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Study of the primary process of NO2 photolysis at high pressures
Volume
16, number
1
15 September
CHEMICAL PHYSICS LETTERS
1972
STI JDY OF THE PRIMARY PROCESS OF NO, PHOTOLYSIS AT HIGH PRESSI JRES H. GAEDTKE,
H. HIPPLER
and J. TROE
hstitut de Chitnie-Physiaue de :‘Ecole Polytechniqique Fe2e:ale de Lausanrre, Lausanne, Switzerland Received
22 June 1972
The quantum yield of the 802 pkotclysis in the carrier gas N2 has been measured 4100 A at pressures up to 1000 atm. Photcdissociation rates of NO2 are discussed.
1. Introduction proThe NO, *NO + 0 system *and its elementary cesses have been studied in great detail. The impor-
tance of this system in atmospheric photochemistry well known. From the experimental point of view,
is
this system provides exceptionally easy access to a large number of elementary kinetic processes, under thermal as well as photochemical conditions. Thermal dissociation was followed from the second order up to the first order limit [l] , the reverse thermal combination from the third up to the second order limit [2]. Fluorescence lifetimes and rates of collisional de-
in the wavelength
range 3100-
pressure dependence of the quantum yield of the NOa photolysis. From the obtained Stern-Volmer plots, we wanted to derive information on the photodissociation
rates of NO,
at different
excitation
wave-
lengths, at least as far as this is possible. Our interest originated from studies of unimolecular reactions, for which the measurement of ener,oy resolved specific rate constants k(Ej in a small molecule would be of considerable use [ 17,181. For this purpose, the NO2 molecule apparently is the ideal case, where mixing of states is so strong, that thermal kinetics in the electronic ground state and photochemical kinetics are intimately related.
activation following light absorption, have been measured at excitation energies up to the dissociation limit at 3979 i% [3,4-81. sible absorption
The complex
spectrum
structure
and apparent
of the vi-
anomalies
2. Experimental
and results
be-
tween measured and calculated fluorescence lifetimes, stimulated much discussion in terms of new formulations of the theory of radiationless transitions [9-l I]. The photolysis of NO2 at 3471 A has been studied using photo-fragment spectroscopy [ 121. Quantum yields of the photolysis in the range 3 130-4050 9, were measured at low pressures in [ 131. Preliminary results on the pressure dependence of the primary quantum yields were reported in refs. [2,14]. The chemiluminescence of the 0 -+ NO + NO, recombina-
The quantum yields of the NO, photolysis were measured using high-pressure Xe or Xe-Hg lamps as light sources and wavelength selection by interference filters or a monochromator. The NO, photolysis at 3660 A and 6 torr without added carrier gas served as internal actinometer. Experiments with added carrier gases always used 6 torr NO,. N, 2s carrier gas was compressed up to 1000 atm in an oil-free membrane compressor. Experimental details are described in [ 2] . The quantum yields obtained are shown in fig. 1.
tion could be followed to very low total pressures [ 151. Knowledge of the electronic structure of the molecule could be considerably extended by recent quantum-chemical calculations [ 161. In the present work, we have studied in detail the
They are represented in a [Q(P= O)/@(P)] - 1 versus pressure plot, where P gives the carrier gas pressure. As shown in ]2] , the quantum yield Q(P) is composed of two contributions, the primary quantum yield b,(P), which may depend on wavelength and pressure, 177
Volume 16, number 1
CHEMICAL PHYSICS LETTERS
a(X) and kQ
15 September 1972 is not as straightforward
as one would
like. Firstly, excitation
the finite spectral resolution of the used light has to be taken into account, in @,l(P=O)and in a(h). This may easily be done and checked by comparison of the results from the Xe and thz Xe-Hg lamps, for which good agreement is obtained. Secondly, the thermal distribution of rotational en-
ergy ci NO? (togzther with some vibrational excitation) is carried during light absorption into the excited state. This energy is known to contiibute to the P(N,l
[n:m]
Fig. 1. Quantum yields of NO2 photolysis in N2. P(NO2) = 6 torr,~:h=3130A,o=3660_~,+=3855A,@=3920A, e = 3940 A, A = 3990 A, <3= 4050 A, v = 4160 A.
