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Hot-atom—laser-induced-fluorescence experiments on the reaction of H(2S) with CO2
Volume 104, n u m b e r 2,3
CHEMICAL PHYSICS LETTERS
3 February 1984
HOT-ATOM-LASER-INDUCED-FLUORESCENCE EXPERIMENTS ON THE REACTION OF H(2S) WITH CO 2 K. KLEINERMANNS and J. WOLFRUM Physikalisch-Chemisches Institut der Ruprecht-Karls-Universit~lt Heidelberg, Im Neuenheimer Feld 253, 6900 Heidelberg, West Germany Received 17 September 1 9 8 3 ; i n final form 3 November 1983
The nascent rotational and fine-structure state distributions in OH (o = 0) produced in the e n d o t h e r m i c reaction H + CO2 OH + CO have been measured with hot hydrogen a t o m s from the photodissociation o f HBr at 193 n m . Considerable OH rotational excitation broadly peaked at around K = 11 is observed. The OH spin doublets are produced statistically (_+10%). The partitioning between the h doublets is non-statistical with a strong preference for the 7r+ c o m p o n e n t (o(n+)/a(n -) = 3.0 _+ 1.0), consistent with a planar HOCO intermediate whose O C - O H b o n d breaks in that plane.
1. Introduction The conversion of CO into C O 2 in the elementary reaction O H + C O - - C O 2 +H ,
AH = 25.4 kcal/mol,
(1)
is the dominant source of carbon dioxide in the oxidation of hydrocarbons [ 1], and plays an important role in the chemistry of the upper atmosphere [2.3 ] and the formation of photochemical smog [4]. Various measurements of the thermal rate constants of reaction (1) over a wide temperature range (200 < T < 2500 K) show an upward curvature for the rate data in the Arrhenius diagram [5]. Several authors [3,6] have applied transition-state theory to explain the curvature in the In k I versus lIT plot;however, quite different activated-complex configurations and barrier heights in the exit channel of (1) can describe the k 1(T) data as well. In addition, reaction (1) has been the subject of several state-resolved kinetic studies, partly in order to explain this behaviour. A rate enhancement of less than a factor of two was measured after vibrational excitation of OH [7], while vibrational excitation of CO even results in a decrease of the rate constant [8]. Evidence for a possible HO-CO complex formation in reaction (1) comes from matrix-isolation studies [9] and the cata0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
lytic decomposition of HCOOH [10]. It has been shown [11] that reaction (1) must occur on a 2A' hypersurface, which is also that of the HOCO ground state. Investigation of the endothermic reverse reaction H + CO 2 ~ OH + CO ( - 1 ) provides another clue to the microscopic details of this interesting reactive system. Mixtures of HI and CO 2 have been photolyzed, and mass spectrometry and gas chromatography' were used to detect the CO product. Total reaction probabilities of ~2% were obtained for the 60 kcal/ mol H atoms of this experiment. The initial collision energy was varied by choice of the photolysis wavelength; a reaction threshold energy close to the reaction endothermicity was obtained [12]. In a recent experiment [13], laser4nduced fluorescence was used to detect the OH product, but the authors did not give nascent OH state distributions because of high pressures and longer probe times. In the present paper, we report nascent rotational and fine-structure state distributions in OH(v = 0) from the reaction of hot H atoms with CO 2 molecules.
