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Measurement of low kinetic energy loss in collision experiments using a conventional double-focussing mass spectrometer
International Journal
of Mass Spectromety and Ion Processes, 57 (1984) 19-26 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
19
MEASUREMENT OF LOW KINETIC ENERGY LOSS IN COLLISION DOUBLEFOCUSSING EXPERIMENTS USING A CONVENTIONAL MASS SPECTROMETER
J.M. CURTIS
and R.K. BOYD
Guelph -Waterloo Centre for Graduate Ont. Nl G 2 WI (Canada)
Work in Chemistry,
University
of Guelph, GueZph,
(First received 16 August 1983; in final form 17 October 1983)
ABSTRACT A technique is described whereby kinetic energy loss peaks, arising from inelastic coBisions of ions which do not result in chemical fragmentation, can be Observed without interference from the relatively very intense main-beam peak. The method extends the lower limit of the range of energies for such processes which can be studied using a conventional double-focussing mass spectrometer, but leaves unchanged the effective energy resolution. The usefulness of the technique is illustrated through a study of I+ collisions with helium.
INTRODUCTION
Studies of inelastic collisions between beams of small ions in the kilovolt energy range and stationary (thermal) gas targets have been done in specialised apparatus [1,2] to investigate electronic and vibrational energy levels. It has been possible [3] to obtain some information of this type using doublefocussing mass spectrometers designed for chemical analysis, but these earlier data were limited by the energy resolution available. More recent work [4,5] has clarified the role of angular collimation of the beam, both before and after collision, in permitting the intrinsic energy resolution of such instruments to be achieved. Very recently f6], an effective energy resolution of 0.1 eV was achieved using a double-focussing mass spectrometer of reversed configuration (ZAB-2F, manufactured by VG Analytical Ltd., Wythenshawe, Manchester, England). In this large-scale instrument [7], in which the magnet precedes the electric sector, the long flight-path lends itself to good angular collimation if the slits are all reduced [6]. 0168-1176/84,‘$03.00
0 1984 Elsevier Science Publishers B.V.
20 EXPERIMENTAL
The instrument available for the present work (VG Analytical 7070F) is a double-focussing mass spectrometer of conventional configuration (electric sector preceding magnet). This instrument is of much smaller scale than that used previously [6] and, as a result of the configuration of the sectors, ion kinetic energy spectroscopy is restricted to events occurring in the first field-free region if any mass specificity is to be applied [83. For both of these reasons, the degree of angular collimation which can be applied to the beam, particularly before collision, is limited. Since this instrument is in heavy daily use to provide an analytical service, it was not possible to install additional collimating slits to improve the effective angular resolution. It thus appeared that it would be impossible to observe and measure a kinetic energy loss less than about 3 eV, since the relatively very intense main beam would blank out any such kinetic energy loss peaks. Here we describe a technique which greatly alleviates these limitations. The scattering of I+ ions off helium was investigated as a test system, since this was one of the systems studied previously [6]. However, since the use of I, as precursor [6] would not be conducive to the maintenance of an analytical service, methyl iodide was used instead as precursor of I+ using electrons of nominal 70 eV energy. Kinetic energy spectra were obtained by scanning the acceleration voltage, V, with both the electric sector field, E, and magnetic field, B, fixed [9]. The energy-resolving slit (beta slit, between electric and magnetic sectors) was reduced until the energy width of the main-beam peak was not limited by slit widths. Figure 1 shows the results obtained for I+/He as the magnetic field was progressively de-tuned upwards from the value which maximized the intensity of the main beam in the absence of collision gas. Figure 2 shows the corresponding results obtained as the magnetic field was de-tuned downwards. The maximum extent of de-tuning corresponded to a mass difference of about 0.1 amu. DISCUSSION
Before discussing the effects observed, it is necessary to establish that the ,peaks observed in the spectra are not artifacts of one kind or another, as comprehensively described for instruments of the present type by Lacey and Macdonald [lo]. This is preferably done by exploring appropriate regions of the [I, p, ~1 surface [lo]. Here, I is the ion current at the detector, p is defined as (E/E,,)/{ V/V,) and p is defined as B2/( E/E,), where subscript 0 denotes values appropriate to normal operation of the instrument in double-focussing mode. The most likely artifacts in the present cases appear to be those due to chemical fragmentations occurring in the electric sector;
21
Fig. 1. Kinetic energy spectra for I+ from CH,I in collision (3.95 keV) with helium. (a) Spectrum obtained at optimum tuning of magnet; (b)-(d) spectra obtained as magnet is progressively de-tuned upwards to a maximum corresponding to - 0.1 u.
