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Optical excitation function of CO by electron impact near threshold
Volume 44, number
I
CIIEMCAL
PHYSICS LETf’ERS
1.5 November
1976
OPTICAL EXCITATION FUNCTION OF CO BY ELECTRON iMPACT NEAR THRESHOLD Kiyoshi FUKUI, Takahiro
HIROTSU, Keiji KUWATA
Department of Chemistry, Faculty of Science, Osaka Ut~iuer~sity, Toyotzuka. Osuka, 560 >apan and Iwao
FUJITA
Osaha ~teclro-Contttttctiicatiott Neyagff wQ, Osaka, 5 72 Japan Rccewed
University.
22 July 1976
TOWCI~I~SSKJI~ S~CC~~UIIIfor the 113Ze + B 3nr, d 3Ai 4 d 311r, nnd I3 ‘IS* + A *II transitions of CO WZI;observed in low cncrgy electron impact and the cxuitatzon ftrnctIon on CO& 3~> war, tramincd near the excitation threshold. The strong 2nd weak resonance peaks near the ~rcsllold were assigned to the corc-cuatcd shape rcsonanccs at IO.39 and 10.7 eV, rcspcctwcly,
which
h,nc
been ob%xvcd
in an
clcctron
scattering
1. Introduction Re~;o~anccs in electron Impact on di~t~lni~ molecules have been mvcst~gatcd using different methods by many rcscarchcrs. The rcsuits have been inclusively rcvicwcd by Schulz [1,2]. In clcctron scattering expcrimcnts Mazeau et al. [3,4] and Swanson et al. [5] have measured the differential excitation function for the b 3x?:; = 0) level at 10.39 eV above the ground state of CO, in which a few resonance structures have been c bscrvcd near the threshold. The core-excited resonanccs have been obscrvcd at 10.34 [3], 11.3, and 12.2 e V [4] , arrd the resonance at 10.04 eV has been assigned to the 2Zi state [3]. The c&e-excited shape resonances have been observed at 10.7 and 11.2 eV [4], and the resonance at 10.7 cV has been assigned to the 211 state [4J. So %r Skubenich [6] has obtained the excitation functions for the various excited states of CO by the obscrvltion of Ii&t emission from these states, howcvcr he could not observe such resonances in the oxcitation function of the b 3Z+ (u’=O) state near the threshold, because of the poor energy resolution of the incident electron. In the present study the excitatio;l function of the b SE+ state was finely cx-
experiment.
amincd near the threshold by the observation of light emission, and a few resonances were observed which correspond to the resonances observed by Mazeau et al. and Swanson et al. in electron scattering experiments. The excitation function obtained in this work relates to the total excitation cross section of the b 3Ei state, and the rcIation between the resonance excitations of CO observed by Mazearr ct aI. and the subsequent prccess of emission from tfle b 3 Zf state is discllsscd in this study.
A detailed description of the apparatus used in the present study has been given elscwherc-- [7]. The collision chamber and the chamber for eIecfrcln gun were evacuated separately. The pressure in eacfl chamber was measured by an ionimtion gauge equipped with the w-a!1of each chamber. These chambers were evacuated at 2 X lo-’ torr before the introduction of target gas. The emitted light was focuzsed by a quartz lens on the inlet slit of a Nahmli RM-23 grating monochromator with a Ihmamatsti R585 photomulti_nlier, 13
Voiumc 44, number 1
IS November 1976
CHEMICAL PHYSICS LE’ITERS
and ‘the output was measured by photoncounting. The energy coriection of the sfectron beam was carried out observing the peak position in the excitation function on NZ(C 311,) which has been accurately determined before by Finn et al. [8]. The energy spread of the electron beam was 0.3 eV in the present study.
3. Results and discussion
(0.24 A) of the equ~ib~um internuciear separations between these states and the ground state fl I]. The vibrational state associated with the vertical excitation is in the vicinit= of u’ = 12, and the state lies by about 9.2 eV above the ground state [5,1 I]. In fact the observed threshold energy at each vibrational band was around the value of 9.2 ev. The (0, u”) vibrational band for the B lC+ -+ A 111 state may bc due to the small differences
transition appears in the longer wavelength region of the spectrum shown in fig. 1, and the (0, u”) vibra-
tional band for the b 3Z+ --t a 3fIr transition in the shorter wavelength. The excitation function of the b 3Z+ state was only observed in the present work ar,d the excitation functions of the d 3Ai and B II? states in the higher energy region of the impact electron could not be separately measured due to the overlap rJf the emissions from other excited species.
