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Translational energy loss spectrometry of molecular dications from methane
Chemical
447
Physics 103 (1986) 447-459
North-Holland,
Amsterdam
TRANSLATIONAL OF MOLECULAR D. MATHUR,
ENERGY LOSS SPECTROMETRY DICATIONS FROM METHANE
C. BADRINATHAN,
F.A. RAJGARA
and U.T. RAHEJA
Ta~a Institute of Fundamenral Research. Homi Bhabha Road Bombay 400005. India Received 28 October
1985
Single electron loss processes have been studied in collisions of Singly charged CHZ and air at impact energies in the 0.5-5 ions the double ionization CHf’
keV range. By means of translational
energies of CH.
could be detected. Comparison Agreement
CH,
and CH,
with Kr. CH,.
N,
of product molecular
molecules have been deduced. No signal corresponding
to
is made between the present results and data obtained by means of Auger spectroscopy.
double charge transfer and charge stripping orbital calculations.
CH,.
ions (for n = l-5)
energy loss spectrometry
in a conventional
between experiment
mass spectrometer
as well as with recent ab initio molecular
and theory as well as between different
experiments
is generally found to
be unsatisfactory.
1. Introduction
A description of collision processes which result in the ejection of two or more electrons from the valence shell of atoms and molecules requires consideration of many-body interactions. For instance, in the case of double ionization of molecules by particle impact, the energetic incident particle interacts with only a single target electron at a time; two-electron ejection in a single collision situation must therefore be regarded as a manifestation of electronic many-body effects. In the case of diatomic and polyatomic molecules, theoretical descriptions of double or multiple ionization are complicated by the existence of a large density of possible electronic configurations which make it difficult to model electronelectron correlations in a quantitative fashion.Even recent advances in quantum-chemical techniques, such as the complete active space self-consistent field [ 1,2] method - which includes several hundred configuration state functions - are found to be of limited scope because of the problems associated with adequate description of correlation effects which can be significant between states of radically different character. Intuitively, a doubly charged molecular ion can 0301-0104/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
be visualised as consisting of two singly charged species interacting only via a purely repulsive coulombic force. It may, however, be possible to envisage a situation where a single electron promoted to a strongly bonding orbital in the course of a collision can counteract the coulombic repulsion to the extent that a shallow potential well is created in the potential energy surface describing a doubly charged molecule; this can result in the formation of either a weakly bound or metastable state of the molecular dication. Hypotheses in which perturbations of the potential energy surfaces pertaining to diatomic cations AB2+ by A + B2+ bonding orbitals were first put forward more than twenty years ago [3]. However, in the majority of practical cases there exists a large number of dissociation limits between the asymptote of the charge polarization states A + B2+ and those of lower-lying AB2+ states; as a result, due consideration has to be given to the multiplicity of avoided curve crossings between diabatic strongly bound charge polarization states and coulombic repulsive states. Consequently, potential energy surfaces become highly mixed in character and necessitate recourse to extremely complex molecular wavefunctions. Over the last ten years or so a number of theoretical B.V.
448
D. Math
et al. / Dications of CH,, (n = I - 5)
techniques have been developed which locate and calculate the widths of local minima in molecular dication potential energy surfaces [4-61; such techniques have recently been applied in ab initio calculations [7] of several electronic states of CO*+ using configuration interaction procedures supplemented by perturbation techniques [S-lo] which obviate the need for the usual brute-force methods which are extremely time consuming (and expensive). The earliest survey of multiply charged molecular positive ions was published over twenty years ago [ 111 and listed the appearance energies of over 50 diatomic and polyatomic dications. Since then, molecular dications have been detected in a number of different experimental configurations involving photon impact as well as charged-particle collisions. Doubly charged diatomic and triatomic species produced by HeIcv and He& radiation (12,131 have been observed in photoionization mass spectrometry experiments. It was found that more doubly charged ions relative to singly charged ions are produced by photon impact than by electronic collisions at the same energy, the contrast being most marked in the case of small polyatomic species. Recently, doubly charged SO:+ ions have been produced by monochromatized synchrotron radiation [ 141 and photoion-photoion coincidence experiments have been carried out to enable the study of fragmentation of individual electronically excited states of the parent dication. In the case of electron impact, most dication data has been obtained at 70 eV collision energy in ion sources in commercial mass spectrometers. A survey of the available data indicates that although the electron impact mass spectra of diatomic and polyatomic species indicate the existence of less than 1% dication abundance [15-211, corresponding abundances of up to 20% have been observed in the case of large aromatic hydrocarbons 122-241 and organometallic complexes 125-27). Appearance potentials and dissociation energies of molecular dications have also been measured by other experimental techniques, such as double charge transfer [28-301, charge stripping in double focusing mass spectrometers [31-341, Auger spec-
troscopy [35-381, field ionization [39], ion confinement in an electron beam [40,41] and laser photodissociation [42].
2. Doubly charged methane ions The methane molecule continues to attract much experimental as well as theoretical attention. In spite of this, certain fundamental properties of this molecule remain unclear. For instance, experiments on single ionization and dissociation of CH, by both electrons and photon impact continue to yield data which cannot be unambiguously interpreted [43,44]. In the case of double ionization of methane there also exists conflicting information (table 1). CH:’ ions were first observed by Spohr et al. [45] in an Auger spectroscopic measurement on methane using the ESCA technique [37] where high-energy electrons are used to create a vacancy in the carbon K-shell. The hole in the K-shell is filled by a valence electron and, simultaneously, another valence electron is emitted, and its kinetic energy is measured. The highest KLL Auger line in CH, was determined by Spohr et al. to occur at 250.0 f 0.3 eV; subtracting the carbon K-shell binding energy of 290.7 f 0.7 eV yielded a value of 40.7 eV for the double ionization of CH,. Subsequently, double charge transfer experiments, H’+CH,+H-+CH;+,
0)
were carried out by Appell (301 at 4 keV impact energy, using a double focusing mass spectrometer with a collision cell inserted between the electric and magnetic sectors. A value of 38.9 eV was determined to be the vertical double ionization energy for methane in these experiments. More recently, Ast et al. [46] have carried out charge stripping experiments at 8 keV collision energy, using a commercial reversed geometry double focusing mass spectrometer, and have determined the double ionization energy of CH, to be only 30.6 eV. Most recently, Dujardin et al. [47] have deduced a double ionization energy of 35 eV by studying the dissociation of CH:’ produced by monochromatized synchrotron radiation; by
449
D. Mathur et al. / Dicarions of CH, (n = I - 5) Table 1 Summary of existing information
on the double ionization
energy of methane
Experimental data method
ref.
AE(CH:+
Auger spectroscopy using ESCA double-charge transfer charge stripping photoion-photoion coincidence charge stripping
1451 [301
40.7 38.9b’ 30.6 ” 35.0 38.97 =Jt’
I&l I471 this work
) ‘) (eV)
Theoretical calculations geometry
method
ref.
AE(CH:+
Td
Hartree-Fock Hartree-Fock SCF, MC SCF and MC CI MO using 6-31 G* basis set
I481 I491 I491
38.8 singlet I$;’ 36.9 triplet 41;’ I&’ 32.3 32.4
D 4h ‘) b, ‘) d,
PO1
) ‘) (ev)
Appearance energy of doubly charged methane ions. kO.7 eV. Assumes a single ionization energy for CH, of 12.7 eV. * 0.25 eV.
means of the photoion-photoion coincidence technique the lowest-energy dissociation channel has been identified as hv + CH, -+ CH;’
+ CH_; + H’.
(2)
The wide discrepancy in the experimental results is further emphasized by the differing predictions of the lifetime for CH:‘. The Auger spectroscopy experiments [45] indicated an extremely short lifetime, which is also in conformity with the lack of observation of the CH:+ ion in any ordinary mass spectrum. On the other hand, ob. servation of CH:+ tons in charge stripping experiments implies that the ion survives for at least the transit time from the collision chamber to the ion detector, which is of the order of a few ps. In an attempt to resolve the discrepancy and to measure the double ionization energies of other related simple hydrocarbons, we have carried out high-resolution translational energy loss measurements of doubly charged product molecular ions resulting from charge stripping collisions of the type CH,: + X + CHZ’
+ X + e - AE,
(3)
where n = l-5. X represents atomic and molecular neutral target gases and AE is the energy defect which is a measure of the single ionization energy of CH,;
3. Experimental
apparatus and procedures
Measurements of the energy defect, AE, were carried out in an ionic collisions apparatus (fig. 1) which has been briefly described in a recent report on state-diagnosed single electron capture cross section measurements [51] in collisions of ground state Kr2+ with Hz. Briefly, the ion source used is a low-voltage arc type of source capable of producing ions from gaseous species as well as alkali metals; the latter case uses a built-in oven capable of reaching temperatures of the order of 500°C. In the present case, singly charged CH,: ions from research grade methane (Mathesons) were extracted by a three-element cylindrical electrostatic lens at energies in the 0.5-5.0 keV range, and focused onto the entrance plane of a region of crossed electric and magnetic fields (Wien filter), where separation on the basis of charge-to-mass ratio occurred. The Wien filter was chosen in preference to the more usual sector devices because of its relatively simple in-line geometry coupled with flexibility of easily variable dispersion. Filtered ions then passed through a double differentially pumped static gas target maintained at room temperature. The gas pressure in the collision zone was monitored by a capacitance manometer (Leybold-Membronovac). In the post-colli-
D. Mathur er al. / Dications of CH, (n = I - 5)
450
DPl
DP2
IG
Fig. 1. Schematic diagram of ICE - ionic collisions experiment. IS: ion source, CH: solid charge holder, L: cylindrical electrostatic lenses, D: electrostatic deflection plates, WF: Wien filter, CC: collision chamber, CM: capacitance manometer, IP: triode ion pump BPM: movable beam profile monitor, PP: electrostatic parallel plate analyzer, CEM: channel electron multiplier, MCP: microchannel plate array, IG: ionization gauge, DP: oil diffusion pump.