22
Fig. 2. As Figure 1, but magnet de-tuned
below the optimum setting used to obtain (a).
23
such events are known [lo] to give rise to diagonal ridges of ion intensity, of negative slope in the [p, ~1 plane. It is’possible [lo] to estimate the values of (V,/V,)( - p-l) at which any such postulated artifact should appear for a given mass setting of the magnet ( - p>. For the case of ion fragmentations arising from CH,I, with the magnetic field set close to the value appropriate to transmission of ion of m/z = 127, it does not appear to be possible to account for any of the observed peaks in terms of such artifacts. However, the peak observed at (V/V,) = 1.008 when the magnet is not detuned [Figs. l(a) and 2(a)] clearly arises from the fragmentation HI+‘+ I+ + H’ occurring in the first field-free region. The peak II, which appears at (V/V,) -K 1 in Figs. l(a) and 2(a), corresponds to I + ions of higher kinetic energy than main-beam I+ ions. It is believed that these fast I+ ions arise from superelastic collisions of excited I+, but in principle they could also arise from some process originating from doubly charged ions. Again, however, it is difficult to devise such a process which could account for peak II. Thus, if peaks I and II arise from inelastic collisions of I+ ions with He atoms, the trends evident in Figs. 1 and 2 must be accounted for. If the presence of the electric sector field E (fixed at the double-focussing value E,,) is ignored for now, a group of ions normally transmitted at (V, B) will also be transmitted at (Vi, B,) provided that (B:/V,) = ( B2/V). Thus, the loci of positions of peaks on a [V, B*] plot should be linear. Since B2 is proportional to magnet current iM; to a reasonable approximation, the plot of the peak positions on a [V, iM] diagram should also be approximately linear, as shown in Fig. 3. Deviations from linearity will occur if the ( B2/iM) ratio is not exactly constant and for other reasons described below. However, ions which were inelastically scattered in the first field-free region must also be transmitted by the electric sector in order to be detected. If Vis changed to Vi in order to satisfy the momentum ( - B,) requirement, the kinetic energy requirement (El/E,) = (VI/Y) will not be satisfied if E is held fixed at E,. The only I + ions which will be successfully transmitted under these circumstances will be those which are scattered through a laboratory angle just sufficient to compensate for the incremental deflection arising from the kinetic energy mismatch to the electric sector field. Thus, the effect of detuning the magnet is to select ions scattered through larger laboratory angles. The technique is a scattering-angle selection method appropriate to accelerating voltage scans [lo], analogous to an early [ll] scattering-angle selection method appropriate to simple B-scans. Such de-tuning acts [ll] as a high-pass filter for scattering angle. A larger scattering angle results from a more violent collision, so that the relative importance of higher-energy processes should simultaneously increase [compare (b) and (c) of Fig. 11. Further, a larger projectile ion scattering angle implies a larger momentum transfer to the target and thus a larger contribution to the total kinetic
24
Fig. 3. Variation of peak positions on (V/V,)
scale with magnet current ( = B').