The lower excited states of CO are the following [9,10] : AIR (the adiabatic threshold efiergy of 8.06 ev), B lZ+ (10.78), C 1Z+(l 1.39); a 311, (6.03), a’ 3x+ (6.93), d 3”1i (7.72), b 3Z+ (1 O-39), e? (11.72), and c 3E;’ (11.55). The ground-state configuration of CO is represented by (lo)*(20)2(30)2(40)2~l~)~(50)2;
X ‘Z+ .
3.2. The excitatim function of the b 3ZZ+state for the b 3 C+ + a 3ll, truns~t~on
The a 311, and A IfI states associate with the electron con~guration . . . (l~)4(So)(2~), aqd the d 3Ai state . . . (1n)3(5a)2(27r). The lowest Rydberg states of b 3Ec+ and B lx+ have the electron configuration of .. . ( ln)4(5uj(3sa) f3-51 . The emission bands in the 270 to 580 nm region at the electron energy of 1 I .3 eV were asrigned to the b 3C+ --f a 3nr, d 3Ai +a311 and B’ZZ++Alfl transi:ions [9,10] , ixsshown& fig. i. The emission bands from the highly excited vibrational states of the d 3Ai state are observed in a wide range of the emission spectrum. The formation of these states by a vertical excitation from the ground CO ( b%‘-
The dependencies of the emission intensity of the b 3Z;c + a 311, transition on both the pressure and the beam current at the energy of the impact electron of 3 1.4 eV near the peak of the excitation function is studied. Linear relationships between the emission intensity and the electron beam current were found up to 3 PA at the pressures of 5 and 2 X 10B5 tori, and upto7pAat5X10-6 torr, respectively. Linear relationships between the emission intensity and the Fessurc in the collision chamber were found up to CO(8'&- All-T)
a3n 1
= 0
z d _I
CO __-.____ _ (v’ (d3.4~-a3nT)
I--- --
300
350
400 WAVELENGTH
450
( nm
L-_---
1
9 _ -
v”)
.--
500
1
550
Fig. 1. Emission spectrum of the CO molecuis at an electron-~rnp~ct energy of 11.3 eV, beam current of 5.0 PA, and pressure in the collision chamber of 2.5 X IO-’ torr. Bandpass of the monochromator was I.0 nm.
14
Volume 44, number
1
CHEMICAL
PHYSICS LETTERS
8 X IO-5 torr. Thus, ah far as thcsc linear relationships exist the excited species are expected to be formed through single collision of the CO molecule with an incident clcctron. In the measurement of the cxcitation function of the b 3X+ state the intensity of the clcctron beam was fixed at the optimum value of 1 .O PA for the best resolution. The excitation function of the b 3C+ state from the threshold energy up to 24 cV is shown in fig. 2. The excitation function has a very sharp rise at the threshold and then a rapid fall mamly due to a resonance excitation, and a plateau in the liiglicr energy region due to a direct excitation. The excitation function was more tincly measured near the threshold as shown in fig. 3. The fine StiUCtUrCS of the excitation function near the threshold, which have not been fourld in the excitation function obtained by Skubcnich [6], corrcspends fairly well to what Ma.zeau et al. have observed in an clcctron scattering expcrimcnt. The core-cxcitcd resonance of the 2Z? state at 10.04 CV lies just below the energy level of 10.39 eV for the parent state of b 3ZZ+,and consequently the molcculc in such a corecxcitcd resonnncc below 10.39 eV decays into other lower excited states than the b 3Z+ state of CO. The very steep rise at 10.39 eV in the excitation function of the b 3Z? state IS caused by the tall of the core-excited resonance peak of the 2Z? ~._
-
_ -__. CO (b-a) 4x10-‘Torr I.OpA
.
.
.. .
. .-. -0 -..
h 6
II
I
8
I
IO
I
I
I
12
kLECTRON 1;~. 2. Excitation
II
I
I4
16
IMPACT
’
I I8
I
I 20
ENERGY
I
’ 22
COfb-al
t I-107s
11
’
24
(eV1
function of CO(b 3 Z+) for the b 3 Z+ - ;1311r transition. Beam current was 1 .O /.LAand pressure in the collision chamber was 4 X low6 torr.
4 I: 10e5 Torr
* ‘1
=._ F : :< s
[-OPA
12.2 F .
-
.
I -. - . -. -*-_ _ ..
. . . . - . _:_..-
..
--_.