sion region the scattered ions were analysed by an electrostatic parallel plate analyzer. Ion detection was by means of a channel electron multiplier operating in the particle counting mode using conventional pulse handling techniques. The ion source, lenses and mass filter were housed in a bakeable stainless steel chamber pumped by an oil diffusion pump to a base pressure of 5 x lo-* Torr. Typical operating pressures were of the order of 5 X lo-’ Torr with gas and heat load, and at an ion source pressure of = 5 X lo-* Torr. The pressure in the gas cell was maintained in the 10e4 Torr range, well within “single collision conditions”. In order to quickly pump away collision chamber gas effusing from the exit slit of CC (see fig. 1) a 20 / s-’ triode ion pump was positioned a few centimetres from the CC exit slit. In conjunction with a second oil diffusion pump, the post-collision vacuum chamber was maintained at a pressure of = 1 x lo-’ Torr with ion beam and gas load. The post-collision chamber also had a moveable beam profile monitor and a microchannel plate detector assembly (BPM and MCP in fig. 1); these items were not used in the present measurements except for preliminary operating tests concerned with beam intensities, shapes and beam attenuation measurements to determine the presence, or otherwise, of long-lived electronic excited states in the incident ion beam (51,521. The apparatus was controlled on-line by means of a microprocessor controlled multichannel analyzer via an interface comprising, amongst other units, a voltage ramp generator whose output was
used to control various programmable power supplies. During measurement of energy loss spectra, a voltage ramp was applied across the parallel plate analyser and the output pulses of the channel electron multiplier were synchronously stored in memory on a multiple scanning basis. After scanning the analyser voltage a sufficient number of times to yield an adequate signal-to-noise ratio, data was transferred to another microcomputer for off-line analysis and a floppy-disc hard copy. Typical singly charged ion current at the entrance of the collision zone was lo-’ A. The elastic peak to background ratio following analysis by the parallel plate analyzer was better than 10’ (typically = 2500); background counts were generally non-fluctuating over very wide energy ranges. The angular resolution in the present measurements was 0.14 radian. The following procedure was adopted for determining the energy defect, AE. If the incident CH,+ ions were transmitted through the energy analyzer when a voltage U was applied across the parallel plates, the corresponding doubly charged ions of the same kinetic energy would be transmitted at a voltage (O.SU - AU), where AU is related to the energy defect in (1) by a factor determined by the geometry of the analyzer. In the present measurements U was taken to be the voltage at the onset of the singly charged peak. By measuring the voltage displacement of the onset of the doubly charged peak from O.SU, one can determine the minimum energy defect, AE,,,i,,, and hence the energy threshold for ionizing a singly charged ion. There are a number of assumptions
D. Marhur et al. / Dicarions of CH, (n = I - 5)
implicit in such a deduction; these have been extensively discussed elsewhere (31). The accuracy and reproducibility of the energy defect measurements were largely determined by the accuracy with which the accelerating voltage could be measured and its stability. The power supply used for applying the acceleration voltage (Spellman Model RHR-10) is stated to have a stability of better than *0.15 V at 3 kV. A programmable power supply of similar specifications was used to scan the analyzer voltage; the error and non-linearity introduced by the computer-controlled ramp generator used for programming this power supply were negligible. Voltage measurement was by means of a 4$ digit voltmeter coupled to a precision resistance chain. Peaks corresponding to doubly charged ions formed by electron loss collisions were all found to have rather sharp onsets and produced tailing on the high-AE side. Moreover, this onset was relatively unaffected by the nature and pressure of the collision gas. Although the signal-to-noise characteristics of the spectra presented in the following section are not good, and CHi+ peaks observed were of the order of 10 eV wide, the position of the relatively sharp onset was found to be usually reproducible to within 0.5 eV on most occasions over a period of more than six months. This method of determining onset energies has been used in the ion kinetic energy experiments of Ast et al. [53] before.
spectroscopically determined [54] value for the ionization energy of He+ (53.403 eV) to yield a difference of 0.37 eV between our data and the spectroscopically defined value of AE,,,. We attribute this difference to contact potentials and use it as a correction factor in all the values of AE,,, reported below. In order to obtain some quantitative information on the systematic errors in our experimental arrangement we have carried out single electron stripping experiments on C’ ions obtained when CH, was put in the ion source. The process studied was C’+X+C”+X+e-AE,
4.2. Production of CH dications
CH’+Kr+CH”+Kr+e-AE
4.1. Calibration of the energy loss scale In order to calibrate the energy scale to account for any contact potentials and electric field nonuniformities in the vicinity of the energy analyzer, a small amount of research grade helium was leaked together with methane into the ion source and AE,i” measured for the electron loss process He’+Kr+He2++Kr+e-AE.
(4)
The energy loss spectrum obtained at 3 keV impact energy is shown in fig. 2. The measured onset for the He’+ inelastic peak was compared with the
(5)
where the collision chamber gas X was Kr, CH, and laboratory air. A typical energy loss spectrum obtained at 3 keV impact energy with Kr collision gas is shown in fig. 3. The average value of A E,,, was 24.76 f 0.30 eV. This may be compared with the known ionization energy of C’ (24.383 eV) established by ultraviolet spectroscopy [54]. The difference of = 0.4 eV is probably a representation of the order of systematic error in the present measurements. The only other threshold data appears to be the ion trap measurement of Redhead [55] on ionization of confined C’ ions, which yielded a value of A E,,, of 31.0 eV.
A typical energy loss spectrum 4. Results and discussion
451
for the collision (6)
is shown in fig. 4 for an impact energy of 3 keV. CH,, N, and laboratory air were also used as target gases; although the ratio of the abundance of the dication to that of the monocation varied with different coliision gases, the value of AE,,, was found to maintain a constant value of 23.07 eV within a reproducibility of f0.30 eV. Taken in conjunction with the spectroscopically determined [56] ionization energy of CH (10.64 + 0.01 eV) we obtain a value of 33.71 f 0.3 eV for the double ionization of CH. The singly charged CH’ ion has an electronic configuration (~IJ)~(~u)~(~u)‘. Ejection of an electron from the outermost 30 orbital yields an electronic configuration and overall sym-
D. Mathur et al. / Dications of CH, (n = I - S)
452
I
He’
tit!+.
3 J,
I
5
ii
I He’ +
iI)848
X-He
++
+
x
856
064
012
l
e
-J 1790
1780
ENERGY ANALYZER VOLTAGE Fig. 2. Energy -_ loss spectrum showing electron loss from He the onset for He’+
l
1800
(Volts)
ions colliding with Kr at 3 keV impact energy. Solid vertical line marks
iormation.
metry of (1~)~(2a)~(3e), 2Z+ for the ground state of CH2+. The only other experimental data is from the charge stripping measurements of Ast et al. [46] which yield a double ionization energy of 33.4 eV which is in excellent agreement with the present result. Theoretically the first investigation into the structure and stability of CH2+ dications was by Pople et al. [50]. Using ab initio molecular orbital methods with both the 3-21G as well as the 6-31G* basis sets, they showed that there is no minimum in the Hartree-Fock potential energy curve for CH’+. They concluded that the ground state potential curve is purely repulsive and that dissociation into C+ and H’ proceeds extremely rapidly. Interestingly, the ionization energies of neutral C and CH calculated by Pople et al. using the same method and basis sets yields values which are in good agreement with experimental data. Ab initio calculations of potential energy curves of three zZ+ states of CH+ leading to dissociation into
C+(2D)+Hf, C+(‘P”)+H+ and C2’+ H have also been carried out by Heil et al. [57] using configuration interaction methods [58] and, later, by Wetmore et al. [59] who carried out restricted Hartree-Fock SCF calculations (601 on the ground state of CH+ in order to generate the molecular orbitals used in subsequent configuration interaction calculations [10,61]. Both sets of ab initio calculations indicate that the predominantly repulsive CH2+ potential energy curves are seriously perturbed by quasi-bound states. As a result of avoided curve crossings between the C’--H’ repulsive potential energy curve and the C2+-H curve which has a significant attractive component due to the strong attraction of the hydrogen atom through polarization by the doubly charged carbon atom, small potential dips are created which may support a metastable state of CH2+. Unfortunately, the question whether the depth of such potential minima are sufficient to support at least one vibrational level of the CH2+ molecular ion
D. Mathur el al. / Dications of CH, (II = I - 5)
453
CH+
CH*
t Kr -+
CH"
l
Kr t e
r*+
L
A
x
I
870
I
880
I
890
64
I
i
/
1,/----J--l
L
1790
sod’
ENERGY ANALYZER VOLTAGE
1800
(Volts)
Fig. 3. Energy loss spectrum showing electron loss from C+ ions colliding with Kr at 3 keV energy. Onset for C2+ formation is indicated by the solid vertical line.
cannot be answered by current initio methods of generating curves.