energy loss; this further complicates the predicted. iM vs. (V/V,) curves (Fig. 3). However, interpolation of the observed peak positions to the “perfectly tuned” condition should correspond to a small range of scattering angles centred about zero and thus a minimum contribution of the target to the observed kinetic energy loss. The general behaviour of peaks I and II has thus been accounted for. The relatively intense signal, which survives at (V/V,) = 1 as the magnet is detuned, remains to be considered. In the absence of collision gas (analyser ion gauge recording 1.5 x lop8 torr), the maximum degree of magnet detuning used to record Figs. 1 and 2 resulted in a decrease of main-beam signal (peak height at V= V,) by a factor of 3 X 104. With collision gas present at the same pressure as that used to obtain Figs. 1 and 2, the corresponding factor was only 3 X 10 *. This collision-broadening arises from relatively gentle elastic collisions. These peak maxima occur at the double focussing condition, so the detuned magnet can still accept, with high efficiency, some fraction of the energy-dispersed beam transmitted through the beta slit. The effect of collision gas is to increase this fraction of the beam which meets both velocity and direction focussing requirements. The inelastically scattered ions which give rise to peaks I and II, however, are observed well away from the double-focussing condition and are transmitted only as a result of relatively large angular deflections, as discussed above. It is for this reason
25
that peaks I and II shift as the magnet is detuned, while the central peak does not. Peak II, if genuine, represents a superelastic collision of I+ ions with an energy increment of more than 20 eV. This is remarkable in view of the fact that the ionisation energy of I+ is 19 eV. Analogpus metastable states of alkali metal atoms, with lifetimes in the microsecond range, have been known for some time [12-141; for example the [(ls)(2~)(2p)]~P,,, state of Li, with a measured lifetime of 5.8 ps, lies 52 eV above the ionisation limit corresponding to the [(~s)~]‘S state of Li+. Final interpretation of peak II, in terms of such an I+ state metastable with respect to both radiation and autoionisation, clearly requires further work. The main point of the present work, however, is the successful observation of peak I which was completely buried under the main-beam peak when the instrument was properly tuned [Figs. l(a) and 2(a)]. The measured energy loss of (0.5 + 0.5 eV) is consistent with excitation of 3P2 ground state to one or both of the 3P, and 3P0 states (0.80 and 0.88 eV, respectively); transitions from 3P0 or 3PI to ID, would also give rise to a peak at about this position, as noted previously [6]. It is of interest to note that the peak due to collision-induced dissociation of HI+ [Figs. l(a) and 2(a)J does not survive the detuning procedure, but falls off more rapidly in intensity than does the central peak at (V/V,) = 1, (no double focussing). This suggests that such processes occur with but little momentum transfer, a result normally used [15] in the- interpretation of angular distributions of product ions. Such distributions usually reflect the kinetic energy release associated with the dissociation, rather than the collisional activation event [15,16]. In summary, the present note has described an experimental method which permits observation of kinetic energy loss (or gain) peaks sufficiently close to the main beam that they would not normally be observable_ Of course, the effective kinetic energy resolution remains limited mainly by lack of angular collimation and more suitable apparatus [1,6] will always be superior in this regard. However, the present work shows that a conventional analytical mass spectrometer can be made to provide useful data. We are currently completing work on the translational spectroscopy of C+ ions. ACKNOWLEDGEMENT
This work was funded by the Natural Sciences and Engineering Council (Canada). REFERENCES 1 J.H. Moore, Jr., J. Chem. Phys., 55 (1971) 2760. 2 R.G. Cooks (Ed.), Collision Spectroscopy, Plenum Press, New York, 1978.
Research
26 3 R.G. Cooks, R.M. CaprioIi, G.R. Lester and J.H. Beynon, Metastable Ions, Elsevier, Amsterdam, 1973. 4 A.G. Brenton, C.J. Proctor and J.H. Beynon, Adv. Mass Spectrom., 8B (1980) 1689. 5 A. Mendez-Amaya, A.G. Brenton, J.E. Szulejko and J.H. Beynon, Proc. R. Sot. London, Ser. A, 373 (1980) 13. 6 A.J. Illies and M.T. Bowers, Chem. Phys., 65 (1982) 281. 7 R.P. Morgan, J.H. Beynon, R.H. Bateman and B.N. Green, Int. J. Mass Spectrom. Ion Phys., 28 (1978) 171. 8 R.K. Boyd, Spectrosc. Int. J., 1 (1982) 169. 9 M. Barber and R.M. Elliott, 12th Conf. Mass Spectrom. Allied Topics, ASTM Committee E-14, Montreal, 1964, Paper 22. 10 M.J. Lacey and C.G. MacdonaId, Org. Mass Spectrom., 13 (1978) 253. 11 T. Ast, D.T. Terwilliger, R.G. Cooks and J.H. Beynon, 3. Phys. Chem., 79 (1975) 708. 12 P. Feldman and R. Novick, Phys. Rev. Lett., 11 (1963) 278. 13 R. Novick, G. Sproa and T. Lucatorto, Phys. Rev. A, 14 (1976) 272. 14 J.R. Willison, R.W. Falcone, J.F. Young and S.E. Harris, Phys. Rev. Lett., 47 (1981) 1827. 15 J. Los and T. Govers, in R.G. Cooks (Ed.), Collision Spectroscopy, Plenum Press, New York, 1978. 16 P.J. Todd, R.J. Warmack and E.J. McBay, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 299.
