Attachment of electrons to molecules at submillielectronvolt resolution

Volume 189, number

43

CHEMICAL

PHYSICS

LETTERS

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1992

Attachment of electrons to molecules at submillielectronvolt resolution D. Klar, M.-W. Ruf and H. Hotop Fachbereich Physik, Universitiit Kaiserslautern, W-6750 Kaiserslautern, Germany

Received

18 September

199

I ; in final form 28 October I99 1

Using a novel laser photoelectron method, we have measured electron attachment to neutral molecules with sufftciently high energy resolution to investigate the limiting E - ‘I2 behaviour of the cross section, predicted for s-wave attachment at electron energies E approaching zero. Our results for SF6, obtained with an effective energy width of about 0.2 meV in the energy range O-200 meV, show that the E- ‘I2 dependence is only reached at very low energies ( 5 I meV) and that the cross section decreases more strongly towards higher energies, exhibiting a pronounced downward cusp at the threshold for Y, vibrational excitation of SF6. At the lowest energies, the experimental rate constant is compatible with the theoretical value for capture due to the e--SF6 polarization potential.

1. Introduction The attachment of slow free electrons to molecules (cross section ae), e-(E)fXYS

XY-

(X-+Y)

)

(1)

with formation of either a long-lived ion XY- or dissociated products X- + Y (dissociative attachment ) is an important process in connection with the dielectric breakdown strength of gases [ 1 ] and has been actively studied by several methods, including swarm [ l-41 and beam techniques [ 4-7 1. Similar to the probability for neutron capture by nuclei at low collision velocities u [ 81, the cross section for attachment of slow, s-wave electrons to molecules should diverge as v- ’ - E - “’ in the limit of vanishing electron velocity v or energy E [g-lo]. Whereas in neutron collisions, s-wave scattering dominates at collision energies below about lo6 eV, higher partial waves can be important in electron scattering even at energies below 1 eV, especially for systems which exhibit the Ramsauer effect. Due to the increasing importance of the centrifugal barrier towards lower energies, s-wave electron scattering will eventually dominate for E-t0 and correspondingly, electron 448

0009-2614/92/$

capture processes should reveal the E -‘D energy dependence at millielectronvolt energies, if s-wave attachment is allowed by the symmetries of the neutral molecule XY and the formed negative ion XY-. At these low energies, the rate constant k, = a,v should, therefore, be constant (i.e. independent of v and E). So far, the most advanced studies of the energy dependence of electron attachment cross sections have been carried out by Chutjian and colleagues [ 4,6,11], who used photoelectrons of variable energy in the range O-200 meV with typically 6 meV energy width (fwhm) by photoionizing Kr atoms in the vicinity of the Kr+ ( 2P,,2) threshold. Their data exhibited a resolution-limited peak at zero electron energy for electron attachment to many different target molecules, including long-lived SF; formation from SF6 and dissociative attachment processes such as Clproduction from Ccl, [ 61. Analysis of their data showed the peak shape to be compatible with an E-II2 energy dependence for energies 5 10 meV. Using various Rydberg atoms A*( nl) as a source of low-energy electrons, Dunning [ 121 showed that the rate constant k,, for Rydberg electron attachment, A**(nl) +XY knl

A+ +XY-

05.00 0 1992 Elsevier Science Publishers

(X-+Y)

,

(2)

B.V. All rights reserved.

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CHEMICAL PHYSICS LETTERS

to several polyatomic molecules is independent of n for n z 40 (i.e. electron energies E x IE,,, I < 10 meV). Within the framework of the quasi-free electron model for A** (nl) collisions [ 12,13 1, which predicts k,,/at high n to be equal to the rate constant k, for s-wave attachment of free electrons, the results of Dunning support an E- ‘I2 behaviour of the s-wave attachment cross section at energies E < 10 meV. In spite of the important information provided by the earlier work, it is highly desirable to study the attachment of free electrons at very low energies with the highest possible resolution ( -c 1 meV) in order to make a clear observation of the s-wave threshold law possible and to quantify the deviations from the E - ‘I2 behaviour towards higher energies. The use of very high resolution is also required in connection with the possible observation of resonances and with threshold effects due to onsets for vibrational excitation [ lo]. In connection with our recent work on the electron transfer from state-selected Rydberg atoms to molecules [ 141 and clusters [ 15,161 and based on our experience with resonant two-step photoionization of metastable rare gas atoms [ 17,181, we have developed a crossed-beams photoionization method to study the attachment of free, monoenergetic electrons to molecules and clusters at low energies (O-200 meV) with submillielectronvolt resolution. The principle of the method is similar to that of Chutjian and Alajajian [6], but the use of lasers leads to substantially higher resolution and allows one to carry out sensitive in situ tests for the influence of residual electric fields in the reaction region. In this Letter, we briefly describe our method, report and discuss first results for electron attachment to XY = SF6 and mention some perspectives for future developments.