state-of-the-art ab potential energy
4.3. Production of CH, and CH, dications The triatomic dication CH:+ possesses Da,, symmetry and, according to the molecular orbital calculations of Pople et al. (501, should have a long lifetime. The ionization energy of CH: is computed by these authors to be 21.1 eV. This dication has also been theoretically investigated by Siegbahn [49] who has carried out large basis set MC SCF and CI calculations using the complete active space SCF method [62,63]. This technique has yielded a CH: ionization energy of 21.0 eV. In our present experiment, CH:’ ion formation was observed for Kr, CH,, N, and laboratory air collision gases and a value of AE,,, of 20.8 f 0.25 eV was measured. This compares very well with both the theoretically deduced values. The only other experimental data is from the charge stripping experiments of Ast et al. [46] who obtained a considerably smaller A E,i, value of 19.6
840
860
880
1790
ENERGY ANI\LVZER VOLTAGE
1eocl (Volts)
Fig. 4. Typical energy loss spectrum showing single electron loss from CH+ ions colliding with Kr at 3 keV incident energy. Solid vertical line indicates the onset for CH2+ formation. k 0.3 eV. A possible explanation for this discrepancy might he in the fact that the double focusing mass spectrometer used by Ast et al. utilises an oscillating electron impact type of ion source whereas in our apparatus the incident ions are produced in a low-voltage arc type of ion source. In the latter type of source ions are extracted from a relatively low-temperature plasma and it is known that electronically and vibrationally “cool” ions are produced. The conventional electron impact source, on the other hand, is known to produce a mixture of ground and metastable ions with a fairly broad vibrational level population distribution [64]. The lower value of A E,,, obtained by Ast et al. may therefore be a reflection of the fact that the incident CH,’ ions emanating from their ion source are vibrationally excited. Taken in conjunction with the spectroscopically determined value for the single ionization energy of the CH, radical [65] of 10.396 f 0.003 eV, our present data yields a value of 31.20 * 0.25 eV for the double ionization energy of CH,. In the case of the methyl dication CH:+ there is an interesting disagreement between the theoret-
D. Mathw et al. / Dications of CH, (n = I - S)
454 CH,,+ +
86h
868
X -CH
872
l
4
+
+
876
X
+
e
880 ”
1790
ENERGY ANALYZER VOLTAGE
la00
(Volts)
Fig. 5. Energy loss spectrum following collision of 3 kcV CH: ions with Kr. The width of the singly charged molecular peak is of the order of 7.5 eV fwhm. Solid vertical line marks the onset for CH:+ formation.
ical predictions of Pople et al. (501 and the observations of Ast et al. [46]. According to the calculations of Pople et al., the energy depth of a potential energy surface minimum in the CH:+ which could support the formation of the dication through such a is only = 150 meV; tunnelling small potential barrier will be expected to occur extremely rapidly. Consequently, Pople et al. conclude that CHj’ should have an insignificant lifetime. On the other hand, Ast et al. report clear evidence for a CH:’ peak in their charge stripping spectrum and obtain a AE,,, value of 18.9 It 0.3 eV. In our present measurements we have attempted to locate the CH:’ peak in energy loss spectra measured with both atomic as well as molecular target gases, and at CH; impact energies ranging from 1 keV to 5 keV; however, no peak corresponding to the methyl dication could be detected. There appear to be two possible explanations: either the lifetime of the CH:+ ions is considerably shorter than the 3.3 f 0.17 t~s transit time for 3 keV ions to travel from the collision chamber to the detector in our apparatus or the electron loss cross section even at our highest impact energy (5 keV) is extremely small. The former explanation is favoured by the conclusions
reached by Pople et al. The latter explanation appears to be unlikely as the measurements of Ast et al. were also conducted at 5 keV energy and the detection sensitivity in the two experiments is unlikely to be vastly different; this is borne out by the fact that the relative cross section for charge stripping in the case of CH’ in the experiment of Ast et al. is measured to be nearly a factor of 20 less than in the case of CH; and our experiments are clearly able to detect the CH” dication. 4.4. Production of CH, dications A typical inelastic spectrum produced by multiple scans of the parallel plate analyzer voltage is shown in fig. 5 for the collision CH:
+Kr+CH:’
+Kr+e-AE.
(7)
The energy defect, AEmin, is found to be 26.26 k 0.25 eV. In a preliminary report [66] on the CH:’ dication we have presented details of the precautions taken to ensure that the data presented in fig. 5 pertains only to electron loss from CH: ions and not impurity 0 + ions possessing the same mass-to-charge ratio. The electronic configuration of ground state methane in tetradral symmetry group notation is
D. Muthur et al. / Dicarions of CH, (n = I - 5)
(la,)‘(2a,)2(1t2)h, ‘A,. Removal of a single electron from the most loosely bound 1 t z orbital breaks its triple degeneracy; more than one final state becomes possible and hence the onset for single ionization is not sharp. High-sensitivity crossed electron-molecular beam measurements [43] using monoenergetic electrons have yielded a value of 12.71 eV as an upper limit for the first vertical ionization energy. Taken in conjunction with our measured value of AE,,, we obtain a value of 38.97 f 0.25 eV for the double ionization energy of CH,. Removing two electrons from the 1 t, orbitals will give rise to ‘A,, ‘E,, ‘T, and ‘T, states of CH:’ . However, the present measurements do not enable unambiguous identification of the overall symmetry of this dication. Hartree-Fock calculations 1481 have yielded values of 38.80 and 39.11 eV for formation of CH:’ into singlet and triplet states, respectively. It has been established [67-691 that two-electron removal from molecules with six valence electrons possessing the general formula AH, results in a dication which has a lower energy in the square planar geometry (D,,, symmetry) rather than a tetrahedral shape. Despite the large exoergicity of the reaction leading to proton loss, the ab initio MO calculations of Pople et al. [50] indicate a large potential barrier towards dissociation. Consequently, a long lifetime is predicted for CH$‘, and a value of 19.7 eV is calculated for AE ,,,,,,. Siegbahn’s SCF, MC SCF and MC Cl calculations [49] also predict a value of 19.6 eV for AE ,,,,,,. A summary of the experimental and theoretical is shown in table 1. It is information on CH:’ seen that although the Auger spectroscopy data of Spohr et al. [45] and the double charge transfer results of Appell et al. [30] are in reasonable accord with the present measurements; the recent photoionization experiments of Dujardin et al. [47], using synchrotron radiation, yield a value for double ionization energy which is = 3 eV lower than our value. The electron loss process in ion-atom collisions involves essentially the same type of Franck-Condon transition as in electron impact ionization of molecules. Thus, ionization energies deduced from electron stripping collisions may be considered to
455
be vertical rather than adiabatic in nature insofar as the existence of a bound state of the methane dication is assumed. The difference between the photoionization data and the present result may therefore be a reflection of the difference between the adiabatic and vertical double ionization energy. In making such a comparison, the following pertinent points need to be considered. In principle, a threshold experiment measures the lowest possible energy for which a particular inelastic process has a non-zero transition moment. In practice, however, technical constraints (such as limited sensitivity) lead to a situation where the observed signal is largely due to inelastic processes which have maximum probability, usually a transition from a classical turning point rather than an adiabatic one. When onsets are estimated by extrapolation of the dication signal from its value at some energy which is higher than the threshold energy to zero level, the result is some indeterminate compromise which is taken account of in the energy calibration procedure only if the shapes of the threshold curves are the same for the calibrant as for the dication of interest. In the present measurements we have attempted to minimise these problems by avoiding use of extrapolation techniques and, instead, adopting the use of the vanishing current method using singleparticle counting. Attention has already been drawn to the charge stripping data of Ast et al. [46] who obtained a very low value of 17.9 eV for AE,,, (table 1). Siegbahn has attempted to explain this low A E,,, value by considering the difference in geometry between neutral and doubly ionized methane. It is shown that square planar (D4,,) CH:’ has a much lower energy than tetrahedral (Td) CH: + . The energy difference between the Td and D,,,, geometries is calculated to be much higher than the barrier for dissociation into CH: + H+. which is found to be = 1 eV. Due to the different geometry, the Franck-Condon factors between T,, CH, and D,, CH:’ would be almost zero and, hence, D4,, structure would -not be observed in Auger spectroscopy experiments. Siegbahn therefore concludes that the results of Spohr et al. pertain not to the ground state of the methane dication but to some excited state which lies roughly 10 eV higher
D. Marhw et al. / Dicutions of CH, (n = I - 5)
456
in energy. In the case of charge stripping experiments, on the other hand, Siegbahn postulates that the excess internal energy obtained upon ionization of CHf ions can be released in collisions with other molecules so that the dication formed can adiabatically shift to square planar geometry where it has a long lifetime. In the light of the present results, and the earlier double charge transfer measurements of Appell [30], it appears less likely that geometrical differences can account for the low double ionization energy obtained by Ast et al. In any case a discrepancy still exists with the double ionization energy calculated specifically for D4,, geometry to an accuracy which is claimed to be better than 0.3 eV [49,50]. Another possible explanation in terms of internal vibrational excitation of CH: appears unlikely in view of the rather large difference in the AE,,, value between the results of Ast et al. and the other experiments (table 1). although the possibility that the incident ions produced in the low-pressure ion source used by Ast et al. (presure = typically 2 x 10m6 Torr) were electronically excited to metastable states cannot be excluded. In contrast, the ion source pressures in our experiments (typically low2 Torr) are such that the incident ions are likely to be electronically and vibrationally cool. 4.5. Production
ofprotonated
methane dications
Protonated methane, CH; (CHi-H,), the prototype for five-coordinate carbocations, is known in the gas phase [70] and is reported to be of significance in reactions involving superacids [71] and in electrophilic reactions of methane [72]. At relatively high CH, pressures in our ion source Torr) CHf (5: 10-l were formed by the ion-molecule reaction CH;
+ CH, -+ CH;
+ CH,.
(8)
A prominent peak corresponding to CH:+ was observed and the value of AE,,, was measured to be 21.0 f 0.25 eV. There appears to be no measurement of the ionization energy of the CH, radical [15,73] so deduction of the second ionization energy on the basis of our A E,i, measurements is not possible at present.
