.H. post-Doctoral Fellow 1972-73. 162 ..
the time nanoseconds have elapsed following excitation of I mr*. The study of fast transients by Rentzepis et al. [3-S] gave different relaxation times following excitation of two different regions of the 1m-r* state, and these were interpreted as being caused by non-uniform coupling between 1nn* and 3n,*. These states are within 2000 cm-l of one another and it was calculated [I] that the rate of intersystem crossing, Inn* + 3nrr* with energy conservation, may fluctuate up and down with increasing vibronic energy of excitation in In,*_ The linewidth measurements [2] for excited vibronic states in the It-m* manifold seemed to indicate that relaxation of these states increased with increasing energy of excitation_ The causes of the line-broadening, hence the pathways for relaxation, are not known. In order to provide additional information about the dynamical processes in aromatic ketones in the condensed phase and perhaps to clarify some of the : unexplained processes alluded to above, we have devised experiments to study the appearance of the lowest triplet, 3ntr*, of benzophenone withii a few picaseconds following the excitation of lnn*. Such measurements had previously [6,71 been performed in the tens of nanosecond time regime for aromatic hy.‘. drocarbons. For_aromatic ketones the triplet build-up .. .__ _c~~curs on the time scale of tens of picoseconds and this paper represents the first rep.ort of such a study:. .. ;
Volume 24, number 2
CHEMICAL PHYSICS LETTERS
in the time regime where electronic, vibrational and
over = 45” of its length using a Korad K-l laser head.’ Approximately 213 of the laser shots were well modelocked trains. A single pulse was switched out of the mode-locked train using a deuterated KDP Pockels cell which was between crossed glan laser prisms. The pulse was switched out when a half-wave voltage pulse from a coaxial capacitor, discharged through a laser initiated spark gap breakdown, was applied to the Pockels cell (see fig. 1).
perhaps chemicai relaxation can occur simultaneously. In addition we have measured an environmental effect by studying the triplet build-up in two solvents.
2. Experimental details 2. I. Sample preparation Benzophenone was obtained from Eastman Kodak and zone refined in 40 passes, The solvents, benzene (Baker, Spectrophotometric) and ethanol (Pharmco, U.S.P. 200 Proof) were used without further purification. The solutions were not degassed and contained 0.40 moles/liter of benzophenone in benzene and 0.47 moles/liter of benzophenone in ethanol.
The polarization of the 10600 A pulse was rotated IT/Zby a half-wave plate. This pulse was amplified by a 6” Brewster-Brewster Nd3*:glass laser rod (k” diameter ED-2 core with l/16” ED-5 cladding, Owens-.
Illinois) pumped over a 44” length using a Korad K-l laser head. The capacitor voltage for this flashlamp was 4.9 C 0.1 kV (4800 f 200 J), and was fired simultaneously with the laser flashlamp. The net single pass gain of the amplifier was 7.1 times with a standard deviation of 1.1 in a series of shots. A “good” single pulse at 10600 A from the amplifier had an energy of typitally 50 mJ and a two photon fluorescence spot width of I .5 mm in ethanol, which corresponds to a gaussian pulsewidth of 5 psec. The 10600 A single p&e was frequency doubled to 5300 A using an angle phase matched KDP crystal. (Typically, a single pulse of 5300 A had an energy content of 4 mJ.) Mixing 1@600 A with 5300 A in a type II KDP crystal [8] produced
2.2. The laser An Nd3+ glass laser was mode-locked and Q-switched using a Brewster angle, 1 cm cell of Q-switch solution EK-9860 in 1,ZdichIoroethane having OD(10600 A) = 0.20 at.1 incidence. The 0.85 m laser cavity was formed by a spherical rear dielectric reflector (R = 99.9%, 10m radius) and a planar front reflector (R = 40%). The 8” X 4” diameter Brewster-Brewster laser rod (ED-2 glass, Owens-Illinois) was flashlamp pumped Ml Q
N,
M2G
P
G
H
N,
15 January 1974
KI
K, F, UR
DL
L, /F
A, ,!
4
;
.. Fig.1. Experikental arrangement: Mz mirror; Q: @itch
L
._ .:
cell; Nrbeodymium rod;_G:@an laser @sm, P; P&keli cell; SG: +a& _ _.-
gap; I-iv: high voltage; Hz half-wave plate; K: KI?P crystal; F: filter; UR; ul&avide~re~e&g,
d@lectric &r&bL.:
v+bie
: :cd delay; L: lens; A: aperture; D; diffuser; BS:,i+m splitieri S::sa@e; PD: photodiode; OX: osciIkwope~... : .-_ ,_. ‘; _; -: ; : -. .‘;.--~_I _. _;- ::. _I_._. ; -: .. : ... -. : ‘,~,..-._I :. . . ._ ..~,., 5, ,-., : ;:. ..’ I. : : ..Y., .: ‘:- ; _,.,:. _.. .,._,. ,-- _ : ___.._‘,. .- .:.: ,;_.;.:;:.._;
o&i:
.-
.: ’ .,.: .: _ -.I ..’_;;:.; i6j’.
