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The predissociation of molecular oxygen cations O2+(B̃ 2Σg−)
Chemical Physics 61 (1981) 215-219 North-Holland Publishing Company
THE
PREDISSOCIATiON
Rolf BOMBACH, Physikalisch-Chemisches
Andreas
OF MOLECULAR SCHMELZER
OXYGEN
and Jean-Pierre
Znstitut der Uniwrsirzt Bawl, CH-4056
CATIONS
O;&‘Z,,
STADELMANN
Bawl, Switzerland
Received 20 May 1981
Using the fixed wavelength (He-k) photoelectron-photoion coincidence technique, we observed that molecular oxygen cations generated in their 6 %‘_su = 4 vibronic state decay competitively via the fragmentation channels O’(‘S) t O?P) and O’(‘S)+O(‘D), the ratio being [O’(CS)+O(‘P)]:[Oi(iS)iO!‘D)]=0.75:0.25.
1. Introduction The unimolecular fragmentation behaviour of excited 0; has been the subject of numerous investigations reviewed in refs. [l-3]. Using the threshold photoelectron-photoion coincidence technique it has been shown that oxygen parent cations in the o 34 vibrational levels of their b ‘S, electronic state relax competitively by predissociation according to the reaction [4] 0;
+ O’CS)
i O(‘P),
(1)
and by radiative transition into the B ‘II, state [S]. The predissociating states have been recently identified to be the r ‘I& and predominantly the 4 ‘Xp’ electronic states of 0; [6 3. The absence of detectable emission [S] indicates that the B *Xi state of 0: is fully predissociated, and leads to the above specified products [7], thus generating O*(%) fragment ions carrying a translational energy of the order ET = 0.8 eV. As the v = 4 vilorational level of the B ‘Xi state of 0; (I = 20.815 ev) lies 0.115 eV above the threshold (1= 20.700 eV [Sl, for the reaction 0; -a O+(%) + O(‘D), this process has been considered compete with reaction (1) [9]. 03Ol-0104/81/0000-0000/S02.50
(2) possibly to
@ North-Holland
The object of this work is to investigate the fragmentation pathway (2) using the fixed wavelength photoelectron-photoion coincidence technique.
2. Experimental The photoeiectron-photoion coincidence spectrometer employed in this work is schematically displayed in fig. 1. Photoelectrons and photoions are generated at the centre of the source by photon impact (He-Ia, 21.22 eV) on the sample gas effusing from an inlet capiltary. If a source field is used, the potentials of the five parallel plates and the inlet capillary are individuahy tuned to provide a homogeneous and axially symmetric electrostatic field, the ionization region being at ground potential. The photoelectrons are energy analysed by means of a hemispherica analyser, equipped with triple tube lenses, working in the constant energy mode. Low energy photoelectrons (e.g. those corresponding to the B ‘X-i state of 0:) are ener,7 analysed by scanning the potentials applied to the hemispheres, in order to reduce the noise electron rate otherwise observed at ionization energies I > 20.5 eV. The photoelectrons are detected by a channel electron multiplier. The electron transmission coefficient has been measured as fc = 10T3.
216
kiFEi%FN DE&OR TRIPLE ION
DE :TECTOR
TUBE
TIME-OF-FLiGKT
LENS
ANALYSER
HEMISPHERES DRIFT
TUBE
GASp;NLET
0
LAa-J_Y SCALE
Fig.
1.
I cm
Scheme of the photoelectron-photoion
dependent flight time of the electron (between 0.2 and 0.5 CI_Sin our experiment and at the energy range under consideration) to obtain the total time-of-flight of the icn. The performance of the apparatus was tested with a FORTRAN IV program based on the algorithm summarized in ref. [13]. After the evaluation of the electrostatic potential field in the source and the TOF analyser, ions of a chosen mass and initial speed (thermal energy neglected in our case) were assumed to start isotropically in the centre of the source. From their flight paths, the TOF distribution as well as the transmission probability were evaluated.
