Mass Spectrometry Volume 8

A Review of the Recent Literature Published between July 1982 and June 1984

By M. E. Rose

The Royal Society of Chemistry

Copyright © 1985 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-328-3

Contents

Chapter 1 Ionization Processes and Ion Dynamics By I. Powis,
Chapter 2 Structures and Reactions of Gas-phase Organic Ions By M. A. Baldwin,
Chapter 3 Photoelectron-Photoion Coincidence Spectroscopy By J. Dannacher and J.-P. Stadelmann,
Chapter 4 Developments and Trends in Instrumentation By T. R. Kemp,
Chapter 5 Applications of Computers and Microprocessors in Mass Spectrometry By J. R. Chapman,
Chapter 6 Fourier-transform Ion Cyclotron Resonance By N. M. M. Nibbering,
Chapter 7 Reactions of Organic Negative Ions in the Gas Phase By J. H. Bowie,
Chapter 8 Fast-atom-bombardment Mass Spectrometry: Applications to Solution Chemistry By R. M. Caprioli,
Chapter 9 Gas Chromatography/Mass Spectrometry and High-performance Liquid Chromatography/Mass Spectrometry By M. E. Rose,
Chapter 10 Drug Metabolism, Pharmacokinetics, and Toxicity By D. J. Harvey,
Chapter 11 Metal-containing and Inorganic Compounds Investigated by Mass Spectrometry By J. Charalambous,

CHAPTER 1

Ionization Processes and Ion Dynamics

BY I. POWIS

1 Introduction

Research into the basic processes in which ions are produced and interact continues to be an active and expanding area. Traditional methods for the generation and monitoring of ions are being extended and complemented, most notably by the introduction of new sources of ultraviolet radiation and molecular-beam technology. With such experimental advances new questions can be addressed and old problems tackled in fresh ways.

For the reviewer, however, the old problem of presenting a critical account of developments that are embodied in a large number of publications remains to be tackled, as before, by a selective citation of the literature. My objective, therefore, has been to identify work in which fundamental aspects of the behaviour of molecular ions are featured. Whatever criteria one employs, difficult choices remain to be made, and it is reasonable to acknowledge that in such circumstances the ultimate arbiter is perhaps a personal preference.

2 Ionization Processes

Molecular Photo ionization. — Of all the processes leading to positive-ion formation, photoionization is the best able to be characterized at a fundamental level in a general manner applicable to all systems. The pursuance of this aim is a vigorous, developing field at present, as regards both theoretical and experimental advances. Apart from the immediate advantage of being able to model photoionization processes such as are encountered in a wide variety of situations, photoionization represents the simplest electron-scattering process and is of considerable intrinsic interest. Studies of photoionization provide an important means for the investigation of molecular electronic structure and dynamics.

An idealized experimental objective would be the measurement of partial cross-sections for the production of specified quantum states of the molecularion products and the electron, as a function of photon energy and relative orientations. Indeed, most of these requirements are currently feasible and have been attained, at least for some specific systems. A full description of the photoelectron includes its spin polarization, and whilst this has previously been considered in atomic systems it is just now receiving attention in connection with molecular systems including, recently, the case of chiral molecules. Most attention, however, focuses upon the various resonances that appear with changing photon wavelength in measured cross-sections and, often even more markedly, in the photoelectron angular distributions. Although pseudo-photon dipole (e, 2e) experiments have been used to establish a substantial body of absolute photoionization cross-section data and continue to be used for systems such as HF and HCl, the bulk of recently published experimental data has been obtained using tunable synchrotron radiation sources. It is the consequent ability to scan continuously from below threshold up to photon energies of ~100eV with high intensity which allows the rich dynamical structure, apparent with highly differentiated cross-section measurements, to be uncovered.

Concomitant with the growth in quantity and quality of experimental data is the development of theoretical treatments for the understanding of such processes. The relationship between theory and experiment here appears to be particularly close, with approximately simultaneous treatments for a given system being quite common. Naturally enough, first-row diatomics such as HCl, N2, and O2 and triatomics such as N2O, CS2, and COS receive attention as theoretically tractable systems, but increasingly larger polyatomics can be tackled in this manner: examples include acetylene, cyanogen, ethylene, and even benzenoid compounds.