and a factor F(P), which gives the influence of secondary reactions to the overall quantum yield d and which is independent of A: Q(P) = &(P)F(P). Inorder to separate F(P) from Q*(P), a wavelength is needed, where Q,(p) = 1 in the whole pressure range. One may show [Z] that this is the case for h = 3 130 A. Therefore, using the 3 130 .&curve in fig. 1, for all other wavelengtlls the primary quantum yields QI\(P) are obtained. Essentially linear Stern-Volmer plots are found with the following slopes from the expression 4-1 x
=@,l(P=O)(l+ap):
a(X= 4160 A) = 10.3 X 1Om3 atm-l . ~(4050) = 8.6 x I o-3, a(4020) = 7.7 X 1o-3, a(3990) = 4.2 X 10-3, ~(3940) = 3.0 X 10-3, ~(3920) = 2.0 X 10-3, a(3855)= 1.4 >: 10-3, a(3815) = 0.7 X 10-3; a(3660) = 0.54 x 10-3, a(3 130) < 0.2x 10-3. In addition to the Stern-Volmer constants (I, the low pressure quantum yields Q;l(P= 0) were measured and found to agree with the results of [ 131. The bshaviour of the fluorescence quantum yields is complementary [ 191.
3. Discussion ‘The Stem-Volmer constants a(h) contain infor.mation on specific rate constants It(E) of the photodissocintion
ofNO2.
Unfortunately,
the relation
betwesn
dissociation process [ 13,191. The width X-Tof this distribution determines the energy resolution of the A-(E)values obtained. Thirdly, the Stern-Volmer constants a-l(E) are given by the ratio of the specific photolysis rate constants FiQ and the rate constants kdeac (f?) of collisional deactivation of activated NO2 molecules.
The approximation of k,,,,(E) by the gas kinetic collision number, which was used in [2,14], is inadequate.
This is shown
from fluorescence studies, which up to excitation energies only slightly below the energies used in the photolysis experiments [4-S, 1.51. From Stem-Volmer curves of the fluorescence yieids, overall rate constants for collisional deactivation near the gas kinetic collision number were derived. However, it was shown that the average deactivation step size was about 500 cm-l in pure NO? under excitation to 4050 .& [6] . If the use of these low pressure results is still justified up to the high pressures of the present study, one finds that one collision should be sufficient to deactivate molecules excited at 4050 A, whereas several collisions are required to deactivate after excitation at 3600 A. The discussed uncertainties in the calibration of the time scale, given by kdeac (E), may introduce errors of a factor of 2 in the k(E)-curve. However, the order of magnitude and the shape of the curve should essentially be correct. By identifying_!? with the light energy in excess of 3979 A plus the average rotational enermy, the k(E)-curve in fig. 2 is obtained. Remarkably good agreement is found with the estimate of a lower limit for k(E) from laser photofragment spectroscopy [ 121, Also given is a theoretical k(E)-curve from the statistical theory of unimolecular processes. In this calculation, the detailed Iocalisation of activated complexes from [20] was used. It was also assumed that,
could be conducted
Volume
16, number
1
CHEMICAL PHYSICS LET-IERS
1.5 September
1972
ischer Nationaifonds and the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References
111J. Tree, Ber. Bunscnges. Physik. Chem. 73 (!969) 144. I21 J. Troe, Ber. Bunserges. Physik. Chcm. 73 (1969) 906.
FioD. 2, Specific rate constants k(E) and population of states f(E) in the dissociation of NOa. Full curve: k(E) calculated [21], e: k(E) determined from photolysis (this work), 0: k(E) from photofragmcnt spectroscopy [ 121, dotted curves: f(E) in different
dissociation
systems.
of strong mixing of states [ 10,l I] , photolyby the large volume of phase space of the electronic ground state. The latter assumption is strongly supported by the following fact. If the k(E)curve from photolysis experiments, which agrees with the calculated curve, is multiplied by the thermal distribution of states, agreement within a factor of 2 with the high pressure results on thermal dissociation and thermal recombination is found [21]. It is therefore not unlikely that the energy-resolved k(E)-curve from photolysis and the thermally averaged rate constants of thermal dissociation and recombination at high pressures correspond to the same elementary kinetic process. because
sis is governed
Acknowledgement Generous
financial
support
from the Schweizer-
[3] A.E. Dou_~Ias and K.P. Huber, Can. J. Phys. 43 (1965) 74. [41 C.H. Myers, D.Xf. Silver and F. Kaufman, J. Chem. Phys. 44 (1966) 718. I51 L.F. Kcyser, F. Kaufman and E.C. Zipf. Chem. Phys. Letters 2 (1968) 523. 161 L.F. Keyser, S.Z. Lcvinc and F. Kaufman, 3. Chem.