I -: ‘,.. -. .- : :, ,: :.,_ :. .-:.. .-. :._ _y..’‘: .:::.- :. _.:_y: ,.L .: :: ;: : : ._..: _,_, _i,. j ..z. : --‘.‘Y ;.- X’ I_,;:.-_.:... .. _.I ..‘:i .--.,:_:r-;:..,:i;. : ,.i:’ _;...~ _.,.;.:,;-_.‘. .,...“x’-- .r. ,_;. - .,>_ _:;..._:;.:, ,..;;,.. _-...:. .?. ,.L...:~*. . . .. .,
Volilme 24, number 2
CkIEhtICAL PHYSICS LRTTERS
a pulse at 3545 A with typically spectral width of 130 cm-r.
1 mJ of energy and
2-3. Merhori of rneanrremenr The principle of the experimental method shown in fig-l was used for measuring at room temperature the build-up of absorption at 5300 BLof benzophenone in ethr?nol solution and in benzene solution. Modifications of this apparatus that were also used included one in which both the W excitation pulse and the green probe pulse were collimated at the sample position and the variable delays were achieved by moving the sample parallel to the pulse paths. The samples were contained in 1 mm path suprasil cells. After faltering out the 10600 a pulse, the time coincident 5300 A and 3545 R pulses split into separate paths by a dielectric mirror. The 3545 W pulse intensity is monitored off a beam splitter by il fast photodiode, and, after a fwed delay is focused on the sample f&rough a 1.09 mm aperture. MeanwhiIe, the 5300 A pulse undergoes a variable delay and is attenuated, focused on a diffuser, recollimated, monitored off a beam splitter, focused on the W excited portion of the sampIe, recollimated again, and finally measured with the fast photodiode. The 3 pulses (UV, reference, and probe) measured with the fast photodiode (Hadron, 1053, S-20 photocathode) are sent through supplementary delay lines so that they arrive at the photodiode at Q=2 nsec intervals. AIso, different filters are used in the path of each pulse to balance the beams. The output voltage of the fast photocliode was monitored with a fast oscilloscope (Tektronix, 519), and the oscillosco~ye traces were recorded using Polaroid type 410 film. The butid-up of the absorption of 5300 A light as a function of time following excitation at 3545 A was observed in our experiment by changing the variable deIay of the 5300 .& pulse so that it arrived at the sample either just before, during or at various fured times following excitation by the 3545 A pulse. The 3545 A and the 5300 A pulses had perpendicular poIarization. Usually 4 or more laser shots were collected for a UV excited sample and at Ieast 1 shot with the W pulse blocked for each delay time Investigated. The height of each pulse Was measured from the Polar&d picture using a precision spectroscopic cornparator. The combined,& (and fallj time of the photodiode-oscilloscope combination p 0.3 nsec) is long 164 -
15 January 1974
compared with the actual pulse duration and therefore the height of each pulse is proportional to the integral of the actual pulse intensity_ The 3 pulses are sufficiently separated in arrival time at the photodiode so that they are completely resolved on the photographs. 2.4. Experimental results From the measured p&e heights, we calculated each photograph,j, the following ratio:
.. -.. ,‘-.. . .. :‘.... ‘. .~ ,;..,.. .. _. ......\ >: ‘~.. ‘.
,_ ..
_. .:
c :
.
..
for
(0 whereH(r,j,,uj) is the height of the 5300 A probe pulse which depends on the delay time, t; the laser shot, jr and the W pulse height for shot j, Uj; and Ho (;i) is the height of the 5300 A reference pulse. We assume R depends only on t and uj. (Note: Only “‘good” laser shots with no evidence of double puising or other earlier pulses were used in the analysis.) From our data, it was reasonable to assume that when the W pulse was blacked, R(t, 0) =R, was independent oft and j_ Hence an average “unpumped ratio”, R,, was calculated for each of the 2 experiments we report. The standard deviations inRO were 3.9% and 1.7% for the experiments on benzophenone in ethanol and benzene, respectively. For each j, we calculated
o(t,j) = log [(&,/R(t+)]
ui”
and in what follows we assume that the D(f, j) is due to triplet-triplet absorption 191 and is proportional to the number of triplet benzophenone molecules in the sample produced after delay t by a normalized W excitation pulse. The experimental points, $, shown in fig.2 give the average D(r) of ah shots at each delay and the error bars give the standard deviations in the averages. 2.5. Treatment of data We have chosen the simplest passibIe kinetic scheme to explain our experimental results. Although the details of the relaxation of the initially excited benzophenone molecules may be more complex, We will see below that this simple model adequately accounts for our exp&-ime&al results. We issumi ‘that the initiaIIy~
excited singlet state (Si) dicays in a single rate..determining step; with. iate k, to the loprest triplet state, ?I, .
