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Vibrational relaxation by electronic energy transfer in glyoxal vapours
Volume 23, number 2
I.5 November
CHEMICAL PHYSICS LETTERS
VIBRATIONAL
1973
RELAXATION BY ELECTRONIC ENERGY TRANSFER IN GLYOXAL VAPOURS A. FRAD and A. TRAMER
Laboraroire de Photophysique
Moliculoire CNRS, Univertiti 91405 Ormy. France
Paris-Sud,
Received 30 August 1973
Fluorescence spectn of the isotopic mixtures: glyoxnl42 - glyoxakfz were studied upon selective single-vibroniclevel excitation. It is shown that the electionic-energy transfer between isotopic species is efficient md determines the rate of the vibrational relaxation.
1. Introduction ,CollisionaUy induced vibrational relaxation in the phase is generally considered as a stepwise process (see, e.g., ref. [l]); and recent fluorescence studies at single-vibronic-level (SVL) excitation [2,3] confirm this model. At low gas pressures (a few collisions per lifetime of the excited electronic state - in the hardsphere approximation), the emission spectrum is composed of the resonance fluorescence from directly excited levels and of bands originating from a few neighbour levels. A large number of collisions is necessary for thermal equilibration of the excited species, the amount of energy removed in one collision not exceeding several hundred cm-l [3-S]. An important anomaly was observed by Parmenter et al. [6] in the case of glyoxal vapours excited to the 8172 tevel (vibrational energy excess A,!& = 940 cm-l) of the first singlet IA, state: even at the lowest gas pressures ( 10m2 torr) the fluorescence from the G vibronic level was almost as strong as the resonance gas
and 72 levels (bEvib = 232 and 463 cm-l) having a non-negligible equilibrium population. The intensity of bands originating from intermediate leveis is extremely low. For example, in the emission excited in the 8; band at a pure glyoxal pressureof 2.5 X 10m2 .torr the integrated intensity of the relaxed fluorescence amounts to ca. 3070 (fig. 1). Since the average time between collisions (the hard-sphere collision diameter being assumed to be u = 4A [8,9] corre-
sponding to k = 7.6 X lo6 collisions/set torr) is of the order of 6 psec while the zero-pressure lifetime of the 8l level is T = 0.8 ~sec [7], the probability of more than one collision per lifetime is negligible. We would be thus obliged to assume that the predominating f I (arbitrary
units) I
fluorescence. In our sludy of the luminescence
of glyoxal vapours at SVL excitation (8l; 2l; 8’4’ levels with AEtib = 735, 1390 and 1680 cm-‘, respectively) [7] we found that this’phenomenon is quite general. The emission spectrum is a superposition of the resonance emission from the initially excited level and of thermally equilibrated fluorescence from @level and lowest 71
I
L
22000
20000
Vcm-t
Fig. 1. Emission spectrum (uncorrected) of glyoxalSr*, p = The bands from O-level are
2.5 x 10m2 torr, 8; excitaticn. marked with asterisks.
297
.._
.
,.
‘.
:,,.
.,
..
Volume 23, number 2
CHEMICAL PHYSICS LETTERS
process is a direct relaxation to the vibrationless level in a single collision. From the pressure dependence of the relative yields of the resonance and relaxed fluorescence we can evaluate the rate constant of this process: we obtain (radiative Lifetimes of the 0 and 8l Ievels being assumed as identical) k, = 8 X lo6 see-l torr-’ which is nearly equal to the total colLision rate [7]. Such a mechanism involving the transfer of the total vibrational energy between two similar molecules With the probability @ = 1) seems, however, implausible. A strong deviation from equipartition of the excess of vibrational ene:gy between colliding molecules is improbable and contradictory to our knowledge of relaxation processes. This objection will be ruled out if it is assumed that the vibrational relaxation takes place not by vibrational-energy (V-V) transfer: M*(ll=n)
+ M’(v=O) = M*(u=n-1)
I5 November 1973
density measurements. The isotopic shifts of the O-O transition and of uh and v{ frequencies are of the order of 50 cm-l. This allows for the selective excitation of the O-O and 2h transitions in glyoxal-ha and of the 8; one in gIyoxal+ using ca. 2 A spectral slits (for the second component of every isotopic doublet the overlapping of bands is stronger)_ Several em%sion bands of both species may be well resolved with ca. 8 A spectral slits. The experimental set-up has been previously described [ 131.
