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Vibrational heating effects in the flash photolysis of ozone
Volu;;_c 36, number 5
VIBRATIONAL J.A. HAhvEY Dcpartmeut
15 Decenbcr
CHEMICAL PHYSICS LETTERS
HEATING
EFFECTS
IN THE FLASH
PHOTOLYSIS
1975
OF OZONE
and W.D. McGRATH
of Chemistry,
iW Queen’s University of Belfasr, Belfast,
UK
Received 28 July 1975
The vibralional rcmperaturc oi ozone hns been estimated during the flash photolvsis of ozone under non-isothermal nnd strictly isothermal conditions, utilisin:! the variation in extinction coefficient with temperature 31 two lv’avelen$hs. Vibrational tcmpera!urcs me;isurcd in the range 300-970 K have been attributed to energy transfer from 0t(3~J to ozone and the effect cussed.
of this non-equilibrium
vibrational
cscitatioll
1. Introduction
on the secondary
reactions
of vibrational
The importance of non-equilibrium distributions of vibration2 energy in reactive systems has recently become a subject of intense int zrest for both practical and theoretical workers in the fields of gas phase kinetics, energy transfer and trajectory studies, the thzoretica! aspect being particularly well investigated by Polanyi and co-workers [I] . These studies suggest that vibrational excitation (V) in reactive species is much more effective in surmounting activation barriers than ei’Lher translational (Tj or rotational (R) excitations even when the reagent energies are several times greater than the barrier height. This is true both for endothermic reactions and for those exothermic reactions with energies ofactivation, the distribution of excess energy- in reactants being maintained in Phe products, viz., AV’ + AV and AT’ 4 AT (where the superscript refers to reactants).
with
excitation
non-zero
activation
cncrgies
in the ozone system
is dis-
[3-T]
Non-equilibrium vibrational distributions have been discussed by Bair et al. [8,9] for ozone systems under conditions in which photolytic explosion occurred with adiabatic temperatures in the regicn of 4500 K and also under near isothermal conditions where thermal effects were unimportant. Ultraviolst flush photolysis studies and analysis of the kinetics of ozone destruction using computer simulation of the reaction scheme by the present authors [IO] emphasize the role played by the reIatively high concentrations of vibrationally excited 02(3Ci) produced rapidly in the following reactions (the symbol $ indicates vibrational excitation): O(‘D)i03+0$
+02., Ax
o(3P)+03
-0:
20(3P) to,.
(1’) + 02 )
(1”) (2)
Recent experimenta and theoretical studies have shown that intermolecular V + V energy transfer is extremely efficient when ne;tr resonant conditions exist. Laser induced vibrational energy studies in ozone systems have prcvided elucidation of Ihe rates of energy transfer between vibrational modes in ozone [?I, and enhancements of reaction rates by orders cf magnitude in reactions involving non-equiiibrium
vibrational energy distributions in ozonehave effectiveIy illustrated
‘564
.’
the particular
importance
of the role
The experiments described in the present work are an attempt to estimate the vibrational content of ozone immediately after the peak of the photolysis flash in order to elucidate the mechanism of heating
of the ozone. Extension of the observations to a time of 500 p’s was also done to allow appraisal of the role of non-equilibrium vibrational excitation in the rate enhancement of secondary reactions occurring in the ultraviolet photolysis of ozone.
Volume
36,
number
5
2. Experiment31
116
The absorption spectrum of ozone in the region of the Hartley continuum was recorded photograpically using a Hilger Littrow spectrograph (E742) for several mixtures of ozone with N, and He in up to 1000.fold excess. A conventional flash photolysis technique was used, the spectra being recorded at delay times of 10 and 25 ~.ts for photolysis flash energies of 270 and 550 J. The Lyman monitoring flash, fired at 92 J, provided a single shot absorption spectrum from which intensity measurements were made at several wavelengths using a modified Hi&r microdensitometer (H481). Light from a quartz iodine lamp, transmitted through t- . photographic plate with a band pass of 0.2 nm, was incident on a photomultiplier tube (RCA lP38A) the output of which was recorded on a nanoammeter. For the purpose of this study the spectral intensities at two wavelengths, 290 and 300 nm, were compared by means of the relative absorbances. This is identical to a comparison of extinction coefficients viz., w = .+00/~290 = ln(Io/1300)/1n(lo/1790). In all experiments in the present work ozone was prepared and puriiied as described elsew!lcre [lo] and when
monitoring
the absorprion
spectrum
at
1975
Ablml 300
r
A 270
.