5. Summary and conclusions The minimum energy required to ionize singly charged species C’, CH+, CH:, CH: and CH: has been measured by means of high-resolution translational energy loss spectrometry of product dications in collisions of CH,’ (n = l-5) ions with Kr, N,, CH, and laboratory air at impact energies in the 0.5-5 keV range. The single ionization energies measured in these experiments are summarized in table 2. Although our data agrees reasonably well with the charge stripping results of Ast et al. [46] in the case of CH+ and CH:, there are serious discrepancies in the case of CH,+ and CH:. The present measurements failed to reveal any signal corresponding to CH, 2+ dication formation with either atomic or molecular target gases. On the other hand, Ast et al. report a reasonably prominent CH:’ peak in their inelastic energy loss spectrum and deduce an ionization energy of 18.9 eV. Ab initio molecular orbital calculations of Pople et al. (501 also suggest an insignificant lifetime for CH:+. In the case of CH:+ there are a number of different experimental and theoretical results available for comparison (see table 1). Although there is reasonable agreement between the experimental measurements of Spohr et al. [45], Appell [30] and the present experiments, the results of Ast et al. [46] yield a double ionization energy for CH, which is of the order of 10 eV lower. On the theoretical front, the Hartree-Fock calculations of Clementi and Popkie [48] are in reasonable accord with the present data for CH,; ab initio molecular orbital calculations yield double ionization energies which are somewhat lower Table 2 Ionization energies (eV) of C+ and CH; Species
This work
C+ CH’ CH; CH; CH; CH;
24.16 f 0.30 23.07 f 0.30 20.8 kO.25 s, 26.26 rt 0.25 21.0 *0.25
‘) No signal corresponding to CH:’
(n = l-5) ions
Previous work ref. [46] 22.8 19.6 18.9 17.9 _ could be detected.
D. Marhur er al. / Dications o/ CH, In = I - 5)
than the present data but are still higher than the results of Ast et al. Another interesting discrepancy concerns experimental and theoretical results on the formation of CH2+ dications; whilst the calculations of Pople et al. indicate a purely repulsive CH2+ potential energy curve, with no possibility of any bound or metastable dication state, the findings of Ast et al. as well as the present measurements indicate that CH” exists for at least a lifetime of the order of 3 ps. More recent calculations by Wetmore et al. [59] indicate that for states correlating with two singly charged atomic ions at the dissociation limit, the combination of a coulombic repulsive C’-H’ curve with an attractive potential well induced by polarization of H by C2+ can lead to a net potential energy curve in which a potential energy maximum separates the well from the dissociation limit; it appears that such a situation may hold even for zero angular momentum. These calculations, as well as those of Heil et al. [57], therefore indicate that quasi-bound states of CH2+ may well exist for sufficiently long times to be detected in charge stripping experiments. Unfortunately, neither calculation is able to report a value for the double ionization energy. Investigations of doubly charged molecular ions may hitherto have been inhibited due to the general belief that dissociation into two singly charged species would be extremely rapid. Recent evidence indicates that whilst many small dications are thermodynamically unstable towards dissociation, the fragmentation process can encounter considerable activation barriers [33,74]. Charge stripping and other methods have recently led to the observation of a large number of molecular dications, including even small molecular species where coulombic repulsion is expected to be substantial. However, the present experimental techniques appear to suffer from two major drawbacks. Firstly, as has been shown in the present instance of molecular dication formation from species derived from methane, there exist considerable discrepancies between different sets of experimental data. Even in those cases where agreement is reasonably good, the accuracy with which the double ionization energy can be deduced leaves considerable room for improvement. In the present experiments the
45-l
peaks obtained in the inelastic spectra were all rather wide (of the order of 8 eV). Whilst great care was taken to calibrate the energy scale and many measurements were undertaken to check reproducibility, such broad energy widths are known to lead to considerable difficulties in establishing highly accurate onsets for charge striping peaks. This is an area where improvements in experimental technique are called for, particularly in relation to detection sensitivity and instrumental energy resolution. Secondly, none of the present experimental methods is suitable for determining the structural details of the dications detected. Hence, high-level ab initio molecular orbital techniques have to be refined in order that molecular dications can be characterised in a more quantitative and reliable fashion than appears to be possible at present. Acknowledgement We wish to express our appreciation to Professors S.K. Bhattacherjee and SK. Mitra for laboratory facilities and encouragement to pursue these studies. We are most thankful to Professor E. Lindholm for his illuminating comments and suggestions and to Professor R.K. Boyd for useful correspondence.
Note added in proof We have been recently informed that the HAM/3 semi-empirical MO method may be used to calculate energies of doubly charged molecular ions [75]. In a recent monograph [76] Ni+ is studied as an example. For methane [77] in Td geometry the calculated energies are: 37.6 eV for singlet ‘p;’ and 36.9 eV for average 0; ’ (pi I, in good agreement with the values in table 1. For D4,, geometry the HAM energy is 28.9 eV, somewhat smaller that the values in table 1. References [I] P.R. Taylor, Mol. Phys. 49 (1983) 1297. [2] P. Siegbahn, A. Heibexg, B. Roos and B. Levy, Physica Scripta 21 (1980) 322.
458
D. Morhur et 01. / Dicotions oJ CH, (n = I - 5)
[3] F.H. Dorman and J.D. Morrison. J. Chem. Phys. 39 (1963) 1806. [4] R.J. LeRoy and R.B. Bernstein, J. Chem. Phys. 54 (1971) 5114. [5] R.J. LeRoy and W.K. Liu, J. Chem. Phys. 69 (1978) 3622. [6] G.A. &gal, R.W. Wetmore and K. Wolf, Chem. Phys. 30 (1978) 269. [7] R.W. Wetmore, R.J. LeRoy and R.K. Boyd, J. Phys. Chem. 88 (1984) 6318. [8] R.J. Buenker and S.D. Peyerimhoff, Theoret. Chim. Acta 39 (1975) 217. [9] K.F. Freed, Chem. Phys. 3 (1974) 463. [IO] R.W. Wetmore and G.A. Segal, Chem. Phys. Letters 36 (1975)478. [ll] F.L. Mohler, NBS Technical Note 243 (Nat]. Bur. Std.. Washington, 1964). [IZ] B.P. Tsai and J.H.D. Eland, Intern. J. Mass Spectrom. Ion Phys. 36 (1980) 143. 1131 T. Masuoka and J.A.R. Samson. J. Chim. Phys. 77 (1980) 623. [14] G. Dujardin. S. Leach, 0. Dutuit, P.-M. Guyon and M. Richard-Viard, Chem. Phys. 88 (1984) 339. [IS] H.M. Rosenstock, K. Draxl, B.W. Steiner and J.T. Herron, J. Phys. Chem. Ref. Data 6 (1977) 1. 1161 T.D. Mark, F. Egger and M. Cheret, J. Chem. Phys. 67 (1977) 3795. [17] E. Hille and T.D. Mark. J. Chem. Phys. 69 (1978) 2492, 4600. [18] J.A.R. Samson, P.C. Kemeny and G.N. Haddad, Chem. Phys. Letters 51 (1977) 75. 1191 A.S. Newton and A.F. Sciamanna. J. Chem. Phys. 52 (1970) 327. [ZO] V.H. Dibeler, J.A. Walker, K.E. McCulloh and H.M. Rosenstock, Intern. J. Mass Spectrom. Ion Phys. 7 (1971) 209. [21] M.J. Weiss and G.M. Lawrence. J. Chem. Phys. 53 (1970) 214. [22] R. Engel, D. Halpern and B.A. Funk, Org. Mass Spectrom. 7 (1973) 177. [23] P.A. Clark, F. Brogli and E. Heilbronner, Helv. Chim. Acta 55 (1972) 1415. 1241 R. Boschi, J.N. Murrell and W. Schmidt, Faraday Discussions Chem. Sot. 54 (1972) 116. 1251 J.W. Rabalais, L.O. Werme, T. Bergmark. L. Karlson, M. Hussain and K. Siegbahn. J. Chem. Phys. 57 (1972) 1185. 1261 G.M. Begun and R.N. Compton, J. Chem. Phys. 58 (1973) 2271. 127) M.R. Litaow and T.R. Spalding, Mass spectrometry of inorganic and organometallic compounds (Elsevier, Amsterdam, 1973). 1281 C. Benoit and J.A. Horsley, Mol. Phys. 30 (1975) 557. [29] J. Appell and J.A. Horsley, J. Chem. Phys. 60 (1974) 3445. 1301 J. Appell, in: Collision spectroscopy, ed. R.G. Cooks (Plenum Press, New York, 1978) p. 244. 1311 R.G. Cooks, T. Ast and J.H. Beynon. Intern. J. Mass Spectrom. Ion Phys. 11 (1973) 490. 1321 C.J. Proctor, C.J. Porter, T. Ast, P.D. Bolton and J.H. Beynon. Org. Mass Spectrom. 16 (1981) 454.