,- _-, ..
.:
:
.. .
..:
.:
,
.’
.. ._,.:
‘.~,._:.,‘:
-y.. ...
_;
1.
,.
:-;..
i..
;-..;
_::
Volume 24, number 2
.8
’
!
CHEhfICAL PHYSICS LETTERS ,
,
r
,
,
I
-
15 January 1974
ta for t > 70 psec (ethanol) and t > 125 psec (benzene). The smooth curves in fig.2 correspond to values of k-l = 16.5 psec (ethanol) and k-l = 30 psec (benzene). For benzophenone in ethanol the data were not reproduced within experimental error by choices of k-1 = 12 psec or k-1 = 21 psec. For benzophenone in benzene the data were not reproduced within error by experimental choices of k-l = 25 psec or k-l = 35 psec. It seems most improbable that the best fit values of k-l couId be in error by more than +- 3 psec (ethanol) or -C5 psec (benzene)_ There is an indication from some of OUTdata around t = 0 that there may be a weakly absorbing, decaying transient, for example as would be caused by a relatively weak absorption from Si. At present, the pro-
,
.6
portion of such a transient is small enough that improved data will be required to expose it.
b) 3. Discussion
TIME
(OS)
Fig.2. Optical densitiesat 5300 A due to triplet benzophenone formation as a function of delay followingsinglet state excitation as measuredin (a) 0.40 moles/liter of benzophcnone in benzene, and (b) O-47 moles/liter of benzophenone in ethanol. The84 are the normalizedopticaI densities as described in the
text, and the smooth curves are the calculated D(r) alsodescribed in the text. or
SilT,.
(3)
The Inn* state of benzophenone in solution originates [IO, Ii] at about 26200 cm-l_ The vibrations active in the spectrum [12, 131 are the carbonyl stretching mode at I170 cm-l and a torsional mode at 90 cm-l _This torsional mode is only 102 cm-1 in the
ground state thus one can expect significant hot-band absorption, involving also the remaining low frequency modes, in spectra recorded at ambient temperatures. Excitation of benzophenone with 3545 A is therefore expected to produce vibronic states spanning a significant energy range. The region of 2000 cm-1 excess energy over zero-point will be reached by absorption from the zero-point level of the ground state, and the
We further assume that the T, state has a significant-
region of 2 1 b0 cm-l excess energy over zero-point will be reached by absorption of 3545 A from ther-
ly greater extinction coefficient at 5300 A than Sj or any other possible intermediate state. Finally, we as-
mann factors of about 0.6. Thus a significant propor-
sume that alJ exciting and probing pulses are gaussian with a fixed duration (fwhm) of 5 psec. On the basis of these assumptions it is possible to expressD(t) as a numerically integrable function of k, the pulsewidth, and the experimentally obtainable values of DexP (-). The curve of D(r) versus t was calculated for various values of k until the experiments were reproduced. D (a) was obtained by averaging da-
mally populated
levels near 100 cm-l having Boltz-
tion of the levels reached by 3545 A excitation should lie below the vibronic state 0 + 2 X 1170 cm-l. In addition, the levels reached are determined. by the spectral width of the 3545 A pulse. Otir experiments determine the rate of appearance of the lowest triplet State Tl which may be reached by two basic pathways from the initially excited singlet state Sj:
-..