2. Experimental In our experiment (see fig. 1 ), free electrons of well-determined and variable energy are produced by two-step photoionization of metastable rare gas atoms [ 17,18 1. A single-mode dye laser transversely excites Ar+ (4s 3PZ) atoms, present in a collimated beam from a differentially pumped dc discharge source, to the Ar(4p 3D3) intermediate level, thus yielding a substantial quasi-stationary population of

ph

Art

(*PI,,)

Ed, J = 4 (2,3)

Es.J=2

Ar+ (*P,,,) LASER 2 3p54p,J=3

r

3p5 45 J = 2

\... ELECTRON “Y.. IM PACT -i ., -

0AT 3p6, J = 0

IFF. PUMPED

Fig. 1. Illustration of the principle and the realization of the novel laser photoelectron attachment method.

(polarized) excited atoms; the latter are excited or ionized by a multimode intracavity dye laser (mode spacing ~40 MHz; typical intracavity power 2 W; diameter in reaction region x 1 mm), which is tunable over the range 475-425 nm with a birefringent filter (typical optical resolution 30-40 GHz = 0.15 meV). The second laser either produces Rydberg atoms, when tuned to wavelengths larger than 462 nm, or free electrons of variable energy otherwise. The excitation/ionization occurs in a field-free region (static magnetic fields S 6 x lo-’ T) for time intervals of = 3 ps with a repetition rate of x lo5 s-l; for this purpose, the first laser is appropriately switched on and off by an acousto-optical modulator. After a short delay, a pulsed electric field of ~23 V/cm is applied to the reaction chamber, which accelerates the positive and negative ions due to photo-excitation/-ionization and to collision processes of the Rydberg/free electrons with a suitable target molecule (here SF6) in opposite directions. Either the positive or the negative ions are injected into a quadrupole mass spectrometer, mass analyzed, and detected by an electron multiplier. The raw data consist of a wavelength scan of the second laser, simultaneously sampling the mass-analyzed ion count 449

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rate, the laser intensity, and the transmission-fringes of a calibrated Fabry-Perot ttalon (free spectral range 199 GHz, finesse z 15), which establishes the relative energy scale. The absolute wavelength scale is fixed by excitation of A?* (ns, nd) Rydberg levels at low n (around 20)) yielding clear and strong ion signals due to the formation of ion pairs Ar+ + SF:. Test experiments showed that losses of SF; ions, resulting from molecular collisions or from photodetachment by the lasers, do not occur at a significant level (60.5%).

3. Results and discussion Fig. 2 shows the ion-yield spectrum for SF< ions in the electron energy range -36 to +24 meV. At negative energies (bound electrons), one observes resolved excitation of low n Rydberg states; towards higher 12, a practically constant quasi-continuum is observed due to the non-resolved excitation of high n Rydberg levels. The step at - 4 meV is due to field ionization of Rydberg levels with n > 60 in the pulsed

I---20d ,p

I

Ok,, -40

-30

J=4 22s J=2

,

,

;

,

-20

-10

0

10

ENERGY

[meV]

ELECTRON

,

, 20

,

$ 30

Fig. 2. Ion-yield spectrum for the production of SF; ions in collisions of SF6 molecules ( T= 300 K) with AP’( ns, nd) Rydberg atoms (E 0). In this experiment, the intracavity laser was tuned from 468 to 458 nm.