The photoions are registered by a particle multiplier after traversing a time-of-flight (TCF) analyser. The ion transmission coefficient was measured as f;-0.3 for thermal 0; molecular ions at a constant source field of Fs = 3 V/cm. Since in one part of the experiments no source field was applied, only those ions with an initial motion towards the ion detector have been monitored, and the ion transmission coefficient was therefore degraded by approximately a factor of ten. Under these conditions (Fs= 0),the electron energy resolution was sufficient (AE = 50 meV) to reproduce the vibrational fine structure (cf. fig. 2) observed in the high resolution photoelectron (PE) spectrum of O2 [lO] in the energy range considered. More details of the experimental setup will be given elsewhere 1111. The
TOF spectra were accumulated by means of a multichannel analyser described in ref. [12]. Tile accumulation times were varied from 12 to 48 h depending on the photoe!ectron signal intensity. The time resolution was set to 40 ns per channel. Since in the coincidence experiments of this type the registration of an electron defines the ion creation time, the time scale in figs. 2 and 3 should be corrected by the energy
coincidencespectrometer.
3. Results and discussion
.
The section of the PE spectrum of 02, which corresponds to the b $5, and the B ‘I;; electronic states of 0; is displayed in fig. 2 (no source field) and fig. 3 (source field FS = 1.95V/cm). The calibration is based on the high resolution data of Lindholm et al. [lo]. Bearing in mind that reaction (I) is the only energetically possible fragmentation pathway for ot@ 5;; u 6 2) the following results are
R. Bombach et al. / Predissociation of Ol@
‘XJ
217
1
Fig. 2.
Part of the photoelectron spectrumof 02 (left; horizontal axis: count rate in arbitrary units) and the timeof-f&@ spectra of 0’ (right: vertical axis: coincidence
count rate in arbitrary units) measured at the indicated
ionization energies with a source field of FS = 0. The arrows
denote the threshold energies of the two framentation pathwaysaccessiblewith He-b radiation. The time scale
in
this and the subsequent figure is not corrected for the energy dependent time-of-flight of the electrons (see text).
derived from an analysis of the 0’ TOF spectra presented in fig. 2. The sharp signals recorded at o = 0 (I = 20.30 eV), 0 = 1 (I = 20.43 eV) and u = 2 (I = 20.57 ev) are easily assigned to Ot ions with an initial speed of us = 3.07 x 103, 3.20 x IO3 and 3.32 X lo3 m/s, respectively, starting in the direction of the ion detector. The time-of-fight of ~5.5 ws is in accordance with the values obtained from the trajectoij calculations. For O;@ “CJ with t, = 3 (i= 20.69eV, t). = 3.44 x IO3 m/s) reaction (1) is the only reaction pathway detected, although the pathway (2) is energetically accessible for rotationa&jr excited 0; c43. The TOF spectrum obtained at I = 20.82 eV (g ‘2,; t, =4) exhibits two signals (cf. fig. 2).
Fig. 3. Part of the PE spectrum of Oz measured using a source field of FS = 1.95 V/tin and the TOF spectra of 0‘ obtained at the indicated ionization energies.
The peak at = 5 J.LSis due to fast O* ions (u, = 3.54 x lo3 m/s) from reaction (l), whereas the broad signal at 7-9 ~LSis assigned to the reIatively slow 0’ ions (u, = 0.83 X 10’ m/s) generated in the course of reaction (2). This assignment is confirmed by noting that the excess energy of Of (6 ‘Xi; u = 4, I = 20.815 eV [lo]) relative to the threshold of reaction (2) (I = 20.700 eV [S]) is equal to the excess energy of 0; (g ‘Xi; v = 5, I = 18.848 eV [lo]) relative to the threshold of reaction (1) (I = 18.733 eV [S]) [4]_ Therefore identical 0“ TOF distributions are expected at I = 18.85 eV [reaction (l)] and I = 20.82 eV [reaction (211 in accordance with the present observations (cf. fig. 2). The trajectory calculations reveal that with Fs = 0 11% of all 0’ ions with us = 3.54 X lo3 m/s have a chance to reach the ion detector, whereas for the slower O+ ions with us= 0.83 X lo3 m/s this probability is 14%. From the integrated TOF spectrum at I = 20.82 eV and appropriate correction for the different
transmission probabilities, a ratio of [UT(S) +O(3P)] : [Cl+(%) +O(‘D)]
= 0.76 : 0.24,
