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Formation of CN(B2Σ) by the electron impact dissociation of HCN. measurements near threshold
CHEMJC_&LPI-ZYSICSLmERS
Volume 62. number 3
FORMATION
OF CN(B 28)
BY THE ELECTRON
MEASUREMENTS
NEAR THRESHOLD
Iwao NISHIYAMA,
Tamotsu KONDOW
Departmar
IMPACT DISSOCIATION
IS April 1979
OF HCN.
and Kozo KUCHITSU
of Clremirtq’. Faeuky of Science, Z%e (Inib emit-vof Tokyo. BrmX-,O-X-U,Tokyo 113. Japan
Received 17 January 1979
Emission spectra of the CN violet band system (B *Z- X *S) were observed by the eIectron impact on HCN with seberal impact ener@s near me thieshold. The formation of CN(B) by the dissotiti~ excitation of HCN m invest&t&- The -hoId eneey weed essentiaUywith that obt&ned by the photodissociation measurements W Ohbe et al- The Ed=&tion function and the dependence of the vibrational populations of CN(B) on the electron energy were obtained. These reSU& swest tit an optimuy aUowed state corn&t&s to the formation of CN(B) from HCN as a main precursor.
1. Introduction The formation ofCN(B ‘_‘) by the dissociative excitation of HCN has been studied by v.~rious experimental and theoretiL;Il methods [ 1 -!O]. Ultraviolet photons [Z-6]. metastable argon 3toms [7,SJ, aud electrons [9.10] have been used as excitation sources. The impact of electrons h3s_3 Iarge cross section of dissociative excitation_ In our previous study, the vibrationa1 populations of CN(B) from HCN obtained by electron impact were found to be significantiy different from those obtained by impact of photons or metast3bIe argon atoms [9]_ The rotational distributions of C%(B) were approximated by superpositions of two BoItzmann functions [ iOj. These mesurements were made in the energy range of about 100-400 eV, where the observed distnbutions were independent of the excitation energy. A serious problem in electron-impact excitation, in contrast to photodissociation. is that the energy transmitted from the erectrons to the molecule is not clearly deHned_ Therefore, no arawers have yet been given to such problems
as how many precursors participate
in
whether the precursors are optically allowed or forbidden from the ground state of HCN, and whether the precursors in the photodissoc&ion and the eIectron-impact dissociation are the same_ In order to provide information on these problems, the formation
of CN(B),
the eIectron-impact dissociative ehcitrttion of HCN has been studied near the threshold energy_ The excitation function and the threshold energy of the formation of CN(B) have been measured. The emission spectra have aIso been anaIyzed to obtain the vlbrationai distributions.
Fig. 1 shows 3 schemstic dr3wing of the 3ppJratus constructed for the present electron-impact study- An electron gun of 3 modified So3 type wds constructed for measurements at low eiectron energy. The apparstus is composed of 3n electron gun, a collision chamber and an optical detection system_ The collision chamber and the electron source were evacuated separately by two oil diffusion pumps to typicaiIy 1 X IO-7 torr. The sample g3s was introduced through a multichannel nozzle and condensed on 3 liquid nitrogen trap. The pressure of the sample gas in the coIIision region during the emission measurement was controIIed to IO- 3-104 torr by 3 variabie leak v&e, whereas the electron source chamber was maintained below 1 X 10W6 torr by differential pumping and cryopumping. The electron gun was composed of 3 filament. focusing eiectrodes, and two slits. The fiiament was m3de of iridium wire of 0.2 mm diameter coated with thoria_
15 Apnll979
CHEMICAL PHYSICS LJ?l-TERS
Volume 62, number 3
mated to be less than lo%_ The threshold energy of the N2(C-B) emission, I I .O eV, was used to calibrate the impact energy_ The emission spectra of CN(B-X) were measured with a beam current of about 100 ,uA. because a high intensity and a good S/IV ratio were required to analyze the vibrational populations_
3. Results and discussion 3-L Exciratioll jimcriorl
<=1
to pump
..