2. Experimental The OH state distribution is measured by combining 157
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CHEMICAL PHYSICS LETTERS
exciplex laser photolysis of rapidly flowing HBr and 0 2 mixtures with laser-induced fluorescence detection of the OH product at a short time after the photolysis pulse. The apparatus used has been described in detail elsewhere [14]. A flow reactor is equipped with a baffle system to reduce the scattered light from the ArE-exciplex-laser (Lambda Physik EMG 102) photolysis pulse at 193 nm (maximum energy 200 mJ/pulse, 15 ns pulse duration) and a Nd-YAG(Quanta Ray) pumped dye-laser (Lambda Physik) analysis pulse. The dye laser operates with rhodamine 640 and a frequency-doubling KDP crystal to generate pulses in the 306-311 nm region (0.2 cm -1 linewidth, 12 ns pulse duration, 1-10 mJ pulse energy) to probe OH( 211) radicals by laser4nduced fluorescence (LIF). The fluorescence light is detected by a photomultiplier (EMI 9659 QB) through imaging optics and a filter transmitting between 240 and 390 nm. The photomultiplier current is measured by a boxcar integration system (PAR 162/165). The timing sequence of the laser pulses originates with a signal pulse emitted by the N d - Y A G laser, which starts the exciplex laser discharge and the Q switch of the probe4aser system. The time delay is continuously adjustable with jitter <5 ns. The delay is adjusted by observing scattered light pulses from the two lasers with a storage oscilloscope. Typically, OH spectra were taken at probe times of 4 0 - 1 0 0 ns, partial pressures around 5 mTorr of HBr, and 5 0 - 2 0 0 mTorr of CO2 . At a probe time of 40 ns and 55 mTorr pressure, less than one collision should have occurred on the average between the hot hydrogen atoms and the gas for a translational cooling cross section ~<60 A 2, and less than 0.1 collisions between the OH product molecules and the gas at typical pressures and analysis times. Interference from CO 2 photodissociati0n at 193 nm can be neglected since the absorption cross section for CO 2 [15] is approximately five orders of magnitude smaller than for HBr [16] at 193 nm. Interference from the reactions H + HBr ~ H 2 + Br and OH + HBr ~ H20 + Br is negligible because of the short analysis times and the low partial pressures of HBr used here. Relative peak heights of the LIF signals were converted to relative OH densities by dividing the intensity by the laser power and the appropriate H 6 n l London [17] and Franck-Condon factors [18]. In 158
3 February 1984
some of the spectral scans, however, the OH absorption was driven to saturation so that the LIF intensity is independent of variations of the laser power and directly proportional to OH density. Both methods gave identical results.
3. Results and discussion The absorption of HBr between 2500 and 1600 A is continuous. Photodissociation of HBr at 193 nm produces ~ 15% spin-orbit-excited bromine atoms [ 19], resulting in a bimodal distribution of initial Hatom translational energies. The translational energy of the photofragments in the c.m. system is E = h v D0(HBr) + Eint(HBr) [-E*(Br(2P1/2))] = 61.9 and 51.3 kcal/mol (~ 15% of the fragments), of which the H atoms carry 80/81. Accordingly, the velocity of the H atoms is 2.26 × 106 and 2.06 × 106 cm/s. The relative velocity in the c.m. system of the hot hydrogen atoms colliding with room-temperature CO 2 molecules is g 2 = u 2 + o 2 0 2 _ 2oHOCO 2 c o s 0 ~ O2 ,
because VCO: ~ vH . Accordingly, with well defined relative velocities, we have initially mono-energetic collision energies of 59.9 and 49.8 kcal/mol (~ 15% of the collisions). Fig. 1 presents the rotational state distribution from the RA branch in OH(v = 0) produced in the reaction H 1 + CO 2 -~ OH + CO under approximately
A
g
0.1
II
0.05
n
1
,
,
,
t
5
,
,
,
,
t
10
,
a
,
,
a
15
t
i
L
i
i
20
K Fig. 1. Rotational state distribution from t h e R 1 branch in OH (o = 0) produced in the reaction H + CO2 ~ OH + CO. K is the rotational q u a n t u m n u m b e r and the ordinate is the relative cross section normalized such that t h e total scattering into OH (o = 0) is unity. 5 m T o r r HBr, 200 mTorr COz, probe time 40 ns.