[33] M. Rabrenovic, C.J. Proctor, T. Ast, C.G. Herbert, A.G. Brenton and J.H. Beynon, J. Phys. Chem. 87 (1983) 3305. [W] D. Stahl and F. Maquin, Chem. Phys. Letters 108 (1984) 613. [35] T.A. Carlson. Photoelectron and Auger spectroscopy (Plenum Press, New York. 1975). 1361 C.D. Wagner, in: Handbook of X-ray and UV photoelectron spectroscopy, ed. D. Briggs (Heyden, Philadelphia. 1977). [37] K. Siegbahn, C. Nordling, G. Johansson, J. Hedman, P.F. Heden. K. Hamrin, U. Gelius. T. Bergmark, L.O. Werme, R. Manne and Y. Boer, ESCA applied to free molecules (North-Holland, Amsterdam, 1971) pp. 62-69. 1381 W.E. Moddeman, T.A. Carlson, M.O. Krause, B.P. Pullen, W.E. Bull and G.K. Schmeitzer, J. Chem. Phys. 55 (1971) 2317. 139) H.D. Beckey, M.D. Migahed and F.W. Roellgen, Intern. J. Mass Spectrom. Ion Phys. 11 (1973) 490. (401 P.A. Redhead, Can. J. Phys. 45 (1%7) 1791. [41] J. Cuthbert, J. Farren and B.S. Prahallada Rao, Proc. Phys. Sot. (London) 9 (1967) 63. 1421 P.C. Cosby, R. Moller and H. Helm, Phys. Rev. A28 (1983) 766. [43] D. Mathur, J. Phys. B13 (1980) 4703. [44] U.T. Raheja, F.A. Rajgara, C. Badrinathan and D. Mathur, J. Phys. B. to be published. 1451 R. Spohr. T. Bergmark, N. Magnusson, L.O. Werma, C. Nordling and K. Siegbahn. Phys. Series 2 (1970) 31. (46) T. Ast, C.J. Porter, C.J. Proctor and J.H. Beynon. Chem. Phys. Letters 78 (1981) 439. 1471 G. Dujardin. L. Hellner, S. Leach and D. Winkoun, Proc. XIV Intern. Conf. Phys. Electronic At. Collisions, Stanford (1985) p. 30. [48] E. Clementi and H. Popkie. J. Am. Chem. Sot. 94 (1972) 4057. [49] P.E.M. Siegbahn, Chem. Phys. 66 (1982) 443. [SO] J.A. Pople, B. Tidor and P. von R. Schleyer, Chem. Phys. Letters 88 (1982) 533. 1511 D. Mathur, C. Badrinathan, F.A. Rajgara and U.T. Raheja, J. Phys. B 18 (1985) 4795. 1521 D. Mathur, in: Atomic and molecular physics, ed. S.C. Mukherjee (Indian Association for the Cultivation of Science. Calcutta, 1984) p. 54. 1531 T. Ast, J.H. Beynon and R.G. Cooks, J. Am. Chem. Sot. 94 (1972) 6611. [SS] C.E. Moore. ed., Atomic energy levels, NBS Circular 467 (Nat]. Bur. Std., Washington, 1971). 1551 P.A. Redhead, Can. J. Phys. 47 (1%9) 2449. 1561 G. Hetzberg and J.W.C. Johns, Astrophys. J. 15B (1969) 399. 1571 T.G. Heil. S.E. Butler and A. Dalgarno. Phys. Rev. A27 (1983) 2365. [SS] S.E. Butler, Phys. Rev. A20 (1979) 2317. [59] R.W. Wetmore, R.K. Boyd and L.J. LeRoy. Chem. Phys. 89 (1984) 329. 1601 H. King and M. Dupuis, J. Chem. Phys. 65 (1976) 111. 1611 G.A. Segal, R.W. Wetmore and K. Wolf, Chem. Phys. 30 (1978) 269.
D. Mathur et al. / Dicarions of CH, (n = 1 - 5) 1621 P.E.M. Siegbahn, J. Almlof, A. Heiberg and B.O. Roos, J. Chem. Phys. 74 (1981) 2384. [63] B.O. Roos, P.R. Taylor and P.E.M. Siegbahn, Chem. Phys. 48 (1980) 157. [64] J.B. Hasted, Physics of atomic collisions, 2nd Ed. (Buttenvorths. London, 1972). [65] G. Her&erg, Can. J. Phys. 39 (1961) 1511. [66] U.T. Raheja, F.A. Rajgara, C. Badrinathan and D. Mathur, J. Phys. B, to be published. [67] B.M. Gimarc, Tetrahedron Letters (1977) 1862. [68] J.B. Collins, P. van R. Schleyer, J.B. Binkley and J.A. Pople, J. Am. Chem. Sot. 98 (1976) 3436. 1691 K. Krogh-Jespersen, D. Cremer, D. Poppinger, J.A. Pople, P. von R. Schleyer and J. Chandrasekhar, J. Am. Chem. Sot. 101 (1979) 4843. 1701 J.L. Franklin, in: Interactions between ions and molecules, ed. P. Ausloos (Plenum Press, New York, 1975) p.1.
459
171) G. Olah. Y. Halpem, J. Shen and Y.K. MO, J. Am. Chem. Sot. 93 (1971) 1251. [72] G. Olah, Y.K. MO and J.A. Olah, J. Am. Chem. Sot. 95 (1973) 4939. 1731 R.D. Levin and S.G. Lias, Ionization potential and appearance potential measurements, NSRDS-NBS 71 (1982). 1741 K. Lammertsma, M. Bat-z&u, G.A. Olah, J.A. Pople, A.J. Kos and P. van R. Schleyer, J. Am. Chem. Sot. 105 (1983) 5252. 1751 L. Asbtink, C. Fridth and E. Lindholm, Intern. J. Mass Spectrom. Ion Phys. 32 (1979) 93. 1761 E. Lindholm and L. Asbrink, Molecular orbitals and their HAM method energies, studied by the semiempirical (Springer, Berlin, 1985). [77] E. Lindholm, private communication.
447
Physics 103 (1986) 447-459
North-Holland,
Amsterdam
TRANSLATIONAL OF MOLECULAR D. MATHUR,
ENERGY LOSS SPECTROMETRY DICATIONS FROM METHANE
C. BADRINATHAN,
F.A. RAJGARA
and U.T. RAHEJA
Ta~a Institute of Fundamenral Research. Homi Bhabha Road Bombay 400005. India Received 28 October
1985
Single electron loss processes have been studied in collisions of Singly charged CHZ and air at impact energies in the 0.5-5 ions the double ionization CHf’
keV range. By means of translational
energies of CH.
could be detected. Comparison Agreement
CH,
and CH,
with Kr. CH,.
N,
of product molecular
molecules have been deduced. No signal corresponding
to
is made between the present results and data obtained by means of Auger spectroscopy.
double charge transfer and charge stripping orbital calculations.
CH,.
ions (for n = l-5)
energy loss spectrometry
in a conventional
between experiment
mass spectrometer
as well as with recent ab initio molecular
and theory as well as between different
experiments
is generally found to
be unsatisfactory.
1. Introduction
A description of collision processes which result in the ejection of two or more electrons from the valence shell of atoms and molecules requires consideration of many-body interactions. For instance, in the case of double ionization of molecules by particle impact, the energetic incident particle interacts with only a single target electron at a time; two-electron ejection in a single collision situation must therefore be regarded as a manifestation of electronic many-body effects. In the case of diatomic and polyatomic molecules, theoretical descriptions of double or multiple ionization are complicated by the existence of a large density of possible electronic configurations which make it difficult to model electronelectron correlations in a quantitative fashion.Even recent advances in quantum-chemical techniques, such as the complete active space self-consistent field [ 1,2] method - which includes several hundred configuration state functions - are found to be of limited scope because of the problems associated with adequate description of correlation effects which can be significant between states of radically different character. Intuitively, a doubly charged molecular ion can 0301-0104/86/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
be visualised as consisting of two singly charged species interacting only via a purely repulsive coulombic force. It may, however, be possible to envisage a situation where a single electron promoted to a strongly bonding orbital in the course of a collision can counteract the coulombic repulsion to the extent that a shallow potential well is created in the potential energy surface describing a doubly charged molecule; this can result in the formation of either a weakly bound or metastable state of the molecular dication. Hypotheses in which perturbations of the potential energy surfaces pertaining to diatomic cations AB2+ by A + B2+ bonding orbitals were first put forward more than twenty years ago [3]. However, in the majority of practical cases there exists a large number of dissociation limits between the asymptote of the charge polarization states A + B2+ and those of lower-lying AB2+ states; as a result, due consideration has to be given to the multiplicity of avoided curve crossings between diabatic strongly bound charge polarization states and coulombic repulsive states. Consequently, potential energy surfaces become highly mixed in character and necessitate recourse to extremely complex molecular wavefunctions. Over the last ten years or so a number of theoretical B.V.
448
D. Math
et al. / Dications of CH,, (n = I - 5)
techniques have been developed which locate and calculate the widths of local minima in molecular dication potential energy surfaces [4-61; such techniques have recently been applied in ab initio calculations [7] of several electronic states of CO*+ using configuration interaction procedures supplemented by perturbation techniques [S-lo] which obviate the need for the usual brute-force methods which are extremely time consuming (and expensive). The earliest survey of multiply charged molecular positive ions was published over twenty years ago [ 111 and listed the appearance energies of over 50 diatomic and polyatomic dications. Since then, molecular dications have been detected in a number of different experimental configurations involving photon impact as well as charged-particle collisions. Doubly charged diatomic and triatomic species produced by HeIcv and He& radiation (12,131 have been observed in photoionization mass spectrometry experiments. It was found that more doubly charged ions relative to singly charged ions are produced by photon impact than by electronic collisions at the same energy, the contrast being most marked in the case of small polyatomic species. Recently, doubly charged SO:+ ions have been produced by monochromatized synchrotron radiation [ 141 and photoion-photoion coincidence experiments have been carried out to enable the study of fragmentation of individual electronically excited states of the parent dication. In the case of electron impact, most dication data has been obtained at 70 eV collision energy in ion sources in commercial mass spectrometers. A survey of the available data indicates that although the electron impact mass spectra of diatomic and polyatomic species indicate the existence of less than 1% dication abundance [15-211, corresponding abundances of up to 20% have been observed in the case of large aromatic hydrocarbons 122-241 and organometallic complexes 125-27). Appearance potentials and dissociation energies of molecular dications have also been measured by other experimental techniques, such as double charge transfer [28-301, charge stripping in double focusing mass spectrometers [31-341, Auger spec-
troscopy [35-381, field ionization [39], ion confinement in an electron beam [40,41] and laser photodissociation [42].