290
A___
L 30
50
50
60
so
70
230
90
700
t-c>
T
Fig. I. Tcmpcraturc dcpcndcncc of ozone absorption coefficient at sclccted wwelcnSths in the Iiartley continuum. with those of Vigrous [ 1 I ] at lower temperatures and those of Jones and Davidson [ 121 at higher ternperatures in fig. 2 which indicates the shift in the absorption coefficient with temperature. Using the information contained in fig. 1 the valtizs tbr the shift in the absorption spectrum for l!iger temperatures at 300 nm bined
were obtained by interpolation
of the data of Jones
et al. and a plot of the ratio w = E~O~/E,~~ against temperature (fig. 3) was obtained. From fig. 3, using 15
elevated
temperature?, oxygen was admixed with ozone in > lOOO-fool11excess to assist in rapid attainment of thermodyliamic equilibrium. The heated cell was a 50 cm lonlr quartz tube jacketed in a copper cylinder with several copper/constantan thermocouples along its length for accurate monitoring of the temperature which could be quickly raised, lowered or maintained uniformly by use of a tempgature-controlled fast air flow. The whole assembly was contained in a thermally insulated cylinder incorporating inlet and outlet vents. The monitoring beam was attenuated by means of a Hilger monochromator (D323) which also permitted wavelength preselection into a narrow band pass Bausch and Lomb double grating monochromator (33-86-66-01) on which the photomultiplier tube (E.M.I. type 9526B) was mounted. Optical absorption measurements were made on a nanoammeter. The absorption coefficient of ozone (e), in cm-l atmel base 10 units, was measured throughout the Haitley absorption continuum for temperatures between 295 and 373 K. The precentage variation in E with temperature at several wavelengths is shown in fig.1. The present results at 290 nm have been com-
15 Deccrnbcr
.
CHEhllCAL P~IYSICS LETTERS
1-c
r -
1.3
-
o -Jones
8
Davidson
a - Presen: D - V1qroux
1.2 iCEI 1.1 -
lo-
0.9
-
O.Bb
’
:
I 600
300
(
Temperature
Variation 300 nm.
Fig.
of/.(e) 2s 3 function
7.
0 70
L&l
06
-
0.5
-
-Jones
e
900
of
K)
tcmpemturo
at A =
oo’ri~n
A -?Presen1 3
-
Viqrour
O“ao-3 0.2 -
0.J
7
8
loo
m
300
400
m
600
700
800
so0
T f r0
Fig.
3.
Variation
of w as a function
of temperature. 565
Volume 36, number 5 the ratio w measured
as described above from fiash phorolysis experiments, an estimate of the equilibrium temperature corresponding to the “vibrarional temperature” of the ozone under specific conditions of inert gas ratio, flash intensity and delay time can be obtained.
3. Results The method of measurement of w described above removes any errors normally found in flash photolysis studies due to inconsistencies in the output of the spectroscopic flash and the emulsion density of the photographic plate. The ratios would seem to be internafly consistent and any errors arising must be due to the interpolation technique employed or to the nonapplicability of the assum::d jntramolrcular vibrational energy equilibration in the ozone. More detailed work on the variation of E with temperature Over a wider temperature range is required and experiments on these lines, using a modified expe~menta1 set-up, are presently being pursued. At the two flash energizs employed in these experiments, 270 and 550 J, the fraction of ozone dissociated [ !O] by the photolysis flash alone was 0.12 and U.3 respcctivety. The results reported in table 1 represent measurements of the “vibrational temperature” (T,) in ozone for essentially eight ratios of inert gas to ozone. For the higl~r flash intensities the shift in the absorption spectrum of ozone vias found to b2 greatest in the shortest delay time after the peak of phoiolysis flash. With helium as added gas the initial
Table 1 Vibration~I temperatures wirnated Initial
iemperatures,
as expected [ lC] , decrease with increase in the mixture ratio which aIso corresponds to an increased rate of vibrational quenching in ozone. In the nitrogen sample under similar conditions of Rash intensity the “vibrational temperature” increase was found to be approximately half that observed with helium, in agreement with the Fact that about 30% of the 0(’ D) atoms produced are quenched to the ground state by nitrogen. In contrast to the above pattern, the vibrational heating under conditions of low flash energy occurs with an apparent ind~iction period, the maximum effect being observed after 25 I-(;. The value of Tvlb. for the Q/O, mixture at the lower flash energy appears to be anomalously high but more experimental data are required in order to attach any significance to this. The extent of vibrational heating at longer times was studied in an esperiment with an He/O3 mixture (170 : 1) flash photolysed at 5.50 J and monitored in the range 1%-500 (rts delay. The experiments, the results of which are recorded in table 2, were designed to correspond to kinetic absorption spsctrometry of an identical ozone sample photolysed under similar conditions and rnonitored between 20 ps and I ms after t, photolysis flash. The average temperature in the present work for the time segment 25-500 gs is 360 K, as measured by the method described above. The ozone vibratiorlal tem?crature peak, corresponding to LIT= 122 K, occurs at about 130 PS after the firirlg of the photolysis flashand is vi&u&y coincident with the maGnum of the 0: profile [ 131 The significance of this fact will be d&ssed later.
