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Positron ionization mass spectrometry: Ionization of organic molecules by positronium formation
CHEMICALPHYSICSLETTERS
Volume 168,number 1
20 April 1990
POSITRON IONIZATION MASS SPECIROMETRY: IONIZATION OF ORGANIC MOLECULES BY POSITRONIUM FORMATION * D.L. DONOHUE, L.D. HULETT Jr., B.A. ECICENRODE, S.A. McLUCKEY and G.L. GLISH AnalyticalChemistryDivision,OakRidge NationalLaboratory,P.0. Box 2008, MS-6142, OakRidge, TN 37831-6142. USA
Received 8 January 1990
Low-energy( -c 10 eV) positrons have been used to ionize organic molecules in the gas phase followedby mass spectrometric analysis. High-energypositrons from a linear electron accelerator-basedfacility are re-moderatedto less than 3 eV and trapped in a miniature Penning trap where they interact with the target molecules.Ions thus formed are mass analyzed in a time-of-flight mass spectrometer.The predominant ionization mechanism in this energyregime involves positronium formation and the spectra are comparable to electron impact spectra at energiesa few eV above the ionization energyof the molecules.
1. Introduction The literature
on positron
interactions
with gases
is rich and can only be briefly summarized here [ l71. A large number of workers have specifically studied the formation
of positronium
in gases [g-14].
Other workers have presented theoretical calculations of positronium formation [ 15 ] and positron attachment processes [ 16,17 1. Related work in which low-energy positron ionization has been studied with mass spectrometry has been reported by Surko and co-workers [ 18,191. Previous work in our laboratory has been reported which describes the positron source [20], the mass spectrometry instrumentation [ 211 and the goals of our research [ 221. This report describes mass spectral results of organic molecules ionized by the positronium (Ps) formation process at positron energies well below the ionization energy of the molecules.
2. Experimental Pulses of 3000 eV positrons 10 to 20 ns wide impinge on a 1000 A thick tungsten (W) film at a rep* Research sponsored by US Department of Energy, Office of EnergyResearch,under contract DE-AC0584OR21400with Martin Marietta EnergySystems,Inc.
etition rate of 400 to 800 s-l. An estimated lo3 fast positrons enter the active area of the film per pulse and are re-moderated and ejected from the back side of the film with an eficiency approaching 10% [ 231 and a distribution of energies from near 0 to 3 eV. This distribution can be shifted to higher energy by applying a bias voltage of 0 to + 10 V to the W film reIative to the ionization chamber at ground potential. The positrons enter the ionization chamber through a molybdenum (MO) grid (85% transmissive) and are guided by a 50 G axial magnetic field, Potentials of + 5 and +24 V applied to the MO grids on either side of the ionization chamber serve to trap the positrons in a volume of 1 cm3 where they interact with the sample gas present at a pressure of 1 X 10M6to 1 X 10e5 Torr. After a trapping period of 20 to 30 l.~s,the ionic products are pulsed out of the ionization volume and accelerated to 3000 eV for time-of-flight mass analysis with a 1 m flight length. The design and operating parameters of the trapping ion source ensure that only positrons of low energy are allowed to interact with the sample gas, i.e. electrons, ions or positrons of higher energy do not have a stable trajectory in the trap. Any ions formed when the 3000 eV positrons arrive at the moderator film are expected to drift out of the ionization chamber during the 20-30 ps trapping period. Further experiments with trapped electrons indicate that no ac-
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
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Volume 168, number 1
CHEMICAL PHYSICS LETTERS
celeration of the positrons occurs which would bring their energies above the ionization energy of the molecules. Ion detection is accomplished with a dual-plate (chevron) channel electron multiplier array, operated in the pulse-counting mode at 1000 V per plate, followed by time-to-digital conversion. Up to 8 ion events per pulse can be detected at 64 ns per channel resolution (5 12 channels total). A typical mass spectrum involves acquiring data for 100000 pulses in a 250 s period (at 400 pulses s-l). Samples were introduced into the vacuum system through precision metering valves. All compounds were reagent-grade purity and were used as received from the vendor. Pressure was measured by a Bayard-Alpert-type ionization gauge and was corrected for differential sensitivity [ 241.