2. Doubly charged methane ions The methane molecule continues to attract much experimental as well as theoretical attention. In spite of this, certain fundamental properties of this molecule remain unclear. For instance, experiments on single ionization and dissociation of CH, by both electrons and photon impact continue to yield data which cannot be unambiguously interpreted [43,44]. In the case of double ionization of methane there also exists conflicting information (table 1). CH:’ ions were first observed by Spohr et al. [45] in an Auger spectroscopic measurement on methane using the ESCA technique [37] where high-energy electrons are used to create a vacancy in the carbon K-shell. The hole in the K-shell is filled by a valence electron and, simultaneously, another valence electron is emitted, and its kinetic energy is measured. The highest KLL Auger line in CH, was determined by Spohr et al. to occur at 250.0 f 0.3 eV; subtracting the carbon K-shell binding energy of 290.7 f 0.7 eV yielded a value of 40.7 eV for the double ionization of CH,. Subsequently, double charge transfer experiments, H’+CH,+H-+CH;+,
0)
were carried out by Appell (301 at 4 keV impact energy, using a double focusing mass spectrometer with a collision cell inserted between the electric and magnetic sectors. A value of 38.9 eV was determined to be the vertical double ionization energy for methane in these experiments. More recently, Ast et al. [46] have carried out charge stripping experiments at 8 keV collision energy, using a commercial reversed geometry double focusing mass spectrometer, and have determined the double ionization energy of CH, to be only 30.6 eV. Most recently, Dujardin et al. [47] have deduced a double ionization energy of 35 eV by studying the dissociation of CH:’ produced by monochromatized synchrotron radiation; by
449
D. Mathur et al. / Dicarions of CH, (n = I - 5) Table 1 Summary of existing information
on the double ionization
energy of methane
Experimental data method
ref.
AE(CH:+
Auger spectroscopy using ESCA double-charge transfer charge stripping photoion-photoion coincidence charge stripping
1451 [301
40.7 38.9b’ 30.6 ” 35.0 38.97 =Jt’
I&l I471 this work
) ‘) (eV)
Theoretical calculations geometry
method
ref.
AE(CH:+
Td
Hartree-Fock Hartree-Fock SCF, MC SCF and MC CI MO using 6-31 G* basis set
I481 I491 I491
38.8 singlet I$;’ 36.9 triplet 41;’ I&’ 32.3 32.4
D 4h ‘) b, ‘) d,
PO1
) ‘) (ev)
Appearance energy of doubly charged methane ions. kO.7 eV. Assumes a single ionization energy for CH, of 12.7 eV. * 0.25 eV.
means of the photoion-photoion coincidence technique the lowest-energy dissociation channel has been identified as hv + CH, -+ CH;’
+ CH_; + H’.
(2)
The wide discrepancy in the experimental results is further emphasized by the differing predictions of the lifetime for CH:‘. The Auger spectroscopy experiments [45] indicated an extremely short lifetime, which is also in conformity with the lack of observation of the CH:+ ion in any ordinary mass spectrum. On the other hand, ob. servation of CH:+ tons in charge stripping experiments implies that the ion survives for at least the transit time from the collision chamber to the ion detector, which is of the order of a few ps. In an attempt to resolve the discrepancy and to measure the double ionization energies of other related simple hydrocarbons, we have carried out high-resolution translational energy loss measurements of doubly charged product molecular ions resulting from charge stripping collisions of the type CH,: + X + CHZ’
+ X + e - AE,
(3)
where n = l-5. X represents atomic and molecular neutral target gases and AE is the energy defect which is a measure of the single ionization energy of CH,;
3. Experimental
apparatus and procedures
Measurements of the energy defect, AE, were carried out in an ionic collisions apparatus (fig. 1) which has been briefly described in a recent report on state-diagnosed single electron capture cross section measurements [51] in collisions of ground state Kr2+ with Hz. Briefly, the ion source used is a low-voltage arc type of source capable of producing ions from gaseous species as well as alkali metals; the latter case uses a built-in oven capable of reaching temperatures of the order of 500°C. In the present case, singly charged CH,: ions from research grade methane (Mathesons) were extracted by a three-element cylindrical electrostatic lens at energies in the 0.5-5.0 keV range, and focused onto the entrance plane of a region of crossed electric and magnetic fields (Wien filter), where separation on the basis of charge-to-mass ratio occurred. The Wien filter was chosen in preference to the more usual sector devices because of its relatively simple in-line geometry coupled with flexibility of easily variable dispersion. Filtered ions then passed through a double differentially pumped static gas target maintained at room temperature. The gas pressure in the collision zone was monitored by a capacitance manometer (Leybold-Membronovac). In the post-colli-
D. Mathur er al. / Dications of CH, (n = I - 5)
450
DPl
DP2
IG
Fig. 1. Schematic diagram of ICE - ionic collisions experiment. IS: ion source, CH: solid charge holder, L: cylindrical electrostatic lenses, D: electrostatic deflection plates, WF: Wien filter, CC: collision chamber, CM: capacitance manometer, IP: triode ion pump BPM: movable beam profile monitor, PP: electrostatic parallel plate analyzer, CEM: channel electron multiplier, MCP: microchannel plate array, IG: ionization gauge, DP: oil diffusion pump.
sion region the scattered ions were analysed by an electrostatic parallel plate analyzer. Ion detection was by means of a channel electron multiplier operating in the particle counting mode using conventional pulse handling techniques. The ion source, lenses and mass filter were housed in a bakeable stainless steel chamber pumped by an oil diffusion pump to a base pressure of 5 x lo-* Torr. Typical operating pressures were of the order of 5 X lo-’ Torr with gas and heat load, and at an ion source pressure of = 5 X lo-* Torr. The pressure in the gas cell was maintained in the 10e4 Torr range, well within “single collision conditions”. In order to quickly pump away collision chamber gas effusing from the exit slit of CC (see fig. 1) a 20 / s-’ triode ion pump was positioned a few centimetres from the CC exit slit. In conjunction with a second oil diffusion pump, the post-collision vacuum chamber was maintained at a pressure of = 1 x lo-’ Torr with ion beam and gas load. The post-collision chamber also had a moveable beam profile monitor and a microchannel plate detector assembly (BPM and MCP in fig. 1); these items were not used in the present measurements except for preliminary operating tests concerned with beam intensities, shapes and beam attenuation measurements to determine the presence, or otherwise, of long-lived electronic excited states in the incident ion beam (51,521. The apparatus was controlled on-line by means of a microprocessor controlled multichannel analyzer via an interface comprising, amongst other units, a voltage ramp generator whose output was
used to control various programmable power supplies. During measurement of energy loss spectra, a voltage ramp was applied across the parallel plate analyser and the output pulses of the channel electron multiplier were synchronously stored in memory on a multiple scanning basis. After scanning the analyser voltage a sufficient number of times to yield an adequate signal-to-noise ratio, data was transferred to another microcomputer for off-line analysis and a floppy-disc hard copy. Typical singly charged ion current at the entrance of the collision zone was lo-’ A. The elastic peak to background ratio following analysis by the parallel plate analyzer was better than 10’ (typically = 2500); background counts were generally non-fluctuating over very wide energy ranges. The angular resolution in the present measurements was 0.14 radian. The following procedure was adopted for determining the energy defect, AE. If the incident CH,+ ions were transmitted through the energy analyzer when a voltage U was applied across the parallel plates, the corresponding doubly charged ions of the same kinetic energy would be transmitted at a voltage (O.SU - AU), where AU is related to the energy defect in (1) by a factor determined by the geometry of the analyzer. In the present measurements U was taken to be the voltage at the onset of the singly charged peak. By measuring the voltage displacement of the onset of the doubly charged peak from O.SU, one can determine the minimum energy defect, AE,,,i,,, and hence the energy threshold for ionizing a singly charged ion. There are a number of assumptions
D. Marhur et al. / Dicarions of CH, (n = I - 5)
implicit in such a deduction; these have been extensively discussed elsewhere (31). The accuracy and reproducibility of the energy defect measurements were largely determined by the accuracy with which the accelerating voltage could be measured and its stability. The power supply used for applying the acceleration voltage (Spellman Model RHR-10) is stated to have a stability of better than *0.15 V at 3 kV. A programmable power supply of similar specifications was used to scan the analyzer voltage; the error and non-linearity introduced by the computer-controlled ramp generator used for programming this power supply were negligible. Voltage measurement was by means of a 4$ digit voltmeter coupled to a precision resistance chain. Peaks corresponding to doubly charged ions formed by electron loss collisions were all found to have rather sharp onsets and produced tailing on the high-AE side. Moreover, this onset was relatively unaffected by the nature and pressure of the collision gas. Although the signal-to-noise characteristics of the spectra presented in the following section are not good, and CHi+ peaks observed were of the order of 10 eV wide, the position of the relatively sharp onset was found to be usually reproducible to within 0.5 eV on most occasions over a period of more than six months. This method of determining onset energies has been used in the ion kinetic energy experiments of Ast et al. [53] before.
spectroscopically determined [54] value for the ionization energy of He+ (53.403 eV) to yield a difference of 0.37 eV between our data and the spectroscopically defined value of AE,,,. We attribute this difference to contact potentials and use it as a correction factor in all the values of AE,,, reported below. In order to obtain some quantitative information on the systematic errors in our experimental arrangement we have carried out single electron stripping experiments on C’ ions obtained when CH, was put in the ion source. The process studied was C’+X+C”+X+e-AE,
4.2. Production of CH dications
CH’+Kr+CH”+Kr+e-AE
4.1. Calibration of the energy loss scale In order to calibrate the energy scale to account for any contact potentials and electric field nonuniformities in the vicinity of the energy analyzer, a small amount of research grade helium was leaked together with methane into the ion source and AE,i” measured for the electron loss process He’+Kr+He2++Kr+e-AE.