Phys. 54 (1971) 355. ]71 S.E. Schwartz and H.S. Johnston, J. Chem. Phys. 51 (1969) 1286. 181 K. Sakurai and H.P. Broida, J. Chem. Phys. 50 (1969) 2404. [91 A.E. Douglas, J. Chcm. Phys. 45 (1966) 1007. IlO1 D.B. Hartley and B.A. Thrush, Discussions Faraday Sot. 37 (1964) 220. 1111 J. Jortncr, S.A. Rice and R.&I. Hochstrasscr, Advan. Photochem. 7 (i969) 149. 1121G.E. Busch and K.R. Wilson, J. Chem. Phys. 56 (1972) 3626,3638,3655. I131 J.N. Pitts, J.H. Shz:p and S.I. Chsn, J. Chem. Phys. 42 (1964) 3655. 1141 H. Gaedtke and J. Trot, 2. Naturforsch. 35n (1970) 789. [ISI K.H. Becker, W. Groth and D. Thran, Chem. Phys. Letters 6 (1970) 583. [I61 R.A. Gangi and L. Burnellc, J. Chcm. Phys. 55 (1971) 8.51. Physik. Chem. 72 (1965) 908. r171 J. Tree, Ber. Bunsengs. [I81 J. Tree, in: Physica chemistry, an advanced treatise, ed. W. Jost (Aadcmic Press, New York, 1972). 1191 E.K.C. Lee and W.M. Uselman, Discussions Faraday Sot., to be published. [201 D.L. Bunker and M. Pattengill, J. Chcm. Phys. 48 (1968) 7’72. [21] IL. Gnedtke and I. Troe, Ber. Bunsengcs. Physik. Chcm., to be published.
179
16, number
1
15 September
CHEMICAL PHYSICS LETTERS
1972
STI JDY OF THE PRIMARY PROCESS OF NO, PHOTOLYSIS AT HIGH PRESSI JRES H. GAEDTKE,
H. HIPPLER
and J. TROE
hstitut de Chitnie-Physiaue de :‘Ecole Polytechniqique Fe2e:ale de Lausanrre, Lausanne, Switzerland Received
22 June 1972
The quantum yield of the 802 pkotclysis in the carrier gas N2 has been measured 4100 A at pressures up to 1000 atm. Photcdissociation rates of NO2 are discussed.
1. Introduction proThe NO, *NO + 0 system *and its elementary cesses have been studied in great detail. The impor-
tance of this system in atmospheric photochemistry well known. From the experimental point of view,
is
this system provides exceptionally easy access to a large number of elementary kinetic processes, under thermal as well as photochemical conditions. Thermal dissociation was followed from the second order up to the first order limit [l] , the reverse thermal combination from the third up to the second order limit [2]. Fluorescence lifetimes and rates of collisional de-
in the wavelength
range 3100-
pressure dependence of the quantum yield of the NOa photolysis. From the obtained Stern-Volmer plots, we wanted to derive information on the photodissociation
rates of NO,
at different
excitation
wave-
lengths, at least as far as this is possible. Our interest originated from studies of unimolecular reactions, for which the measurement of ener,oy resolved specific rate constants k(Ej in a small molecule would be of considerable use [ 17,181. For this purpose, the NO2 molecule apparently is the ideal case, where mixing of states is so strong, that thermal kinetics in the electronic ground state and photochemical kinetics are intimately related.