3. Results The compounds studied are listed in table 1. Relative ionization efficiencies for these compounds were measured at 0 V bias on the moderator film. Table 1 Relative positron ionization efftciencies for selected organic compounds Compound
IE ”
Rel. eff. ‘)
1 n-butane 2 n-hexane 3 n-heptane 4 n-decane 5 cyclohexane 6 cyclohexanone 7 1-hexene 8 cyclohexene 9 1$hexadiene 10 1,rl-cyclohexadiene I1 benzene I2 aniline 13 nitrobenzene 14 n-butylbcnzene
10.6 10.2 9.9 9.1 9.9 9.1 9.3 8.9 9.0 8.8 9.2 7.7 9.9 8.7
0.11 0.034 0.035 0.022’ 0.059 0.12 0.34 0.34 2.1* 1.0 1.2 5.@ 0.10* 1.3’
a1Adiabatic ionization potentials from ref. [ 251. b, Rel. eff.=total ions collected per 1000 pulses corrected to 1 kW electron accelerator power and a sample pressure of 1 x 1O-6 Tot-r. Ionization gauge readings corrected with measured values from ref. [ 241; values marked with * indicate use of correlations data based on 2 (total number of electrons) from table 2 in ref. [ 24 ] _
38
20 April 1990
Thus, the positrons were present at energies from 0 to 3 eV (the intrinsic energy spectrum from this 1000 8, moderator film). The ion signals were corrected for the number of pulses collected, for the number of positrons per pulse (based on the power level of the electron accelerator), and for the pressure of the sample (assuming a linear relationship). The relative ionization efficiency was found to be inversely related to the ionization energy (IE) of the molecule: alkanes have a high IE and low positron ionization signal whereas aromatics with lower IEs show much higher positron ionization efficiencies. Due to the spread of positron energies and to uncertainties in several other parameters, no attempt was made to accurately determine the cross section for the ionization process. However, the ion signals observed are consistent with cross sections in the range 0.1 to 1 AZ, in agreement with ref. [ 9 1. The effect of raising the moderator bias, and therefore the average positron energy is shown in fig. 1 for hexane. In the range of 0 to + 5 V the total ion signal from hexane ions increases by a factor of 150. Also shown in fig. 1 are theoretical curves obtained by convolving our positron energy spectrum with step functions at 4.0 and 5.0 V to simulate the positronium formation threshold. This theoretical approach resulted in a calibration line relating the experimental data to the voltage position of the step function. This approach yielded for hexane a Ps formation threshold of 4.1 eV, compared to the ex-
25
I
I
I
I
I
HEXANE
Moderator
Voltage
(V)
Fig. 1. Total ion signal for hexane at 1 X 10m5Torr as a function of bias voltage applied to the W film m-moderator. Theoretical curves result from convolution of measured positron energy distribution with step functions at + 4 and + S V.
Volume 168, number 1
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CHEMICAL PHYSICS LETTERS
petted value of 3.4 eV (given by the IE - 6.8 eV) . A similar study was carried out for N2 with 0 to + 10 V bias applied to the moderator, yielding a Ps threshold of 8.9 eV, compared to the expected value of 8.8 eV. The mass spectra observed with low-energy positrons show little fragmentation near the Ps formation threshold with increasing fragmentation occurring as the moderator voltage is increased. This is observed clearly for the alkanes as well as for alkylsubstituted benzenes. For benzene and toluene at 0 V bias on the moderator, only the molecular ion is observed. An experiment with normal and deuterated benzene and toluene clearly demonstrated that there was no loss of H, either from the ring or the methyl group on toluene. Fig. 2 illustrates the dramatic increase in the fragment ions at m/z 91 and 92 compared to the molecular ion signal at m/z 134 for n-butylbenzene as the moderator bias is changed from 0 to + 5 V. Positron ionization spectra of several compounds studied have been compared to low-energy electron impact spectra [ 261 as well as to data acquired with the same apparatus (under electron trapping conditions) and were found to be quite similar although the energy of the ionizing particle was different by about 6.8 eV. Thhs, positron ionization at 1 eV above the Ps formation threshold gives similar fragmentation to electron impact at 1 eV above the IE of the compound. The correspondence between low-energy positron ionization and low-energy electron impact ionization is believed to be due to the similar time scales for the two processes; both are rapid femtosecond and therefore involve similar Franck-Condon transitions. For instance, a 3 eV positron travels 1 A in 0.1 fs whereas a 10 eV electron travels only 1.8 times faster. Molecular vibrations, by comparison, take place on a picosecond time scale. The difference in absolute energy at which the ionization occurs is due to the local Coulombic field of the positron which creates a potential well 6.8 eV deep into which a valence electron can be captured, thus lowering the barrier for removal of such an electron.