(4)
The energy loss spectrum obtained at 3 keV impact energy is shown in fig. 2. The measured onset for the He’+ inelastic peak was compared with the
(5)
where the collision chamber gas X was Kr, CH, and laboratory air. A typical energy loss spectrum obtained at 3 keV impact energy with Kr collision gas is shown in fig. 3. The average value of A E,,, was 24.76 f 0.30 eV. This may be compared with the known ionization energy of C’ (24.383 eV) established by ultraviolet spectroscopy [54]. The difference of = 0.4 eV is probably a representation of the order of systematic error in the present measurements. The only other threshold data appears to be the ion trap measurement of Redhead [55] on ionization of confined C’ ions, which yielded a value of A E,,, of 31.0 eV.
A typical energy loss spectrum 4. Results and discussion
451
for the collision (6)
is shown in fig. 4 for an impact energy of 3 keV. CH,, N, and laboratory air were also used as target gases; although the ratio of the abundance of the dication to that of the monocation varied with different coliision gases, the value of AE,,, was found to maintain a constant value of 23.07 eV within a reproducibility of f0.30 eV. Taken in conjunction with the spectroscopically determined [56] ionization energy of CH (10.64 + 0.01 eV) we obtain a value of 33.71 f 0.3 eV for the double ionization of CH. The singly charged CH’ ion has an electronic configuration (~IJ)~(~u)~(~u)‘. Ejection of an electron from the outermost 30 orbital yields an electronic configuration and overall sym-
D. Mathur et al. / Dications of CH, (n = I - S)
452
I
He’
tit!+.
3 J,
I
5
ii
I He’ +
iI)848
X-He
++
+
x
856
064
012
l
e
-J 1790
1780
ENERGY ANALYZER VOLTAGE Fig. 2. Energy -_ loss spectrum showing electron loss from He the onset for He’+
l
1800
(Volts)
ions colliding with Kr at 3 keV impact energy. Solid vertical line marks
iormation.
metry of (1~)~(2a)~(3e), 2Z+ for the ground state of CH2+. The only other experimental data is from the charge stripping measurements of Ast et al. [46] which yield a double ionization energy of 33.4 eV which is in excellent agreement with the present result. Theoretically the first investigation into the structure and stability of CH2+ dications was by Pople et al. [50]. Using ab initio molecular orbital methods with both the 3-21G as well as the 6-31G* basis sets, they showed that there is no minimum in the Hartree-Fock potential energy curve for CH’+. They concluded that the ground state potential curve is purely repulsive and that dissociation into C+ and H’ proceeds extremely rapidly. Interestingly, the ionization energies of neutral C and CH calculated by Pople et al. using the same method and basis sets yields values which are in good agreement with experimental data. Ab initio calculations of potential energy curves of three zZ+ states of CH+ leading to dissociation into
C+(2D)+Hf, C+(‘P”)+H+ and C2’+ H have also been carried out by Heil et al. [57] using configuration interaction methods [58] and, later, by Wetmore et al. [59] who carried out restricted Hartree-Fock SCF calculations (601 on the ground state of CH+ in order to generate the molecular orbitals used in subsequent configuration interaction calculations [10,61]. Both sets of ab initio calculations indicate that the predominantly repulsive CH2+ potential energy curves are seriously perturbed by quasi-bound states. As a result of avoided curve crossings between the C’--H’ repulsive potential energy curve and the C2+-H curve which has a significant attractive component due to the strong attraction of the hydrogen atom through polarization by the doubly charged carbon atom, small potential dips are created which may support a metastable state of CH2+. Unfortunately, the question whether the depth of such potential minima are sufficient to support at least one vibrational level of the CH2+ molecular ion
D. Mathur el al. / Dications of CH, (II = I - 5)
453
CH+
CH*
t Kr -+
CH"
l
Kr t e
r*+
L
A
x
I
870
I
880
I
890
64
I
i
/
1,/----J--l
L
1790
sod’
ENERGY ANALYZER VOLTAGE
1800
(Volts)
Fig. 3. Energy loss spectrum showing electron loss from C+ ions colliding with Kr at 3 keV energy. Onset for C2+ formation is indicated by the solid vertical line.
cannot be answered by current initio methods of generating curves.
state-of-the-art ab potential energy
4.3. Production of CH, and CH, dications The triatomic dication CH:+ possesses Da,, symmetry and, according to the molecular orbital calculations of Pople et al. (501, should have a long lifetime. The ionization energy of CH: is computed by these authors to be 21.1 eV. This dication has also been theoretically investigated by Siegbahn [49] who has carried out large basis set MC SCF and CI calculations using the complete active space SCF method [62,63]. This technique has yielded a CH: ionization energy of 21.0 eV. In our present experiment, CH:’ ion formation was observed for Kr, CH,, N, and laboratory air collision gases and a value of AE,,, of 20.8 f 0.25 eV was measured. This compares very well with both the theoretically deduced values. The only other experimental data is from the charge stripping experiments of Ast et al. [46] who obtained a considerably smaller A E,i, value of 19.6
840
860
880
1790
ENERGY ANI\LVZER VOLTAGE
1eocl (Volts)
Fig. 4. Typical energy loss spectrum showing single electron loss from CH+ ions colliding with Kr at 3 keV incident energy. Solid vertical line indicates the onset for CH2+ formation. k 0.3 eV. A possible explanation for this discrepancy might he in the fact that the double focusing mass spectrometer used by Ast et al. utilises an oscillating electron impact type of ion source whereas in our apparatus the incident ions are produced in a low-voltage arc type of ion source. In the latter type of source ions are extracted from a relatively low-temperature plasma and it is known that electronically and vibrationally “cool” ions are produced. The conventional electron impact source, on the other hand, is known to produce a mixture of ground and metastable ions with a fairly broad vibrational level population distribution [64]. The lower value of A E,,, obtained by Ast et al. may therefore be a reflection of the fact that the incident CH,’ ions emanating from their ion source are vibrationally excited. Taken in conjunction with the spectroscopically determined value for the single ionization energy of the CH, radical [65] of 10.396 f 0.003 eV, our present data yields a value of 31.20 * 0.25 eV for the double ionization energy of CH,. In the case of the methyl dication CH:+ there is an interesting disagreement between the theoret-
D. Mathw et al. / Dications of CH, (n = I - S)
454 CH,,+ +
86h
868
X -CH
872
l
4
+
+
876
X
+
e
880 ”
1790
ENERGY ANALYZER VOLTAGE
la00
(Volts)
Fig. 5. Energy loss spectrum following collision of 3 kcV CH: ions with Kr. The width of the singly charged molecular peak is of the order of 7.5 eV fwhm. Solid vertical line marks the onset for CH:+ formation.
ical predictions of Pople et al. (501 and the observations of Ast et al. [46]. According to the calculations of Pople et al., the energy depth of a potential energy surface minimum in the CH:+ which could support the formation of the dication through such a is only = 150 meV; tunnelling small potential barrier will be expected to occur extremely rapidly. Consequently, Pople et al. conclude that CHj’ should have an insignificant lifetime. On the other hand, Ast et al. report clear evidence for a CH:’ peak in their charge stripping spectrum and obtain a AE,,, value of 18.9 It 0.3 eV. In our present measurements we have attempted to locate the CH:’ peak in energy loss spectra measured with both atomic as well as molecular target gases, and at CH; impact energies ranging from 1 keV to 5 keV; however, no peak corresponding to the methyl dication could be detected. There appear to be two possible explanations: either the lifetime of the CH:+ ions is considerably shorter than the 3.3 f 0.17 t~s transit time for 3 keV ions to travel from the collision chamber to the detector in our apparatus or the electron loss cross section even at our highest impact energy (5 keV) is extremely small. The former explanation is favoured by the conclusions
reached by Pople et al. The latter explanation appears to be unlikely as the measurements of Ast et al. were also conducted at 5 keV energy and the detection sensitivity in the two experiments is unlikely to be vastly different; this is borne out by the fact that the relative cross section for charge stripping in the case of CH’ in the experiment of Ast et al. is measured to be nearly a factor of 20 less than in the case of CH; and our experiments are clearly able to detect the CH” dication. 4.4. Production of CH, dications A typical inelastic spectrum produced by multiple scans of the parallel plate analyzer voltage is shown in fig. 5 for the collision CH:
+Kr+CH:’
+Kr+e-AE.