at fC and 25 ps after I~~sl~in~of O,;N, _ and 03!He misturcs Flash energy
Ratio
WI0
w25
270 550
0.4688 0.4234
0.4.557 0.3773
553 503
540 453
(joules)
tixture %/03
15 December 1975
CHEhIICAL PHYSICS LETTERS
25
:1
T,,
09
r,,
WI
---__I
I-MO,
5:l
210 550
0.3677 0.7073
0.649 1 0.506
444 970
852 606
He/O,
17O:l
270 550
0.3737 0.6757
0.4564 0.1419
449 813
541 300
He/O:,
1000:1
210 550
0.1614 0.215r!
0.1996 0.1947,
c ?OO 300
--
-
”
--
._
c 300 < 300 .-
Volume 36, nur~lber 5 Table 2 Vibrational
temperature Delay
wt T(K)
~-.--
CHEhIlCAL
of ozone ns time
a function
PHYSICS
of time after
flashing
LETTERS
an He/O,
15 December
1975
mixture (ratio 170 : 1) _--_-----
bs)
10
25
IO
17-O
0.6257 813
0.2419 300
0.3014 385
0.347 422
4. Discussion The time dependence of the vibrational temperature of ozone after the initial peak, which occurs within the duration of the flash, can give important information about the influence of vibrational excitation on secondary reactions having activation energies. Under the conditions of the experiment reported in table 2 the vibrational temperature is significantly higher than room temperature, rising to a peak (AT= 122 K) at about 120 ps after flashing. Theoretical studies of an identical system carried out by the present authors [lo] indicated that enhacements in the rate coefficients of the reactions oFO(~ P) and 02(’ Og) with ozone of 8 + 5 and 60 5 25 respectively were required to explain the observed ozone decay profile. These enhancements correspond to a non-equilibrium vibrational energy distribution in ozone represented by an average thermodynamic temperature of 350 K in excellent agreement with the average vibrational temperature of 360 K reported here.
5. The source of vibrational
heating
Calculated of the possible equilibrium thermodynamic temperature rise in the W photolysis of ozone shows conclusively that, in the experiments described here, the shift in the’W absorption spectrum of ozone cannot be attributed to thermal heating due to nonisothermal conditions. Studies reported in another publication [lo] a 1so rule out the possibility of direct heating of the ozone by absorption of infrared radiation from the photolysis flash. These facts are supported by investigations carried out in these laboraiories on the production and rate of quenching of 05 produced in the flash photolysis of ozone. The Iatter studies indicate, for example, that the rate of production of 0; is dependent not on the thermalizing efficiency of
1
200
500
0.2549 340
0.2646
__-_--
355
the added gas but on its efficiency in quenching 0: and 0;. From what has been said above it therefore follows that in the present experiments the vibrational energy distribution in 03 is non-equilibrium in nature, or we can talk about a “vibrational temperature*’ which is higher than the “translational temperature” of the system. It may also be that the “rotational temperature” is higher than the calculated thermodynamic equilibrium temperature of the system but we have no expei-imental data which could confirm or disprove this. The only remaining source ofvitrational excitation in this system is energy transfer from a vibrationally excited oxygen molecule. A full discussion of this hypothesis, given elsewhere [lo] , attributes the observ:d vibrational temperature to a transfer of energy from Oi(‘Xi), produced in the secondary reactions of O(3P) and O(lD) atoms with ozone. A very important question remains to be answered and that is whether or not the “vibrational temperature” of ozone is in fact identical to a thermodynamic temperature in which all the vibrational modes of ozone are in equilibrium with each other. The importance of this