is obtained (estimated error 1-0.05) for the fragmentation pathways under investigation. Generally the transmission of ions is enhanced by applying an electrostatic field to the source region, which unfortunately degrades the electron energy resolution (compare the PE spectra in figs. 2 and 3). In contrast to the relatively high fields (Fs = 10-20 V/cm [12,14]) needed to collect all the fast OI ions produced by reaction (1) at 20.82 eV, a source field of Fs= 1.95 V/cm was calculated to be sufficient to yield a theoretical transmission of 100% for the slow ions generated in the course of reaction (2) at this very energy. In fig. 3 the TOF spectra obtained with Fs = 1.95.V/cm are dispiayed. In addition to a small shift towards shorter flight times, the signals of the fast Oi ions [us= (3.32-3.54) x lo3 m/s] are slightly broader than those obtained with Fs = 0. This is in agreement with the higher transmission of 17% calculated for these ions with Fs = 1.95 V/cm. The TQF distribution of the slow 0” ions (u, = 0.83 X lo3 m/s, I = 20.82 eV and I = 18.85 eV) reflects the calculated transmission of 100% and shows the typical trapeziform shape [l, 41. The integration of the TOF spectrum at I = 20.82 eV (cf. fig. 3) corrected for the calculated transmission probabilities yields a ratio [0+(‘S) +G(3P)]: [0+(‘S) +O(‘D)]
= 0.74 : 0.26,
in accordance with the ratio determined for Fs=O. A further note concerning the 6 ‘XE, L)= 3 level is in order. Although the calculated transmission of the slow 0’ ions is enhanced by a factor of ~7 by the application of a source field of 1.95 V/cm, no reliable signal corresponding to reaction pathway (2) is detected. The weak feature seen in the baseline between 6.0 and 7.5 p.S (cf. fig. 3) may .as well originate from a shadow of the neighhoming level u =4 due to the degraded electron energy resolution (A_??= 80 meV for FS = 1.95 V/cm). For the higher vibrational levels u 5 5 of 0; B %g, no data have been obtained because of the vanishing
electron count rate at I z 20.9 eV due to the low Franck-Condon intensities and to apparatus characteristics. The present study supports that 0,‘ molecular cations generated in their 6 “Z, electronic state with u c 3 fragment into 0+(&S) + O(3P) [4,7,9,12,14]. For o =4, competition between this reaction pathway and the reaction yielding 0’(4S) and the excited O(‘D) is observed, the ratio being [O+(“S) + 0(3P)]: [G+(4S) + O(‘D)] = 0.75 : 0.25. Similar competition between different fragmentation pathways has been observed for the higher electronic states of 0: [4,9,14]. Acknowledgement This work is part C 18 of Project No 2.622080 of the Schweizerischer Nationalfonds zur Fiirderung der wissenschaftlichen Forschung. Financial support by Ciba-Geigy SA, Sandoz SA and F. Hoffmann-La Roche & Co. SA, Base1 is gratefully acknowledged.
References [l] T. Baer, in: Gas phase ion chemistry, Vol. 1, ed. M. Bowers (Academic Press, New York, 1979) p. 153. [Z] 3. Be:kowita, Photoabsorption, photoionization and photoelectron spectroscopy (Academic Press, New York, 1979). [3] J.HD. Eland, in: Electron spectroscopy: theory, technique and applications, Vol. 3, eds. CR Brnndle and AD. Baker (Academic Press, New York, 1979) p. 231. [4] P.M. Guyon, T. Baer, LFA. Ferreirk I. Nenner. A. Tabch&Fouhaill& R. Batter and T.R. Govern, J. Phys. B. At. Mol. Phys. 11 (1978) Ll41. [5] P.H. Krupenie, J. Phys. Chem. Ref. Data i (1972) 423. [6] H. Helm, P.C. Cosby aid D.L. Hues&, J._Chem. Phys. 73 (1980) 2629, and references therein. [7] C.J. Dauby and J.H.D. Eland, Intent. J. M&s Specrrom. ion Phys. 8 (1972) 153. CS] K.-P. Huber and G. Her&erg, Molecular spectra and molecul& structure. Vol. 4, Constants of diatomic molecules Nan Nostrand, Princetcn. 19791. [9] TM. Guyon, T. Baer, L.F.A. Fen&m, I. Penner. A. Tabeht-Fouhailli, R. Bolter a&d T. Govei-s, Rapport d’actbit6 Lime (1977).
R. Bomback et ni. j Predissociation of O;(* (101 0. Edqvist, E. Lindholm, L.E. Win and L. &brink, Phys. Scripta 1 (1970) 25. [II] J.L. Beauchamp, R. Bornbach. J. Dznnacher, J.-P. Stadelmann. L. Theme and J. Vogt, to be published.
[l2]
‘Xi)
219
J. Dannacher, J.-P. Stadelmann and J. Vogt, Intern. J. Mass Spectrom. Ion Phys. 37 (1981) 203. [13] 0. Klemperer, Electron optics (Cambridge Univ. Press, London, 1971). [14] J.H.D. Eland, J. Chem. Php. 70 (1979) 2926.