0
to pump Fii_ 1. Schematic drtte
of the apparatus-
Slits made of molybdenum sheets of 0.1 mm thickness were set in front of the filament and on the exit of the electron beam. The current and the convergence of the electron beam were monitored by a Faraday cup and ZIslrt set in front of the Faraday cup. A total beam current up to about 200 DA at 10 eV impact energy was obtained_ As a test of the apparatus, the second positive (C-B) band system of the nitrogen molecule was observed by electron impact on N,. The excitation function of the formation of N, (C),6 =O, measured with a beam current controlled to about IO--2OOyA. was in fair agreement with that reported by Finn et al. [l 11. However, a slight discrepancy was observed in the steepness of the excitation function in the threshold region. The energy spread of the incident electrons was esthnated to be less than 2 eV on the basis of this steepness. This energy spread is probably caused by the potential difference across the filament. The emission intensity, measured under different conditions of beam convergence. was found to be in good proportion to the total beam current_ T!lis observation guaranteed that the whole excitation region was always viewed by the monochromator, and the uncertainty in the normalization of the emission intensity by the total beam current ~3s esti-
When the escitstion function of the CN(B-X) emission (fig. 2) was measured the wavelengtli WJS set at 388.3 nm corresponding to the P-branch head of the O-O band- The band pass was set to about 02-l -0 nm. so that the light from only the U’ = 0 state of CN(B)W;IS detected by the monochromator. Hence, the excitation function shown in fig. Z corresponds to the Formation process of CN(B) in the ground vibrational state from HCN. Several repeated measurements gave reproducible excitation functions (within lo%)_ The threshold energy was estimated to be 8_Sk 1 .OeV. The essential source of the uncertainty is the systematic error in the energy scale. Okabe et al. measured this threshold energy by the photodissociation of HCN to be S-4 eV [2]. The threshold energy calculated from the data obtdined by thermod>rnamicaI measurements [I?!] is 5.7 eV. Thus the threshold energy obtained from electron impact dissociation agrees with the existing values within the experimental uncertainty_ This threshold energy corresponds to the following processHCN * H(ls)
-I-CN(B)
Other dissociation
_
(1)
processes with different
-I 2’
threshold
tn
z
HCN -
CN(B?Sl
E b”
10
20
30
40
Gt/eV Fii_ 2. The
excitation function of the CN(B-X)
O-O emission. 463
energies such as HCN + H(2s, Zp) f CN(B),
18.6 eV
(2)
-and HCN + Hf + CN(B),
15 Apd 1979
CHEMICAL PHYSICS LJDTERS
VoIume 62, number 3
23.0 eV ,
(3)
are also possible_ However, no clear indication of the existence of such additional onsets has been observed in the present excitation function. Therefore, processes such as (2) and/or (3) are probably minor in comparison with (I) in the eiectron impact dissociation_ The overall shape of the excitation function shows that J major precursor of CN(BB) is an optically allowed stare in-regard to the tmnsition from the ground state of HCN- A simiiar conckion was reached in our Fano-piot analysis of data taken with higher electron energies (IOO-1000 eVj [13!. 3-2 Vibt-ariottal and rorarional dLW-ibr~rio?u of CiVfB) The au = 0, CI sequences of the CN violet band (B-X) were observed_ The emission spectra of the 4~ = 0 sequence. which gave the most intense emission,
HCN-
e
were observed with electron energies From the threshoh to 40 eV, and the dependence of the emission spectra on the impact energy was investigated (fig_ 3). The spectral resolutions were 0.075 and 0.15 nm for the main bands and the tail bands, respectively_ The relatiw intensities of each band head changed with the impact energy, in contrast to our previous observation above 100 eV that the relative intensities were essentially independent of the efectron energy [9]. When the impact energy was decreased, the relative intensities of the band heads for higher vibrational states became weaker. The vibrational popuIations were obtained from the reIative intensities of each viira:ionaI band by using the Fran&-Condon factors calculated by Spindler [ 14]_ The relative vibrational populations, P,ajP,, , obtained at four impact energies are Iisted in table I _ For the main band, which includes information on the populations for u’ = O-9, the vibrational populations were estimated by a band envelope analysis [S,9], where the vibrationa populations and the rotational temperatures obtained at 1OOeV 1131 were used 3s initial pammeters. The rotsltionai distributions for V’ = O-2 were approximated by superpositions of two