is obvious, since the exact contributions of the vibrational modes to the W absorption spectrum are not known, i.e., excitation of one or more of the vibrational modes in such a way as to result in a vibrational distribution which is different from the Boltzmann equilibium distribution found in “thermally” heated ozone will affect the absorption spectrum in some unpredictable fashion. In such case it would not be justifiable to equate the ratios of extinction ccefficients at two different wavelengths for the “thermally” and “photochemically” heated systems in order to obtain a value of the vibrationar temperature. The assumption is made in this work and further discussed elsewhere [lC] that equilibration between the ~2 and the vl and u3 modes is sufficiently fast to keep pace with the rate of energy transfer into the u2 mode in :, 567
Volume 36, number 5
15 December
CHERlICAL PHYSICS LETTERS
---XV from the upper vibrational levels of OZ(~ CR). VU_._> This being so rte vibrational mcdes will be equilibrated and the “vibrational temperature”’ of the ozor!e is accurately represented by the thermodynamic tem.peratlire which gives rise to the corresponding shift in the UV absorption spectrum of ozone. The difftisz nature of the ozone UV absorption
bands and their sensitivity to temperature fluctuations increase the uncertainty in essignment of the vibra-
photolysis experiments where appreciable heating
1975
vibrational
may occur due to the sequence
o(3P)+03+o$+02, o;1F;co3
+o;
+o,
v
Evidence in support of this has been reviewed by Hanvey and McGrath [IO] .
tional structure. Simons ct al. [141 ,in atttmpting to predict the effect of temperature on the shape of the
Hartley continuum, have illustrated the possible comFlexity of the spectrum. In
Anomalously high vibrational temperature might also be expected to arise in the thermal decomposition of ozone and in the photolysis using visible radiation of A> 600 nm. In both these situations only ground state oxygen atoms are involved in the reaction scheme and would yield 03 in levels suitable for populating
oniy the v1 and v3 vibrational modes of ozorie [IO]. Because o.f the slow intramolecular coupling between ‘@l , v3) and v7_ [2] and the apparent necessity for UT excitation in order to bring about enhancement of ozone reactivity [5], it n;ay be concluded that 0; in both these processes will lw/e little effect or, the rates of reaction between 0c3P) and 02(1 ~3~) with 03, as long as the absolute concentration of oxygen atoms is very small in comparison with the ozone concentration. These conditions do not normally apply in flash
References [ I ] D.S. Perry, J.C. Polanyi and C.W. Wilson Jr.! Chem. Phys. 3 (1974) 317. [2] K.-K. Hui, D.I. Rosen and T.A. Cool, C!xm. Phys. Letters 32 (1975) 141. [ 3 ] R.J. Gorden and X1.C. Lin, Chem. Phys. Letters 22 (1973) 262. [4] h1.J. Kurylo, W. Braun and A. Kaldor, Chcm. Phys. Letters 27 (1974) 249. Freund and 151 M.J. Kurylo, W. Braun, A. Kaldor, S.hl. R.P. Wayne, J. Photochcm. 3 (1974) 71. [61 W. Braun, NJ. Kurylo, A. Kaldor and R.P. Wayne, J. Chem. Phys. 61 (19743461. [71 M.J. Kurylo, W. Braun and C.N. Xuan, J. Chem. Phys. 62 (1975) 2065. ISI V.D. Baiamonte, G.R. Snclling nnd E.J. Bair, J. Chcm. Phys. 44 (1966) 673. 191 V.D. Baiamontc, L.G. Hartshorn and E.J. Bair, J. Chem. Phys. 55 (1971) 3617. 1101 J.A. Hanvcy nnd W-D. McCrath, J. Chhcrn. Sot. Faraday II, to be published. IllI E. Viprous, Ann. Phys. 6 (1953) 709. 1121 W.hI. Jones and N. Davidson, J. Am. Chem. Sot. 84 (1962) 2868. 1131 S.A. Slonn, Ph.D. Thesis, The Queen’s University, Belfast (1968). 1141 J.W. Simons, R.J. Panr, H.A. WcSster and E.J. Bair, J. Chcm. Phys. 59 (1973) 1203.