3. Results The preliminary results may be summarized in the following way. (1) In the case of excitation of the glyoxalk2 O-vibronic level, the bands of glyoxal-d2 appear with a relative intensity increasing with the d/h ratio and for a constant molar fraction - with the total gas pressure. (2) In the case of excitation of higher vibronic levels (the S1 level of the dz- and the 2’ level of the hzspecies: (a) the resonance emission (8: and 8; bands in fig. 2) consists almost uniquely of bands emitted by the directly excited species (M), the relative intensity of the M’ emission does not exceed l/IO even at highest pressures (1 torr); (b) in the relaxed emission (in fig. 2 th; 8; bands broadened by overlap with 8:7: and 8,7, bands) the relative intensities of both isotopic species depend strongly on their mole fractions and slightly on the total gas pressure; (c) the intensity ratio of M and M’ relaxed emissions remains - for a given mole fraction - roughly the same in the case of a quasi-selective excitation of either M or M’ species, as well as in the case of a broad-band (M + M’) excitation. me results may be treated in the framework of a simple kinetic model (fig. 3). We consider the intensities of fluorescence emitted from the O-levels of isotopic species M and M’ contained in a mixture with molar fractions a and (I-a), respkctively. The radiative lifetimes of O-levels kF and kb are supposed to be identical. k, and ki are the zero-pressure decay constants, kq and kh the selfquenching constants assuming equal quenching efficiency of M-M and M-M’
+ M’(Y= I),
but by the resonance transfer of the purely electronic excitation, (E-E) transfer: M*(Y=~) + M’(v=O) = M(v=n) + M’*(v=O), The !atter mechanism was first evidenced in the case of NO by Klemperer et al. [IO, 1 I), by a study of the
fluorescence spectra of isotopic mixtures upon the selective excitation of one isotopic species. While in the case of the (V-V) transfer mechanism the electronic excitation remains localized at the directly excited species M, the (E-E) transfer leads to its redistribution between M and M’ species. T3e emission spectrum would thus contain the M and M’ bands with the intensity ratio depending on the concentration ratio and the total gas pressure. This technique was applied to glyoxal using glyoxa& and gly0xa1-d~ mixtures.
2. Experimental Giyoxal-h;? and glyoxald2 were obtained by oxidation of ethylene and ethylene-d4 with an SeO,-P,05 mixture and purified by fractional distillation in vacua. The isotopic exchange is slow enough to prevent the formation of the mixed (hd) species even after several hours of irradiation. The gas pressures were measured ivith calibrated Pirani gauges and controlled by optical-. 298
: : 1
.‘._
.: .;
,::,
..‘,.>,-,
:
; -.,
2,
‘,,
y
:
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:
: :.:
:
..,
;,
.
..:i:
__
..
.,
.
,.
: -1
,:
:_, -.
15 November 1973
CHEMICAL PHYSICS LETTERS
Volume 23, number 2
d2 1
81
relaxation by the (V-V) 1. In the case of a direct M we obtain:
mechanism. excitation OF the 0-Ievel of
dz
(1) and if the transfer constants Boltzmann factor:
differ only by the
ktfk; = exp(AElkT),
2. On the other hand, if the O-levels are populated by the collisional relaxation front a higher vibronic level ofM:
21600
Fig. 2. Fart of the emission spectrum of the mixture: glyosalh2 (p = 0.2 tarr) + gIyoxal+ (p = 0.1 torr) excited in the S& band of the deutecared species. T&e electronicenergy transfer between the isotopic species without a change in vibrational quart. turn number is characterized by k;, k, for the vibrationless level and by kr, k;* for higher vibronic levels. In view of the weakness of,the M’ resonance emission (see 2a). the twostep rehtxation involving the M(v=l) -M’(u=l) transfer is neglected. k, and k; characterize the relaxation by the (E-E) transfer between identical and isotopic molecules; k, may include t&o the collisions.