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
single-collision conditions. Probe times and total pressures were varied between 40 and 140 ns and 50 and 200 mTorr without noticeable change in the form o f the spectra due to rotational relaxation effects. As shown in fig. 1, there is considerable OH rotational excitation. The nascent rotational distribution in OH(v = 0) is broadly peaked at rotational quantum numbers around 11 and falls to zero around K = 18. In fig. 1, relative cross sections instead o f densities could be given, due to the time resolution o f our experiment. In crossed-molecular-beams-laser-inducedfluorescence experiments, a f l u x - d e n s i t y transformation is necessary to obtain cross sections from product densities measured b y LIF. At equal cross sections for their production, high-velocity product molecules lead to smaller densities than low-velocity species because of their faster disappearance from the analysis zone. For the transformation, detailed internal state and angular distributions have to be known. In contrast, in our experiments, even the fastest OH molecules are still within the analysis area at typical probe times. S p i n - o r b i t and o r b i t a l - r o t a t i o n interactions in the 2[I OH radical cause fine-structure splittings for each rotational level. Each o f these f'me-structure levels can be probed b y different rotational sub-bands, i.e. 11~ by P1 or R1, Hi- by Q1,11~ b y P2 or R 2 and II~- b y Q2 [20], so that cross reactions for production o f the sub4evels can be determined separately. The 2111/2 and 2II3/2 spin states were, within experimental error, equally populated (-+10%). The X doublet fine-structure states showed a clear preference for the lower-energy X component which is probed b y excitation o f R 1 lines in our experiment. The preference increases with K and approaches o(n+)/o(zr - ) = 3.0 -+ 1.0 at the highest populated K. For high rotational states, the electron density o f the singly occupied OH orbital forms lobes perpendicular to the OH angular m o m e n t u m vector JOH ( II+ states) or parallel t O J o n ( I I - states) [21]. The experimental result shows that the breakup o f the HOCO complex generates forces in a plane containing the bond to be broken (the C - O H bond), so that a torque on OH is exerted with JOH perpendicular to that bond, i.e. asymptotically perpendicular to the OH unpaired p electron. This picture is consistent with a planar HOCO intermediate whose C - O H bond breaks in that plane.
3 February 1984
4. Conclusions Further experimental work will be directed towards the study o f the Doppler profiles o f the hot hydrogen atoms with a tunable L~ laser light source [22], and measurement o f the nascent state distribution and the total reaction cross section o f OH at different collision energies by varying the photolysis wavelength and the hot atom precursor. Acknowledgement The financial support o f the Deutsche Forschungsgemeinschaft is gratefully acknowledged. [1 ] B. Lewis and G. Elbe, Combustion, flames and explosion of gases (Academic Press, New York, 1961) p. 90. [2] P.M. Banks and G. Kockarts, Aeronomy (Academic Press, New York, 1973) p. 362. [3] I. Smith and R. Zellner, J. Chem. Soc. Faraday I1 69 (1973) 1617, and references therein. [4] H. Niki, E.E. Daby and B. Weinstock, Advan. Chem. 113 (1972) 16. [5 ] R. Zellner, J. Phys. Chem. 83 (1979) 18, and references therein; R.A. Ravishankara, to be published (1983). [6] D.M. Golden, J. Phys. Chem. 83 (1979) 108. [7] J.E. Spencer, H. Endo and G.P. Glass, Sixteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1977) p. 829. [8 ] T. Dreier and J. Wolfrum, Eighteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1981) p. 801. [9] D.E. Milligan, J. Chem. Phys. 54 (1971) 927. [10]~D.E. Tevault, M.C. Lin, M.E. Umstead and R.R. Smadzewski, Intern. J. Chem. Kinetics 11 (1979) 445. [ 11 ] I.W.M. Smith Chem. Phys. Letters 49 (1977) 112. [12] G.A. Oldershaw and D.A. Porter, Nature 223 (1969) 490. [ 13 ] C.R. Quick Jr. and J.J. Tiel, to be published. [14] P. Andresen, A. Jacobs, K. Kleinermanns and J. Wolfrum, Nineteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1982) pp. 11-22. [15 ] H. Okabe, Photo chemistry of small molecules (Wileylnterscience, New York, 1978) p. 208. [16] B.J. Huebert and R.M. Martin, J. Phys. Chem. 72 (1968) 3046. [17] J.L. Chidsey and D.R. Crosley, J. Quant. Spectrosc. Radiat. Transfer 23 (1980) 187. [18] B. Lin, private communication. [19] F. Magnotta, D.J. Nesbitt and S.R. Leone, Chem. Phys. Letters 83 (1981) 21. [20] M.H. Crosswhite and G.H. Dieke, J. Quant. Spectrosc. Radiat. Transfer 2 (1962) 97. [21 ] W.D. Gwinn, B.E. Turner, W.M. Goss and G.L. Blackman, Astrophys. J. 179 (1973) 789. [22] R. Schmiedl, H. Dugan, W. Meier and K.H. Welge, Z. Physik A304 (1982) 137. 159
CHEMICAL PHYSICS LETTERS
3 February 1984
HOT-ATOM-LASER-INDUCED-FLUORESCENCE EXPERIMENTS ON THE REACTION OF H(2S) WITH CO 2 K. KLEINERMANNS and J. WOLFRUM Physikalisch-Chemisches Institut der Ruprecht-Karls-Universit~lt Heidelberg, Im Neuenheimer Feld 253, 6900 Heidelberg, West Germany Received 17 September 1 9 8 3 ; i n final form 3 November 1983
The nascent rotational and fine-structure state distributions in OH (o = 0) produced in the e n d o t h e r m i c reaction H + CO2 OH + CO have been measured with hot hydrogen a t o m s from the photodissociation o f HBr at 193 n m . Considerable OH rotational excitation broadly peaked at around K = 11 is observed. The OH spin doublets are produced statistically (_+10%). The partitioning between the h doublets is non-statistical with a strong preference for the 7r+ c o m p o n e n t (o(n+)/a(n -) = 3.0 _+ 1.0), consistent with a planar HOCO intermediate whose O C - O H b o n d breaks in that plane.