VIBRATIONAL J.A. HAhvEY Dcpartmeut
15 Decenbcr
CHEMICAL PHYSICS LETTERS
HEATING
EFFECTS
IN THE FLASH
PHOTOLYSIS
1975
OF OZONE
and W.D. McGRATH
of Chemistry,
iW Queen’s University of Belfasr, Belfast,
UK
Received 28 July 1975
The vibralional rcmperaturc oi ozone hns been estimated during the flash photolvsis of ozone under non-isothermal nnd strictly isothermal conditions, utilisin:! the variation in extinction coefficient with temperature 31 two lv’avelen$hs. Vibrational tcmpera!urcs me;isurcd in the range 300-970 K have been attributed to energy transfer from 0t(3~J to ozone and the effect cussed.
of this non-equilibrium
vibrational
cscitatioll
1. Introduction
on the secondary
reactions
of vibrational
The importance of non-equilibrium distributions of vibration2 energy in reactive systems has recently become a subject of intense int zrest for both practical and theoretical workers in the fields of gas phase kinetics, energy transfer and trajectory studies, the thzoretica! aspect being particularly well investigated by Polanyi and co-workers [I] . These studies suggest that vibrational excitation (V) in reactive species is much more effective in surmounting activation barriers than ei’Lher translational (Tj or rotational (R) excitations even when the reagent energies are several times greater than the barrier height. This is true both for endothermic reactions and for those exothermic reactions with energies ofactivation, the distribution of excess energy- in reactants being maintained in Phe products, viz., AV’ + AV and AT’ 4 AT (where the superscript refers to reactants).
with
excitation
non-zero
activation
cncrgies
in the ozone system
is dis-
[3-T]
Non-equilibrium vibrational distributions have been discussed by Bair et al. [8,9] for ozone systems under conditions in which photolytic explosion occurred with adiabatic temperatures in the regicn of 4500 K and also under near isothermal conditions where thermal effects were unimportant. Ultraviolst flush photolysis studies and analysis of the kinetics of ozone destruction using computer simulation of the reaction scheme by the present authors [IO] emphasize the role played by the reIatively high concentrations of vibrationally excited 02(3Ci) produced rapidly in the following reactions (the symbol $ indicates vibrational excitation): O(‘D)i03+0$
+02., Ax
o(3P)+03
-0:
20(3P) to,.
(1’) + 02 )
(1”) (2)
Recent experimenta and theoretical studies have shown that intermolecular V + V energy transfer is extremely efficient when ne;tr resonant conditions exist. Laser induced vibrational energy studies in ozone systems have prcvided elucidation of Ihe rates of energy transfer between vibrational modes in ozone [?I, and enhancements of reaction rates by orders cf magnitude in reactions involving non-equiiibrium
vibrational energy distributions in ozonehave effectiveIy illustrated
‘564
.’
the particular
importance
of the role
The experiments described in the present work are an attempt to estimate the vibrational content of ozone immediately after the peak of the photolysis flash in order to elucidate the mechanism of heating
of the ozone. Extension of the observations to a time of 500 p’s was also done to allow appraisal of the role of non-equilibrium vibrational excitation in the rate enhancement of secondary reactions occurring in the ultraviolet photolysis of ozone.
Volume
36,
number
5
2. Experiment31
116
The absorption spectrum of ozone in the region of the Hartley continuum was recorded photograpically using a Hilger Littrow spectrograph (E742) for several mixtures of ozone with N, and He in up to 1000.fold excess. A conventional flash photolysis technique was used, the spectra being recorded at delay times of 10 and 25 ~.ts for photolysis flash energies of 270 and 550 J. The Lyman monitoring flash, fired at 92 J, provided a single shot absorption spectrum from which intensity measurements were made at several wavelengths using a modified Hi&r microdensitometer (H481). Light from a quartz iodine lamp, transmitted through t- . photographic plate with a band pass of 0.2 nm, was incident on a photomultiplier tube (RCA lP38A) the output of which was recorded on a nanoammeter. For the purpose of this study the spectral intensities at two wavelengths, 290 and 300 nm, were compared by means of the relative absorbances. This is identical to a comparison of extinction coefficients viz., w = .+00/~290 = ln(Io/1300)/1n(lo/1790). In all experiments in the present work ozone was prepared and puriiied as described elsew!lcre [lo] and when
monitoring
the absorprion
spectrum
at
1975
Ablml 300
r
A 270
.
