The fragmentation of CH4+ ions from photoionization between 12 and 40 eV

JOURNAL OF ELECTRON SPECTROSCOPY and Related Phenomena

ELSEVIER

Journal of Electron Spectroscopyand Related Phenomena 73 (1995) 209-216

The fragmentation of CH - ions from photoionization between 12 and 40 eV Thomas

A. Field, John H.D. Eland*

Physical Chemistry Laboratory, South Parks Road, Oxford OX1 3QZ, UK

First received 17 October 1994; in final form 3 November 1994

Abstract

The fragmentation of the methane ion has been measured as a function of ion internal energy in the range 12-40 eV using the fixed wavelength photoelectron photoion coincidence (PEPICO) technique, with He Ia and He IIa light. We confirm that the fragmentation of methane ions in the A state is not well represented by statistical calculations. In the satellite states between the A state and 33 eV, the ion breaks into small fragments; in satellite states above 33 eV the fragmentation is related to the behaviour of doubly-charged methane. Keywords." CH4+ ion; Coincidence method; Fragmentation; Photoionization

I. Introduction

The unimolecular dissociations of large molecular ions, the basis of mass spectrometry, are usually modelled by the statistical theory of unimolecular reactions. In most cases this theory is successful in modelling the data, but there are notable exceptions. Even ions which dissociate statistically at low internal energies may change over to non-statistical behaviour at high energy. Methane is at the lower size limit of molecules which m a y be expected to dissociate in a statistical manner, so the dynamics of its ionic fragmentation are of great interest. The energies and vibrational structures of the X and A states of the methane ion were determined ¢r Dedicated to the memory of Professor W.C. (Bill) Price. * Corresponding author.

long ago by Price and coworkers, who recorded resolved photoelectron spectra [1]. The earliest experiment on the fragmentation of energyselected methane ions was that of von Koch in 1964 [2], in which the ions were generated by charge exchange. More recently the photoelectron photoion coincidence (PEPICO) technique [3], particularly the form using zero energy or threshold electrons (TPEPICO), has been used to examine the fragmentation of energy-selected methane ions [4-6]. The ground state has been studied in detail and the statistical R R K M theory has been successful in explaining the experimental results [4]. The metastable loss of H from the parent ion above 14.3 eV is attributed to tunnelling through a centrifugal (rotational) barrier [7]. The A state of the ion has received less attention, partly because of the high energy needed for its formation. Dissociative photoionization cross-

0368-2048/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0368-2048(94)02282-8

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sections of methane have been measured over an energy range which includes the A state [8], and Dutuit et al. have recorded TPEPICO spectra from the ionization threshold of methane to 28 eV [5] and reported non-statistical behaviour in the Astate region. In the present study we examine the behaviour of methane ions in the whole energy region using the fixed wavelength PEPICO method and extend measurements to higher energy where weak satellite states are present. The importance of a comparison between the two methods is that indirect ionization, i.e. autoionization via highly excited neutral states, is present and may dominate in TPEPICO, but is absent in PEPICO. Any difference in behaviour of ions of the same energy prepared via different routes is a direct contradiction of the model of free energy flow, which is a key assumption of statistical theories. During the preparation of this paper another TPEPICO study of methane was reported by Furuya et al. who examined the A state and Rydberg states below it [6]. They compared the fragmentation pathways of ions formed purely through nearby Rydberg states and those formed in the A state, and reported significant differences. Satellite states lying above the A state have been observed in e,2e spectroscopy [9] and have also been investigated theoretically [10]. They have recently been seen in photoelectron spectroscopy using X-ray and 65 eV synchrotron light excitation [11]. These states arise from two-electron processes in which one electron is removed from the molecule and a second is excited within it. Carlsson G6the et al. have assigned the satellite states to processes in which two electrons are removed from bonding orbitals, one being ejected and the second promoted into an antibonding orbital [11]. The minimum photon energy for double ionization, which is also expected in the high energy region, has been measured in photon impact experiments as 35.0 eV [12] or 36.5 + 0.5eV [13] and the ratio of double to single ionization at 30.4 nm is reported to be 0.005 [12]. The major products of double ionization at 30.4 nm and their appearance potentials (APs) are C H ~ - + H + (AP35.0 or 36.5+0.5), C H ~ - + H + (AP 38.1 4-0.6) and CH2~ + H + (AP38.1 + 0.6eV) [13,141.