THE
PREDISSOCIATiON
Rolf BOMBACH, Physikalisch-Chemisches
Andreas
OF MOLECULAR SCHMELZER
OXYGEN
and Jean-Pierre
Znstitut der Uniwrsirzt Bawl, CH-4056
CATIONS
O;&‘Z,,
STADELMANN
Bawl, Switzerland
Received 20 May 1981
Using the fixed wavelength (He-k) photoelectron-photoion coincidence technique, we observed that molecular oxygen cations generated in their 6 %‘_su = 4 vibronic state decay competitively via the fragmentation channels O’(‘S) t O?P) and O’(‘S)+O(‘D), the ratio being [O’(CS)+O(‘P)]:[Oi(iS)iO!‘D)]=0.75:0.25.
1. Introduction The unimolecular fragmentation behaviour of excited 0; has been the subject of numerous investigations reviewed in refs. [l-3]. Using the threshold photoelectron-photoion coincidence technique it has been shown that oxygen parent cations in the o 34 vibrational levels of their b ‘S, electronic state relax competitively by predissociation according to the reaction [4] 0;
+ O’CS)
i O(‘P),
(1)
and by radiative transition into the B ‘II, state [S]. The predissociating states have been recently identified to be the r ‘I& and predominantly the 4 ‘Xp’ electronic states of 0; [6 3. The absence of detectable emission [S] indicates that the B *Xi state of 0: is fully predissociated, and leads to the above specified products [7], thus generating O*(%) fragment ions carrying a translational energy of the order ET = 0.8 eV. As the v = 4 vilorational level of the B ‘Xi state of 0; (I = 20.815 ev) lies 0.115 eV above the threshold (1= 20.700 eV [Sl, for the reaction 0; -a O+(%) + O(‘D), this process has been considered compete with reaction (1) [9]. 03Ol-0104/81/0000-0000/S02.50
(2) possibly to
@ North-Holland
The object of this work is to investigate the fragmentation pathway (2) using the fixed wavelength photoelectron-photoion coincidence technique.
2. Experimental The photoeiectron-photoion coincidence spectrometer employed in this work is schematically displayed in fig. 1. Photoelectrons and photoions are generated at the centre of the source by photon impact (He-Ia, 21.22 eV) on the sample gas effusing from an inlet capiltary. If a source field is used, the potentials of the five parallel plates and the inlet capillary are individuahy tuned to provide a homogeneous and axially symmetric electrostatic field, the ionization region being at ground potential. The photoelectrons are energy analysed by means of a hemispherica analyser, equipped with triple tube lenses, working in the constant energy mode. Low energy photoelectrons (e.g. those corresponding to the B ‘X-i state of 0:) are ener,7 analysed by scanning the potentials applied to the hemispheres, in order to reduce the noise electron rate otherwise observed at ionization energies I > 20.5 eV. The photoelectrons are detected by a channel electron multiplier. The electron transmission coefficient has been measured as fc = 10T3.
216
kiFEi%FN DE&OR TRIPLE ION
DE :TECTOR
TUBE
TIME-OF-FLiGKT
LENS
ANALYSER
HEMISPHERES DRIFT
TUBE
GASp;NLET
0
LAa-J_Y SCALE
Fig.
1.
I cm
Scheme of the photoelectron-photoion
dependent flight time of the electron (between 0.2 and 0.5 CI_Sin our experiment and at the energy range under consideration) to obtain the total time-of-flight of the icn. The performance of the apparatus was tested with a FORTRAN IV program based on the algorithm summarized in ref. [13]. After the evaluation of the electrostatic potential field in the source and the TOF analyser, ions of a chosen mass and initial speed (thermal energy neglected in our case) were assumed to start isotropically in the centre of the source. From their flight paths, the TOF distribution as well as the transmission probability were evaluated.
The photoions are registered by a particle multiplier after traversing a time-of-flight (TCF) analyser. The ion transmission coefficient was measured as f;-0.3 for thermal 0; molecular ions at a constant source field of Fs = 3 V/cm. Since in one part of the experiments no source field was applied, only those ions with an initial motion towards the ion detector have been monitored, and the ion transmission coefficient was therefore degraded by approximately a factor of ten. Under these conditions (Fs= 0),the electron energy resolution was sufficient (AE = 50 meV) to reproduce the vibrational fine structure (cf. fig. 2) observed in the high resolution photoelectron (PE) spectrum of O2 [lO] in the energy range considered. More details of the experimental setup will be given elsewhere 1111. The
TOF spectra were accumulated by means of a multichannel analyser described in ref. [12]. Tile accumulation times were varied from 12 to 48 h depending on the photoe!ectron signal intensity. The time resolution was set to 40 ns per channel. Since in the coincidence experiments of this type the registration of an electron defines the ion creation time, the time scale in figs. 2 and 3 should be corrected by the energy
coincidencespectrometer.