of Mass Spectromety and Ion Processes, 57 (1984) 19-26 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
19
MEASUREMENT OF LOW KINETIC ENERGY LOSS IN COLLISION DOUBLEFOCUSSING EXPERIMENTS USING A CONVENTIONAL MASS SPECTROMETER
J.M. CURTIS
and R.K. BOYD
Guelph -Waterloo Centre for Graduate Ont. Nl G 2 WI (Canada)
Work in Chemistry,
University
of Guelph, GueZph,
(First received 16 August 1983; in final form 17 October 1983)
ABSTRACT A technique is described whereby kinetic energy loss peaks, arising from inelastic coBisions of ions which do not result in chemical fragmentation, can be Observed without interference from the relatively very intense main-beam peak. The method extends the lower limit of the range of energies for such processes which can be studied using a conventional double-focussing mass spectrometer, but leaves unchanged the effective energy resolution. The usefulness of the technique is illustrated through a study of I+ collisions with helium.
INTRODUCTION
Studies of inelastic collisions between beams of small ions in the kilovolt energy range and stationary (thermal) gas targets have been done in specialised apparatus [1,2] to investigate electronic and vibrational energy levels. It has been possible [3] to obtain some information of this type using doublefocussing mass spectrometers designed for chemical analysis, but these earlier data were limited by the energy resolution available. More recent work [4,5] has clarified the role of angular collimation of the beam, both before and after collision, in permitting the intrinsic energy resolution of such instruments to be achieved. Very recently f6], an effective energy resolution of 0.1 eV was achieved using a double-focussing mass spectrometer of reversed configuration (ZAB-2F, manufactured by VG Analytical Ltd., Wythenshawe, Manchester, England). In this large-scale instrument [7], in which the magnet precedes the electric sector, the long flight-path lends itself to good angular collimation if the slits are all reduced [6]. 0168-1176/84,‘$03.00
0 1984 Elsevier Science Publishers B.V.
20 EXPERIMENTAL
The instrument available for the present work (VG Analytical 7070F) is a double-focussing mass spectrometer of conventional configuration (electric sector preceding magnet). This instrument is of much smaller scale than that used previously [6] and, as a result of the configuration of the sectors, ion kinetic energy spectroscopy is restricted to events occurring in the first field-free region if any mass specificity is to be applied [83. For both of these reasons, the degree of angular collimation which can be applied to the beam, particularly before collision, is limited. Since this instrument is in heavy daily use to provide an analytical service, it was not possible to install additional collimating slits to improve the effective angular resolution. It thus appeared that it would be impossible to observe and measure a kinetic energy loss less than about 3 eV, since the relatively very intense main beam would blank out any such kinetic energy loss peaks. Here we describe a technique which greatly alleviates these limitations. The scattering of I+ ions off helium was investigated as a test system, since this was one of the systems studied previously [6]. However, since the use of I, as precursor [6] would not be conducive to the maintenance of an analytical service, methyl iodide was used instead as precursor of I+ using electrons of nominal 70 eV energy. Kinetic energy spectra were obtained by scanning the acceleration voltage, V, with both the electric sector field, E, and magnetic field, B, fixed [9]. The energy-resolving slit (beta slit, between electric and magnetic sectors) was reduced until the energy width of the main-beam peak was not limited by slit widths. Figure 1 shows the results obtained for I+/He as the magnetic field was progressively de-tuned upwards from the value which maximized the intensity of the main beam in the absence of collision gas. Figure 2 shows the corresponding results obtained as the magnetic field was de-tuned downwards. The maximum extent of de-tuning corresponded to a mass difference of about 0.1 amu. DISCUSSION
Before discussing the effects observed, it is necessary to establish that the ,peaks observed in the spectra are not artifacts of one kind or another, as comprehensively described for instruments of the present type by Lacey and Macdonald [lo]. This is preferably done by exploring appropriate regions of the [I, p, ~1 surface [lo]. Here, I is the ion current at the detector, p is defined as (E/E,,)/{ V/V,) and p is defined as B2/( E/E,), where subscript 0 denotes values appropriate to normal operation of the instrument in double-focussing mode. The most likely artifacts in the present cases appear to be those due to chemical fragmentations occurring in the electric sector;
21
Fig. 1. Kinetic energy spectra for I+ from CH,I in collision (3.95 keV) with helium. (a) Spectrum obtained at optimum tuning of magnet; (b)-(d) spectra obtained as magnet is progressively de-tuned upwards to a maximum corresponding to - 0.1 u.