N - BUTYLBENZENE Cl0 H,, MW - 134
0 VOLTS
100
t-h&“lmbwa.sb 10
20 304050
100
150
200
250 300
+5 VOLTS
Fig. 2. Time-of-flight mass spectra of n-butylb-enzene at 2x 10e6 Torr (corrected for ionizationgaugesensitivity).Top spectrum taken with 0 V bias on the moderator; bottom spectrum with t 5 V bias on moderator.
4. Conclusions Ionization by low-energy positrons has been observed and can be explained by invoking the Ps formation process which is energetically allowed in the so-called Ore gap starting at 6.8 eV below the IE. The onset of this process has been studied for hexane and nitrogen and found to occur at approximately the correct energy. Large changes in ionization efficiency are observed
for hexane over a relatively
small
change in positron energy as the Ps formation threshold is exceeded. Chemical selectivity is ob; served resulting from the choice of positron energy relative to the Ps formation threshold (which is de39
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CHEMICAL PHYSICS LETTERS
pendent on the IE of the compound). Fragmentation is generally minor, but increases dramatically over a small energy range in a similar fashion to low-energy electron impact. Future work will seek to control the positron energy more closely to obtain better estimates of the ionization cross sections and to extend the positron energy range to below 0.5 eV where the possibility of positron attachment exists.
References [ 1] T.C. Griffith andG.R. Heyland, Phys. Rept. 39 (1978) 169. [2] H.S.W. Massey, Phys. Today (March 1976) 42. [3] W. Raith, in: Positron scattering in gases, eds. J.W. Humbenton and M.R.C. McDowell (Plenum Press, New York. 1984) o. 1. [ 41 WE. Kauppila and T.S. Stein, in: Positron scattering in gases, eds. J.W. Humberston and M.R.C. McDowell (Plenum Press, New York, 1984) p. 15. [ 5 ] R.P. McEachran, in: Positron scattering in gases, eds. J.W. Humberston and M.R.C. McDowell (Plenum Press, New York, 1984) p. 27. [6] T.C. Griffith, M. Charlton, G. Clark, G.R. Hey1andandG.L. Wright, in: Positron annihilation, eds. P.G. Coleman, SC. Sharma and L.M. Diana (North-Holland, Amsterdam, 1982) p. 95. [ 71 W.E. Kauppila and T.S. Stein, Can. J. Phys. 60 ( 1982) 471, [ 8 ] P.G. Coleman, T.C. Griffith, G.R. Heyland and T.L. Killeen, I. Phys. B g (1975) L 185. [9] M. Charlton, T.C. GriEfth, G.R. Heyland and K.S. Lines, J.Phys.B12(1979)L633. [lo] M. Charlton, T.C. Griffith, G.R. Heyland, K.S. Lines and G.L. Wright, J. Phys. B 13 (1980) L 757.