M
M’
a
[J-al
II-al k!a
Fig. 3. Scheme of relaxation and mnsfer processes.
[k;k; +a(k,-k;)k;
tk,k@ fk,kk c[k,k; + (l-a)(k,-k~)kt+k~k,]p+k:ks
a
4
-ff = I-a
If it is supposed
m(4)
that k, = ki (4,) is reduced to:
and if k: = 0 (corresponding to relaxation of the higher vibronic level of M by the vibrational-energytransfer mechanism, i.e., without M-M’ transfer of the electronic excitation), eq. (4) would be reduced to eq. (1). It is to be noted that if eq. (5j is valid, the intensity ratios of the relaxed fluorescence are identical for any excited vibronic level of either isotopic species. As mentioned above (2c), we are not very far from this limiting case. Since the decay and quenching rate constants have been previously determined [7] as: k, = 4.9 X lo5 set-t, k, = 3 X 106 se~-~ torr-l for the hydrogenated and kS = 1.9 X 10s set-l, kq = 2.6 x 106 see-t torr-t for the deuterated species, k, for the h,-dz fransfer may be determined from the pressure dependence of the I,lr[ ratios in the case of the selective excitation of the O-level of glyoxal+. From the linear pilots similar to that represented in fig. 4, kt(h2 + $1 is estimated as (7.5 k 1.5) X Id set-1 tosr-l, and, if eq. (2) is valid k,(d3+h,) would be of the order of 9 X 10s. By substituting this v&e into eq. (4) one can t lhtse data, obtained by the single-pholqnkounting tech. nique ate JighUy difSerent (especially kq) froth those of Yardley (81 and Beyer [Y) whdused dye-laser excitaiion. ‘ .I 299. .,’ .
.‘. .’‘
,. .(
..‘.
”
,., ‘
,.
L-L’...,,
.
_..
..’
:
.’ 1.
,. ;*,.
.:,
._._
f
1.5 November 1973
CHEMICAL PHYSICS LETTERS
Volume 23, number 2
similar to eq. (3). In the case of the 8; excitation we
fmd k: < 106 see-l torr-! _
4. DiscusGon
0
I.
In the case of NO, as shown in ref. [ II], the rapid electronic-energy transfer is due to the long-range dipole-dipole interactions. For glyoxal, rough estimations of the probability of (E-E) transfer indicate that the observed rate constants are compatible with the Fiirster mechanism [14], if the value of the overlap integral:
Vpllari’l e
5
15
IO
Fig. 4. Pressure dependence
of intensity ratios of the 8: emie species: excitation of hZ species the O-O band, hz mole fraction a = 0.14 (0) and 0.21 (0).
sion bands of the h2 and ds in
calculate ‘he expected intensity ratios upon gL and 2’ excitation in the limiting cases: k, = ki and ki = 0. As can be seen in fig. 5, the experimental points ap-
where 4(v) and f(v) are absorption and fluorescence spectra normalized in the wavenumber scale, is of the order of 10d3 - 10e4. A surprising difference between NO and glyoxal is the high efficiency of the relaxing energy transfer as compared to that not involving the change in the vibrational quantum number: in glyoxal, k, is higher by one order of magnitude than k, and k:. This effect may be tentatively explained by a difference in the absorption-fluorescence overlap; in a polyatomic molecule the value of the F, f integral may be higher for the fluorescence originat’ing from excited vibronic levels. On the other hand, its value is certainly different in the case of energy transfer between identical molecules and between two isotopic species:
proach closely the curve corresponding to eq. IS), slight deviations may be due to a non-negligible role of the (V-V) transfer and/or to a higher probability of the (E-E) transfer between identical molecules (see below). Because of the weakness of the M’ resonance enission k: may be only roughly estimated from a relation
h. 0
i
and deviations from the equalities (2) and k, = k: may be expected. One can suppose that the relaxation by (E-E) transfer is a quite general phenomenon. In most molecules its effects are probably hidden by an efficient relaxation due to the (V-V) transfer mechanism. A particular feature of glyoxal is the high value of its self-quenching constant, especially for higher vrbronic levels [7] : “hard” collisions necessary for ordinary relaxation processes quench the fluorescence and the relaxation due to long-range interactions is accentuated. This work is being continued and full results will be published elsewhere.
a ._--_I -__L__l_r_,_
--o-
pftorrl
Fig. 5. Pressure dependence of the intensity ratios of the 137 bands. 8: excitation of glyoxald, at a constant fractional pressure of, 0.1 tort. Experimental points and curwes calculated by: (a), eq. (5) and (b), eq. (3). 300
:_ . .