o-o X10
cNm*E-x*cl
11
O-0
11-11
K-K
13-13
14-U
1545
5 5 380
302
384
386
388
nm
390
200
mi
410
415tm-l
Fig_ 3_ Emission spectra of the CN(B-X) violet bad system measured nez the th.reshoId: feft, III& bands, mt, tail bands. The vertiul scale of 4ch band is asbitmry- Note that the lines cmsed by the rohtional perturbation, which appeared strongly in the mGn and tail bmds in the impactof me-table argonatom on .zyanides (ret [?1). are too weali.to be observedin the present spe 464
Volume 62, number 3
CHEMICAL.PHYSICS LETTERS
Table I Relative vibrational populations of CN(B) produced by the electron impact on HCW) v’
31 eV
24ev
14 eV
1oev
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.00 0.58 o-43 0.30 0.25 (O-20) (0.16) (0.12) (O-10) (0.09) 0.093 0.100 0.081, 0.072 0.041 0.035
1.00 0.55 0.40 0.26 021 (0.15) (O-12) (0.10) (0.09) (O-09) 0085 0.081 0.064 0063 0.032 0.029
1.00 0.46 0.33 0.18 0.15 (0.10) (0.0s) (0.07) (00.06) (0.06) 0054 0.048 0.041 0.037 0.025 0.014
1.00 0.37 0.23 0.10 0.06 (0.02) (0.01)
EWWJ)
023
020
0.16
O-12
a) The energies correspond to those of incident electrons. The uncertainties =e estimated to be CL IO, 20,30 and 403 for U’= 1,2.3 and 4, respectively, and CL 20% for IJ’-a 10. Values listed in parentheses are crude estmtes; see text_ b, Average rotational energy (%0.05 eV) for U’= 0 esttited from a bsnd-enveIope analysis: see text_ Boltzmann distributions and those for ti 2 3 by ir single Bohzmann distrrbution; these approximations were based on the distributions determined at higher impact energies [ 131, where the emission spectra were observed with higher resohrtion (0.01 nm). The vibrational populations for u’ G 4 were estimated by trial and error. On the other hand_ only crude estimates of P,. could be obtained for the headless bands, u’ = 5-9, since the vibrational structures overlapped one another_ The populations for hig!rer vibr&ional St&es, IJ’= 10-15. which constitute the tail band,were obtLned from the areas under the band contours, since their vibrational structures were observed almost separately. The emission bands for 16 B u’ f 19 were also observed significantly_ Since they were weak and overlapped with other bands, their populations could not be obtained. However, there was no indication of remarkable population disorder. The relative vibrational population obtained at 31 eV agreed essentially with that obtained at energies higher than 100 eV, where the vsbrational excitation
lSA&.%nll979
was reported to be higher than that in the photodissociation obtained at about 10 eV photon energy [3.6] As the electron energy was decreased. the relative vibrational popuiations for u’ 2 I decreased regularly_ The vibrational populations for 10 eV are similar to those obtained by photodissociation. Although the present analysis did not provide fully quantitative estimates of the rotational distributions, the following trends were made cIe,ur The rotationa temperatures for the 31 eV case agree essentirdly with those for the 100 eV case, and the rotational temperatures decrease uniformly as the impact enem is lowered. The average rotational energies estimated for r~‘= 0 are listed in table I _ The results obtained in the above analysis suggest that the difference between electron-impact dissociation md photodissociation may be explained primarily by the difference in the energy transmitted to the parent molecule- The main precursor contributing to the formation of CN(B) in the case of electron impact is probably the same as that in the photodissociation. On the other hand, the vibrational excitation of CN(B) produced in the reaction of rnetastable argon atoms with HCN is reported to be much hi&er than that in the electron- or photon-impact cases Furthermore. in the metastable-unpsct case the branching ratio forming the CN(A’ll) state. as observed in the enhancement of the perturbed rotational lines, is reported to be much higher [7] _These marked differences indicate a strong interaction of the met,rstable argon atom with the dissoclatinS molecule.
Acknowledgement The present study 11,~sbeen supported by a grant from the Kurata Foundation.