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electric-ion extraction field, resulting in a decrease of SF; ions, since the effective Rydberg density is reduced. The ionization limit (zero energy) is marked by the vertical dashed line. Its position agrees to within 0.1 meV, i.e. within the optical resolution, with the true ionization limit, which we determined by laser spectroscopy. From this agreement, we conclude that the electric fields in the reaction region are at most 2 mV/mm. This finding was corroborated by test experiments with a second single-mode laser, with which resolved excitation of Rydberg levels up to at least n=250 was observed. A careful analysis of the threshold region, based on the known laser lineshape and energy dependence of the Ar* (4p ‘Dj ) photoionization cross section and on the assumption of an E -‘I2 threshold behaviour for the free-electron attachment cross section, suggests that the effective energy resolution of this experiment was better than 0.4 meV. The data in fig. 2 were taken with an SF6 density of z 3 x 10” cm-3; the corresponding SF; counting rate at E= - 1 meV was typically 4x lo3 s-‘. In order to extend the energy range, data were accumulated towards higher energies with an increased SF6 density around 3 x 10” cmp3. Tests demonstrated the linearity of the SF; detector for the described conditions. Fig. 3 presents the summed data in the energy range 0.8-190 meV, as corrected for the wavelength dependence of the photon flux and of the Ar* (4p 3D3) photoionization cross section [ 191; the latter is nearly constant in the relevant range for the chosen parallel linear laser polarizations. The data have been placed on an absolute cross-section scale (relevant at E< 170 meV) by normalization to the T=300 K rate constant k,=(2.27f0.09)~10-’ cm3/s due to Petrovic and Crompton [ 21, who presented a critical discussion of the measured rate constants. We note that the contribution of SF; formation to the negative-ion signals is so small that it could be neglected in the calculation of the thermally averaged rate constant. The ratio of the SF, to the SF; signals was observed to be < low3 in selected Rydberg atom experiments (n= 18, 28, 43) and for attachment of free electrons at E= 1 meV. At E= 170 meV, the SF< fraction was found to be below 10% (i.e. lower than the fractions reported in ref. [ 3 ] ). Apart from the steep rise of the cross section towards E = 0, two characteristic features, labelled ( I )

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Volume 189, number 4,5

I

0

I

20

I

40

I

I

60

1

60

ELECTRON

-II

100

ENERGY

I

11 120

l

140

1

160

1

’ 160

11

[meV]

Fig. 3. SF; ion yield due to attachment of free electrons to SF6 molecules (T=300 K) in the range 0.8-190 meV. At energies above 175 meV, the opening of the Ar+ (ZPI,Z) photoionization channel influences the data (see also insert and text). The absolute cross-section scale (valid for E< 170 meV) was obtained through normalization to the accurate T= 300 K rate constant of Petrovic and Crompton [2]. The features (I), (II) and (III) are discussed in the text.

and (II) in fig. 3, are especially noteworthy. Feature (I) is a clear downward cusp with an onset at the energy of one quantum of the vi vibration in SF, (E(v,) ~95.4 meV); the occurrence of such structure in the SF, electron attachment cross section has been predicted by Gauyacq and Herzenberg [ lo], but not been observed before. Feature (II) is a sharp asymmetric peak, located at E ccE, ,* = 177.5 meV (see insert in fig. 3), i.e. the energy above which photoionization of A?( 4p 3D3) into the Ar+ ( 2P1,2) + e- continuum becomes possible. The sharp peak reflects the production of SF; ions due to attachment of threshold electrons from this excited-ion channel. The intensity of the peak can be used, when comparing with the SF; production around E=O, to deduce a threshold branching ratio for Ar+ (*Pi ,2 ) /Ar+ (‘P3/2 ) formation of x 1%. This small ratio reflects the fact that - similar to the situation for Ne*( 3p 3D3) [ 17 ] - photoionization of the valence electron proceeds in almost all cases such