:.
.’
..
.,;
: ‘.
.,
:’
..‘..
.
.
‘-,
CHEhfICAL PHYSICS LE-ITERS
Volume 23, number 2
[6] L.G. Anderson, C.S. Farmenter, H&f. Poland and
Acknowledgement The authors
are highly
15 November 1973
indebted
to Dr. K. Freed
whose suggestions were at the origin of this work.
References 111 J.L. Stretton, in: Transfcr and storngc of energy by molecules, Vol. 2, cds. GM. Burnett and A.M. North
OVilry-Intersciencr, New York, 1969). 121H,F. Kemperand hf. Stockburgcr, J. Cbem. Phys. 53 (1970) 268. [3f C.S. Parmenter and AH. White, J. Chem. Phys. 50 (1969) 1631. [41 J.M. Blondcau, Tbcsis, LiBe (1971). [Sl L.M. Logan, I. Buduls and LG. Ross, in: Molecular luminescence, rd. E.C. Ltm (Benjamin, New York, 1969); LG. Ross, Conference on Radiationless Transitions, Boulder (1972).
I.D. Rau. Chem. Phys. Letters 8 (1971) 262; C.S. Parmcnter, to be published. j7j A. Frad and A. Tranrer. to be published. 181 J.T. Yardfey. G.W. HoBeman and J.I. Stcinfeld. Cbem. * . Phys. Let& 10 fIP71) 266. [9] R.A. Bcyer and W.C. Lineberger. Chem. Phys. Letters 20 (1973) 600. [ 101 L.A. Melton and W. KJcmpcrer. J. Cbcm. Phys. 55 (1971) 1468. [ll] R.G. Gordon and Ying-Nan Chin, J. Chem. Phys. 55 (1971) 1469. [121 F.W. Buss, J.M. Brown, A.R.H. Cole, A. Lofthus, S.L.N.G. Krishnamachari, G.A. Osborne, G.A. PaIdus, D.A. Rumsay and I,. Watmann, Can. J. Pbys. 48 (1970) 1230; D.M. Agar, E.J. Bair. F.W. Birss. P. Borrell, PC. Chen. G.N. Curric, A.J. hfcHugh, B.J. Orr, D.A. Ramsay and J.-Y. Roncin, Can. J. Phys. 49 (1971) 323. [I3] A. Frad, F. Labmani, A. Tramer and C. Tric, J. Chem. Phys., submitted for publication. [ 141 Th. Fiirster, in: Modem quantum chemistiy, Vol. 3. cd. 0. Sinano& (Academic Press, New York. 1965).
301
..’
1
,‘--. .:
,..‘, _. .,:
” ‘.
‘, . . .,i ._,.
.,.
I.5 November
CHEMICAL PHYSICS LETTERS
VIBRATIONAL
1973
RELAXATION BY ELECTRONIC ENERGY TRANSFER IN GLYOXAL VAPOURS A. FRAD and A. TRAMER
Laboraroire de Photophysique
Moliculoire CNRS, Univertiti 91405 Ormy. France
Paris-Sud,
Received 30 August 1973
Fluorescence spectn of the isotopic mixtures: glyoxnl42 - glyoxakfz were studied upon selective single-vibroniclevel excitation. It is shown that the electionic-energy transfer between isotopic species is efficient md determines the rate of the vibrational relaxation.