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
-Si
G* v
{Ti}
The constants k,, kk and kk are relaxation rate constants determining the vibrational relaxation in the sirrgIet manifold, electronic and/or vibrational relaxation to Tr from Ti, and also to Tt from Tj, respectively. /csT and kb are rate constants for Intersystem crossing from S, and from other singlet vibronic states {S.} reached by vibrational relaxation from SJ _Ti and T,. may be different electronic states than TJ or just different vibronic levels of T, , but it is known that at least one and probably two ns* triplet states lie close to, but just above, the zero-point level of the singlet nn* state (2,141. If the experimental data is correctly described by the rheoreticaI curves drawn in fig.2 then bottlenecks must exist in both the pathways to T, described above. For example in the path Si + iTi) + T, either k~+k~ork$%k~: thus our measurements refer either to intersystem crossing or to relaxation in the triplet manifold. Recent measurements of vibrational relaxation have yielded upper limits for relaxation times In the range 2-I 1 psec (average about 6 psec) for organic dye molecules in various solutions [ 15191. if vibrational relaxation times were as long as, say, 11 psec then the lower pathway of our kinetic scheme would be favored but this would be at variance with the results of Rentiepis et al. [l, 3-51 who have claimed that the intersystem crossing depends on the wavelength of excitation in the lmr* state. What is important for the present is that we have shown that relatively high precision measurements can.be made of the build-up of triplet states in a situation where the intersystem crossing is known to be iu the picosecond regime: the simplest interpretation is that the -measurements refer to Intersystem crossing rates but further work is needed to fully investigate the rarqifications of the scheme pretinted above. The results show that the build-up of Tt of.benzophenone in ethanol occurs about twice as fast as in benzene solution_ If it were assumed that ihe relaxation without change in total spin was not solvent sensitive, or that it w& fast compared w& the observed :
IS January 1974
rates, then we could conclude that the intersystem crossing step is dependent on solvent. The Inn* and 3nn* states will be shifted to higher energy in ethanol compared with benzene so the initially excited state in ethanol consists of less excess vibrational energy than in benzene: however, there is an increased possibility of reaching a 3nn* state energy due to the shifts to lower energy of these states. Assuming that the spinorbit interaction does not change, there appear to be two obvious possibilities for the unusual (because the processes are so fast) solvent effect that we have discovered: (1) vibrational relaxation (kR) has increased sufficiently in ethanol to cause the intersystem crossing to occur from lower singlet vibronic states having different k,, ; (2) the intimate details of the coupling between S and {Ti} are modified by the change in solvent due to modifications of the widths and locations of energy levels in the two manifolds. At present we are seeking answers to these interesting new questions. In one of the previous experiments [3-51 on benzophenone in solution 3471 A radiation was used for excitation in which case there is sufficient energy to initially excite predominantly levels involving two quanta of the carbonyl stretching mode. The discrepancy between that previous measurement (Lifetime of a fast transient [5] was 20 psec) and our studies iu benzene may be caused by the difference in the excitation wavelength. However, our mkasurements do not necessarily measure the intersystem crossing rates that were claimed to have been determined by Rentzepis et al. [3-S]. These workers studied the disappearance of a “singletsinglet” absorption, we have observed the appearance of a known [9] triplet-triplet absorption. Our results place a lower limit on the intersystem crossing rate.
Acknowledgement We thank Dr. J.E. Wessel for advice regarding the con&&ion of the laser and experimental methods.
References
[ 11 A. Nits=. J. Jortner and P;M. Rent&&, C&m: phyi Liztteis8 (1971) 445. .:
Volume 24, number 2
CHEMICAL PHYSICS LETTERS
[2] S. Dym and R.M. Hochstrasser, J. Chem. Phys. 51 (1969) 2458. [ 31 P.M. Rentzepis, Science 169 (1970) 239. [4] P.M. Rentzepis and C.J. Mitschele, Anal. Chem. 42 (1970) 20. [S] P.M. Renaepis and GE. Busch, Mol. Photochem- 4 (1972) 353. [6] R. Bonneau, J. Faure, I. Joussot-Dubien, L. Lfndqvfst and C. Barthelemy. Compt. Rend. Acad. Sci. (Paris) B267 (1968) 412. [7] R. Bonneau, J. Faure and J. Joussot-Dubien, Chem. Phys. Letters 2 (1968) 65. ]8] hl. Okada and S. leiri, Japan. 3. Appl. Phys. 10 (1971) 808. 191 D.S. McClure and P-L. Hanst, J. Chern. Phys. 23 (1955)
1772_ [IO] R.M. Hochstrasser and J.E. Wesset, Chem. Phys. Letters 19 (1973) 156.
15 January 1974
[ 111 R.E. Brown, L.A. Singer and J.H. Parks. Chem. Phys. Letters 14 (1972) 193. [I21 S. Dym, R.M. Hochstrasser and M. Schafer, J. Chem. Phys. 48 (1968) 646. (131 R.M. Hochstrasser, G.W. Scott and A.H. Zewail, 3. Chem. Phys. SB (1973) 393. [14] M. Batley and D.R. Kearns, Chem. Phys. Letters 2 (1968)
.-_.
423.
1151
P-M_Rentzepis, Chem. Phys. Letters 2
CHEMICAL PHYSICS LETTERS
15 January 1974
THE DYNAMICS OF POPULATING THE LOWEST TRIPLET STATE OF BENZOPHENONE FOLLOWING SINGLET EXCITATION* Robin M. HOCHSTRASSER, Hanspeter LUTZ and Gary W. SCOTT Deparrtnent of Chemistry and Laboratory for Research on the Structure of Matter, The ffniversity of Pennsylvania, Philadelphia, Pennsylvania I Pi 74. USA
Received 19 October 1973
Measurements of the build-up of triplet-triplet absorption at 5300 A have been made during and subsequent to the excitation of benzophenone by 5 psec pulses of 3545 A radiation (frequency tripled Nd*: glass fundamental). The data were fitted to a simple model that included the finite gaussian pulse width and results for the assumed proCeSSSi%Tr are: k-r = 16.5 f 3 p~ec for benzophenone in ethanol; k-’ = 30 t 5 psec for benzophenone &benzene. This new type of solvent effect on triplet build-up is discussed.