14 February 1992

that the rare gas core is not changed. Correspondingly, the excitation of Beutler-Fano resonances Ar*(*Pi,2 nd’, J=2, 3) from Ap(*P3/2 4p3D3) between the Ar+ (*P3,*) and Ar+ (‘P,,*) ionization thresholds is a weak process, as reflected in the measured photoionization cross section to be discussed elsewhere [ 191. Although the oscillator strength for the (weak) excitation of these resonances joins smoothly into the Ar+ (‘P,,*) continuum, the lifetime of the A?( nd’, J= 2,3) resonances towards autoionization is so short [ 201 (even for n as high as 300) that their contribution to SF; formation via Rydberg electron transfer is negligible. The insert in fig. 3 shows the Ar+ (2P,,2) threshold peak in more detail with a linear intensity and an expanded energy scale. One observes a sharp peak with a full-widthat-half-maximum (fwhm) of AE=O.4 meV and a tail towards higher energies. The width of this peak provides a direct upper limit for the effective energy resolution in the present attachment experiment; it is compatible with the value imposed by the analysis of the data at the Ar+ (*P3/2) threshold, but it provides a more definitive measure. Model-fit calculations for the threshold peak, involving convolution of an E-‘I* cross-section behaviour with different spectrometer functions, showed [ 191 that the effective energy resolution was close to 0.2 meV (fwhm). Besides the photon energy bandwidth of the ionizing laser (0.15 meV fwhm Gaussian), residual electric fields mainly contribute to the observed attachment lineshape at threshold. Another feature, labelled (III) in fig. 3, occurs around the threshold for excitation of one quantum of the v3 vibration in SF, (E= 117.5 meV). It is a rather weak structure, but was reproducibly observed. In contrast to the downward cusp at the v1 threshold, the feature at the v3 threshold involves a slight increase of the attachment cross section over a narrow energy range. We note that electron-scattering studies of SF6 [ 2 1 ] have previously revealed prominent threshold peaks associated with the onsets for vi and v3 vibrational excitation. These processes compete with electron attachment, and thereby lead to the structure in the attachment cross section, observed here for the first time. The model calculation of Gauyacq and Herzenberg included the vi vibrational coordinate only, which was considered to promote electron attachment [ lo]. A comparison 451

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of our data with their results will be presented elsewhere. We now discuss the threshold behaviour of the attachment cross section. Some time ago, theoretical predictions were made by Vogt and Wannier [22] and later by Klots [ 23 1, who discussed the capture of a charged particle interacting with a target merely through the polarization potential Q,,,,= -(re2/2r4, where (Y is the target polarizability, e the electron charge, and r the distance between charged particle and target. In the limit of very low collision energies, Vogt and Wannier obtained the following quantummechanical cross section: oc=4na?j(a/2E)‘r2,

(3)

where a0 is the Bohr radius = 52.9 pm, CYand E in au. This expression amounts to twice the classical orbiting cross section [ 221. Formula (3) exhibits the correct threshold behaviour for s-wave capture in potentials decaying faster than 1/r* [ 8,9] and provides an absolute value for the capture rate constant kc = cr,v in the energy range, in which eq. ( 3 ) is valid. From fig. 3 in ref. [ 221, one can infer that eq. (3) is valid for ( 8aE) ‘I4 < 0.5. For XY = SF6, a molecule without a permanent dipole and quadrupole moment, the polarization potential is in fact the dominant long-range e --XY interaction. With (~(SF~)=44.1 ai [24], one obtains k,=5.15xlO-’ cm3 s-’ at energies <3 meV. Based on the work of Vogt and Wannier and Klots [22,23] suggested a simple formula for the s-wave electron-capture cross section, which agrees with eq. ( 3 ) for E-t 0 and interpolates to the de Broglie s-wave cross section oB=di2=aa~/2E (X =A/2n= reduced de Broglie wavelength), uk = (nai/2E)(l

-exp[

-4(2aE)“2]}

.

(4)

In fig. 4 we compare the limiting capture cross section a, of eq. (3), the expression a, of eq. (4), and the s-wave cross section nX* with our experimental results in the range 0.2-90 meV. At the lowest energies ( 5 1 meV), the theoretical values are remarkably close to the experimental cross sections, both in absolute size and slope. Towards higher energies, the experimental results decrease more strongly than OK (by a factor of 1.5 from 1 to 90 meV). From fig. 4, it is clear that the limiting E-‘I2 behaviour is not completely reached even at the lowest energies (see 452

ELECTRON

ENERGY

E [meV]