activation following light absorption, have been measured at excitation energies up to the dissociation limit at 3979 i% [3,4-81. sible absorption
The complex
spectrum
structure
and apparent
of the vi-
anomalies
2. Experimental
and results
be-
tween measured and calculated fluorescence lifetimes, stimulated much discussion in terms of new formulations of the theory of radiationless transitions [9-l I]. The photolysis of NO2 at 3471 A has been studied using photo-fragment spectroscopy [ 121. Quantum yields of the photolysis in the range 3 130-4050 9, were measured at low pressures in [ 131. Preliminary results on the pressure dependence of the primary quantum yields were reported in refs. [2,14]. The chemiluminescence of the 0 -+ NO + NO, recombina-
The quantum yields of the NO, photolysis were measured using high-pressure Xe or Xe-Hg lamps as light sources and wavelength selection by interference filters or a monochromator. The NO, photolysis at 3660 A and 6 torr without added carrier gas served as internal actinometer. Experiments with added carrier gases always used 6 torr NO,. N, 2s carrier gas was compressed up to 1000 atm in an oil-free membrane compressor. Experimental details are described in [ 2] . The quantum yields obtained are shown in fig. 1.
tion could be followed to very low total pressures [ 151. Knowledge of the electronic structure of the molecule could be considerably extended by recent quantum-chemical calculations [ 161. In the present work, we have studied in detail the
They are represented in a [Q(P= O)/@(P)] - 1 versus pressure plot, where P gives the carrier gas pressure. As shown in ]2] , the quantum yield Q(P) is composed of two contributions, the primary quantum yield b,(P), which may depend on wavelength and pressure, 177
Volume 16, number 1
CHEMICAL PHYSICS LETTERS
a(X) and kQ
15 September 1972 is not as straightforward
as one would
like. Firstly, excitation
the finite spectral resolution of the used light has to be taken into account, in @,l(P=O)and in a(h). This may easily be done and checked by comparison of the results from the Xe and thz Xe-Hg lamps, for which good agreement is obtained. Secondly, the thermal distribution of rotational en-
ergy ci NO? (togzther with some vibrational excitation) is carried during light absorption into the excited state. This energy is known to contiibute to the P(N,l
[n:m]
Fig. 1. Quantum yields of NO2 photolysis in N2. P(NO2) = 6 torr,~:h=3130A,o=3660_~,+=3855A,@=3920A, e = 3940 A, A = 3990 A, <3= 4050 A, v = 4160 A.
and a factor F(P), which gives the influence of secondary reactions to the overall quantum yield d and which is independent of A: Q(P) = &(P)F(P). Inorder to separate F(P) from Q*(P), a wavelength is needed, where Q,(p) = 1 in the whole pressure range. One may show [Z] that this is the case for h = 3 130 A. Therefore, using the 3 130 .&curve in fig. 1, for all other wavelengtlls the primary quantum yields QI\(P) are obtained. Essentially linear Stern-Volmer plots are found with the following slopes from the expression 4-1 x
=@,l(P=O)(l+ap):
a(X= 4160 A) = 10.3 X 1Om3 atm-l . ~(4050) = 8.6 x I o-3, a(4020) = 7.7 X 1o-3, a(3990) = 4.2 X 10-3, ~(3940) = 3.0 X 10-3, ~(3920) = 2.0 X 10-3, a(3855)= 1.4 >: 10-3, a(3815) = 0.7 X 10-3; a(3660) = 0.54 x 10-3, a(3 130) < 0.2x 10-3. In addition to the Stern-Volmer constants (I, the low pressure quantum yields Q;l(P= 0) were measured and found to agree with the results of [ 131. The bshaviour of the fluorescence quantum yields is complementary [ 191.
3. Discussion ‘The Stem-Volmer constants a(h) contain infor.mation on specific rate constants It(E) of the photodissocintion
ofNO2.
Unfortunately,
the relation
betwesn
dissociation process [ 13,191. The width X-Tof this distribution determines the energy resolution of the A-(E)values obtained. Thirdly, the Stern-Volmer constants a-l(E) are given by the ratio of the specific photolysis rate constants FiQ and the rate constants kdeac (f?) of collisional deactivation of activated NO2 molecules.
The approximation of k,,,,(E) by the gas kinetic collision number, which was used in [2,14], is inadequate.