1. Introduction The mechanism mation matic
following ketones
and dynamics excitation
of triplet state for-
of singlet states
in the condensed
of aro-
phase have been
studied theoretically and experimentally from many viewpoints. With reference to benzophenone a current idea is that the very fast intersystem crossing is induced by the interaction
of the benzophenone
states
with the
environment
[I]. A number of experiments indicate that in the condensed phase the lntr* state relaxes to some other electronic state in times that are very short compared with the natural radiative rate for Inn* + IA. In addition to the extremely low quantum yield for fluorescence, there have been measurements of linewidths in low temperature high resolution absorption spectra [2], and direct measurements of short-lived transients 13-51 following excitation of J nn*, both of which suggest electronic relaxation times in the range of 10 psec for inn*. It was always assumed that the disappearance of the Inrr* state would yield triplet states of benzophenone: a question arises as to the dynamics of populating the lowest triplet state, 3nrr*, which is known to appear with virtually unit quantum yield by * lhis research was supported by a grant from the National Science Foundation (GP25334) and by the NSF-MRL at the University of Pennsylvania. ’ N.I.H. post-Doctoral Fellow 1972-73. 162 ..
the time nanoseconds have elapsed following excitation of I mr*. The study of fast transients by Rentzepis et al. [3-S] gave different relaxation times following excitation of two different regions of the 1m-r* state, and these were interpreted as being caused by non-uniform coupling between 1nn* and 3n,*. These states are within 2000 cm-l of one another and it was calculated [I] that the rate of intersystem crossing, Inn* + 3nrr* with energy conservation, may fluctuate up and down with increasing vibronic energy of excitation in In,*_ The linewidth measurements [2] for excited vibronic states in the It-m* manifold seemed to indicate that relaxation of these states increased with increasing energy of excitation_ The causes of the line-broadening, hence the pathways for relaxation, are not known. In order to provide additional information about the dynamical processes in aromatic ketones in the condensed phase and perhaps to clarify some of the : unexplained processes alluded to above, we have devised experiments to study the appearance of the lowest triplet, 3ntr*, of benzophenone withii a few picaseconds following the excitation of lnn*. Such measurements had previously [6,71 been performed in the tens of nanosecond time regime for aromatic hy.‘. drocarbons. For_aromatic ketones the triplet build-up .. .__ _c~~curs on the time scale of tens of picoseconds and this paper represents the first rep.ort of such a study:. .. ;
Volume 24, number 2
CHEMICAL PHYSICS LETTERS
in the time regime where electronic, vibrational and
over = 45” of its length using a Korad K-l laser head.’ Approximately 213 of the laser shots were well modelocked trains. A single pulse was switched out of the mode-locked train using a deuterated KDP Pockels cell which was between crossed glan laser prisms. The pulse was switched out when a half-wave voltage pulse from a coaxial capacitor, discharged through a laser initiated spark gap breakdown, was applied to the Pockels cell (see fig. 1).
perhaps chemicai relaxation can occur simultaneously. In addition we have measured an environmental effect by studying the triplet build-up in two solvents.
2. Experimental details 2. I. Sample preparation Benzophenone was obtained from Eastman Kodak and zone refined in 40 passes, The solvents, benzene (Baker, Spectrophotometric) and ethanol (Pharmco, U.S.P. 200 Proof) were used without further purification. The solutions were not degassed and contained 0.40 moles/liter of benzophenone in benzene and 0.47 moles/liter of benzophenone in ethanol.
The polarization of the 10600 A pulse was rotated IT/Zby a half-wave plate. This pulse was amplified by a 6” Brewster-Brewster Nd3*:glass laser rod (k” diameter ED-2 core with l/16” ED-5 cladding, Owens-.
Illinois) pumped over a 44” length using a Korad K-l laser head. The capacitor voltage for this flashlamp was 4.9 C 0.1 kV (4800 f 200 J), and was fired simultaneously with the laser flashlamp. The net single pass gain of the amplifier was 7.1 times with a standard deviation of 1.1 in a series of shots. A “good” single pulse at 10600 A from the amplifier had an energy of typitally 50 mJ and a two photon fluorescence spot width of I .5 mm in ethanol, which corresponds to a gaussian pulsewidth of 5 psec. The 10600 A single p&e was frequency doubled to 5300 A using an angle phase matched KDP crystal. (Typically, a single pulse of 5300 A had an energy content of 4 mJ.) Mixing 1@600 A with 5300 A in a type II KDP crystal [8] produced
2.2. The laser An Nd3+ glass laser was mode-locked and Q-switched using a Brewster angle, 1 cm cell of Q-switch solution EK-9860 in 1,ZdichIoroethane having OD(10600 A) = 0.20 at.1 incidence. The 0.85 m laser cavity was formed by a spherical rear dielectric reflector (R = 99.9%, 10m radius) and a planar front reflector (R = 40%). The 8” X 4” diameter Brewster-Brewster laser rod (ED-2 glass, Owens-Illinois) was flashlamp pumped Ml Q
N,
M2G
P
G
H
N,
15 January 1974
KI
K, F, UR
DL
L, /F
A, ,!