290
A___
L 30
50
50
60
so
70
230
90
700
t-c>
T
Fig. I. Tcmpcraturc dcpcndcncc of ozone absorption coefficient at sclccted wwelcnSths in the Iiartley continuum. with those of Vigrous [ 1 I ] at lower temperatures and those of Jones and Davidson [ 121 at higher ternperatures in fig. 2 which indicates the shift in the absorption coefficient with temperature. Using the information contained in fig. 1 the valtizs tbr the shift in the absorption spectrum for l!iger temperatures at 300 nm bined
were obtained by interpolation
of the data of Jones
et al. and a plot of the ratio w = E~O~/E,~~ against temperature (fig. 3) was obtained. From fig. 3, using 15
elevated
temperature?, oxygen was admixed with ozone in > lOOO-fool11excess to assist in rapid attainment of thermodyliamic equilibrium. The heated cell was a 50 cm lonlr quartz tube jacketed in a copper cylinder with several copper/constantan thermocouples along its length for accurate monitoring of the temperature which could be quickly raised, lowered or maintained uniformly by use of a tempgature-controlled fast air flow. The whole assembly was contained in a thermally insulated cylinder incorporating inlet and outlet vents. The monitoring beam was attenuated by means of a Hilger monochromator (D323) which also permitted wavelength preselection into a narrow band pass Bausch and Lomb double grating monochromator (33-86-66-01) on which the photomultiplier tube (E.M.I. type 9526B) was mounted. Optical absorption measurements were made on a nanoammeter. The absorption coefficient of ozone (e), in cm-l atmel base 10 units, was measured throughout the Haitley absorption continuum for temperatures between 295 and 373 K. The precentage variation in E with temperature at several wavelengths is shown in fig.1. The present results at 290 nm have been com-
15 Deccrnbcr
.
CHEhllCAL P~IYSICS LETTERS
1-c
r -
1.3
-
o -Jones
8
Davidson
a - Presen: D - V1qroux
1.2 iCEI 1.1 -
lo-
0.9
-
O.Bb
’
:
I 600
300
(
Temperature
Variation 300 nm.
Fig.
of/.(e) 2s 3 function
7.
0 70
L&l
06
-
0.5
-
-Jones
e
900
of
K)
tcmpemturo
at A =
oo’ri~n
A -?Presen1 3
-
Viqrour
O“ao-3 0.2 -
0.J
7
8
loo
m
300
400
m
600
700
800
so0
T f r0
Fig.
3.
Variation
of w as a function
of temperature. 565
Volume 36, number 5 the ratio w measured
as described above from fiash phorolysis experiments, an estimate of the equilibrium temperature corresponding to the “vibrarional temperature” of the ozone under specific conditions of inert gas ratio, flash intensity and delay time can be obtained.
3. Results The method of measurement of w described above removes any errors normally found in flash photolysis studies due to inconsistencies in the output of the spectroscopic flash and the emulsion density of the photographic plate. The ratios would seem to be internafly consistent and any errors arising must be due to the interpolation technique employed or to the nonapplicability of the assum::d jntramolrcular vibrational energy equilibration in the ozone. More detailed work on the variation of E with temperature Over a wider temperature range is required and experiments on these lines, using a modified expe~menta1 set-up, are presently being pursued. At the two flash energizs employed in these experiments, 270 and 550 J, the fraction of ozone dissociated [ !O] by the photolysis flash alone was 0.12 and U.3 respcctivety. The results reported in table 1 represent measurements of the “vibrational temperature” (T,) in ozone for essentially eight ratios of inert gas to ozone. For the higl~r flash intensities the shift in the absorption spectrum of ozone vias found to b2 greatest in the shortest delay time after the peak of phoiolysis flash. With helium as added gas the initial
Table 1 Vibration~I temperatures wirnated Initial
iemperatures,
as expected [ lC] , decrease with increase in the mixture ratio which aIso corresponds to an increased rate of vibrational quenching in ozone. In the nitrogen sample under similar conditions of Rash intensity the “vibrational temperature” increase was found to be approximately half that observed with helium, in agreement with the Fact that about 30% of the 0(’ D) atoms produced are quenched to the ground state by nitrogen. In contrast to the above pattern, the vibrational heating under conditions of low flash energy occurs with an apparent ind~iction period, the maximum effect being observed after 25 I-(;. The value of Tvlb. for the Q/O, mixture at the lower flash energy appears to be anomalously high but more experimental data are required in order to attach any significance to this. The extent of vibrational heating at longer times was studied in an esperiment with an He/O3 mixture (170 : 1) flash photolysed at 5.50 J and monitored in the range 1%-500 (rts delay. The experiments, the results of which are recorded in table 2, were designed to correspond to kinetic absorption spsctrometry of an identical ozone sample photolysed under similar conditions and rnonitored between 20 ps and I ms after t, photolysis flash. The average temperature in the present work for the time segment 25-500 gs is 360 K, as measured by the method described above. The ozone vibratiorlal tem?crature peak, corresponding to LIT= 122 K, occurs at about 130 PS after the firirlg of the photolysis flashand is vi&u&y coincident with the maGnum of the 0: profile [ 131 The significance of this fact will be d&ssed later.