Fig. 4. Comparison of experimental and theoretical tions (see text) in the range 0.1-90 meV.

cross sec-

also fig. 5), but our data exhibit the approach to this limit at the low energies, and thereby demonstrate the validity of the theoretical threshold law in a convincing way (note that the data at ES 0.3 meV tend to be too high due to the increasing influence of the Rydberg collision signal ). Differences between theory and experiment are more clearly displayed, when the leading energy dependence E - ‘I2 is removed by presenting energy-dependent rate constants k,(E) = a,( E)v( E) in a linlog-diagram, as shown in fig. 5. The experimental rate constants tend to saturate only at the lowest energies, decrease by a factor of 3.3 from 1 to 90 meV and drop rapidly above the onset for v, vibrational excitation (Ea95.4 meV). A critical analysis of the threshold region showed that the experimental energy scale is uncertain to within kO.05 meV; close to zero energy, this produces a non-negligible systematic error in the experimental rate constants indicated by the error bars. We emphasize that the absolute value for the rate constant k,,,= (4.2 Z!I1) X lo-’ cm3 s-l, reported by Dunning [ 121 for Rydberg electron collisions with SF6 at high n (binding energy range l8 meV) agrees with the present results; in view of their experimental uncertainties, the Rydberg data could not reveal, however, the energy dependence clearly present in our free-electron results. It is difftcult to extract from our data an accurate value for the limiting rate constant k,(E+O). The

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ELECTRON

ENERGY

E [md]

Fig. 5. Comparison of energy-dependent experimental rate constants k,(E) with the limiting capture rate constant k, [ 221 and the rate-constant expression kK [ 231. Note the drastic drop of the experimental rate constant at energies above the onset for u, -vibrational excitation of SF6 (95.4 meV).

major uncertainty stems from the extrapolation to zero energy. Guided by the general behaviour of the Klots rate constant, one arrives at a value for k,(E-+O), which agrees with the theoretical capture rateconstantk,=5.15x10-7cm3s-’ [22] towithin & 10% or better. Finally, as an important point with regard to the overall energy dependence of the measured cross section for SF; formation, we have to mention the problem of the (energy-dependent) lifetime of the SF; ions, created in the electron-attachment process. In our experiment ( T= 300 K), the average time between formation and detection of SF; ions is close to 80 ps. So far, no measurements of the lifetimes for free SF; ions produced by monoenergetic free electrons in the range O-200 meV exist, although data involving SF; formation by attachment of nonmonoenergetic electrons or of Rydberg electrons were reported with conflicting results (see, e.g. refs. [ 25271 and references therein). We note that Rydberg electron-attachment results at high principal quantum numbers (n 2 30, binding energies 5 15 meV), i.e. in the region of validity of the (quasi) free-electron model [ 121, have shown through the identity of the A+ and SF; signals [ 12,141 that the corresponding SF; autodetachment lifetimes are around

14 February 1992

0.5 ms or longer. These findings also demonstrate that the electric fields encountered by the SF; ions after their formation until detection do not lead to ion losses by field detachment. These high n Rydberg results are expected to be equally valid for SF; ions formed in collisions with free electrons at energies Es 15 meV. With rising electron energy, the SF; lifetime will in general decrease (see refs. [ 23,2527 ] ), and thereby the measured cross sections could be smaller than the cross sections for the primary attachment process. In this connection, it is of interest to compare our results in fig. 3 with cross sections derived from swarm experiments [ 3 1, in which collisions with the buffer gas may stabilize the SF; ions. Over the energy range 10-l 70 meV, the absolute cross sections for SF; formation presented by Hunter et al. [ 31, which incorporate the results of ref. [ 61 at lower energies, agree with our values to within f40%. A more detailed comparison of our attachment cross sections with other experimental and theoretical results will be published elsewhere. In the future, we plan to apply the laser photoelectron attachment method presented in this Letter to other molecules of interest (e.g. F2 [ 111). Moreover, we are working on an extension of the method in order to investigate the attachment of monoenergetic low-energy electrons to clusters in a supersonic target beam. Several molecular clusters exhibit a zero energy resonance not present in the attachment to the monomer [ 28 1. Recent data on electron transfer from state-selected rare gas Rydberg atoms to N20 clusters have even revealed an intriguing dependence of the negative-ion spectra on principal quantum number [ 161, corresponding to size-selective negative cluster ion formation at very low electron energies.