40
20 April 1990
[ 111 J.D. McNutt, S.C. Sharma, M.H. Franklin and M.A. Woodall II, Phys. Rev. A20 (1979) 357. [ 12 ] G.J. Celltans and J.H. Green, Proc. Phys. Sot. (London) 83 (1964) 823. [ 131 G.J. Celitans, S.J. Tao and J.H. Green, Proc. Phys. Sot. (London) 83 (1964) 833. [ 14 ] SC. Sharma, J.D. McNutt, A. Eftekhari and Y.J. Ataiiyan, Can. J. Phys. 60 (1982) 610. [ 15 1W.J. Madia, J.S. Schug, A.L. Nichols and H.J. Ache, J. Phys. Chem. 78 (1974) 2682. [ 161 D.M. Schrader and CM. Wang, J. Phys. Chem. 80 (1976) 2507. [ 17 ] D.M. Schrader and R.E. Svetic, Can. J. Phys. 60 (1982) 517. [ 181CM. Surko, A. Passner, M. Leventhal and F.J. Wysocki, Phys.Rev.Letten61 (1988) 1831. [ 19 ] A. Passner, C.M. Surko, M. Leventhal and A.P. Mills Jr., Phys. Rev. 39 (1989) 3706. [20] L.D. Hulett Jr., T.A. Lewis, R.G. Alsmiller, R. Peelle, S. Pendyala, J.M. Dale and T.M. Rosseel, Nucl. Ins& Methods B 24/25 (1987) 905. [2 1 ]D.L. Donahue, L.D. Hulett Jr., S.A. McLuckey, G.L. Glish and H.S. M&own, Intern. J. Mass. Spectrom. Ion Processes, in press. [22] S.A. McLuckey, G.L. Glish,D.L. Don0hueandL.D. Hulett Jr., Intern J. Mass Spectrom. Ion Processes, in press. [23] M.R. Paulsen, M. Charlton, J. Chevallier, B.I. Deutch, EM. Jacobsen and L. Laricchia, in: Positron annihilatoin, eels. L. Dorikens-Vanpraet, M. Dorikens and D. Seegers (World Scientific, Singapore, 1989) p. 597. [24] J.E. BartmessandRM. Georgiadis, Vacuum 33 (1983) 149. [25 ] R.D. Levin and S.G. Lias, Ionization potential and appearance potential measurements 1971-198 1, NSRDS NBS71 (1982). [26] A. Maccoll, Org. Mass Spectrom. 17 ( 1982) 1.
Volume 168,number 1
20 April 1990
POSITRON IONIZATION MASS SPECIROMETRY: IONIZATION OF ORGANIC MOLECULES BY POSITRONIUM FORMATION * D.L. DONOHUE, L.D. HULETT Jr., B.A. ECICENRODE, S.A. McLUCKEY and G.L. GLISH AnalyticalChemistryDivision,OakRidge NationalLaboratory,P.0. Box 2008, MS-6142, OakRidge, TN 37831-6142. USA
Received 8 January 1990
Low-energy( -c 10 eV) positrons have been used to ionize organic molecules in the gas phase followedby mass spectrometric analysis. High-energypositrons from a linear electron accelerator-basedfacility are re-moderatedto less than 3 eV and trapped in a miniature Penning trap where they interact with the target molecules.Ions thus formed are mass analyzed in a time-of-flight mass spectrometer.The predominant ionization mechanism in this energyregime involves positronium formation and the spectra are comparable to electron impact spectra at energiesa few eV above the ionization energyof the molecules.
1. Introduction The literature
on positron
interactions
with gases
is rich and can only be briefly summarized here [ l71. A large number of workers have specifically studied the formation
of positronium
in gases [g-14].
Other workers have presented theoretical calculations of positronium formation [ 15 ] and positron attachment processes [ 16,17 1. Related work in which low-energy positron ionization has been studied with mass spectrometry has been reported by Surko and co-workers [ 18,191. Previous work in our laboratory has been reported which describes the positron source [20], the mass spectrometry instrumentation [ 211 and the goals of our research [ 221. This report describes mass spectral results of organic molecules ionized by the positronium (Ps) formation process at positron energies well below the ionization energy of the molecules.