at fC and 25 ps after I~~sl~in~of O,;N, _ and 03!He misturcs Flash energy
Ratio
WI0
w25
270 550
0.4688 0.4234
0.4.557 0.3773
553 503
540 453
(joules)
tixture %/03
15 December 1975
CHEhIICAL PHYSICS LETTERS
25
:1
T,,
09
r,,
WI
---__I
I-MO,
5:l
210 550
0.3677 0.7073
0.649 1 0.506
444 970
852 606
He/O,
17O:l
270 550
0.3737 0.6757
0.4564 0.1419
449 813
541 300
He/O:,
1000:1
210 550
0.1614 0.215r!
0.1996 0.1947,
c ?OO 300
--
-
”
--
._
c 300 < 300 .-
Volume 36, nur~lber 5 Table 2 Vibrational
temperature Delay
wt T(K)
~-.--
CHEhIlCAL
of ozone ns time
a function
PHYSICS
of time after
flashing
LETTERS
an He/O,
15 December
1975
mixture (ratio 170 : 1) _--_-----
bs)
10
25
IO
17-O
0.6257 813
0.2419 300
0.3014 385
0.347 422
4. Discussion The time dependence of the vibrational temperature of ozone after the initial peak, which occurs within the duration of the flash, can give important information about the influence of vibrational excitation on secondary reactions having activation energies. Under the conditions of the experiment reported in table 2 the vibrational temperature is significantly higher than room temperature, rising to a peak (AT= 122 K) at about 120 ps after flashing. Theoretical studies of an identical system carried out by the present authors [lo] indicated that enhacements in the rate coefficients of the reactions oFO(~ P) and 02(’ Og) with ozone of 8 + 5 and 60 5 25 respectively were required to explain the observed ozone decay profile. These enhancements correspond to a non-equilibrium vibrational energy distribution in ozone represented by an average thermodynamic temperature of 350 K in excellent agreement with the average vibrational temperature of 360 K reported here.
5. The source of vibrational
heating
Calculated of the possible equilibrium thermodynamic temperature rise in the W photolysis of ozone shows conclusively that, in the experiments described here, the shift in the’W absorption spectrum of ozone cannot be attributed to thermal heating due to nonisothermal conditions. Studies reported in another publication [lo] a 1so rule out the possibility of direct heating of the ozone by absorption of infrared radiation from the photolysis flash. These facts are supported by investigations carried out in these laboraiories on the production and rate of quenching of 05 produced in the flash photolysis of ozone. The Iatter studies indicate, for example, that the rate of production of 0; is dependent not on the thermalizing efficiency of
1
200
500
0.2549 340
0.2646
__-_--
355
the added gas but on its efficiency in quenching 0: and 0;. From what has been said above it therefore follows that in the present experiments the vibrational energy distribution in 03 is non-equilibrium in nature, or we can talk about a “vibrational temperature*’ which is higher than the “translational temperature” of the system. It may also be that the “rotational temperature” is higher than the calculated thermodynamic equilibrium temperature of the system but we have no expei-imental data which could confirm or disprove this. The only remaining source ofvitrational excitation in this system is energy transfer from a vibrationally excited oxygen molecule. A full discussion of this hypothesis, given elsewhere [lo] , attributes the observ:d vibrational temperature to a transfer of energy from Oi(‘Xi), produced in the secondary reactions of O(3P) and O(lD) atoms with ozone. A very important question remains to be answered and that is whether or not the “vibrational temperature” of ozone is in fact identical to a thermodynamic temperature in which all the vibrational modes of ozone are in equilibrium with each other. The importance of this