4
;
.. Fig.1. Experikental arrangement: Mz mirror; Q: @itch
L
._ .:
cell; Nrbeodymium rod;_G:@an laser @sm, P; P&keli cell; SG: +a& _ _.-
gap; I-iv: high voltage; Hz half-wave plate; K: KI?P crystal; F: filter; UR; ul&avide~re~e&g,
d@lectric &r&bL.:
v+bie
: :cd delay; L: lens; A: aperture; D; diffuser; BS:,i+m splitieri S::sa@e; PD: photodiode; OX: osciIkwope~... : .-_ ,_. ‘; _; -: ; : -. .‘;.--~_I _. _;- ::. _I_._. ; -: .. : ... -. : ‘,~,..-._I :. . . ._ ..~,., 5, ,-., : ;:. ..’ I. : : ..Y., .: ‘:- ; _,.,:. _.. .,._,. ,-- _ : ___.._‘,. .- .:.: ,;_.;.:;:.._;
o&i:
.-
.: ’ .,.: .: _ -.I ..’_;;:.; i6j’.
I -: ‘,.. -. .- : :, ,: :.,_ :. .-:.. .-. :._ _y..’‘: .:::.- :. _.:_y: ,.L .: :: ;: : : ._..: _,_, _i,. j ..z. : --‘.‘Y ;.- X’ I_,;:.-_.:... .. _.I ..‘:i .--.,:_:r-;:..,:i;. : ,.i:’ _;...~ _.,.;.:,;-_.‘. .,...“x’-- .r. ,_;. - .,>_ _:;..._:;.:, ,..;;,.. _-...:. .?. ,.L...:~*. . . .. .,
Volilme 24, number 2
CkIEhtICAL PHYSICS LRTTERS
a pulse at 3545 A with typically spectral width of 130 cm-r.
1 mJ of energy and
2-3. Merhori of rneanrremenr The principle of the experimental method shown in fig-l was used for measuring at room temperature the build-up of absorption at 5300 BLof benzophenone in ethr?nol solution and in benzene solution. Modifications of this apparatus that were also used included one in which both the W excitation pulse and the green probe pulse were collimated at the sample position and the variable delays were achieved by moving the sample parallel to the pulse paths. The samples were contained in 1 mm path suprasil cells. After faltering out the 10600 a pulse, the time coincident 5300 A and 3545 R pulses split into separate paths by a dielectric mirror. The 3545 W pulse intensity is monitored off a beam splitter by il fast photodiode, and, after a fwed delay is focused on the sample f&rough a 1.09 mm aperture. MeanwhiIe, the 5300 A pulse undergoes a variable delay and is attenuated, focused on a diffuser, recollimated, monitored off a beam splitter, focused on the W excited portion of the sampIe, recollimated again, and finally measured with the fast photodiode. The 3 pulses (UV, reference, and probe) measured with the fast photodiode (Hadron, 1053, S-20 photocathode) are sent through supplementary delay lines so that they arrive at the photodiode at Q=2 nsec intervals. AIso, different filters are used in the path of each pulse to balance the beams. The output voltage of the fast photocliode was monitored with a fast oscilloscope (Tektronix, 519), and the oscillosco~ye traces were recorded using Polaroid type 410 film. The butid-up of the absorption of 5300 A light as a function of time following excitation at 3545 A was observed in our experiment by changing the variable deIay of the 5300 .& pulse so that it arrived at the sample either just before, during or at various fured times following excitation by the 3545 A pulse. The 3545 A and the 5300 A pulses had perpendicular poIarization. Usually 4 or more laser shots were collected for a UV excited sample and at Ieast 1 shot with the W pulse blocked for each delay time Investigated. The height of each pulse Was measured from the Polar&d picture using a precision spectroscopic cornparator. The combined,& (and fallj time of the photodiode-oscilloscope combination p 0.3 nsec) is long 164 -
15 January 1974
compared with the actual pulse duration and therefore the height of each pulse is proportional to the integral of the actual pulse intensity_ The 3 pulses are sufficiently separated in arrival time at the photodiode so that they are completely resolved on the photographs. 2.4. Experimental results From the measured p&e heights, we calculated each photograph,j, the following ratio:
.. -.. ,‘-.. . .. :‘.... ‘. .~ ,;..,.. .. _. ......\ >: ‘~.. ‘.
,_ ..
_. .:
c :
.