2. Experimental Pulses of 3000 eV positrons 10 to 20 ns wide impinge on a 1000 A thick tungsten (W) film at a rep* Research sponsored by US Department of Energy, Office of EnergyResearch,under contract DE-AC0584OR21400with Martin Marietta EnergySystems,Inc.
etition rate of 400 to 800 s-l. An estimated lo3 fast positrons enter the active area of the film per pulse and are re-moderated and ejected from the back side of the film with an eficiency approaching 10% [ 231 and a distribution of energies from near 0 to 3 eV. This distribution can be shifted to higher energy by applying a bias voltage of 0 to + 10 V to the W film reIative to the ionization chamber at ground potential. The positrons enter the ionization chamber through a molybdenum (MO) grid (85% transmissive) and are guided by a 50 G axial magnetic field, Potentials of + 5 and +24 V applied to the MO grids on either side of the ionization chamber serve to trap the positrons in a volume of 1 cm3 where they interact with the sample gas present at a pressure of 1 X 10M6to 1 X 10e5 Torr. After a trapping period of 20 to 30 l.~s,the ionic products are pulsed out of the ionization volume and accelerated to 3000 eV for time-of-flight mass analysis with a 1 m flight length. The design and operating parameters of the trapping ion source ensure that only positrons of low energy are allowed to interact with the sample gas, i.e. electrons, ions or positrons of higher energy do not have a stable trajectory in the trap. Any ions formed when the 3000 eV positrons arrive at the moderator film are expected to drift out of the ionization chamber during the 20-30 ps trapping period. Further experiments with trapped electrons indicate that no ac-
0009-2614/90/$ 03.50 0 Elsevier Science Publishers B.V. (North-Holland)
31
Volume 168, number 1
CHEMICAL PHYSICS LETTERS
celeration of the positrons occurs which would bring their energies above the ionization energy of the molecules. Ion detection is accomplished with a dual-plate (chevron) channel electron multiplier array, operated in the pulse-counting mode at 1000 V per plate, followed by time-to-digital conversion. Up to 8 ion events per pulse can be detected at 64 ns per channel resolution (5 12 channels total). A typical mass spectrum involves acquiring data for 100000 pulses in a 250 s period (at 400 pulses s-l). Samples were introduced into the vacuum system through precision metering valves. All compounds were reagent-grade purity and were used as received from the vendor. Pressure was measured by a Bayard-Alpert-type ionization gauge and was corrected for differential sensitivity [ 241.
3. Results The compounds studied are listed in table 1. Relative ionization efficiencies for these compounds were measured at 0 V bias on the moderator film. Table 1 Relative positron ionization efftciencies for selected organic compounds Compound
IE ”
Rel. eff. ‘)
1 n-butane 2 n-hexane 3 n-heptane 4 n-decane 5 cyclohexane 6 cyclohexanone 7 1-hexene 8 cyclohexene 9 1$hexadiene 10 1,rl-cyclohexadiene I1 benzene I2 aniline 13 nitrobenzene 14 n-butylbcnzene
10.6 10.2 9.9 9.1 9.9 9.1 9.3 8.9 9.0 8.8 9.2 7.7 9.9 8.7
0.11 0.034 0.035 0.022’ 0.059 0.12 0.34 0.34 2.1* 1.0 1.2 5.@ 0.10* 1.3’
a1Adiabatic ionization potentials from ref. [ 251. b, Rel. eff.=total ions collected per 1000 pulses corrected to 1 kW electron accelerator power and a sample pressure of 1 x 1O-6 Tot-r. Ionization gauge readings corrected with measured values from ref. [ 241; values marked with * indicate use of correlations data based on 2 (total number of electrons) from table 2 in ref. [ 24 ] _
38
20 April 1990
Thus, the positrons were present at energies from 0 to 3 eV (the intrinsic energy spectrum from this 1000 8, moderator film). The ion signals were corrected for the number of pulses collected, for the number of positrons per pulse (based on the power level of the electron accelerator), and for the pressure of the sample (assuming a linear relationship). The relative ionization efficiency was found to be inversely related to the ionization energy (IE) of the molecule: alkanes have a high IE and low positron ionization signal whereas aromatics with lower IEs show much higher positron ionization efficiencies. Due to the spread of positron energies and to uncertainties in several other parameters, no attempt was made to accurately determine the cross section for the ionization process. However, the ion signals observed are consistent with cross sections in the range 0.1 to 1 AZ, in agreement with ref. [ 9 1. The effect of raising the moderator bias, and therefore the average positron energy is shown in fig. 1 for hexane. In the range of 0 to + 5 V the total ion signal from hexane ions increases by a factor of 150. Also shown in fig. 1 are theoretical curves obtained by convolving our positron energy spectrum with step functions at 4.0 and 5.0 V to simulate the positronium formation threshold. This theoretical approach resulted in a calibration line relating the experimental data to the voltage position of the step function. This approach yielded for hexane a Ps formation threshold of 4.1 eV, compared to the ex-
25
I
I
I
I
I
HEXANE
Moderator
Voltage
(V)
Fig. 1. Total ion signal for hexane at 1 X 10m5Torr as a function of bias voltage applied to the W film m-moderator. Theoretical curves result from convolution of measured positron energy distribution with step functions at + 4 and + S V.