(7)
The energy defect, AEmin, is found to be 26.26 k 0.25 eV. In a preliminary report [66] on the CH:’ dication we have presented details of the precautions taken to ensure that the data presented in fig. 5 pertains only to electron loss from CH: ions and not impurity 0 + ions possessing the same mass-to-charge ratio. The electronic configuration of ground state methane in tetradral symmetry group notation is
D. Muthur et al. / Dicarions of CH, (n = I - 5)
(la,)‘(2a,)2(1t2)h, ‘A,. Removal of a single electron from the most loosely bound 1 t z orbital breaks its triple degeneracy; more than one final state becomes possible and hence the onset for single ionization is not sharp. High-sensitivity crossed electron-molecular beam measurements [43] using monoenergetic electrons have yielded a value of 12.71 eV as an upper limit for the first vertical ionization energy. Taken in conjunction with our measured value of AE,,, we obtain a value of 38.97 f 0.25 eV for the double ionization energy of CH,. Removing two electrons from the 1 t, orbitals will give rise to ‘A,, ‘E,, ‘T, and ‘T, states of CH:’ . However, the present measurements do not enable unambiguous identification of the overall symmetry of this dication. Hartree-Fock calculations 1481 have yielded values of 38.80 and 39.11 eV for formation of CH:’ into singlet and triplet states, respectively. It has been established [67-691 that two-electron removal from molecules with six valence electrons possessing the general formula AH, results in a dication which has a lower energy in the square planar geometry (D,,, symmetry) rather than a tetrahedral shape. Despite the large exoergicity of the reaction leading to proton loss, the ab initio MO calculations of Pople et al. [50] indicate a large potential barrier towards dissociation. Consequently, a long lifetime is predicted for CH$‘, and a value of 19.7 eV is calculated for AE ,,,,,,. Siegbahn’s SCF, MC SCF and MC Cl calculations [49] also predict a value of 19.6 eV for AE ,,,,,,. A summary of the experimental and theoretical is shown in table 1. It is information on CH:’ seen that although the Auger spectroscopy data of Spohr et al. [45] and the double charge transfer results of Appell et al. [30] are in reasonable accord with the present measurements; the recent photoionization experiments of Dujardin et al. [47], using synchrotron radiation, yield a value for double ionization energy which is = 3 eV lower than our value. The electron loss process in ion-atom collisions involves essentially the same type of Franck-Condon transition as in electron impact ionization of molecules. Thus, ionization energies deduced from electron stripping collisions may be considered to
455
be vertical rather than adiabatic in nature insofar as the existence of a bound state of the methane dication is assumed. The difference between the photoionization data and the present result may therefore be a reflection of the difference between the adiabatic and vertical double ionization energy. In making such a comparison, the following pertinent points need to be considered. In principle, a threshold experiment measures the lowest possible energy for which a particular inelastic process has a non-zero transition moment. In practice, however, technical constraints (such as limited sensitivity) lead to a situation where the observed signal is largely due to inelastic processes which have maximum probability, usually a transition from a classical turning point rather than an adiabatic one. When onsets are estimated by extrapolation of the dication signal from its value at some energy which is higher than the threshold energy to zero level, the result is some indeterminate compromise which is taken account of in the energy calibration procedure only if the shapes of the threshold curves are the same for the calibrant as for the dication of interest. In the present measurements we have attempted to minimise these problems by avoiding use of extrapolation techniques and, instead, adopting the use of the vanishing current method using singleparticle counting. Attention has already been drawn to the charge stripping data of Ast et al. [46] who obtained a very low value of 17.9 eV for AE,,, (table 1). Siegbahn has attempted to explain this low A E,,, value by considering the difference in geometry between neutral and doubly ionized methane. It is shown that square planar (D4,,) CH:’ has a much lower energy than tetrahedral (Td) CH: + . The energy difference between the Td and D,,,, geometries is calculated to be much higher than the barrier for dissociation into CH: + H+. which is found to be = 1 eV. Due to the different geometry, the Franck-Condon factors between T,, CH, and D,, CH:’ would be almost zero and, hence, D4,, structure would -not be observed in Auger spectroscopy experiments. Siegbahn therefore concludes that the results of Spohr et al. pertain not to the ground state of the methane dication but to some excited state which lies roughly 10 eV higher
D. Marhw et al. / Dicutions of CH, (n = I - 5)
456
in energy. In the case of charge stripping experiments, on the other hand, Siegbahn postulates that the excess internal energy obtained upon ionization of CHf ions can be released in collisions with other molecules so that the dication formed can adiabatically shift to square planar geometry where it has a long lifetime. In the light of the present results, and the earlier double charge transfer measurements of Appell [30], it appears less likely that geometrical differences can account for the low double ionization energy obtained by Ast et al. In any case a discrepancy still exists with the double ionization energy calculated specifically for D4,, geometry to an accuracy which is claimed to be better than 0.3 eV [49,50]. Another possible explanation in terms of internal vibrational excitation of CH: appears unlikely in view of the rather large difference in the AE,,, value between the results of Ast et al. and the other experiments (table 1). although the possibility that the incident ions produced in the low-pressure ion source used by Ast et al. (presure = typically 2 x 10m6 Torr) were electronically excited to metastable states cannot be excluded. In contrast, the ion source pressures in our experiments (typically low2 Torr) are such that the incident ions are likely to be electronically and vibrationally cool. 4.5. Production
ofprotonated
methane dications
Protonated methane, CH; (CHi-H,), the prototype for five-coordinate carbocations, is known in the gas phase [70] and is reported to be of significance in reactions involving superacids [71] and in electrophilic reactions of methane [72]. At relatively high CH, pressures in our ion source Torr) CHf (5: 10-l were formed by the ion-molecule reaction CH;
+ CH, -+ CH;
+ CH,.
(8)
A prominent peak corresponding to CH:+ was observed and the value of AE,,, was measured to be 21.0 f 0.25 eV. There appears to be no measurement of the ionization energy of the CH, radical [15,73] so deduction of the second ionization energy on the basis of our A E,i, measurements is not possible at present.
5. Summary and conclusions The minimum energy required to ionize singly charged species C’, CH+, CH:, CH: and CH: has been measured by means of high-resolution translational energy loss spectrometry of product dications in collisions of CH,’ (n = l-5) ions with Kr, N,, CH, and laboratory air at impact energies in the 0.5-5 keV range. The single ionization energies measured in these experiments are summarized in table 2. Although our data agrees reasonably well with the charge stripping results of Ast et al. [46] in the case of CH+ and CH:, there are serious discrepancies in the case of CH,+ and CH:. The present measurements failed to reveal any signal corresponding to CH, 2+ dication formation with either atomic or molecular target gases. On the other hand, Ast et al. report a reasonably prominent CH:’ peak in their inelastic energy loss spectrum and deduce an ionization energy of 18.9 eV. Ab initio molecular orbital calculations of Pople et al. (501 also suggest an insignificant lifetime for CH:+. In the case of CH:+ there are a number of different experimental and theoretical results available for comparison (see table 1). Although there is reasonable agreement between the experimental measurements of Spohr et al. [45], Appell [30] and the present experiments, the results of Ast et al. [46] yield a double ionization energy for CH, which is of the order of 10 eV lower. On the theoretical front, the Hartree-Fock calculations of Clementi and Popkie [48] are in reasonable accord with the present data for CH,; ab initio molecular orbital calculations yield double ionization energies which are somewhat lower Table 2 Ionization energies (eV) of C+ and CH; Species
This work
C+ CH’ CH; CH; CH; CH;
24.16 f 0.30 23.07 f 0.30 20.8 kO.25 s, 26.26 rt 0.25 21.0 *0.25
‘) No signal corresponding to CH:’
(n = l-5) ions
Previous work ref. [46] 22.8 19.6 18.9 17.9 _ could be detected.
D. Marhur er al. / Dications o/ CH, In = I - 5)
than the present data but are still higher than the results of Ast et al. Another interesting discrepancy concerns experimental and theoretical results on the formation of CH2+ dications; whilst the calculations of Pople et al. indicate a purely repulsive CH2+ potential energy curve, with no possibility of any bound or metastable dication state, the findings of Ast et al. as well as the present measurements indicate that CH” exists for at least a lifetime of the order of 3 ps. More recent calculations by Wetmore et al. [59] indicate that for states correlating with two singly charged atomic ions at the dissociation limit, the combination of a coulombic repulsive C’-H’ curve with an attractive potential well induced by polarization of H by C2+ can lead to a net potential energy curve in which a potential energy maximum separates the well from the dissociation limit; it appears that such a situation may hold even for zero angular momentum. These calculations, as well as those of Heil et al. [57], therefore indicate that quasi-bound states of CH2+ may well exist for sufficiently long times to be detected in charge stripping experiments. Unfortunately, neither calculation is able to report a value for the double ionization energy. Investigations of doubly charged molecular ions may hitherto have been inhibited due to the general belief that dissociation into two singly charged species would be extremely rapid. Recent evidence indicates that whilst many small dications are thermodynamically unstable towards dissociation, the fragmentation process can encounter considerable activation barriers [33,74]. Charge stripping and other methods have recently led to the observation of a large number of molecular dications, including even small molecular species where coulombic repulsion is expected to be substantial. However, the present experimental techniques appear to suffer from two major drawbacks. Firstly, as has been shown in the present instance of molecular dication formation from species derived from methane, there exist considerable discrepancies between different sets of experimental data. Even in those cases where agreement is reasonably good, the accuracy with which the double ionization energy can be deduced leaves considerable room for improvement. In the present experiments the
45-l
peaks obtained in the inelastic spectra were all rather wide (of the order of 8 eV). Whilst great care was taken to calibrate the energy scale and many measurements were undertaken to check reproducibility, such broad energy widths are known to lead to considerable difficulties in establishing highly accurate onsets for charge striping peaks. This is an area where improvements in experimental technique are called for, particularly in relation to detection sensitivity and instrumental energy resolution. Secondly, none of the present experimental methods is suitable for determining the structural details of the dications detected. Hence, high-level ab initio molecular orbital techniques have to be refined in order that molecular dications can be characterised in a more quantitative and reliable fashion than appears to be possible at present. Acknowledgement We wish to express our appreciation to Professors S.K. Bhattacherjee and SK. Mitra for laboratory facilities and encouragement to pursue these studies. We are most thankful to Professor E. Lindholm for his illuminating comments and suggestions and to Professor R.K. Boyd for useful correspondence.
Note added in proof We have been recently informed that the HAM/3 semi-empirical MO method may be used to calculate energies of doubly charged molecular ions [75]. In a recent monograph [76] Ni+ is studied as an example. For methane [77] in Td geometry the calculated energies are: 37.6 eV for singlet ‘p;’ and 36.9 eV for average 0; ’ (pi I, in good agreement with the values in table 1. For D4,, geometry the HAM energy is 28.9 eV, somewhat smaller that the values in table 1. References [I] P.R. Taylor, Mol. Phys. 49 (1983) 1297. [2] P. Siegbahn, A. Heibexg, B. Roos and B. Levy, Physica Scripta 21 (1980) 322.
458
D. Morhur et 01. / Dicotions oJ CH, (n = I - 5)
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