22
Fig. 2. As Figure 1, but magnet de-tuned
below the optimum setting used to obtain (a).
23
such events are known [lo] to give rise to diagonal ridges of ion intensity, of negative slope in the [p, ~1 plane. It is’possible [lo] to estimate the values of (V,/V,)( - p-l) at which any such postulated artifact should appear for a given mass setting of the magnet ( - p>. For the case of ion fragmentations arising from CH,I, with the magnetic field set close to the value appropriate to transmission of ion of m/z = 127, it does not appear to be possible to account for any of the observed peaks in terms of such artifacts. However, the peak observed at (V/V,) = 1.008 when the magnet is not detuned [Figs. l(a) and 2(a)] clearly arises from the fragmentation HI+‘+ I+ + H’ occurring in the first field-free region. The peak II, which appears at (V/V,) -K 1 in Figs. l(a) and 2(a), corresponds to I + ions of higher kinetic energy than main-beam I+ ions. It is believed that these fast I+ ions arise from superelastic collisions of excited I+, but in principle they could also arise from some process originating from doubly charged ions. Again, however, it is difficult to devise such a process which could account for peak II. Thus, if peaks I and II arise from inelastic collisions of I+ ions with He atoms, the trends evident in Figs. 1 and 2 must be accounted for. If the presence of the electric sector field E (fixed at the double-focussing value E,,) is ignored for now, a group of ions normally transmitted at (V, B) will also be transmitted at (Vi, B,) provided that (B:/V,) = ( B2/V). Thus, the loci of positions of peaks on a [V, B*] plot should be linear. Since B2 is proportional to magnet current iM; to a reasonable approximation, the plot of the peak positions on a [V, iM] diagram should also be approximately linear, as shown in Fig. 3. Deviations from linearity will occur if the ( B2/iM) ratio is not exactly constant and for other reasons described below. However, ions which were inelastically scattered in the first field-free region must also be transmitted by the electric sector in order to be detected. If Vis changed to Vi in order to satisfy the momentum ( - B,) requirement, the kinetic energy requirement (El/E,) = (VI/Y) will not be satisfied if E is held fixed at E,. The only I + ions which will be successfully transmitted under these circumstances will be those which are scattered through a laboratory angle just sufficient to compensate for the incremental deflection arising from the kinetic energy mismatch to the electric sector field. Thus, the effect of detuning the magnet is to select ions scattered through larger laboratory angles. The technique is a scattering-angle selection method appropriate to accelerating voltage scans [lo], analogous to an early [ll] scattering-angle selection method appropriate to simple B-scans. Such de-tuning acts [ll] as a high-pass filter for scattering angle. A larger scattering angle results from a more violent collision, so that the relative importance of higher-energy processes should simultaneously increase [compare (b) and (c) of Fig. 11. Further, a larger projectile ion scattering angle implies a larger momentum transfer to the target and thus a larger contribution to the total kinetic
24
Fig. 3. Variation of peak positions on (V/V,)
scale with magnet current ( = B').