Acknowledgement This work has been supported by the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich 9 1. We gratefully acknowledge S. Schohl and T. Kraft for their experimental cooperation and K. Zinsmeister for technical help. We thank W. Domcke for several useful discussions, C.E. Klots for a comment on ref. [23], and R.W. Crompton, A. Chutjian, 453

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J.P. Gauyacq, and K.P. Rohr for discussions of their work.

References [ I] LG. Christophorou, ed., Electron molecule interactions and their applications, Vols. 1 and 2 (Academic Press, New York, 1984). [2] Z.Lj. Petrovic and R.W. Crompton, J. Phys. B 18 ( 1985) 2717. [ 31 S.R. Hunter, J.G. Carter and L.G. Christophorou, J. Chem. Phys. 90 (1989) 4879. [ 41 A. Chutjian, Review Lecture presented at XVIIth ICPEAC, Brisbane, 199 1; to be published in: Electronic and Atomic Collisions, Proc. XVII ICPEAC. (51 D. Spence and G.J. Schulz, J. Chem. Phys. 58 (1973) 1800. [6] A. Chutjian and S.H. Alajajian, Phys. Rev. A 31 ( 1985) 2885. [7] T. Oster, A. Kuhn and E. Illenberger, Intern. J. Mass Spectrom. Ion Processes 89 ( 1989) 1. [ 81 H.A. Bethe, Phys. Rev. 47 (1935) 747. [9] E.P. Wigner, Phys. Rev. 73 (1948) 1002. [lo] J.P. Gauyacq and A. Herzenberg, J. Phys. B 17 (1984) 1155. [ II] A. Chutjian and S.H. Alajajian, Phys. Rev. A 35 ( 1987) 4512. [ 121 F.B. Dunning, J. Phys. Chem. 91 (1987) 2244, and references therein.

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[ 131 M. Matsuzawa, J. Phys. Sot. Japan 32 (1972) 1088; 33 (1972) 1108. [ 141 K. Harth, M.-W. Rufand H. Hotop, Z. Physik. D 14 (1989) 149. [ 151 T. Kraft, M.-W. Rufand H. Hotop, Z. Physik. D 14 (1989) 119. [ 161 T. Kraft, M.-W. Rufand H. Hotop, Z. Physik. D 17 ( 1990) 37. [ 17 ] J. Ganz, B. Lewandowski, A. Siegel, W. Bussert, H. Waibel, M.-W. Ruf and H. Hotop, J. Phys. B 15 (1982) L485. [ 181 S. Schohl, D. Klar, T. Kraft, H.A.J. Meijer, M.-W. Ruf, U. Schmitz, S.J. Smith and H. Hotop, Z. Physik. D 2 1 ( 199 1) 25. [ 191 D. KIar, M.-W. Ruf and H. Hotop, to be published. [20] D. Klar, K. Harth, J. Ganz, T. Kraft, M.-W. Ruf and H. Hotop, in: Today and tomorrow in photoionization, Proc. Sem. Leningrad, eds. M.Ya. Amusia and J.B. West, pp. 7882, SERC Daresbury Laboratory, 199 1. [2l]K.Rohr,J.Phys.B10(1977) 1175. [ 221 E. Vogt and G.H. Wannier, Phys. Rev. 95 ( 1954) 1190. [ 231 C.E. Klots, Chem. Phys. Letters 38 ( 1976) 6 1. [24] R.D. Nelson Jr. and R.H. Cole, J. Chem. Phys. 54 ( 1971) 4033. [25] L.G. Christophorou, Advan. Electron. Electron Phys. 46 (1978) 71. [ 26 ] J.P. Astruc, R. Barbe, A. Lagreze and J.P. Schermann, Chem. Phys. 75 (1983) 405. [27] G. Brincourt, S. Rajab Pacha, R. Catella, Y. Zerega and J. Andre, Chem. Phys. Letters 156 ( 1989) 574. [28] T.D. Mark, Intern. J. Mass Spectrom. Ion Processes 107 (1991) 143.