1. Introduction ,CollisionaUy induced vibrational relaxation in the phase is generally considered as a stepwise process (see, e.g., ref. [l]); and recent fluorescence studies at single-vibronic-level (SVL) excitation [2,3] confirm this model. At low gas pressures (a few collisions per lifetime of the excited electronic state - in the hardsphere approximation), the emission spectrum is composed of the resonance fluorescence from directly excited levels and of bands originating from a few neighbour levels. A large number of collisions is necessary for thermal equilibration of the excited species, the amount of energy removed in one collision not exceeding several hundred cm-l [3-S]. An important anomaly was observed by Parmenter et al. [6] in the case of glyoxal vapours excited to the 8172 tevel (vibrational energy excess A,!& = 940 cm-l) of the first singlet IA, state: even at the lowest gas pressures ( 10m2 torr) the fluorescence from the G vibronic level was almost as strong as the resonance gas
and 72 levels (bEvib = 232 and 463 cm-l) having a non-negligible equilibrium population. The intensity of bands originating from intermediate leveis is extremely low. For example, in the emission excited in the 8; band at a pure glyoxal pressureof 2.5 X 10m2 .torr the integrated intensity of the relaxed fluorescence amounts to ca. 3070 (fig. 1). Since the average time between collisions (the hard-sphere collision diameter being assumed to be u = 4A [8,9] corre-
sponding to k = 7.6 X lo6 collisions/set torr) is of the order of 6 psec while the zero-pressure lifetime of the 8l level is T = 0.8 ~sec [7], the probability of more than one collision per lifetime is negligible. We would be thus obliged to assume that the predominating f I (arbitrary
units) I
fluorescence. In our sludy of the luminescence
of glyoxal vapours at SVL excitation (8l; 2l; 8’4’ levels with AEtib = 735, 1390 and 1680 cm-‘, respectively) [7] we found that this’phenomenon is quite general. The emission spectrum is a superposition of the resonance emission from the initially excited level and of thermally equilibrated fluorescence from @level and lowest 71
I
L
22000
20000
Vcm-t
Fig. 1. Emission spectrum (uncorrected) of glyoxalSr*, p = The bands from O-level are
2.5 x 10m2 torr, 8; excitaticn. marked with asterisks.
297
.._
.
,.
‘.
:,,.
.,
..
Volume 23, number 2
CHEMICAL PHYSICS LETTERS
process is a direct relaxation to the vibrationless level in a single collision. From the pressure dependence of the relative yields of the resonance and relaxed fluorescence we can evaluate the rate constant of this process: we obtain (radiative Lifetimes of the 0 and 8l Ievels being assumed as identical) k, = 8 X lo6 see-l torr-’ which is nearly equal to the total colLision rate [7]. Such a mechanism involving the transfer of the total vibrational energy between two similar molecules With the probability @ = 1) seems, however, implausible. A strong deviation from equipartition of the excess of vibrational ene:gy between colliding molecules is improbable and contradictory to our knowledge of relaxation processes. This objection will be ruled out if it is assumed that the vibrational relaxation takes place not by vibrational-energy (V-V) transfer: M*(ll=n)
+ M’(v=O) = M*(u=n-1)
I5 November 1973
density measurements. The isotopic shifts of the O-O transition and of uh and v{ frequencies are of the order of 50 cm-l. This allows for the selective excitation of the O-O and 2h transitions in glyoxal-ha and of the 8; one in gIyoxal+ using ca. 2 A spectral slits (for the second component of every isotopic doublet the overlapping of bands is stronger)_ Several em%sion bands of both species may be well resolved with ca. 8 A spectral slits. The experimental set-up has been previously described [ 131.