energy loss; this further complicates the predicted. iM vs. (V/V,) curves (Fig. 3). However, interpolation of the observed peak positions to the “perfectly tuned” condition should correspond to a small range of scattering angles centred about zero and thus a minimum contribution of the target to the observed kinetic energy loss. The general behaviour of peaks I and II has thus been accounted for. The relatively intense signal, which survives at (V/V,) = 1 as the magnet is detuned, remains to be considered. In the absence of collision gas (analyser ion gauge recording 1.5 x lop8 torr), the maximum degree of magnet detuning used to record Figs. 1 and 2 resulted in a decrease of main-beam signal (peak height at V= V,) by a factor of 3 X 104. With collision gas present at the same pressure as that used to obtain Figs. 1 and 2, the corresponding factor was only 3 X 10 *. This collision-broadening arises from relatively gentle elastic collisions. These peak maxima occur at the double focussing condition, so the detuned magnet can still accept, with high efficiency, some fraction of the energy-dispersed beam transmitted through the beta slit. The effect of collision gas is to increase this fraction of the beam which meets both velocity and direction focussing requirements. The inelastically scattered ions which give rise to peaks I and II, however, are observed well away from the double-focussing condition and are transmitted only as a result of relatively large angular deflections, as discussed above. It is for this reason
25
that peaks I and II shift as the magnet is detuned, while the central peak does not. Peak II, if genuine, represents a superelastic collision of I+ ions with an energy increment of more than 20 eV. This is remarkable in view of the fact that the ionisation energy of I+ is 19 eV. Analogpus metastable states of alkali metal atoms, with lifetimes in the microsecond range, have been known for some time [12-141; for example the [(ls)(2~)(2p)]~P,,, state of Li, with a measured lifetime of 5.8 ps, lies 52 eV above the ionisation limit corresponding to the [(~s)~]‘S state of Li+. Final interpretation of peak II, in terms of such an I+ state metastable with respect to both radiation and autoionisation, clearly requires further work. The main point of the present work, however, is the successful observation of peak I which was completely buried under the main-beam peak when the instrument was properly tuned [Figs. l(a) and 2(a)]. The measured energy loss of (0.5 + 0.5 eV) is consistent with excitation of 3P2 ground state to one or both of the 3P, and 3P0 states (0.80 and 0.88 eV, respectively); transitions from 3P0 or 3PI to ID, would also give rise to a peak at about this position, as noted previously [6]. It is of interest to note that the peak due to collision-induced dissociation of HI+ [Figs. l(a) and 2(a)J does not survive the detuning procedure, but falls off more rapidly in intensity than does the central peak at (V/V,) = 1, (no double focussing). This suggests that such processes occur with but little momentum transfer, a result normally used [15] in the- interpretation of angular distributions of product ions. Such distributions usually reflect the kinetic energy release associated with the dissociation, rather than the collisional activation event [15,16]. In summary, the present note has described an experimental method which permits observation of kinetic energy loss (or gain) peaks sufficiently close to the main beam that they would not normally be observable_ Of course, the effective kinetic energy resolution remains limited mainly by lack of angular collimation and more suitable apparatus [1,6] will always be superior in this regard. However, the present work shows that a conventional analytical mass spectrometer can be made to provide useful data. We are currently completing work on the translational spectroscopy of C+ ions. ACKNOWLEDGEMENT
This work was funded by the Natural Sciences and Engineering Council (Canada). REFERENCES 1 J.H. Moore, Jr., J. Chem. Phys., 55 (1971) 2760. 2 R.G. Cooks (Ed.), Collision Spectroscopy, Plenum Press, New York, 1978.
Research
26 3 R.G. Cooks, R.M. CaprioIi, G.R. Lester and J.H. Beynon, Metastable Ions, Elsevier, Amsterdam, 1973. 4 A.G. Brenton, C.J. Proctor and J.H. Beynon, Adv. Mass Spectrom., 8B (1980) 1689. 5 A. Mendez-Amaya, A.G. Brenton, J.E. Szulejko and J.H. Beynon, Proc. R. Sot. London, Ser. A, 373 (1980) 13. 6 A.J. Illies and M.T. Bowers, Chem. Phys., 65 (1982) 281. 7 R.P. Morgan, J.H. Beynon, R.H. Bateman and B.N. Green, Int. J. Mass Spectrom. Ion Phys., 28 (1978) 171. 8 R.K. Boyd, Spectrosc. Int. J., 1 (1982) 169. 9 M. Barber and R.M. Elliott, 12th Conf. Mass Spectrom. Allied Topics, ASTM Committee E-14, Montreal, 1964, Paper 22. 10 M.J. Lacey and C.G. MacdonaId, Org. Mass Spectrom., 13 (1978) 253. 11 T. Ast, D.T. Terwilliger, R.G. Cooks and J.H. Beynon, 3. Phys. Chem., 79 (1975) 708. 12 P. Feldman and R. Novick, Phys. Rev. Lett., 11 (1963) 278. 13 R. Novick, G. Sproa and T. Lucatorto, Phys. Rev. A, 14 (1976) 272. 14 J.R. Willison, R.W. Falcone, J.F. Young and S.E. Harris, Phys. Rev. Lett., 47 (1981) 1827. 15 J. Los and T. Govers, in R.G. Cooks (Ed.), Collision Spectroscopy, Plenum Press, New York, 1978. 16 P.J. Todd, R.J. Warmack and E.J. McBay, Int. J. Mass Spectrom. Ion Phys., 50 (1983) 299.