Volume 168, number 1
20 April 1990
CHEMICAL PHYSICS LETTERS
petted value of 3.4 eV (given by the IE - 6.8 eV) . A similar study was carried out for N2 with 0 to + 10 V bias applied to the moderator, yielding a Ps threshold of 8.9 eV, compared to the expected value of 8.8 eV. The mass spectra observed with low-energy positrons show little fragmentation near the Ps formation threshold with increasing fragmentation occurring as the moderator voltage is increased. This is observed clearly for the alkanes as well as for alkylsubstituted benzenes. For benzene and toluene at 0 V bias on the moderator, only the molecular ion is observed. An experiment with normal and deuterated benzene and toluene clearly demonstrated that there was no loss of H, either from the ring or the methyl group on toluene. Fig. 2 illustrates the dramatic increase in the fragment ions at m/z 91 and 92 compared to the molecular ion signal at m/z 134 for n-butylbenzene as the moderator bias is changed from 0 to + 5 V. Positron ionization spectra of several compounds studied have been compared to low-energy electron impact spectra [ 261 as well as to data acquired with the same apparatus (under electron trapping conditions) and were found to be quite similar although the energy of the ionizing particle was different by about 6.8 eV. Thhs, positron ionization at 1 eV above the Ps formation threshold gives similar fragmentation to electron impact at 1 eV above the IE of the compound. The correspondence between low-energy positron ionization and low-energy electron impact ionization is believed to be due to the similar time scales for the two processes; both are rapid femtosecond and therefore involve similar Franck-Condon transitions. For instance, a 3 eV positron travels 1 A in 0.1 fs whereas a 10 eV electron travels only 1.8 times faster. Molecular vibrations, by comparison, take place on a picosecond time scale. The difference in absolute energy at which the ionization occurs is due to the local Coulombic field of the positron which creates a potential well 6.8 eV deep into which a valence electron can be captured, thus lowering the barrier for removal of such an electron.
N - BUTYLBENZENE Cl0 H,, MW - 134
0 VOLTS
100
t-h&“lmbwa.sb 10
20 304050
100
150
200
250 300
+5 VOLTS
Fig. 2. Time-of-flight mass spectra of n-butylb-enzene at 2x 10e6 Torr (corrected for ionizationgaugesensitivity).Top spectrum taken with 0 V bias on the moderator; bottom spectrum with t 5 V bias on moderator.
4. Conclusions Ionization by low-energy positrons has been observed and can be explained by invoking the Ps formation process which is energetically allowed in the so-called Ore gap starting at 6.8 eV below the IE. The onset of this process has been studied for hexane and nitrogen and found to occur at approximately the correct energy. Large changes in ionization efficiency are observed
for hexane over a relatively
small
change in positron energy as the Ps formation threshold is exceeded. Chemical selectivity is ob; served resulting from the choice of positron energy relative to the Ps formation threshold (which is de39
Volume 168, number 1
CHEMICAL PHYSICS LETTERS
pendent on the IE of the compound). Fragmentation is generally minor, but increases dramatically over a small energy range in a similar fashion to low-energy electron impact. Future work will seek to control the positron energy more closely to obtain better estimates of the ionization cross sections and to extend the positron energy range to below 0.5 eV where the possibility of positron attachment exists.