2. Experimental method PEPICO spectra were obtained by recording time-of-flight mass spectra in coincidence with energy-selected photoelectrons in spectra taken at the wavelengths of H e I a (58.4nm, 21.2eV) and He IIc~ (30.4nm, 40.8 eV). The mass spectra were analysed for the branching ratios of each fragment ion at each internal energy of CH~-. A schematic diagram of the apparatus is given in Fig. 1. Vacuum ultraviolet (VUV) light from a helium discharge lamp is selected by a toroidal grating monochromator and crossed with sample gas from an effusive source. Ionization occurs in the common source region of a small time-of-flight mass spectrometer and a photoelectron spectrometer, where the electric field is pulsed to optimize electron energy resolution and mass resolution. Electrons are gathered during a nominally fieldfree period when only a small penetrating field is present in the source region from the focusing element of the electron lens. Use of the field-free period eliminates the degradation of electron energy resolution that would follow from use of a wide photon beam in an electric field, and removes the need for very tight light-beam collimation. This strategy is essential when using wavelength-selected HeII excitation, because the intensity of the light would be too much reduced by restricting the light to a narrow beam. The actual electron resolution depends on the pass energy in the hemispherical analyser. The pass energies used, and the resulting resolutions were: 5V pass, 250meV resolution; 10V, 350meV; and 20V, 550meV. High pass energies give low resolution but high collection efficiency, which was necessary for investigating the weak satellite states. A second advantage of the pulsed regime is that it allows the use of a high drawout field giving good mass resolution. In the present set-up, detection of an electron triggers a drawout pulse of 125 V across the 1 cm source to extract ions, giving a mass resolution (M/AM) better than 100 ( F W H M definition). Between drawout pulses, "sweep" pulses of 10V amplitude are applied to the gas needle at 300kHz to remove ions formed in ionization events from which the electron is not detected. Such ions would otherwise accumulate in the

T.A. Field, J.H.D. Eland/Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 209-216

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source until extracted by the next drawout pulse, and would give rise to strong spurious peaks in the PEPICO spectrum. This pulsed method requires particular attention to screening, decoupling and earthing to avoid pickup of pulses by the signal amplifiers or on other electrodes. False coincidences are more difficult to deal with in pulsed PEPICO experiments than where spectra are recorded with static fields. With static fields, false coincidences raise the flat background level of the spectrum, above which true coincidences are observed as structured peaks. In the pulsed regime, false coincidences produce a structured spectrum, superimposed on the spectrum of true coincidences. The false coincidences are of three types: (1) coincidences between a true electron, i.e. a real photoelectron from the target gas, and an ion from a different ionization event; (2) coincidences between a false electron signal, e.g. a noise pulse, and any ion; (3) coincidences between secondary electrons from surface collisions or inelastic scattering in the gas and ions from the same ionization events. The first two types of false coincidence can be removed by subtracting spectra recorded by send-

ing artificial electron pulses to initiate ion drawout. The "artificial" spectra can be generated by a large number of artificial starts, to give good statistics, and must be normalized before subtraction to the numbers of photoelectron starts in the spectrum to be corrected. The normalization factor is exactly equal to the ratio of the numbers of starts in the real and artificial spectra provided that the ionization rate is low, and the light intensity is invariant as a function of time. In practice a small correction factor, determined from the PEPICO spectra of simple mixed gases, can be applied. The third type of false coincidence is much harder to remove, but usually becomes important only at electron energies approaching zero. The method we have adopted to deal with false signals of this type is explained later. A disadvantage of the pulsed PEPICO method compared with the static field method is that it is more difficult to determine kinetic energy releases, particularly in fragmentations involving hydrogen atom or ion formation, because of the effects of the delay between ionization and application of the ion extraction pulse. This delay, which amounts to about 500ns at 5 eV pass energy, consists of the electron flight time plus the time taken to switch on the ion extraction pulse. The delay also has the

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effect that H + and H~ ions, which escape the source rapidly, are detected less efficiently than heavier ions.