._ .:
(Illlrlrlllrl 9
”
IO II ELECTRON
,I2 13 IMPACT
I4 15 ENERGY
.
”
‘I
16 (eV)
17
Fig. 3. EGt.ttton function of CO(b 3X> nczr the threshold. Bean current was 1.0 PA and pressure in the collision chambcl was4 X 10mGtorr. “ S” denotes corcescitcd shape rczonancc, and “F” denotes core-cxcitcd resmmce extends
[31_
beyond
IO.39 cV, the threshold in the part of the corccxcitcd resonance of the 2Zt state above 10.39 cV may decay through a new channc1 to the b 3X+ state by one electron transition. Thus, the excitation function of the b 31Zf state couId have a very steep ris: at 10.39 eV. While, in the electron scattering cxperimcnts the angular dependence of the differential cross section for the peak at 10.39 eV has been ascertamed to be isotropic in agreement with that of the 2IZ* state [3]. Such a cut-off profile with a very Aarp me at the threshold has been frequently seen [2,3 ] and a detailed explanation for the phenomena has been given in refs. [2,12]. The weak peak at 10.7 eV was assigned to the coreexcited shape resonance of 211symmetry, because in the electron scattering angular dependence of thcdifferentia cross section for the resonance shows a curve of p-type behavior with a minimum at 2 scattering angle of 90” [3]. In the compound state of 211 symmc&y of COan incident electron is temporarily trapped in 2 low of the b 3Xf state. The
_---
1976
1 10 4kV) s
4
state which
--
15 Novcmbcr
111olecu1e
15
Volume 44, number
1
CHEMICAL
PHYSICS LETTERS
effective potential barrier caused by the Interaction between the incident p wave electron and the parent state of b SIC+. Therefore the shape resonance at 10.7 eV exists just above the energy !evel of 10.39 eV for the the b 32+ state. The excitation function obtained corresponds to the total excita?ion cross section for the b 3Z+ state. Therefore the excitation function shown in fig. 3 mdicates that the channel to the b 3X+ state through the core-excited shape rcsonancc of the 211 state at 10.7 eV is less possible than the ctire-excited shape resonance at 10.4 eV caused by a tall of the coreexcited resonance of the *Z+ state, though both the decays from the resonance at 10.39 eV and 10.7 eV into the b ‘P’ state are one electron tranntions. Another core-excited shape resonance at 11.2 cV with a d wave character associated with the parent state of b 3E’ has been observed as a relatively weak peak in an electron scattering experiment [3]. In the present work the corresponding resonance peak was not clearly cbscrved though the decay into the b 3X+ state is also a one electron transition. Further, the core-excited resonances at I 1.3 and 12.2 eV, which have been observed z’sweak peaks in 3n electron scattering cxpedment 131, were also very weak in the present work. The core-excited resonance at 11.3 eV may bc associated with t!le parent state of c 311(1 1.41 eVj [3]. The parent state for the core-
16
15
November 1976
excited resonance at 12.2 eV is yet unknown. The decays from these resonances into the b 3E+ state may be considerably unfavorable because these are two electron transitions. In addition, the broad peak centered at about 12 CV may be a broad maximum for the electron-exchange excitation.
2eferences [ I] G.J. Schulz, Rev. Mod. Phys. 45 (1973) 378. [2] G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. [3] J. Mnzeau, F. Gresteau, G. Joycz, J. Rcmhardt and R.I. Hall. J. Phvs. BS (19721 1890. [4] .J. R&tharht, G. j0yeL.I. Mazcau and R-1. Hall. J. Phys. B 5 (1972) 1884. [S ] N. Swanson, C.E. Kuyatt, J.W. Cooper and M. Krauss. Phys. Rev. Letters 28 (1972) 948. [6] Y.V. Skubenich, Opt. Spcctry. 23 (1967) 540. 17) K. Fukui, I. FuJitd and K. Kuwata, Shitsuryo Bunseki (h&g Spectroscopy) 23 (1975) 105. [8] T G. Finn, J.F.M. Aarts and J.P. Docring, J. Chcm. Wys. 56 (1972) 5632. 19) G. Hcnbcrg, Spectra of diatomic molcculcs (Van Nostrand, Prmceton, 1950). [IO] R.W.B. Pcarsc and A.G. Gaydon, The identification of molecular spectra (Chapman and H‘111,London, 1965). [ 1 l] C. !Icrzbcrg dnd T.J. Hugo, Can. J. Phyc. 48 (1970) 3004. [ 121 H.S. Taylor, Advan. Chem. Phys. 18 (1970) 91.