1. Introduction The conversion of CO into C O 2 in the elementary reaction O H + C O - - C O 2 +H ,
AH = 25.4 kcal/mol,
(1)
is the dominant source of carbon dioxide in the oxidation of hydrocarbons [ 1], and plays an important role in the chemistry of the upper atmosphere [2.3 ] and the formation of photochemical smog [4]. Various measurements of the thermal rate constants of reaction (1) over a wide temperature range (200 < T < 2500 K) show an upward curvature for the rate data in the Arrhenius diagram [5]. Several authors [3,6] have applied transition-state theory to explain the curvature in the In k I versus lIT plot;however, quite different activated-complex configurations and barrier heights in the exit channel of (1) can describe the k 1(T) data as well. In addition, reaction (1) has been the subject of several state-resolved kinetic studies, partly in order to explain this behaviour. A rate enhancement of less than a factor of two was measured after vibrational excitation of OH [7], while vibrational excitation of CO even results in a decrease of the rate constant [8]. Evidence for a possible HO-CO complex formation in reaction (1) comes from matrix-isolation studies [9] and the cata0 009-2614/84/$ 03.00 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
lytic decomposition of HCOOH [10]. It has been shown [11] that reaction (1) must occur on a 2A' hypersurface, which is also that of the HOCO ground state. Investigation of the endothermic reverse reaction H + CO 2 ~ OH + CO ( - 1 ) provides another clue to the microscopic details of this interesting reactive system. Mixtures of HI and CO 2 have been photolyzed, and mass spectrometry and gas chromatography' were used to detect the CO product. Total reaction probabilities of ~2% were obtained for the 60 kcal/ mol H atoms of this experiment. The initial collision energy was varied by choice of the photolysis wavelength; a reaction threshold energy close to the reaction endothermicity was obtained [12]. In a recent experiment [13], laser4nduced fluorescence was used to detect the OH product, but the authors did not give nascent OH state distributions because of high pressures and longer probe times. In the present paper, we report nascent rotational and fine-structure state distributions in OH(v = 0) from the reaction of hot H atoms with CO 2 molecules.