Photoionization resonances and structures are conveniently considered as either one-electron phenomena, such as shape resonances (i.e. quasi-bound states where an electron is temporarily trapped by a potential barrier), or two-electron phenomena such as autoionization (where photoionization takes place via excitation into a discrete bound state lying above an ionization threshold followed by decay into the ionization continuum).

As with other photoabsorption processes, photoionization cross-sections depend upon dipole transition amplitudes between initial and final states. The final-state wavefunction describes the continuum electron orbital, and herein lies the computational challenge. Although in principle obtainable from the solution of the Schrödinger equation using a static-exchange potential (i.e. at the Hartree–Fock level of approximation, appropriate for the description of one-electron phenomena), significant difficulties arise in the calculation in this manner of the continuum function. This is due primarily to the non-central nature of the potential in a non-spherical molecule and the inapplicability of the algebraic variational methodology commonly applied to bound-state problems.

One of the more generally successful approaches has been the multiple-scattering method (MSM), in which these difficulties are circumvented by representing the molecular potential as a cluster of spherical potentials positioned on the atomic sites. The MSM also often employs a local exchange approximation. A recent extension to this approach covers the evaluation of the interstitial integrals encountered with such a model potential. The MSM potential is clearly very approximated; nevertheless the MSM formalism is routinely capable of producing qualitatively correct predictions of such features as shape resonances. It thus fulfils its aim of providing a valuable tool for exploratory investigation of these phenomena. A particular advantage of MSM at the present time is its applicability to large molecules such as benzene. In this particular example calculated asymmetry parameters were found to agree with experiment for the first two ionizations, (1e1g)-1 and (3e2g)-1. A disagreement for the third (la) channel was ascribed to vibronic mixing effects, which are not reproduced in the calculations.

Thiel has demonstrated how shape resonances may be visualized by means of radial-density plots derived from MSM continuum functions, using the well known σu resonances in the N2+X2Σ+g and CO2+ [??]2Σ+g ionizations as examples. The results for CO2 (Figure 1) show dramatically enhanced densities in the l = 3 or 5 partial-wave components of the σu channel at the resonant photon energy, close to the atomic centres. Similar plots have been presented for the ionization of cyanogen. The nature of these radial-density plots at resonance, with clear nodal structure and localized around the atoms, is suggestive of discrete valence-like orbitals. Thiel’s work demonstrates strong correlations between the resonant continuum functions and the unoccupied antibonding virtual orbitals that are obtained from minimal-basis-set MO calculations. Other results for ethylene suggest, though, that the correlation may not always be so good as to enable simple predictions of the occurrence and relative energy of shape resonances to be made without resort to full scattering calculations.

MSM calculations provide a good qualitative and even semi-quantitative guide to the occurrence of shape resonances. However, they tend to predict exaggerated features with too narrow a width, as in the (7σ)-1, (6σ)-1 ionizations of N2O. The development of Hartree–Fock level computations capable of quantitative agreement with experiment is therefore of some interest. One such procedure is the moment-theory approach in which the electronic Hamiltonian is diagonalized in a large augmented basis set, using standard L2 methods. The many discrete virtual orbitals lying in the continuum region of the spectrum that are thus obtained are not physical but may be smoothed, typically using Stieltjes or Tchebycheff moment theory, although other procedures are being evaluated. While these calculations may readily be carried out in the static-exchange approximation, they yield no information concerning angular distributions, since the scattering equations are not solved. Moreover, the cross-sections so obtained may be unduly sensitive to the choice of underlying discrete pseudo-spectrum; Γchebycheff results for NO, for example, appear to show spurious resonance structure that is not found in other calculations.

Another approach, potentially the most reliable at the Hartree–Fock level of approximation, is to solve for the continuum function using single-centre expansion techniques. Although these methods are at present only practicable for small molecules such as N2, NO, and CO, the Schwinger variational treatment of Lucchese and McKoy has been successfully applied to linear molecules like CO2 and HCCH. An adaptation of the Schwinger method promises to be particularly useful in the case of highly polar molecules.