3. Results The preliminary results may be summarized in the following way. (1) In the case of excitation of the glyoxalk2 O-vibronic level, the bands of glyoxal-d2 appear with a relative intensity increasing with the d/h ratio and for a constant molar fraction - with the total gas pressure. (2) In the case of excitation of higher vibronic levels (the S1 level of the dz- and the 2’ level of the hzspecies: (a) the resonance emission (8: and 8; bands in fig. 2) consists almost uniquely of bands emitted by the directly excited species (M), the relative intensity of the M’ emission does not exceed l/IO even at highest pressures (1 torr); (b) in the relaxed emission (in fig. 2 th; 8; bands broadened by overlap with 8:7: and 8,7, bands) the relative intensities of both isotopic species depend strongly on their mole fractions and slightly on the total gas pressure; (c) the intensity ratio of M and M’ relaxed emissions remains - for a given mole fraction - roughly the same in the case of a quasi-selective excitation of either M or M’ species, as well as in the case of a broad-band (M + M’) excitation. me results may be treated in the framework of a simple kinetic model (fig. 3). We consider the intensities of fluorescence emitted from the O-levels of isotopic species M and M’ contained in a mixture with molar fractions a and (I-a), respkctively. The radiative lifetimes of O-levels kF and kb are supposed to be identical. k, and ki are the zero-pressure decay constants, kq and kh the selfquenching constants assuming equal quenching efficiency of M-M and M-M’
+ M’(Y= I),
but by the resonance transfer of the purely electronic excitation, (E-E) transfer: M*(Y=~) + M’(v=O) = M(v=n) + M’*(v=O), The !atter mechanism was first evidenced in the case of NO by Klemperer et al. [IO, 1 I), by a study of the
fluorescence spectra of isotopic mixtures upon the selective excitation of one isotopic species. While in the case of the (V-V) transfer mechanism the electronic excitation remains localized at the directly excited species M, the (E-E) transfer leads to its redistribution between M and M’ species. T3e emission spectrum would thus contain the M and M’ bands with the intensity ratio depending on the concentration ratio and the total gas pressure. This technique was applied to glyoxal using glyoxa& and gly0xa1-d~ mixtures.
2. Experimental Giyoxal-h;? and glyoxald2 were obtained by oxidation of ethylene and ethylene-d4 with an SeO,-P,05 mixture and purified by fractional distillation in vacua. The isotopic exchange is slow enough to prevent the formation of the mixed (hd) species even after several hours of irradiation. The gas pressures were measured ivith calibrated Pirani gauges and controlled by optical-. 298
: : 1
.‘._
.: .;
,::,
..‘,.>,-,
:
; -.,
2,
‘,,
y
:
”
,F,.‘<,._
:
: :.:
:
..,
;,
.
..:i:
__
..
.,
.
,.
: -1
,:
:_, -.
15 November 1973
CHEMICAL PHYSICS LETTERS
Volume 23, number 2
d2 1
81
relaxation by the (V-V) 1. In the case of a direct M we obtain:
mechanism. excitation OF the 0-Ievel of
dz
(1) and if the transfer constants Boltzmann factor:
differ only by the
ktfk; = exp(AElkT),
2. On the other hand, if the O-levels are populated by the collisional relaxation front a higher vibronic level ofM:
21600
Fig. 2. Fart of the emission spectrum of the mixture: glyosalh2 (p = 0.2 tarr) + gIyoxal+ (p = 0.1 torr) excited in the S& band of the deutecared species. T&e electronicenergy transfer between the isotopic species without a change in vibrational quart. turn number is characterized by k;, k, for the vibrationless level and by kr, k;* for higher vibronic levels. In view of the weakness of,the M’ resonance emission (see 2a). the twostep rehtxation involving the M(v=l) -M’(u=l) transfer is neglected. k, and k; characterize the relaxation by the (E-E) transfer between identical and isotopic molecules; k, may include t&o the collisions.
M
M’
a
[J-al
II-al k!a
Fig. 3. Scheme of relaxation and mnsfer processes.