References [l] M-F. Freed and Y-B. Band, in: Excited states, Vol. 3 (Academic Press, New York, 1977). [2] D. Davis and H. Okabc, 3. Chem. Phys. 49 (196s) 5526. [3] A.Meleand H.Okabe, J. Chem. Phys. 51 (1969) 4796. [4] hI N.R. Ashfold, WT. Blacpherson and J-P. Simons, Chem. Phys Letters 55 (1978) 84; J. Photocbem. 9 (1978) 134. 465
Volume 62. number 3
FORMATION
OF CN(B 28)
BY THE ELECTRON
MEASUREMENTS
NEAR THRESHOLD
Iwao NISHIYAMA,
Tamotsu KONDOW
Departmar
IMPACT DISSOCIATION
IS April 1979
OF HCN.
and Kozo KUCHITSU
of Clremirtq’. Faeuky of Science, Z%e (Inib emit-vof Tokyo. BrmX-,O-X-U,Tokyo 113. Japan
Received 17 January 1979
Emission spectra of the CN violet band system (B *Z- X *S) were observed by the eIectron impact on HCN with seberal impact ener@s near me thieshold. The formation of CN(B) by the dissotiti~ excitation of HCN m invest&t&- The -hoId eneey weed essentiaUywith that obt&ned by the photodissociation measurements W Ohbe et al- The Ed=&tion function and the dependence of the vibrational populations of CN(B) on the electron energy were obtained. These reSU& swest tit an optimuy aUowed state corn&t&s to the formation of CN(B) from HCN as a main precursor.
1. Introduction The formation ofCN(B ‘_‘) by the dissociative excitation of HCN has been studied by v.~rious experimental and theoretiL;Il methods [ 1 -!O]. Ultraviolet photons [Z-6]. metastable argon 3toms [7,SJ, aud electrons [9.10] have been used as excitation sources. The impact of electrons h3s_3 Iarge cross section of dissociative excitation_ In our previous study, the vibrationa1 populations of CN(B) from HCN obtained by electron impact were found to be significantiy different from those obtained by impact of photons or metast3bIe argon atoms [9]_ The rotational distributions of C%(B) were approximated by superpositions of two BoItzmann functions [ iOj. These mesurements were made in the energy range of about 100-400 eV, where the observed distnbutions were independent of the excitation energy. A serious problem in electron-impact excitation, in contrast to photodissociation. is that the energy transmitted from the erectrons to the molecule is not clearly deHned_ Therefore, no arawers have yet been given to such problems
as how many precursors participate
in
whether the precursors are optically allowed or forbidden from the ground state of HCN, and whether the precursors in the photodissoc&ion and the eIectron-impact dissociation are the same_ In order to provide information on these problems, the formation
of CN(B),
the eIectron-impact dissociative ehcitrttion of HCN has been studied near the threshold energy_ The excitation function and the threshold energy of the formation of CN(B) have been measured. The emission spectra have aIso been anaIyzed to obtain the vlbrationai distributions.
Fig. 1 shows 3 schemstic dr3wing of the 3ppJratus constructed for the present electron-impact study- An electron gun of 3 modified So3 type wds constructed for measurements at low eiectron energy. The apparstus is composed of 3n electron gun, a collision chamber and an optical detection system_ The collision chamber and the electron source were evacuated separately by two oil diffusion pumps to typicaiIy 1 X IO-7 torr. The sample g3s was introduced through a multichannel nozzle and condensed on 3 liquid nitrogen trap. The pressure of the sample gas in the coIIision region during the emission measurement was controIIed to IO- 3-104 torr by 3 variabie leak v&e, whereas the electron source chamber was maintained below 1 X 10W6 torr by differential pumping and cryopumping. The electron gun was composed of 3 filament. focusing eiectrodes, and two slits. The fiiament was m3de of iridium wire of 0.2 mm diameter coated with thoria_
15 Apnll979
CHEMICAL PHYSICS LJ?l-TERS
Volume 62, number 3
mated to be less than lo%_ The threshold energy of the N2(C-B) emission, I I .O eV, was used to calibrate the impact energy_ The emission spectra of CN(B-X) were measured with a beam current of about 100 ,uA. because a high intensity and a good S/IV ratio were required to analyze the vibrational populations_
3. Results and discussion 3-L Exciratioll jimcriorl
<=1
to pump
..