I
CIIEMCAL
PHYSICS LETf’ERS
1.5 November
1976
OPTICAL EXCITATION FUNCTION OF CO BY ELECTRON iMPACT NEAR THRESHOLD Kiyoshi FUKUI, Takahiro
HIROTSU, Keiji KUWATA
Department of Chemistry, Faculty of Science, Osaka Ut~iuer~sity, Toyotzuka. Osuka, 560 >apan and Iwao
FUJITA
Osaha ~teclro-Contttttctiicatiott Neyagff wQ, Osaka, 5 72 Japan Rccewed
University.
22 July 1976
TOWCI~I~SSKJI~ S~CC~~UIIIfor the 113Ze + B 3nr, d 3Ai 4 d 311r, nnd I3 ‘IS* + A *II transitions of CO WZI;observed in low cncrgy electron impact and the cxuitatzon ftrnctIon on CO& 3~> war, tramincd near the excitation threshold. The strong 2nd weak resonance peaks near the ~rcsllold were assigned to the corc-cuatcd shape rcsonanccs at IO.39 and 10.7 eV, rcspcctwcly,
which
h,nc
been ob%xvcd
in an
clcctron
scattering
1. Introduction Re~;o~anccs in electron Impact on di~t~lni~ molecules have been mvcst~gatcd using different methods by many rcscarchcrs. The rcsuits have been inclusively rcvicwcd by Schulz [1,2]. In clcctron scattering expcrimcnts Mazeau et al. [3,4] and Swanson et al. [5] have measured the differential excitation function for the b 3x?:; = 0) level at 10.39 eV above the ground state of CO, in which a few resonance structures have been c bscrvcd near the threshold. The core-excited resonanccs have been obscrvcd at 10.34 [3], 11.3, and 12.2 e V [4] , arrd the resonance at 10.04 eV has been assigned to the 2Zi state [3]. The c&e-excited shape resonances have been observed at 10.7 and 11.2 eV [4], and the resonance at 10.7 cV has been assigned to the 211 state [4J. So %r Skubenich [6] has obtained the excitation functions for the various excited states of CO by the obscrvltion of Ii&t emission from these states, howcvcr he could not observe such resonances in the oxcitation function of the b 3Z+ (u’=O) state near the threshold, because of the poor energy resolution of the incident electron. In the present study the excitatio;l function of the b SE+ state was finely cx-
experiment.
amincd near the threshold by the observation of light emission, and a few resonances were observed which correspond to the resonances observed by Mazeau et al. and Swanson et al. in electron scattering experiments. The excitation function obtained in this work relates to the total excitation cross section of the b 3Ei state, and the rcIation between the resonance excitations of CO observed by Mazearr ct aI. and the subsequent prccess of emission from tfle b 3 Zf state is discllsscd in this study.
A detailed description of the apparatus used in the present study has been given elscwherc-- [7]. The collision chamber and the chamber for eIecfrcln gun were evacuated separately. The pressure in eacfl chamber was measured by an ionimtion gauge equipped with the w-a!1of each chamber. These chambers were evacuated at 2 X lo-’ torr before the introduction of target gas. The emitted light was focuzsed by a quartz lens on the inlet slit of a Nahmli RM-23 grating monochromator with a Ihmamatsti R585 photomulti_nlier, 13
Voiumc 44, number 1
IS November 1976
CHEMICAL PHYSICS LE’ITERS
and ‘the output was measured by photoncounting. The energy coriection of the sfectron beam was carried out observing the peak position in the excitation function on NZ(C 311,) which has been accurately determined before by Finn et al. [8]. The energy spread of the electron beam was 0.3 eV in the present study.
3. Results and discussion
(0.24 A) of the equ~ib~um internuciear separations between these states and the ground state fl I]. The vibrational state associated with the vertical excitation is in the vicinit= of u’ = 12, and the state lies by about 9.2 eV above the ground state [5,1 I]. In fact the observed threshold energy at each vibrational band was around the value of 9.2 ev. The (0, u”) vibrational band for the B lC+ -+ A 111 state may bc due to the small differences
transition appears in the longer wavelength region of the spectrum shown in fig. 1, and the (0, u”) vibra-
tional band for the b 3Z+ --t a 3fIr transition in the shorter wavelength. The excitation function of the b 3Z+ state was only observed in the present work ar,d the excitation functions of the d 3Ai and B II? states in the higher energy region of the impact electron could not be separately measured due to the overlap rJf the emissions from other excited species.