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3. Results and discussion

The process of removing false coincidences of types (1) and (2) above from the spectra is illustrated in Fig. 2; the spectrum shown is taken at 14.79eV in the X state. The fraction of false coincidences in the raw spectra is always kept small, as shown, by using a low ionization rate. This is done, despite the resulting long run times, both to obviate the need for large corrections and to eliminate non-linear paralysis effects in the timeto-amplitude converter which would arise if more

Our measurements on the ground state of CH~with He I a (21.2 eV), shown in Fig. 3, agree closely with those reported previously. Crossover between CH4+ and CH~- is at 14.25 + 0.1 eV; this is in agreement with earlier measurements [2,4,5]. The whole shape of the breakdown diagram agrees with the predictions of the most recent statistical R R K M calculations [5]. 3.2. A state

The photoelectron spectrum and breakdown

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diagram are shown in Fig. 4; measurement points in the PEPICO spectrum were set at the energies of the vibrational lines visible in the photoelectron spectrum. The branching ratio of CH~- is everywhere zero within our experimental error, as is expected both on the basis of previous results [2,5,6] and because CH~- is unstable above 14.4eV. Absence of CH~- signal is a validation of the use of artificial spectra, as described above, to remove false coincidences. Some CH + is observed at the v = 0 and v = 1 line energies, but the branching ratio drops with increasing energy. By contrast, Samson et al. [8] found no discontinuity in the photoionization cross section of CH~- in the region of the A state and concluded that CH + ions were produced only from the ground state of the CH~ion. By comparing the branching ratios of CH~and CH~- in our data with the photoionization cross sections of Samson et al. we estimate that

213

the rise in the CH~- photoionization cross-section due to formation of CH~- in the A state is at most 0.05 Mb which is 0.4% of the total cross section of CH~-. It is not surprising that this small increase in cross section was not detected. Samson et al. observe a rise of 0.3Mb in the photoionization cross section of CH~- across the A state and a rise of 0.1 Mb in the cross section of CH÷; the ratio of these rises is close to the ratio of the percentages of these ions in our data. We have not observed C + from the A state, though the thermodynamic threshold of C + + 2H2 is at 19.36eV. Plessis et al. [15], however, found a very weak C + signal at its expected appearance energy in an extremely sensitive electron impact experiment. The general form of our breakdown diagram in the A-state region is similar to those previously reported from experiments using the TPEPICO technique [5,6]. Dutuit et al. [5] also reported a statistical calculation of the breakdown diagram between 12 and 28 eV. In the A-state region there are marked differences between the calculated and experimental breakdown diagrams. The calculated branching ratio of CH~- is similar to our result, but the branching ratios for CH + and CH~- are different; they are both close to 50% in the calculated data, with that of CH + rising with increasing energy and that of CH2~ falling. Experimentally, the ratios are near 70% (CH~-) and 20% (CH +) and do not change much with energy. The statistical theory in its "strong" form assumes that all internal energy, including electronic energy, may pass freely between all internal modes. This is clearly not the case for the A state and the isoenergetic states of the methane ion populated via high Rydberg states (seen in TPEPICO spectra), which decay more to CH~-. In the more usual "weak" form of the theory, internal energy of nuclear motion is assumed to be freely exchanged, to reach statistical equilibrium among all vibrational and, where appropriate, rotational degrees of freedom within a single electronic state. If the fragmentation is very rapid, however, the internal energy may not equilibrate even between vibrational modes before the molecule has dissociated. Carlsson G6the et al. [11] have reported a high-resolution photoelectron spectrum of the A state, giving the widths of the vibrational lines as

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T.A. Field, J.H.D. Eland/Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 209-216

fitted with Lorentzians. The F W H M of the v = 0 line is 49 meV, and that of the v = 6 line is 200 meV, which correspond to lifetimes of 13fs and 3.3fs respectively. These may be compared with the vibration period of 15.3 fs [11] for the totally symmetric stretch excited in ionization, showing that at most one complete vibration can occur before the loss of phase coherence. The coherence lifetimes of the vibrational states excited in photoionization are not necessarily equal to the lifetimes of the CH~ions before dissociation, but their brevity does provide a possible explanation for the failure of statistical calculations to predict the breakdown diagram. The potential energy surface of CH4~ in the A state is bound in the direction of the totally symmetric vibration vl, which is the only mode strongly excited on ionization; therefore the first step in dissociation must be transfer into other modes which lead to dissociation. The nature of the subsequent steps is much less clear. It is striking that over the energy range viewed, the intensities of CH~- and CH + remain in almost constant ratio. If the photoelectron peak width changes are interpreted as changes of rate, the rate of formation of CH + must change with energy in the same way as the rate of formation of CH +, suggesting a sequential mechanism. The overall reaction of CH + formation must be CH + C H + + H2 + H, as the thermodynamic limit for formation of C H + + 3H (24.2eV) is above the A-state energy range. A mechanism CH4~ -~ CH~- + H 2 (rate determining) followed by CH~- ~ CH + + H (fast for the energized fraction of CH~-) therefore seems a reasonable chemical hypothesis. Because the reaction is so fast a full quantum mechanical description is perhaps needed. Dutuit et al. [5] reported fine structure in the breakdown diagram within the A state. The present data are smooth, though weak structure could be concealed by the statistical uncertainty. Dutuit et al. also reported that CH~- fragment intensity reappears at energies above the A state and has a maximum at the C H ~ - + H ( n = 2 ) threshold 24.5eV. The present data exhibit no