A one-electron phenomenon, other than shape resonance, that is well known in atomic photoionization is the Cooper minimum. This arises when the matrix element describing the transition into one of the dominant continuum channels changes sign. A distinct minimum in the net cross-section can result as this element passes through zero. The effect on the asymmetry parameter, β, may be even more striking. Consider ionization of the 3p orbital of argon. Dipole selection rules permit s and d photoelectron channels, the latter of which changes sign at the Cooper minimum. The d channel will display anisotropy, and at the minimum only the isotropic s channel contributes to the net ionization, causing the β parameters to fall markedly to zero. Arguing that the lone-pair orbitals of S and Cl should behave similarly, Carlson et al. have now obtained experimental evidence of Cooper minima in the molecular photoionization of CS2, CCl4, Cl2, and HCl. Figure 2 shows how the asymmetry parameters for ionization of CCl4 are affected at the Cooper minimum. For all these molecules MSM-type calculations give a good qualitative account of the phenomenon. It is remarkable that even the non-lone-pair orbitals examined (5σg of Cl2, 5σ of HCl, 6t2 and 6a1 of CCl4) show partial Cooper minima at the appropriate photoelectron energies.

In general the electric-dipole selection rules operative in photoionization will permit several continuum channels of alternative symmetries to arise in production of a given ion state. These will be degenerate and hence cannot be distinguished by photoelectron spectroscopy alone, although to do so would be desirable since the differing behaviour of the possible channels is an important facet of theoretical treatments. Nor can photoelectron angular-distribution measurements from a randomly oriented sample distinguish the channels; the necessary averaging over all orientations ‘washes out’ the dynamical detail. A few years ago Dehmer and Dill performed an experiment on H2 that could distinguish between the allowed σg and πg photoelectron channels, providing a more rigorous test for theory. This was achieved using effectively ‘fixed’ H2, the orientation of the molecular axis at the time of photoionization being determined by observing the direction of axial recoil dissociation fragments from the H2+ produced initially. An alternative procedure for determining molecular alignment is the measurement of the degree of polarization of fluorescence of the resulting ion target. This approach has been applied to [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] photoionization. Although fair agreement is found with theoretical calculations for CO, the N2 data display quite poor agreement with Hartree–Fock calculations of the channel ratio. It has been suggested that one problem with these calculations may be the restriction to a frozen-core hole state to represent the excited ion. A different technique for distinguishing degenerate continua makes use of spin-polarization photoelectron spectroscopy, and its use for the ionization of CH3Br has been described.

Like the above one-electron effects, autoionization resonances may be manifest as structure in ionization cross-sections as for H2, H2S, SO2, HgCl2, HgBr2, and HgI2, as well as the pronounced non-Franck-Condon vibrational distributions seen in photoelectron spectra of molecules such as HCl, CO2, and fluorobenzenes. A theoretical description of autoionization has to address the nature of the discrete-continuum-state interaction. Multi-channel quantum defect theory (MQDT) has been combined with ab initio determination of electronic parameters to treat autoionization of the Hopfield Ryberg series of N2 converging to the B2Σ+u state of the ion. These calculations are successful in showing the X2Σ+u ionization to be more affected by this autoionization than the A2Πu state.

A complete understanding of ionization resonances requires, as well as a full treatment of electron correlation, a consideration of vibronic interactions and competing neutral predissociation processes. Simultaneous vibrational autoionization and electronic predissociation in NO have been treated using an MQDT approach. It might be expected that Δv > 1 autoionization would compete less successfully with predissociation than Δv = 1. However, this is not found to be the case, and the calculation shows that it is the II Rydbergvalence-state interactions that both induce Δv > 1 autoionization and determine the resonance widths. Vibronic interactions play an important role in vibrational autoionization of N2. Vibrational branching ratios for a number of autoionizing features in the photoion-yield curve of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (v) between 800 and 760 Å, using charge exchange to distinguish v = 0 and v = 1 and 2, have been reported. High-resolution data between the v = 0 or 1 thresholds reveal considerable structure corresponding to autoionizing npπu Rydbergs that converge to the [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] state of the ion. Ionization from the v” = 1 state of the neutral species gives strong regular structure due to good Franck–Condon overlap, but ionization from v” = 0 gives weaker irregular features as a result of the interactions with Rydberg states converging to N2+A2Πu. Competition between vibrational and electronic autoionization of the series converging to B2 Σ+u (v = 1) has been described.

(Continues…)Excerpted from Mass Spectrometry Volume 8 by M. E. Rose. Copyright © 1985 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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