0
to pump Fii_ 1. Schematic drtte
of the apparatus-
Slits made of molybdenum sheets of 0.1 mm thickness were set in front of the filament and on the exit of the electron beam. The current and the convergence of the electron beam were monitored by a Faraday cup and ZIslrt set in front of the Faraday cup. A total beam current up to about 200 DA at 10 eV impact energy was obtained_ As a test of the apparatus, the second positive (C-B) band system of the nitrogen molecule was observed by electron impact on N,. The excitation function of the formation of N, (C),6 =O, measured with a beam current controlled to about IO--2OOyA. was in fair agreement with that reported by Finn et al. [l 11. However, a slight discrepancy was observed in the steepness of the excitation function in the threshold region. The energy spread of the incident electrons was esthnated to be less than 2 eV on the basis of this steepness. This energy spread is probably caused by the potential difference across the filament. The emission intensity, measured under different conditions of beam convergence. was found to be in good proportion to the total beam current_ T!lis observation guaranteed that the whole excitation region was always viewed by the monochromator, and the uncertainty in the normalization of the emission intensity by the total beam current ~3s esti-
When the escitstion function of the CN(B-X) emission (fig. 2) was measured the wavelengtli WJS set at 388.3 nm corresponding to the P-branch head of the O-O band- The band pass was set to about 02-l -0 nm. so that the light from only the U’ = 0 state of CN(B)W;IS detected by the monochromator. Hence, the excitation function shown in fig. Z corresponds to the Formation process of CN(B) in the ground vibrational state from HCN. Several repeated measurements gave reproducible excitation functions (within lo%)_ The threshold energy was estimated to be 8_Sk 1 .OeV. The essential source of the uncertainty is the systematic error in the energy scale. Okabe et al. measured this threshold energy by the photodissociation of HCN to be S-4 eV [2]. The threshold energy calculated from the data obtdined by thermod>rnamicaI measurements [I?!] is 5.7 eV. Thus the threshold energy obtained from electron impact dissociation agrees with the existing values within the experimental uncertainty_ This threshold energy corresponds to the following processHCN * H(ls)
-I-CN(B)
Other dissociation
_
(1)
processes with different
-I 2’
threshold
tn
z
HCN -
CN(B?Sl
E b”
10
20
30
40
Gt/eV Fii_ 2. The
excitation function of the CN(B-X)
O-O emission. 463
energies such as HCN + H(2s, Zp) f CN(B),
18.6 eV
(2)
-and HCN + Hf + CN(B),
15 Apd 1979
CHEMICAL PHYSICS LJDTERS
VoIume 62, number 3
23.0 eV ,
(3)
are also possible_ However, no clear indication of the existence of such additional onsets has been observed in the present excitation function. Therefore, processes such as (2) and/or (3) are probably minor in comparison with (I) in the eiectron impact dissociation_ The overall shape of the excitation function shows that J major precursor of CN(BB) is an optically allowed stare in-regard to the tmnsition from the ground state of HCN- A simiiar conckion was reached in our Fano-piot analysis of data taken with higher electron energies (IOO-1000 eVj [13!. 3-2 Vibt-ariottal and rorarional dLW-ibr~rio?u of CiVfB) The au = 0, CI sequences of the CN violet band (B-X) were observed_ The emission spectra of the 4~ = 0 sequence. which gave the most intense emission,
HCN-
e
were observed with electron energies From the threshoh to 40 eV, and the dependence of the emission spectra on the impact energy was investigated (fig_ 3). The spectral resolutions were 0.075 and 0.15 nm for the main bands and the tail bands, respectively_ The relatiw intensities of each band head changed with the impact energy, in contrast to our previous observation above 100 eV that the relative intensities were essentially independent of the efectron energy [9]. When the impact energy was decreased, the relative intensities of the band heads for higher vibrational states became weaker. The vibrational popuIations were obtained from the reIative intensities of each viira:ionaI band by using the Fran&-Condon factors calculated by Spindler [ 14]_ The relative vibrational populations, P,ajP,, , obtained at four impact energies are Iisted in table I _ For the main band, which includes information on the populations for u’ = O-9, the vibrational populations were estimated by a band envelope analysis [S,9], where the vibrationa populations and the rotational temperatures obtained at 1OOeV 1131 were used 3s initial pammeters. The rotsltionai distributions for V’ = O-2 were approximated by superpositions of two