2. Experimental The OH state distribution is measured by combining 157
Volume 104, n u m b e r 2,3
CHEMICAL PHYSICS LETTERS
exciplex laser photolysis of rapidly flowing HBr and 0 2 mixtures with laser-induced fluorescence detection of the OH product at a short time after the photolysis pulse. The apparatus used has been described in detail elsewhere [14]. A flow reactor is equipped with a baffle system to reduce the scattered light from the ArE-exciplex-laser (Lambda Physik EMG 102) photolysis pulse at 193 nm (maximum energy 200 mJ/pulse, 15 ns pulse duration) and a Nd-YAG(Quanta Ray) pumped dye-laser (Lambda Physik) analysis pulse. The dye laser operates with rhodamine 640 and a frequency-doubling KDP crystal to generate pulses in the 306-311 nm region (0.2 cm -1 linewidth, 12 ns pulse duration, 1-10 mJ pulse energy) to probe OH( 211) radicals by laser4nduced fluorescence (LIF). The fluorescence light is detected by a photomultiplier (EMI 9659 QB) through imaging optics and a filter transmitting between 240 and 390 nm. The photomultiplier current is measured by a boxcar integration system (PAR 162/165). The timing sequence of the laser pulses originates with a signal pulse emitted by the N d - Y A G laser, which starts the exciplex laser discharge and the Q switch of the probe4aser system. The time delay is continuously adjustable with jitter <5 ns. The delay is adjusted by observing scattered light pulses from the two lasers with a storage oscilloscope. Typically, OH spectra were taken at probe times of 4 0 - 1 0 0 ns, partial pressures around 5 mTorr of HBr, and 5 0 - 2 0 0 mTorr of CO2 . At a probe time of 40 ns and 55 mTorr pressure, less than one collision should have occurred on the average between the hot hydrogen atoms and the gas for a translational cooling cross section ~<60 A 2, and less than 0.1 collisions between the OH product molecules and the gas at typical pressures and analysis times. Interference from CO 2 photodissociati0n at 193 nm can be neglected since the absorption cross section for CO 2 [15] is approximately five orders of magnitude smaller than for HBr [16] at 193 nm. Interference from the reactions H + HBr ~ H 2 + Br and OH + HBr ~ H20 + Br is negligible because of the short analysis times and the low partial pressures of HBr used here. Relative peak heights of the LIF signals were converted to relative OH densities by dividing the intensity by the laser power and the appropriate H 6 n l London [17] and Franck-Condon factors [18]. In 158
3 February 1984
some of the spectral scans, however, the OH absorption was driven to saturation so that the LIF intensity is independent of variations of the laser power and directly proportional to OH density. Both methods gave identical results.
3. Results and discussion The absorption of HBr between 2500 and 1600 A is continuous. Photodissociation of HBr at 193 nm produces ~ 15% spin-orbit-excited bromine atoms [ 19], resulting in a bimodal distribution of initial Hatom translational energies. The translational energy of the photofragments in the c.m. system is E = h v D0(HBr) + Eint(HBr) [-E*(Br(2P1/2))] = 61.9 and 51.3 kcal/mol (~ 15% of the fragments), of which the H atoms carry 80/81. Accordingly, the velocity of the H atoms is 2.26 × 106 and 2.06 × 106 cm/s. The relative velocity in the c.m. system of the hot hydrogen atoms colliding with room-temperature CO 2 molecules is g 2 = u 2 + o 2 0 2 _ 2oHOCO 2 c o s 0 ~ O2 ,
because VCO: ~ vH . Accordingly, with well defined relative velocities, we have initially mono-energetic collision energies of 59.9 and 49.8 kcal/mol (~ 15% of the collisions). Fig. 1 presents the rotational state distribution from the RA branch in OH(v = 0) produced in the reaction H 1 + CO 2 -~ OH + CO under approximately
A
g
0.1
II
0.05
n
1
,
,
,
t
5
,
,
,
,
t
10
,
a
,
,
a
15
t
i
L
i
i
20
K Fig. 1. Rotational state distribution from t h e R 1 branch in OH (o = 0) produced in the reaction H + CO2 ~ OH + CO. K is the rotational q u a n t u m n u m b e r and the ordinate is the relative cross section normalized such that t h e total scattering into OH (o = 0) is unity. 5 m T o r r HBr, 200 mTorr COz, probe time 40 ns.