The lower excited states of CO are the following [9,10] : AIR (the adiabatic threshold efiergy of 8.06 ev), B lZ+ (10.78), C 1Z+(l 1.39); a 311, (6.03), a’ 3x+ (6.93), d 3”1i (7.72), b 3Z+ (1 O-39), e? (11.72), and c 3E;’ (11.55). The ground-state configuration of CO is represented by (lo)*(20)2(30)2(40)2~l~)~(50)2;
X ‘Z+ .
3.2. The excitatim function of the b 3ZZ+state for the b 3 C+ + a 3ll, truns~t~on
The a 311, and A IfI states associate with the electron con~guration . . . (l~)4(So)(2~), aqd the d 3Ai state . . . (1n)3(5a)2(27r). The lowest Rydberg states of b 3Ec+ and B lx+ have the electron configuration of .. . ( ln)4(5uj(3sa) f3-51 . The emission bands in the 270 to 580 nm region at the electron energy of 1 I .3 eV were asrigned to the b 3C+ --f a 3nr, d 3Ai +a311 and B’ZZ++Alfl transi:ions [9,10] , ixsshown& fig. i. The emission bands from the highly excited vibrational states of the d 3Ai state are observed in a wide range of the emission spectrum. The formation of these states by a vertical excitation from the ground CO ( b%‘-
The dependencies of the emission intensity of the b 3Z;c + a 311, transition on both the pressure and the beam current at the energy of the impact electron of 3 1.4 eV near the peak of the excitation function is studied. Linear relationships between the emission intensity and the electron beam current were found up to 3 PA at the pressures of 5 and 2 X 10B5 tori, and upto7pAat5X10-6 torr, respectively. Linear relationships between the emission intensity and the Fessurc in the collision chamber were found up to CO(8'&- All-T)
a3n 1
= 0
z d _I
CO __-.____ _ (v’ (d3.4~-a3nT)
I--- --
300
350
400 WAVELENGTH
450
( nm
L-_---
1
9 _ -
v”)
.--
500
1
550
Fig. 1. Emission spectrum of the CO molecuis at an electron-~rnp~ct energy of 11.3 eV, beam current of 5.0 PA, and pressure in the collision chamber of 2.5 X IO-’ torr. Bandpass of the monochromator was I.0 nm.
14
Volume 44, number
1
CHEMICAL
PHYSICS LETTERS
8 X IO-5 torr. Thus, ah far as thcsc linear relationships exist the excited species are expected to be formed through single collision of the CO molecule with an incident clcctron. In the measurement of the cxcitation function of the b 3X+ state the intensity of the clcctron beam was fixed at the optimum value of 1 .O PA for the best resolution. The excitation function of the b 3C+ state from the threshold energy up to 24 cV is shown in fig. 2. The excitation function has a very sharp rise at the threshold and then a rapid fall mamly due to a resonance excitation, and a plateau in the liiglicr energy region due to a direct excitation. The excitation function was more tincly measured near the threshold as shown in fig. 3. The fine StiUCtUrCS of the excitation function near the threshold, which have not been fourld in the excitation function obtained by Skubcnich [6], corrcspends fairly well to what Ma.zeau et al. have observed in an clcctron scattering expcrimcnt. The core-cxcitcd resonance of the 2Z? state at 10.04 CV lies just below the energy level of 10.39 eV for the parent state of b 3ZZ+,and consequently the molcculc in such a corecxcitcd resonnncc below 10.39 eV decays into other lower excited states than the b 3Z+ state of CO. The very steep rise at 10.39 eV in the excitation function of the b 3Z? state IS caused by the tall of the core-excited resonance peak of the 2Z? ~._
-
_ -__. CO (b-a) 4x10-‘Torr I.OpA
.
.
.. .
. .-. -0 -..
h 6
II
I
8
I
IO
I
I
I
12
kLECTRON 1;~. 2. Excitation
II
I
I4
16
IMPACT
’
I I8
I
I 20
ENERGY
I
’ 22
COfb-al
t I-107s
11
’
24
(eV1
function of CO(b 3 Z+) for the b 3 Z+ - ;1311r transition. Beam current was 1 .O /.LAand pressure in the collision chamber was 4 X low6 torr.
4 I: 10e5 Torr
* ‘1
=._ F : :< s
[-OPA
12.2 F .
-
.
I -. - . -. -*-_ _ ..
. . . . - . _:_..-
..
--_.
._ .:
(Illlrlrlllrl 9
”
IO II ELECTRON
,I2 13 IMPACT
I4 15 ENERGY
.
