Mass Spectrometry Volume 10

A Review of the Recent Literature Published between July 1986 and June 1988

By M. E. Rose

The Royal Society of Chemistry

Copyright © 1989 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85186-348-1

Contents

Chapter 1 Ionization Processess and Ion Dynamics By C. Lifshitz, 1,
Chapter 2 Structures and Reactions of Gas-phase Organic Ions By P.C. Burgers and J.K. Terlouw, 35,
Chapter 3 Developments and Trends in Instrumentation By F.A. Mellon, 75,
Chapter 4 Application of Computers and Microprocessors in Mass Spectrometry By J.R. Chapman, 118,
Chapter 5 Organic Negative Ions: Structure, Reactivity, and Mechanism By R.A.J. O’Hair and J.H. Bowie, 145,
Chapter 6 Analysis of Mixtures by Mass Spectrometry Part I: Developments in Gas Chromatography/Mass Spectrometry By R.P. Evershed, 181,
Chapter 7 Analysis of Mixtures by Mass Spectrometry Part II: Techniques Other than Gas Chromatography/Mass Spectrometry By D.A. Catlow and M.E. Rose, 222,
Chapter 8 Mass Spectrometry Applied to Natural Products: Nucleosides, Nucleotides and Nucleic Acids By P. Vigny and A. Viari, 253,
Chapter 9 The Use of Mass Spectrometry in Studies of Drug Metabolism and Pharmacokinetics By D.J. Harvey, 273,
Chapter 10 Metal-containing and Inorganic Compounds Investigated by Mass Spectrometry By J. Charalambous and K.W.P. White, 323,
Chapter 11 High-temperature Mass Spectrometric Studies of Inorganic Systems By E.R. Plante and J.W. Hastie, 357,
Subject Index, 379,
Author Index, 388,

CHAPTER 1

Ionization Processes and Ion Dynamics

BY C. LIFSHITZ

1. Introduction

Covering the literature concerning ionization processes and ion dynamics over a period of two years and doing justice to the field has become a formidable task, in view of the enormous expansion in these areas. It would perhaps be advisable to divide in the future the topic into two sub topics, one dealing with basic aspects of ionization processes (including dynamic aspects) and the other dealing with basic aspects of ion fragmentations and ion-molecule reactions. I have concentrated in this chapter on ionization processes. My task has been, as in the last review to present work related to fundamental aspects of the behavior of relatively simple molecular ions. Other reviews of the mass spectrometry literature are concentrating on the more applicative nature of ionization processes, unimolecular fragmentations and ion-molecule reactions with emphasis on the general trend in analytical mass spectrometry today, namely towards larger and larger molecular ions of biological importance.

2. Ionization Processes

2.1 Molecular Photoionization. – The emphasis in recent years has been on photoionization dynamics. Great progress has been made in experiments due to the development of synchrotron radiation giving continuously variable photon energies and in theory, since theoreticians are now able to perform calculations of the electronic continuum in the molecular field. Several review articles have appeared on molecular photoionization. Recent research has dealt with resonances which are quasi discrete states embedded in the ionization continuum and characterized by a finite lifetime. These resonances lead to peaks or dips in the ionization cross section. Three major categories have been recognized as explained previously: Shape resonances, autoionization resonances and Cooper minima. Recently, each of these phenomena has been discussed separately. Molecular shape resonances in diatomic molecules, which have no counterpart in atoms, are due to the existence of the σ* antibonding molecular orbital. Two diabatic states, the Rσ Rydberg and a σ* valence state mix and give rise to two adiabatic states. One of these adiabatic states, which has σ* character at low intemuclear distance and becomes a Rydberg orbital at large distance, constitutes the shape resonance, reached by ionization from a bound σ orbital. Shape resonances have been studied recently experimentally and theoretically for several diatomic and polyatomicmolecules. The investigation of the valence shells of benzene has allowed a comprehensive evaluation of the ability to predict the photoelectron dynamic properties of a moderately complex polyatomic system. The energy of the maximum in the ionization cross section due to a shape resonance is strongly dependent on the intemuclear distance. A search for a quantitative relationship between shape-resonance energies and bond lengths has been made with the aim of developing a new analytical method competitive with EXAFS (extended X-ray absorption fine structure).