Volume 104, number 2,3
CHEMICAL PHYSICS LETTERS
single-collision conditions. Probe times and total pressures were varied between 40 and 140 ns and 50 and 200 mTorr without noticeable change in the form o f the spectra due to rotational relaxation effects. As shown in fig. 1, there is considerable OH rotational excitation. The nascent rotational distribution in OH(v = 0) is broadly peaked at rotational quantum numbers around 11 and falls to zero around K = 18. In fig. 1, relative cross sections instead o f densities could be given, due to the time resolution o f our experiment. In crossed-molecular-beams-laser-inducedfluorescence experiments, a f l u x - d e n s i t y transformation is necessary to obtain cross sections from product densities measured b y LIF. At equal cross sections for their production, high-velocity product molecules lead to smaller densities than low-velocity species because of their faster disappearance from the analysis zone. For the transformation, detailed internal state and angular distributions have to be known. In contrast, in our experiments, even the fastest OH molecules are still within the analysis area at typical probe times. S p i n - o r b i t and o r b i t a l - r o t a t i o n interactions in the 2[I OH radical cause fine-structure splittings for each rotational level. Each o f these f'me-structure levels can be probed b y different rotational sub-bands, i.e. 11~ by P1 or R1, Hi- by Q1,11~ b y P2 or R 2 and II~- b y Q2 [20], so that cross reactions for production o f the sub4evels can be determined separately. The 2111/2 and 2II3/2 spin states were, within experimental error, equally populated (-+10%). The X doublet fine-structure states showed a clear preference for the lower-energy X component which is probed b y excitation o f R 1 lines in our experiment. The preference increases with K and approaches o(n+)/o(zr - ) = 3.0 -+ 1.0 at the highest populated K. For high rotational states, the electron density o f the singly occupied OH orbital forms lobes perpendicular to the OH angular m o m e n t u m vector JOH ( II+ states) or parallel t O J o n ( I I - states) [21]. The experimental result shows that the breakup o f the HOCO complex generates forces in a plane containing the bond to be broken (the C - O H bond), so that a torque on OH is exerted with JOH perpendicular to that bond, i.e. asymptotically perpendicular to the OH unpaired p electron. This picture is consistent with a planar HOCO intermediate whose C - O H bond breaks in that plane.
3 February 1984
4. Conclusions Further experimental work will be directed towards the study o f the Doppler profiles o f the hot hydrogen atoms with a tunable L~ laser light source [22], and measurement o f the nascent state distribution and the total reaction cross section o f OH at different collision energies by varying the photolysis wavelength and the hot atom precursor. Acknowledgement The financial support o f the Deutsche Forschungsgemeinschaft is gratefully acknowledged. [1 ] B. Lewis and G. Elbe, Combustion, flames and explosion of gases (Academic Press, New York, 1961) p. 90. [2] P.M. Banks and G. Kockarts, Aeronomy (Academic Press, New York, 1973) p. 362. [3] I. Smith and R. Zellner, J. Chem. Soc. Faraday I1 69 (1973) 1617, and references therein. [4] H. Niki, E.E. Daby and B. Weinstock, Advan. Chem. 113 (1972) 16. [5 ] R. Zellner, J. Phys. Chem. 83 (1979) 18, and references therein; R.A. Ravishankara, to be published (1983). [6] D.M. Golden, J. Phys. Chem. 83 (1979) 108. [7] J.E. Spencer, H. Endo and G.P. Glass, Sixteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1977) p. 829. [8 ] T. Dreier and J. Wolfrum, Eighteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1981) p. 801. [9] D.E. Milligan, J. Chem. Phys. 54 (1971) 927. [10]~D.E. Tevault, M.C. Lin, M.E. Umstead and R.R. Smadzewski, Intern. J. Chem. Kinetics 11 (1979) 445. [ 11 ] I.W.M. Smith Chem. Phys. Letters 49 (1977) 112. [12] G.A. Oldershaw and D.A. Porter, Nature 223 (1969) 490. [ 13 ] C.R. Quick Jr. and J.J. Tiel, to be published. [14] P. Andresen, A. Jacobs, K. Kleinermanns and J. Wolfrum, Nineteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1982) pp. 11-22. [15 ] H. Okabe, Photo chemistry of small molecules (Wileylnterscience, New York, 1978) p. 208. [16] B.J. Huebert and R.M. Martin, J. Phys. Chem. 72 (1968) 3046. [17] J.L. Chidsey and D.R. Crosley, J. Quant. Spectrosc. Radiat. Transfer 23 (1980) 187. [18] B. Lin, private communication. [19] F. Magnotta, D.J. Nesbitt and S.R. Leone, Chem. Phys. Letters 83 (1981) 21. [20] M.H. Crosswhite and G.H. Dieke, J. Quant. Spectrosc. Radiat. Transfer 2 (1962) 97. [21 ] W.D. Gwinn, B.E. Turner, W.M. Goss and G.L. Blackman, Astrophys. J. 179 (1973) 789. [22] R. Schmiedl, H. Dugan, W. Meier and K.H. Welge, Z. Physik A304 (1982) 137. 159