reappearance of the CH~- ion in the region leading up to an energy of 24.5eV. There are several possible reasons for the discrepancy between our data and those of Dutuit et al. First, CH~- might reappear by a mechanism involving superexcited states of neutral CH 4 which eventually release a threshold electron and give the product CH3~; such superexcited states would not be produced by fixed wavelength excitation. Alternatively, the discrepancy might be due to "false" coincidences of the type (3) described above, involving secondary or inelastically scattered electrons from lower states being detected as threshold electrons. We are not able to measure the fragmentation at exactly 24.5 eV because with 30.4nm light the photoelectron signal from CH 4 at this energy is too weak. However, von Koch measured the breakdown at exactly 24.5 eV by charge exchange with He + and observed no CH~- fragment ions [2]. 3.3. Satellite states

Measurements with the electron hemisphere operating at two different pass energies, 20 and 10eV, are combined in Fig. 5 with the photoelectron spectrum recorded at 10eV pass energy. Dominant in the photoelectron spectrum is a rising background of electrons which has its peak at zero energy. The apparent fraction of true coincident ions per electron drops towards low electron energy as this background rises, showing that most electrons in the rising background are not produced by gas-phase photoionization; they may be derived from ionization by scattered light at surfaces. This electron background obscures the satellite states in the photoelectron spectrum, but has little effect on the coincidence spectra, which effectively select true gas-phase photoelectrons and ions. The PEPICO spectra are not free from interference, however, as they contain false coincidences of the third type noted above. There is an apparent non-zero branching ratio for the CH~- ion, which increases towards zero energy. We attribute this signal entirely to false coincidences with secondary and inelastically scattered electrons. To remove these false coincidences we make two assumptions. First, we assume that stable CH~ions are formed exclusively in the cation ground

T.A. Field, J.H.D. Eland/Journal of Electron Spectroscopy and Related Phenomena 73 (1995) 209-216

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215

state so that all CH~- ions observed at higher energy are from false coincidences. Secondly, we assume that the mass spectrum of ions in coincidence with scattered electrons is not dependent on the energy of the electrons and is the same as the spectrum recorded with artificial electron starts. With these assumptions (whose validity is not certain) the extra false coincidences could be removed by an additional subtraction of the artificial spectrum from the raw data using the CH + branching ratio to determine a scaling factor. The results corrected in this manner are presented in Fig. 5. The error bars are long at the low electron energies because the fraction of true electron signals is low. In the energy range between the A state and about 32 eV, the trend of the breakdown diagram towards smaller fragments at higher energy is continued; we find a crossover between CH~- and CH + at 29 + 1 eV, in reasonable agreement with an estimate of 27.5eV from differentiating the crosssection data of Samson et al. [8]. The continuity of the breakdown diagram in this region indicates that fragmentation from the low-energy satellite states probably produces the same ground-state products as that of the valence states. Above 32eV there is a rise in the branching ratios of CH~ and CH~- which is in striking contrast to the lower energy behaviour. It indicates that either doubly ionized or electronically excited products must be formed. The onset is well below the appearance energies of CH3~ (35 or 36.5± 0.5eV) and CH~- (38.1 ± 0.6eV) in double ionization [12,13], so actual double ionization is not involved there. In agreement with this, Samson et al. observed clear discontinuities in the partial photoionization cross-sections of CH + at 33.2 eV, of CH + at 35.2eV, and of C + at 42.7eV, which they attribute to formation of excited states below the double ionization thresholds. We therefore conclude that the rises in the branching of CH~- and CH~- in our data are not residual artifacts but real. The photoionization cross section of CH~- is 50 times that of CH~- at 33 eV so it is not significant that no similar discontinuity in the cross section of CH~- was observed by Samson et al. Below the threshold for double photoionization, Rydberg states with a doubly ionized core and one excited electron are formed; their cores will decay

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in the same way as doubly charged ions, by charge separation into two singly charged fragments, with a high probability of electronic excitation. According to this "ion core model" [16], one member of the resulting ion pair captures the excited electron, being thereby neutralized, while the other escapes as an ion. Processes of this general sort give rise to high Rydberg fragments [16], high kinetic energy fragment ions [17] and excited products which emit fluorescence [18,19]. The model explains why CH~and CH~, the main products from double ionization, are also found as the main products of single ionization around the double ionization threshold. H ÷ and H f ions, which are probably also produced in this way, are poorly detected in our apparatus because of their high kinetic energy.