This is shown
from fluorescence studies, which up to excitation energies only slightly below the energies used in the photolysis experiments [4-S, 1.51. From Stem-Volmer curves of the fluorescence yieids, overall rate constants for collisional deactivation near the gas kinetic collision number were derived. However, it was shown that the average deactivation step size was about 500 cm-l in pure NO? under excitation to 4050 .& [6] . If the use of these low pressure results is still justified up to the high pressures of the present study, one finds that one collision should be sufficient to deactivate molecules excited at 4050 A, whereas several collisions are required to deactivate after excitation at 3600 A. The discussed uncertainties in the calibration of the time scale, given by kdeac (E), may introduce errors of a factor of 2 in the k(E)-curve. However, the order of magnitude and the shape of the curve should essentially be correct. By identifying_!? with the light energy in excess of 3979 A plus the average rotational enermy, the k(E)-curve in fig. 2 is obtained. Remarkably good agreement is found with the estimate of a lower limit for k(E) from laser photofragment spectroscopy [ 121, Also given is a theoretical k(E)-curve from the statistical theory of unimolecular processes. In this calculation, the detailed Iocalisation of activated complexes from [20] was used. It was also assumed that,
could be conducted
Volume
16, number
1
CHEMICAL PHYSICS LET-IERS
1.5 September
1972
ischer Nationaifonds and the Deutsche Forschungsgemeinschaft is gratefully acknowledged.
References
111J. Tree, Ber. Bunscnges. Physik. Chem. 73 (!969) 144. I21 J. Troe, Ber. Bunserges. Physik. Chcm. 73 (1969) 906.
FioD. 2, Specific rate constants k(E) and population of states f(E) in the dissociation of NOa. Full curve: k(E) calculated [21], e: k(E) determined from photolysis (this work), 0: k(E) from photofragmcnt spectroscopy [ 121, dotted curves: f(E) in different
dissociation
systems.
of strong mixing of states [ 10,l I] , photolyby the large volume of phase space of the electronic ground state. The latter assumption is strongly supported by the following fact. If the k(E)curve from photolysis experiments, which agrees with the calculated curve, is multiplied by the thermal distribution of states, agreement within a factor of 2 with the high pressure results on thermal dissociation and thermal recombination is found [21]. It is therefore not unlikely that the energy-resolved k(E)-curve from photolysis and the thermally averaged rate constants of thermal dissociation and recombination at high pressures correspond to the same elementary kinetic process. because
sis is governed
Acknowledgement Generous
financial
support
from the Schweizer-
[3] A.E. Dou_~Ias and K.P. Huber, Can. J. Phys. 43 (1965) 74. [41 C.H. Myers, D.Xf. Silver and F. Kaufman, J. Chem. Phys. 44 (1966) 718. I51 L.F. Kcyser, F. Kaufman and E.C. Zipf. Chem. Phys. Letters 2 (1968) 523. 161 L.F. Keyser, S.Z. Lcvinc and F. Kaufman, 3. Chem.
Phys. 54 (1971) 355. ]71 S.E. Schwartz and H.S. Johnston, J. Chem. Phys. 51 (1969) 1286. 181 K. Sakurai and H.P. Broida, J. Chem. Phys. 50 (1969) 2404. [91 A.E. Douglas, J. Chcm. Phys. 45 (1966) 1007. IlO1 D.B. Hartley and B.A. Thrush, Discussions Faraday Sot. 37 (1964) 220. 1111 J. Jortncr, S.A. Rice and R.&I. Hochstrasscr, Advan. Photochem. 7 (i969) 149. 1121G.E. Busch and K.R. Wilson, J. Chem. Phys. 56 (1972) 3626,3638,3655. I131 J.N. Pitts, J.H. Shz:p and S.I. Chsn, J. Chem. Phys. 42 (1964) 3655. 1141 H. Gaedtke and J. Trot, 2. Naturforsch. 35n (1970) 789. [ISI K.H. Becker, W. Groth and D. Thran, Chem. Phys. Letters 6 (1970) 583. [I61 R.A. Gangi and L. Burnellc, J. Chcm. Phys. 55 (1971) 8.51. Physik. Chem. 72 (1965) 908. r171 J. Tree, Ber. Bunsengs. [I81 J. Tree, in: Physica chemistry, an advanced treatise, ed. W. Jost (Aadcmic Press, New York, 1972). 1191 E.K.C. Lee and W.M. Uselman, Discussions Faraday Sot., to be published. [201 D.L. Bunker and M. Pattengill, J. Chcm. Phys. 48 (1968) 7’72. [21] IL. Gnedtke and I. Troe, Ber. Bunsengcs. Physik. Chcm., to be published.
179