”
‘I
16 (eV)
17
Fig. 3. EGt.ttton function of CO(b 3X> nczr the threshold. Bean current was 1.0 PA and pressure in the collision chambcl was4 X 10mGtorr. “ S” denotes corcescitcd shape rczonancc, and “F” denotes core-cxcitcd resmmce extends
[31_
beyond
IO.39 cV, the threshold in the part of the corccxcitcd resonance of the 2Zt state above 10.39 cV may decay through a new channc1 to the b 3X+ state by one electron transition. Thus, the excitation function of the b 31Zf state couId have a very steep ris: at 10.39 eV. While, in the electron scattering cxperimcnts the angular dependence of the differential cross section for the peak at 10.39 eV has been ascertamed to be isotropic in agreement with that of the 2IZ* state [3]. Such a cut-off profile with a very Aarp me at the threshold has been frequently seen [2,3 ] and a detailed explanation for the phenomena has been given in refs. [2,12]. The weak peak at 10.7 eV was assigned to the coreexcited shape resonance of 211symmetry, because in the electron scattering angular dependence of thcdifferentia cross section for the resonance shows a curve of p-type behavior with a minimum at 2 scattering angle of 90” [3]. In the compound state of 211 symmc&y of COan incident electron is temporarily trapped in 2 low of the b 3Xf state. The
_---
1976
1 10 4kV) s
4
state which
--
15 Novcmbcr
111olecu1e
15
Volume 44, number
1
CHEMICAL
PHYSICS LETTERS
effective potential barrier caused by the Interaction between the incident p wave electron and the parent state of b SIC+. Therefore the shape resonance at 10.7 eV exists just above the energy !evel of 10.39 eV for the the b 32+ state. The excitation function obtained corresponds to the total excita?ion cross section for the b 3Z+ state. Therefore the excitation function shown in fig. 3 mdicates that the channel to the b 3X+ state through the core-excited shape rcsonancc of the 211 state at 10.7 eV is less possible than the ctire-excited shape resonance at 10.4 eV caused by a tall of the coreexcited resonance of the *Z+ state, though both the decays from the resonance at 10.39 eV and 10.7 eV into the b ‘P’ state are one electron tranntions. Another core-excited shape resonance at 11.2 cV with a d wave character associated with the parent state of b 3E’ has been observed as a relatively weak peak in an electron scattering experiment [3]. In the present work the corresponding resonance peak was not clearly cbscrved though the decay into the b 3X+ state is also a one electron transition. Further, the core-excited resonances at I 1.3 and 12.2 eV, which have been observed z’sweak peaks in 3n electron scattering cxpedment 131, were also very weak in the present work. The core-excited resonance at 11.3 eV may bc associated with t!le parent state of c 311(1 1.41 eVj [3]. The parent state for the core-
16
15
November 1976
excited resonance at 12.2 eV is yet unknown. The decays from these resonances into the b 3E+ state may be considerably unfavorable because these are two electron transitions. In addition, the broad peak centered at about 12 CV may be a broad maximum for the electron-exchange excitation.
2eferences [ I] G.J. Schulz, Rev. Mod. Phys. 45 (1973) 378. [2] G.J. Schulz, Rev. Mod. Phys. 45 (1973) 423. [3] J. Mnzeau, F. Gresteau, G. Joycz, J. Rcmhardt and R.I. Hall. J. Phvs. BS (19721 1890. [4] .J. R&tharht, G. j0yeL.I. Mazcau and R-1. Hall. J. Phys. B 5 (1972) 1884. [S ] N. Swanson, C.E. Kuyatt, J.W. Cooper and M. Krauss. Phys. Rev. Letters 28 (1972) 948. [6] Y.V. Skubenich, Opt. Spcctry. 23 (1967) 540. 17) K. Fukui, I. FuJitd and K. Kuwata, Shitsuryo Bunseki (h&g Spectroscopy) 23 (1975) 105. [8] T G. Finn, J.F.M. Aarts and J.P. Docring, J. Chcm. Wys. 56 (1972) 5632. 19) G. Hcnbcrg, Spectra of diatomic molcculcs (Van Nostrand, Prmceton, 1950). [IO] R.W.B. Pcarsc and A.G. Gaydon, The identification of molecular spectra (Chapman and H‘111,London, 1965). [ 1 l] C. !Icrzbcrg dnd T.J. Hugo, Can. J. Phyc. 48 (1970) 3004. [ 121 H.S. Taylor, Advan. Chem. Phys. 18 (1970) 91.