[k;k; +a(k,-k;)k;
tk,k@ fk,kk c[k,k; + (l-a)(k,-k~)kt+k~k,]p+k:ks
a
4
-ff = I-a
If it is supposed
m(4)
that k, = ki (4,) is reduced to:
and if k: = 0 (corresponding to relaxation of the higher vibronic level of M by the vibrational-energytransfer mechanism, i.e., without M-M’ transfer of the electronic excitation), eq. (4) would be reduced to eq. (1). It is to be noted that if eq. (5j is valid, the intensity ratios of the relaxed fluorescence are identical for any excited vibronic level of either isotopic species. As mentioned above (2c), we are not very far from this limiting case. Since the decay and quenching rate constants have been previously determined [7] as: k, = 4.9 X lo5 set-t, k, = 3 X 106 se~-~ torr-l for the hydrogenated and kS = 1.9 X 10s set-l, kq = 2.6 x 106 see-t torr-t for the deuterated species, k, for the h,-dz fransfer may be determined from the pressure dependence of the I,lr[ ratios in the case of the selective excitation of the O-level of glyoxal+. From the linear pilots similar to that represented in fig. 4, kt(h2 + $1 is estimated as (7.5 k 1.5) X Id set-1 tosr-l, and, if eq. (2) is valid k,(d3+h,) would be of the order of 9 X 10s. By substituting this v&e into eq. (4) one can t lhtse data, obtained by the single-pholqnkounting tech. nique ate JighUy difSerent (especially kq) froth those of Yardley (81 and Beyer [Y) whdused dye-laser excitaiion. ‘ .I 299. .,’ .
.‘. .’‘
,. .(
..‘.
”
,., ‘
,.
L-L’...,,
.
_..
..’
:
.’ 1.
,. ;*,.
.:,
._._
f
1.5 November 1973
CHEMICAL PHYSICS LETTERS
Volume 23, number 2
similar to eq. (3). In the case of the 8; excitation we
fmd k: < 106 see-l torr-! _
4. DiscusGon
0
I.
In the case of NO, as shown in ref. [ II], the rapid electronic-energy transfer is due to the long-range dipole-dipole interactions. For glyoxal, rough estimations of the probability of (E-E) transfer indicate that the observed rate constants are compatible with the Fiirster mechanism [14], if the value of the overlap integral:
Vpllari’l e
5
15
IO
Fig. 4. Pressure dependence
of intensity ratios of the 8: emie species: excitation of hZ species the O-O band, hz mole fraction a = 0.14 (0) and 0.21 (0).
sion bands of the h2 and ds in
calculate ‘he expected intensity ratios upon gL and 2’ excitation in the limiting cases: k, = ki and ki = 0. As can be seen in fig. 5, the experimental points ap-
where 4(v) and f(v) are absorption and fluorescence spectra normalized in the wavenumber scale, is of the order of 10d3 - 10e4. A surprising difference between NO and glyoxal is the high efficiency of the relaxing energy transfer as compared to that not involving the change in the vibrational quantum number: in glyoxal, k, is higher by one order of magnitude than k, and k:. This effect may be tentatively explained by a difference in the absorption-fluorescence overlap; in a polyatomic molecule the value of the F, f integral may be higher for the fluorescence originat’ing from excited vibronic levels. On the other hand, its value is certainly different in the case of energy transfer between identical molecules and between two isotopic species:
proach closely the curve corresponding to eq. IS), slight deviations may be due to a non-negligible role of the (V-V) transfer and/or to a higher probability of the (E-E) transfer between identical molecules (see below). Because of the weakness of the M’ resonance enission k: may be only roughly estimated from a relation
h. 0
i
and deviations from the equalities (2) and k, = k: may be expected. One can suppose that the relaxation by (E-E) transfer is a quite general phenomenon. In most molecules its effects are probably hidden by an efficient relaxation due to the (V-V) transfer mechanism. A particular feature of glyoxal is the high value of its self-quenching constant, especially for higher vrbronic levels [7] : “hard” collisions necessary for ordinary relaxation processes quench the fluorescence and the relaxation due to long-range interactions is accentuated. This work is being continued and full results will be published elsewhere.
a ._--_I -__L__l_r_,_
--o-
pftorrl
Fig. 5. Pressure dependence of the intensity ratios of the 137 bands. 8: excitation of glyoxald, at a constant fractional pressure of, 0.1 tort. Experimental points and curwes calculated by: (a), eq. (5) and (b), eq. (3). 300
:_ . .
:.
.’
..
.,;
: ‘.
.,
:’
..‘..
.
.
‘-,
CHEhfICAL PHYSICS LE-ITERS
Volume 23, number 2
[6] L.G. Anderson, C.S. Farmenter, H&f. Poland and
Acknowledgement The authors
are highly
15 November 1973
indebted
to Dr. K. Freed
whose suggestions were at the origin of this work.
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