..
for
(0 whereH(r,j,,uj) is the height of the 5300 A probe pulse which depends on the delay time, t; the laser shot, jr and the W pulse height for shot j, Uj; and Ho (;i) is the height of the 5300 A reference pulse. We assume R depends only on t and uj. (Note: Only “‘good” laser shots with no evidence of double puising or other earlier pulses were used in the analysis.) From our data, it was reasonable to assume that when the W pulse was blacked, R(t, 0) =R, was independent oft and j_ Hence an average “unpumped ratio”, R,, was calculated for each of the 2 experiments we report. The standard deviations inRO were 3.9% and 1.7% for the experiments on benzophenone in ethanol and benzene, respectively. For each j, we calculated
o(t,j) = log [(&,/R(t+)]
ui”
and in what follows we assume that the D(f, j) is due to triplet-triplet absorption 191 and is proportional to the number of triplet benzophenone molecules in the sample produced after delay t by a normalized W excitation pulse. The experimental points, $, shown in fig.2 give the average D(r) of ah shots at each delay and the error bars give the standard deviations in the averages. 2.5. Treatment of data We have chosen the simplest passibIe kinetic scheme to explain our experimental results. Although the details of the relaxation of the initially excited benzophenone molecules may be more complex, We will see below that this simple model adequately accounts for our exp&-ime&al results. We issumi ‘that the initiaIIy~
excited singlet state (Si) dicays in a single rate..determining step; with. iate k, to the loprest triplet state, ?I, .
,- _-, ..
.:
:
.. .
..:
.:
,
.’
.. ._,.:
‘.~,._:.,‘:
-y.. ...
_;
1.
,.
:-;..
i..
;-..;
_::
Volume 24, number 2
.8
’
!
CHEhfICAL PHYSICS LETTERS ,
,
r
,
,
I
-
15 January 1974
ta for t > 70 psec (ethanol) and t > 125 psec (benzene). The smooth curves in fig.2 correspond to values of k-l = 16.5 psec (ethanol) and k-l = 30 psec (benzene). For benzophenone in ethanol the data were not reproduced within experimental error by choices of k-1 = 12 psec or k-1 = 21 psec. For benzophenone in benzene the data were not reproduced within error by experimental choices of k-l = 25 psec or k-l = 35 psec. It seems most improbable that the best fit values of k-l couId be in error by more than +- 3 psec (ethanol) or -C5 psec (benzene)_ There is an indication from some of OUTdata around t = 0 that there may be a weakly absorbing, decaying transient, for example as would be caused by a relatively weak absorption from Si. At present, the pro-
,
.6
portion of such a transient is small enough that improved data will be required to expose it.
b) 3. Discussion
TIME
(OS)
Fig.2. Optical densitiesat 5300 A due to triplet benzophenone formation as a function of delay followingsinglet state excitation as measuredin (a) 0.40 moles/liter of benzophcnone in benzene, and (b) O-47 moles/liter of benzophenone in ethanol. The84 are the normalizedopticaI densities as described in the
text, and the smooth curves are the calculated D(r) alsodescribed in the text. or
SilT,.
(3)
The Inn* state of benzophenone in solution originates [IO, Ii] at about 26200 cm-l_ The vibrations active in the spectrum [12, 131 are the carbonyl stretching mode at I170 cm-l and a torsional mode at 90 cm-l _This torsional mode is only 102 cm-1 in the
ground state thus one can expect significant hot-band absorption, involving also the remaining low frequency modes, in spectra recorded at ambient temperatures. Excitation of benzophenone with 3545 A is therefore expected to produce vibronic states spanning a significant energy range. The region of 2000 cm-1 excess energy over zero-point will be reached by absorption from the zero-point level of the ground state, and the
We further assume that the T, state has a significant-
region of 2 1 b0 cm-l excess energy over zero-point will be reached by absorption of 3545 A from ther-
ly greater extinction coefficient at 5300 A than Sj or any other possible intermediate state. Finally, we as-
mann factors of about 0.6. Thus a significant propor-
sume that alJ exciting and probing pulses are gaussian with a fixed duration (fwhm) of 5 psec. On the basis of these assumptions it is possible to expressD(t) as a numerically integrable function of k, the pulsewidth, and the experimentally obtainable values of DexP (-). The curve of D(r) versus t was calculated for various values of k until the experiments were reproduced. D (a) was obtained by averaging da-
mally populated
levels near 100 cm-l having Boltz-
tion of the levels reached by 3545 A excitation should lie below the vibronic state 0 + 2 X 1170 cm-l. In addition, the levels reached are determined. by the spectral width of the 3545 A pulse. Otir experiments determine the rate of appearance of the lowest triplet State Tl which may be reached by two basic pathways from the initially excited singlet state Sj:
-..