3. Results and discussion
.
The section of the PE spectrum of 02, which corresponds to the b $5, and the B ‘I;; electronic states of 0; is displayed in fig. 2 (no source field) and fig. 3 (source field FS = 1.95V/cm). The calibration is based on the high resolution data of Lindholm et al. [lo]. Bearing in mind that reaction (I) is the only energetically possible fragmentation pathway for ot@ 5;; u 6 2) the following results are
R. Bombach et al. / Predissociation of Ol@
‘XJ
217
1
Fig. 2.
Part of the photoelectron spectrumof 02 (left; horizontal axis: count rate in arbitrary units) and the timeof-f&@ spectra of 0’ (right: vertical axis: coincidence
count rate in arbitrary units) measured at the indicated
ionization energies with a source field of FS = 0. The arrows
denote the threshold energies of the two framentation pathwaysaccessiblewith He-b radiation. The time scale
in
this and the subsequent figure is not corrected for the energy dependent time-of-flight of the electrons (see text).
derived from an analysis of the 0’ TOF spectra presented in fig. 2. The sharp signals recorded at o = 0 (I = 20.30 eV), 0 = 1 (I = 20.43 eV) and u = 2 (I = 20.57 ev) are easily assigned to Ot ions with an initial speed of us = 3.07 x 103, 3.20 x IO3 and 3.32 X lo3 m/s, respectively, starting in the direction of the ion detector. The time-of-fight of ~5.5 ws is in accordance with the values obtained from the trajectoij calculations. For O;@ “CJ with t, = 3 (i= 20.69eV, t). = 3.44 x IO3 m/s) reaction (1) is the only reaction pathway detected, although the pathway (2) is energetically accessible for rotationa&jr excited 0; c43. The TOF spectrum obtained at I = 20.82 eV (g ‘2,; t, =4) exhibits two signals (cf. fig. 2).
Fig. 3. Part of the PE spectrum of Oz measured using a source field of FS = 1.95 V/tin and the TOF spectra of 0‘ obtained at the indicated ionization energies.
The peak at = 5 J.LSis due to fast O* ions (u, = 3.54 x lo3 m/s) from reaction (l), whereas the broad signal at 7-9 ~LSis assigned to the reIatively slow 0’ ions (u, = 0.83 X 10’ m/s) generated in the course of reaction (2). This assignment is confirmed by noting that the excess energy of Of (6 ‘Xi; u = 4, I = 20.815 eV [lo]) relative to the threshold of reaction (2) (I = 20.700 eV [S]) is equal to the excess energy of 0; (g ‘Xi; v = 5, I = 18.848 eV [lo]) relative to the threshold of reaction (1) (I = 18.733 eV [S]) [4]_ Therefore identical 0“ TOF distributions are expected at I = 18.85 eV [reaction (l)] and I = 20.82 eV [reaction (211 in accordance with the present observations (cf. fig. 2). The trajectory calculations reveal that with Fs = 0 11% of all 0’ ions with us = 3.54 X lo3 m/s have a chance to reach the ion detector, whereas for the slower O+ ions with us= 0.83 X lo3 m/s this probability is 14%. From the integrated TOF spectrum at I = 20.82 eV and appropriate correction for the different
transmission probabilities, a ratio of [UT(S) +O(3P)] : [Cl+(%) +O(‘D)]
= 0.76 : 0.24,
is obtained (estimated error 1-0.05) for the fragmentation pathways under investigation. Generally the transmission of ions is enhanced by applying an electrostatic field to the source region, which unfortunately degrades the electron energy resolution (compare the PE spectra in figs. 2 and 3). In contrast to the relatively high fields (Fs = 10-20 V/cm [12,14]) needed to collect all the fast OI ions produced by reaction (1) at 20.82 eV, a source field of Fs= 1.95 V/cm was calculated to be sufficient to yield a theoretical transmission of 100% for the slow ions generated in the course of reaction (2) at this very energy. In fig. 3 the TOF spectra obtained with Fs = 1.95.V/cm are dispiayed. In addition to a small shift towards shorter flight times, the signals of the fast Oi ions [us= (3.32-3.54) x lo3 m/s] are slightly broader than those obtained with Fs = 0. This is in agreement with the higher transmission of 17% calculated for these ions with Fs = 1.95 V/cm. The TQF distribution of the slow 0” ions (u, = 0.83 X lo3 m/s, I = 20.82 eV and I = 18.85 eV) reflects the calculated transmission of 100% and shows the typical trapeziform shape [l, 41. The integration of the TOF spectrum at I = 20.82 eV (cf. fig. 3) corrected for the calculated transmission probabilities yields a ratio [0+(‘S) +G(3P)]: [0+(‘S) +O(‘D)]
= 0.74 : 0.26,
in accordance with the ratio determined for Fs=O. A further note concerning the 6 ‘XE, L)= 3 level is in order. Although the calculated transmission of the slow 0’ ions is enhanced by a factor of ~7 by the application of a source field of 1.95 V/cm, no reliable signal corresponding to reaction pathway (2) is detected. The weak feature seen in the baseline between 6.0 and 7.5 p.S (cf. fig. 3) may .as well originate from a shadow of the neighhoming level u =4 due to the degraded electron energy resolution (A_??= 80 meV for FS = 1.95 V/cm). For the higher vibrational levels u 5 5 of 0; B %g, no data have been obtained because of the vanishing
electron count rate at I z 20.9 eV due to the low Franck-Condon intensities and to apparatus characteristics. The present study supports that 0,‘ molecular cations generated in their 6 “Z, electronic state with u c 3 fragment into 0+(&S) + O(3P) [4,7,9,12,14]. For o =4, competition between this reaction pathway and the reaction yielding 0’(4S) and the excited O(‘D) is observed, the ratio being [O+(“S) + 0(3P)]: [G+(4S) + O(‘D)] = 0.75 : 0.25. Similar competition between different fragmentation pathways has been observed for the higher electronic states of 0: [4,9,14]. Acknowledgement This work is part C 18 of Project No 2.622080 of the Schweizerischer Nationalfonds zur Fiirderung der wissenschaftlichen Forschung. Financial support by Ciba-Geigy SA, Sandoz SA and F. Hoffmann-La Roche & Co. SA, Base1 is gratefully acknowledged.
References [l] T. Baer, in: Gas phase ion chemistry, Vol. 1, ed. M. Bowers (Academic Press, New York, 1979) p. 153. [Z] 3. Be:kowita, Photoabsorption, photoionization and photoelectron spectroscopy (Academic Press, New York, 1979). [3] J.HD. Eland, in: Electron spectroscopy: theory, technique and applications, Vol. 3, eds. CR Brnndle and AD. Baker (Academic Press, New York, 1979) p. 231. [4] P.M. Guyon, T. Baer, LFA. Ferreirk I. Nenner. A. Tabch&Fouhaill& R. Batter and T.R. Govern, J. Phys. B. At. Mol. Phys. 11 (1978) Ll41. [5] P.H. Krupenie, J. Phys. Chem. Ref. Data i (1972) 423. [6] H. Helm, P.C. Cosby aid D.L. Hues&, J._Chem. Phys. 73 (1980) 2629, and references therein. [7] C.J. Dauby and J.H.D. Eland, Intent. J. M&s Specrrom. ion Phys. 8 (1972) 153. CS] K.-P. Huber and G. Her&erg, Molecular spectra and molecul& structure. Vol. 4, Constants of diatomic molecules Nan Nostrand, Princetcn. 19791. [9] TM. Guyon, T. Baer, L.F.A. Fen&m, I. Penner. A. Tabeht-Fouhailli, R. Bolter a&d T. Govei-s, Rapport d’actbit6 Lime (1977).
R. Bomback et ni. j Predissociation of O;(* (101 0. Edqvist, E. Lindholm, L.E. Win and L. &brink, Phys. Scripta 1 (1970) 25. [II] J.L. Beauchamp, R. Bornbach. J. Dznnacher, J.-P. Stadelmann. L. Theme and J. Vogt, to be published.
[l2]
‘Xi)
219
J. Dannacher, J.-P. Stadelmann and J. Vogt, Intern. J. Mass Spectrom. Ion Phys. 37 (1981) 203. [13] 0. Klemperer, Electron optics (Cambridge Univ. Press, London, 1971). [14] J.H.D. Eland, J. Chem. Php. 70 (1979) 2926.