References [ 1] T.C. Griffith andG.R. Heyland, Phys. Rept. 39 (1978) 169. [2] H.S.W. Massey, Phys. Today (March 1976) 42. [3] W. Raith, in: Positron scattering in gases, eds. J.W. Humbenton and M.R.C. McDowell (Plenum Press, New York. 1984) o. 1. [ 41 WE. Kauppila and T.S. Stein, in: Positron scattering in gases, eds. J.W. Humberston and M.R.C. McDowell (Plenum Press, New York, 1984) p. 15. [ 5 ] R.P. McEachran, in: Positron scattering in gases, eds. J.W. Humberston and M.R.C. McDowell (Plenum Press, New York, 1984) p. 27. [6] T.C. Griffith, M. Charlton, G. Clark, G.R. Hey1andandG.L. Wright, in: Positron annihilation, eds. P.G. Coleman, SC. Sharma and L.M. Diana (North-Holland, Amsterdam, 1982) p. 95. [ 71 W.E. Kauppila and T.S. Stein, Can. J. Phys. 60 ( 1982) 471, [ 8 ] P.G. Coleman, T.C. Griffith, G.R. Heyland and T.L. Killeen, I. Phys. B g (1975) L 185. [9] M. Charlton, T.C. GriEfth, G.R. Heyland and K.S. Lines, J.Phys.B12(1979)L633. [lo] M. Charlton, T.C. Griffith, G.R. Heyland, K.S. Lines and G.L. Wright, J. Phys. B 13 (1980) L 757.
40
20 April 1990
[ 111 J.D. McNutt, S.C. Sharma, M.H. Franklin and M.A. Woodall II, Phys. Rev. A20 (1979) 357. [ 12 ] G.J. Celltans and J.H. Green, Proc. Phys. Sot. (London) 83 (1964) 823. [ 131 G.J. Celitans, S.J. Tao and J.H. Green, Proc. Phys. Sot. (London) 83 (1964) 833. [ 14 ] SC. Sharma, J.D. McNutt, A. Eftekhari and Y.J. Ataiiyan, Can. J. Phys. 60 (1982) 610. [ 15 1W.J. Madia, J.S. Schug, A.L. Nichols and H.J. Ache, J. Phys. Chem. 78 (1974) 2682. [ 161 D.M. Schrader and CM. Wang, J. Phys. Chem. 80 (1976) 2507. [ 17 ] D.M. Schrader and R.E. Svetic, Can. J. Phys. 60 (1982) 517. [ 181CM. Surko, A. Passner, M. Leventhal and F.J. Wysocki, Phys.Rev.Letten61 (1988) 1831. [ 19 ] A. Passner, C.M. Surko, M. Leventhal and A.P. Mills Jr., Phys. Rev. 39 (1989) 3706. [20] L.D. Hulett Jr., T.A. Lewis, R.G. Alsmiller, R. Peelle, S. Pendyala, J.M. Dale and T.M. Rosseel, Nucl. Ins& Methods B 24/25 (1987) 905. [2 1 ]D.L. Donahue, L.D. Hulett Jr., S.A. McLuckey, G.L. Glish and H.S. M&own, Intern. J. Mass. Spectrom. Ion Processes, in press. [22] S.A. McLuckey, G.L. Glish,D.L. Don0hueandL.D. Hulett Jr., Intern J. Mass Spectrom. Ion Processes, in press. [23] M.R. Paulsen, M. Charlton, J. Chevallier, B.I. Deutch, EM. Jacobsen and L. Laricchia, in: Positron annihilatoin, eels. L. Dorikens-Vanpraet, M. Dorikens and D. Seegers (World Scientific, Singapore, 1989) p. 597. [24] J.E. BartmessandRM. Georgiadis, Vacuum 33 (1983) 149. [25 ] R.D. Levin and S.G. Lias, Ionization potential and appearance potential measurements 1971-198 1, NSRDS NBS71 (1982). [26] A. Maccoll, Org. Mass Spectrom. 17 ( 1982) 1.