o-o X10
cNm*E-x*cl
11
O-0
11-11
K-K
13-13
14-U
1545
5 5 380
302
384
386
388
nm
390
200
mi
410
415tm-l
Fig_ 3_ Emission spectra of the CN(B-X) violet bad system measured nez the th.reshoId: feft, III& bands, mt, tail bands. The vertiul scale of 4ch band is asbitmry- Note that the lines cmsed by the rohtional perturbation, which appeared strongly in the mGn and tail bmds in the impactof me-table argonatom on .zyanides (ret [?1). are too weali.to be observedin the present spe 464
Volume 62, number 3
CHEMICAL.PHYSICS LETTERS
Table I Relative vibrational populations of CN(B) produced by the electron impact on HCW) v’
31 eV
24ev
14 eV
1oev
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
1.00 0.58 o-43 0.30 0.25 (O-20) (0.16) (0.12) (O-10) (0.09) 0.093 0.100 0.081, 0.072 0.041 0.035
1.00 0.55 0.40 0.26 021 (0.15) (O-12) (0.10) (0.09) (O-09) 0085 0.081 0.064 0063 0.032 0.029
1.00 0.46 0.33 0.18 0.15 (0.10) (0.0s) (0.07) (00.06) (0.06) 0054 0.048 0.041 0.037 0.025 0.014
1.00 0.37 0.23 0.10 0.06 (0.02) (0.01)
EWWJ)
023
020
0.16
O-12
a) The energies correspond to those of incident electrons. The uncertainties =e estimated to be CL IO, 20,30 and 403 for U’= 1,2.3 and 4, respectively, and CL 20% for IJ’-a 10. Values listed in parentheses are crude estmtes; see text_ b, Average rotational energy (%0.05 eV) for U’= 0 esttited from a bsnd-enveIope analysis: see text_ Boltzmann distributions and those for ti 2 3 by ir single Bohzmann distrrbution; these approximations were based on the distributions determined at higher impact energies [ 131, where the emission spectra were observed with higher resohrtion (0.01 nm). The vibrational populations for u’ G 4 were estimated by trial and error. On the other hand_ only crude estimates of P,. could be obtained for the headless bands, u’ = 5-9, since the vibrational structures overlapped one another_ The populations for hig!rer vibr&ional St&es, IJ’= 10-15. which constitute the tail band,were obtLned from the areas under the band contours, since their vibrational structures were observed almost separately. The emission bands for 16 B u’ f 19 were also observed significantly_ Since they were weak and overlapped with other bands, their populations could not be obtained. However, there was no indication of remarkable population disorder. The relative vibrational population obtained at 31 eV agreed essentially with that obtained at energies higher than 100 eV, where the vsbrational excitation
lSA&.%nll979
was reported to be higher than that in the photodissociation obtained at about 10 eV photon energy [3.6] As the electron energy was decreased. the relative vibrational popuiations for u’ 2 I decreased regularly_ The vibrational populations for 10 eV are similar to those obtained by photodissociation. Although the present analysis did not provide fully quantitative estimates of the rotational distributions, the following trends were made cIe,ur The rotationa temperatures for the 31 eV case agree essentirdly with those for the 100 eV case, and the rotational temperatures decrease uniformly as the impact enem is lowered. The average rotational energies estimated for r~‘= 0 are listed in table I _ The results obtained in the above analysis suggest that the difference between electron-impact dissociation md photodissociation may be explained primarily by the difference in the energy transmitted to the parent molecule- The main precursor contributing to the formation of CN(B) in the case of electron impact is probably the same as that in the photodissociation. On the other hand, the vibrational excitation of CN(B) produced in the reaction of rnetastable argon atoms with HCN is reported to be much hi&er than that in the electron- or photon-impact cases Furthermore. in the metastable-unpsct case the branching ratio forming the CN(A’ll) state. as observed in the enhancement of the perturbed rotational lines, is reported to be much higher [7] _These marked differences indicate a strong interaction of the met,rstable argon atom with the dissoclatinS molecule.
Acknowledgement The present study 11,~sbeen supported by a grant from the Kurata Foundation.
References [l] M-F. Freed and Y-B. Band, in: Excited states, Vol. 3 (Academic Press, New York, 1977). [2] D. Davis and H. Okabc, 3. Chem. Phys. 49 (196s) 5526. [3] A.Meleand H.Okabe, J. Chem. Phys. 51 (1969) 4796. [4] hI N.R. Ashfold, WT. Blacpherson and J-P. Simons, Chem. Phys Letters 55 (1978) 84; J. Photocbem. 9 (1978) 134. 465