4. Conclusions The breakdown diagram of methane ions formed in the ground state by photoionization is exactly the same whether measured by fixed wavelength P E P I C O or by variable wavelength TPEPICO. This demonstrates with high probability that initial formation effects are absent, and strengthens the conclusion from comparison of the measured and calculated breakdown diagrams that a statistical model is appropriate. The small size of the methane molecule may be offset by strong coupling mediated by the Jahn-Teller effect in the ground state. At the energy of the A state, non-communicating ionic states (isolated states) are present, and even within the A state itself there is clear evidence that the breakdown is non-statistical. We agree with Dutuit et al. on the non-statistical nature of the A state decay, but cannot confirm the reappearance of CH~- at the high energy side and above the A state. The breakdown diagram in the region of satellite states above the A state but below about 32eV shows similar fragmentation to that in the A state and is probably nonstatistical too. Higher energy satellite states below the double ionization limit are characterized by dissociations which mimic the behaviour of doubly charged ions and probably produce electronically excited products. Above the double ionization limit, behaviour characteristic of doubly

charged ions becomes dominant because in this region all the populated states are either doubly charged or have doubly charged ion cores.

Acknowledgements We thank P. Baltzer for providing details of the high-resolution photoelectron spectrum of CH 4 in the A state region. The project was supported by the SERC under grant GR/H28219.

References [1] A.W. Potts and W.C. Price, Proc. R. Soc. London Ser. A, 326 (1972) 165. [2] H. yon Koch, Ark. Fys., 28 (1965) 529. [3] I. Powis, J. Chem. Soc. Faraday Trans. 2, 75 (1979) 1294. B. Brehm and E. yon Puttkamer, Adv. Mass Spectrom., 4 (1968) 591. [4] R. Stockbauer, J. Chem. Phys., 58 (1973) 3800. [5] O. Dutuit, M. Ait-Kaci, J. Lemaireand M. Richard-Viard, Phys. Scr., T31 (1990) 223. [6] K. Furuya, K. Kimura, Y. Sakai, T. Takayanagi and N. Yonekura, J. Chem. Phys., 101 (1994) 2720. [7] C.E. Klots. J. Phys. Chem., 75 (1971) 1526. T. Baer, Adv. Chem. Phys., 54 (1986) 111. [8] J.A.R. Samson, G.N. Haddad, T. Masuoka, P.N. Pareek and D.A.L. Kilcoyne,J. Chem. Phys., 90 (1989) 6925. [9] M.J. van der Wiel, W. Stoll, A. Hamnett and C.E. Brion, Chem. Phys. Lett., 37 (1976) 240. [10] L.S. Cederbaum, W. Domcke,J. Shirmer, W. yon Niessen, G.H.F. Diereksen and W.P. Kraemer, J. Chem. Phys., 69 (1978) 1591. [11] M. Carlsson Grthe, B. Wannberg, L. Karlsson, S. Svensson, P. Baltzer, F.T. Chau and M.-Y. Adam, J. Chem. Phys., 94 (1991) 2536. [12] G. Dujardin, D. Winkoun and S. Leach, Phys. Rev. A, 31 (1985) 3027. [13] P.A. Hatherly, M. Stankiewicz, L.J. Fransinski, K. Codling and M.A. MacDonald, Chem. Phys. Lett., 159 (1989) 355. [14] D.A. Hagan and J.H.D. Eland, Org. Mass Spectrom., 27 (1992) 855. [15] P. Plessis, P. Marmet and R. Dutil, J. Phys. B, 16 (1983) 1283. [16] R.S. Freund, in R.F. Stebbings and F.B. Dunning (Eds.), Rydberg States of Atoms and Molecules, Cambridge University Press, Cambridge, 1983, Chapter 10. [17]J.H.D. Eland, in C.Y. Ng (Ed.), Vacuum Ultraviolet Photoionization, World Scientific, Singapore, . 1991, pp. 332-333. [18] T, Fieldand J.H.D. Eland,Chem.Phys.Lett., 197(1992)542. [19] T. Ogawa, T. Tsuboi and K. Nakashima, Chem. Phys., 156 (1991) 465.