is obvious, since the exact contributions of the vibrational modes to the W absorption spectrum are not known, i.e., excitation of one or more of the vibrational modes in such a way as to result in a vibrational distribution which is different from the Boltzmann equilibium distribution found in “thermally” heated ozone will affect the absorption spectrum in some unpredictable fashion. In such case it would not be justifiable to equate the ratios of extinction ccefficients at two different wavelengths for the “thermally” and “photochemically” heated systems in order to obtain a value of the vibrationar temperature. The assumption is made in this work and further discussed elsewhere [lC] that equilibration between the ~2 and the vl and u3 modes is sufficiently fast to keep pace with the rate of energy transfer into the u2 mode in :, 567
Volume 36, number 5
15 December
CHERlICAL PHYSICS LETTERS
---XV from the upper vibrational levels of OZ(~ CR). VU_._> This being so rte vibrational mcdes will be equilibrated and the “vibrational temperature”’ of the ozor!e is accurately represented by the thermodynamic tem.peratlire which gives rise to the corresponding shift in the UV absorption spectrum of ozone. The difftisz nature of the ozone UV absorption
bands and their sensitivity to temperature fluctuations increase the uncertainty in essignment of the vibra-
photolysis experiments where appreciable heating
1975
vibrational
may occur due to the sequence
o(3P)+03+o$+02, o;1F;co3
+o;
+o,
v
Evidence in support of this has been reviewed by Hanvey and McGrath [IO] .
tional structure. Simons ct al. [141 ,in atttmpting to predict the effect of temperature on the shape of the
Hartley continuum, have illustrated the possible comFlexity of the spectrum. In
Anomalously high vibrational temperature might also be expected to arise in the thermal decomposition of ozone and in the photolysis using visible radiation of A> 600 nm. In both these situations only ground state oxygen atoms are involved in the reaction scheme and would yield 03 in levels suitable for populating
oniy the v1 and v3 vibrational modes of ozorie [IO]. Because o.f the slow intramolecular coupling between ‘@l , v3) and v7_ [2] and the apparent necessity for UT excitation in order to bring about enhancement of ozone reactivity [5], it n;ay be concluded that 0; in both these processes will lw/e little effect or, the rates of reaction between 0c3P) and 02(1 ~3~) with 03, as long as the absolute concentration of oxygen atoms is very small in comparison with the ozone concentration. These conditions do not normally apply in flash
References [ I ] D.S. Perry, J.C. Polanyi and C.W. Wilson Jr.! Chem. Phys. 3 (1974) 317. [2] K.-K. Hui, D.I. Rosen and T.A. Cool, C!xm. Phys. Letters 32 (1975) 141. [ 3 ] R.J. Gorden and X1.C. Lin, Chem. Phys. Letters 22 (1973) 262. [4] h1.J. Kurylo, W. Braun and A. Kaldor, Chcm. Phys. Letters 27 (1974) 249. Freund and 151 M.J. Kurylo, W. Braun, A. Kaldor, S.hl. R.P. Wayne, J. Photochcm. 3 (1974) 71. [61 W. Braun, NJ. Kurylo, A. Kaldor and R.P. Wayne, J. Chem. Phys. 61 (19743461. [71 M.J. Kurylo, W. Braun and C.N. Xuan, J. Chem. Phys. 62 (1975) 2065. ISI V.D. Baiamonte, G.R. Snclling nnd E.J. Bair, J. Chcm. Phys. 44 (1966) 673. 191 V.D. Baiamontc, L.G. Hartshorn and E.J. Bair, J. Chem. Phys. 55 (1971) 3617. 1101 J.A. Hanvcy nnd W-D. McCrath, J. Chhcrn. Sot. Faraday II, to be published. IllI E. Viprous, Ann. Phys. 6 (1953) 709. 1121 W.hI. Jones and N. Davidson, J. Am. Chem. Sot. 84 (1962) 2868. 1131 S.A. Slonn, Ph.D. Thesis, The Queen’s University, Belfast (1968). 1141 J.W. Simons, R.J. Panr, H.A. WcSster and E.J. Bair, J. Chcm. Phys. 59 (1973) 1203.