Autoionization resonances are due to couplings between the continuum and resonant Rydberg states. For light molecules electrostatic coupling terms are important while for heavy molecules it is the spin-orbit interaction which is important. Electrostatic autoionization has been studied in detail for CO and other diatomic molecules. Spin-orbit autoionization has been studied for HBr and HCl. Both types of autoionization have been treated very successfully by multichannel quantum defect theory (MQDT) which treats the autoionization process on the basis of a breakdown of the adiabatic Born-Oppenheimer approximation. A comparison between the calculated photoionization cross section for HBr and the experimental results’ is reproduced in Figure 1. Twenty-four autoionizing resonances appear in the energy region studied, between the 2Π3/2 and 2Π1/2 ionic limits, and their wavelength positions, cross sections, widths and tentative assignments have been determined. Fluorescence has been employed as a probe of molecular autoionization in N2O, CS2 and HCl. When predissociative levels mediate the autoionization process this leads to significant deviations from the vibrational autoionization “propensity rule”, which states that the change in the vibrational quantum number is Δv = –1 or that it is minimal. The fate of core excited molecules, SiH4, HI, CH3I, HBr and CH3Br, has been studied in detail recently. Two-step decay processes have been proposed for valence resonances – a fast dissociation followed by the autoionization of the excited fragment. In general, the dynamics of vibronic autoionization of Rydberg states of polyatomic molecules, including Rydberg-valence vibronic coupling via nontotally symmetric modes is now coming under theoretical scrutiny, as more refined high resolution photoionization and threshold photoelectron spectra become available experimentally for polyatomic molecules.

Many-body effects lead to a spreading of the spectral intensity of the inner-valence states over several lines in photoelectron spectra. This breakdown of the molecular orbital model has been explained in some detail in the previous review. It has been studied further recently, employing high-energy-resolution synchrotron radiation on H2S, NH3, HF and CS2. Many valence satellites, some which have not been seen previously by binary (e,2e) spectroscopy and others which have not been predicted by theory, have been observed. Valence ionization spectra have been calculated for quite complex molecules such as TiCl4, o-Benzyne, cyano-derivatives of organic molecules, first and second row transition metal diatomics, S2N2 and S3, quinone and benzene like molecules, p-nitroaniline by an ab initio Green’s function formalism that takes the effects of electron correlation and relaxation into account. In some cases satellite lines appear at very low energies. Ionization energies are calculated by the extended two-particle-hole Tamm-Dancoff approximation (extended 2ph-TDA) which describes the satellite lines in a photoelectron spectrum by including the mixing of single hole with two hole one particle (2h1p) configurations.

The method called (e,2e) spectroscopy has been reviewed previously together with molecular photoionization. Its lower energy resolution is giving way to using tunable synchrotron radiation as the method of choice for studying configuration interaction states discussed above. It has, however, a unique feature not available to photoionization, which makes it a powerful emerging technique for the study of molecular orbitals and the laboratory investigation of molecular wavefunctions and chemical bonding. The technique is now being called electron momentum spectroscopy (EMS) and has been reviewed recently. EMS measurements provide information on orbital imaging and have thus given an empirical aspect to the orbital pattern concept, which was, until the advent of this method, of a somewhat unreal nature. EMS is based upon the high energy binary (e,2e) reaction. It employs high energy electron impact ionization and measures all the necessary energies and angles of the particles involved, in coincidence. Energy conservation yields information on the binding (ionization) energy of the target and leads to information analogous to photoelectron spectra. Momentum conservation yields information on the momentum p of the bound electron (which is ionized in the collision). The measured (e,2e) cross section is directly proportional to the momentum density ρ(p). Let ψ(r) be a wave function; the quantity |ψ(r)|2 has the interpretation of electron charge (probability) density in position (r) space. EMS measures the momentum probability density,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] I

and the momentum space wave function ψ(p) is related to ψ(r) by the Fourier transform,