CHEMICAL PHYSICS LETTERS
Volume 24, number 2
-Si
G* v
{Ti}
The constants k,, kk and kk are relaxation rate constants determining the vibrational relaxation in the sirrgIet manifold, electronic and/or vibrational relaxation to Tr from Ti, and also to Tt from Tj, respectively. /csT and kb are rate constants for Intersystem crossing from S, and from other singlet vibronic states {S.} reached by vibrational relaxation from SJ _Ti and T,. may be different electronic states than TJ or just different vibronic levels of T, , but it is known that at least one and probably two ns* triplet states lie close to, but just above, the zero-point level of the singlet nn* state (2,141. If the experimental data is correctly described by the rheoreticaI curves drawn in fig.2 then bottlenecks must exist in both the pathways to T, described above. For example in the path Si + iTi) + T, either k~+k~ork$%k~: thus our measurements refer either to intersystem crossing or to relaxation in the triplet manifold. Recent measurements of vibrational relaxation have yielded upper limits for relaxation times In the range 2-I 1 psec (average about 6 psec) for organic dye molecules in various solutions [ 15191. if vibrational relaxation times were as long as, say, 11 psec then the lower pathway of our kinetic scheme would be favored but this would be at variance with the results of Rentiepis et al. [l, 3-51 who have claimed that the intersystem crossing depends on the wavelength of excitation in the lmr* state. What is important for the present is that we have shown that relatively high precision measurements can.be made of the build-up of triplet states in a situation where the intersystem crossing is known to be iu the picosecond regime: the simplest interpretation is that the -measurements refer to Intersystem crossing rates but further work is needed to fully investigate the rarqifications of the scheme pretinted above. The results show that the build-up of Tt of.benzophenone in ethanol occurs about twice as fast as in benzene solution_ If it were assumed that ihe relaxation without change in total spin was not solvent sensitive, or that it w& fast compared w& the observed :
IS January 1974
rates, then we could conclude that the intersystem crossing step is dependent on solvent. The Inn* and 3nn* states will be shifted to higher energy in ethanol compared with benzene so the initially excited state in ethanol consists of less excess vibrational energy than in benzene: however, there is an increased possibility of reaching a 3nn* state energy due to the shifts to lower energy of these states. Assuming that the spinorbit interaction does not change, there appear to be two obvious possibilities for the unusual (because the processes are so fast) solvent effect that we have discovered: (1) vibrational relaxation (kR) has increased sufficiently in ethanol to cause the intersystem crossing to occur from lower singlet vibronic states having different k,, ; (2) the intimate details of the coupling between S and {Ti} are modified by the change in solvent due to modifications of the widths and locations of energy levels in the two manifolds. At present we are seeking answers to these interesting new questions. In one of the previous experiments [3-51 on benzophenone in solution 3471 A radiation was used for excitation in which case there is sufficient energy to initially excite predominantly levels involving two quanta of the carbonyl stretching mode. The discrepancy between that previous measurement (Lifetime of a fast transient [5] was 20 psec) and our studies iu benzene may be caused by the difference in the excitation wavelength. However, our mkasurements do not necessarily measure the intersystem crossing rates that were claimed to have been determined by Rentzepis et al. [3-S]. These workers studied the disappearance of a “singletsinglet” absorption, we have observed the appearance of a known [9] triplet-triplet absorption. Our results place a lower limit on the intersystem crossing rate.
Acknowledgement We thank Dr. J.E. Wessel for advice regarding the con&&ion of the laser and experimental methods.
References
[ 11 A. Nits=. J. Jortner and P;M. Rent&&, C&m: phyi Liztteis8 (1971) 445. .:
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CHEMICAL PHYSICS LETTERS
[2] S. Dym and R.M. Hochstrasser, J. Chem. Phys. 51 (1969) 2458. [ 31 P.M. Rentzepis, Science 169 (1970) 239. [4] P.M. Rentzepis and C.J. Mitschele, Anal. Chem. 42 (1970) 20. [S] P.M. Renaepis and GE. Busch, Mol. Photochem- 4 (1972) 353. [6] R. Bonneau, J. Faure, I. Joussot-Dubien, L. Lfndqvfst and C. Barthelemy. Compt. Rend. Acad. Sci. (Paris) B267 (1968) 412. [7] R. Bonneau, J. Faure and J. Joussot-Dubien, Chem. Phys. Letters 2 (1968) 65. ]8] hl. Okada and S. leiri, Japan. 3. Appl. Phys. 10 (1971) 808. 191 D.S. McClure and P-L. Hanst, J. Chern. Phys. 23 (1955)
1772_ [IO] R.M. Hochstrasser and J.E. Wesset, Chem. Phys. Letters 19 (1973) 156.
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[ 111 R.E. Brown, L.A. Singer and J.H. Parks. Chem. Phys. Letters 14 (1972) 193. [I21 S. Dym, R.M. Hochstrasser and M. Schafer, J. Chem. Phys. 48 (1968) 646. (131 R.M. Hochstrasser, G.W. Scott and A.H. Zewail, 3. Chem. Phys. SB (1973) 393. [14] M. Batley and D.R. Kearns, Chem. Phys. Letters 2 (1968)
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423.
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