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] II

To make comparison with experimental results, one calculates orbital densities by squaring the Fourier transform of wave functions obtained from SCF-MO calculations. EMS is a sensitive probe of phase space corresponding to small p, and therefore to the region of larger r which is particularly important for chemical bonding and reactivity. It has provided information on the methyl inductive effect by indicating appreciable delocalization of electron density away from the nitrogen in methyl amines. It provides a powerful experimental tool in quantum chemistry for the testing, evaluation and design of molecular wavefunctions. Quite a number of diatomic, triatomic and polyatomic molecules have been studied by EMS, including H2, NO, Cl2, Br2, CO, H2O, D2O, CO2, COS and CS2, H2S, NH3, NF2, methyl amines, CF4, CH3F and CH2Cl. Figure 2 reproduces a comparison between experimental and calculated momentum distributions for the 2b1, orbital of H2S. Calculations were carried out at various levels of theory and inclusion of correlation was found to have a minimal effect on the momentum distributions. Many of these (e,2e) studies measured binding energy spectra which demonstrate satellite lines. For example, in H2S, extensive many-body states arise from the 4a1-1, hole state.

Absolute total absorption, photoionization and dissociative photoionization cross sections of ammonia have been measured from 80 to 1120 Å.

2.2 Multi-photon Ionization. – Molecular dynamic photoelectron spectroscopy using resonant multiphoton ionization (REMPI-PES) provides opportunities for the study of the dynamics of excited state photoionization and autoionization processes, the observation of neutral and ionic states that are symmetry forbidden in single photon excitation and the study of photodissociation and intramolecular relaxation. Several new review articles on MPI-PES have appeared recently. Detailed experimental studies of two diatomic molecules, H2 and NO permit a direct comparison with theory. Photoelectron angular distributions have been compared with available theoretical calculations both for MPI and for single photon ionization of H2. The need for substantial progress in understanding the photo-ionization dynamics of even the simplest excited molecular states has been pointed out. Major contributions have come through the study by REMPI-PES of rotational and vibrational branching ratios of excited molecular states. As expected on the basis of Franck-Condon arguments, photoionization via a particular vibrational level v’ of the C1Πu state of H2 leads to a photoelectron spectrum strongly peaked at the ionic vibrational level v+ = v’ Rotational propensity rules in photoionization were studied for H2 and NO, since specific rovibronic levels of different electronic states of the neutrals could be excited. In single photon ionization, such experiments are not possible since resolution is inadequate to resolve rotational structure and ionization takes place from a thermal distribution of rotational levels of the ground state. The electronic excitation of the ion core is retained following photoionization of a molecular Rydberg state; as a result ionization of N2 via the o31Πu state leads exclusively to the A2Πu state of the ion. If the rotational level of the resonant intermediate state is properly chosen, the angular momentum selection and propensity rules for single photon ionization of the excited neutral intermediate indicate that the ionic state might be produced in a single rotational level. Thus, it is possible in the case of N2 to produce an electronically excited state of the ion in a single vibrational and rotational state. The rotational distribution of N+2 formed by REMPI has been recently measured by laser induced fluorescence (LIF) and rotational propensity in the photoionization from the c’ 1Σu+ and c1Πu states was further studied. The autoionization dynamics of individual rotational levels has been studied for NO in the 9dσπ, v = 2 band. The branching ratio into the NO+ v+ = 1 state increases significantly on the autoionizing resonances, in accord with the Δv = –1 propensity rule for vibrational autoionization.

An added advantage of REMPI-PES lies in the ability to determine the ionic energy levels of species that are present in small concentrations in the presence of a species with an interfering PES. REMPI is used to selectively ionize the species of interest while leaving the interfering species unexcited. This has been applied to rare gas dimers such as Xe2 in the presence of a large excess of their monomers, (Figure 3).

MPI ion-current spectra have been obtained as a function of laser wavelength in the U.V./visible region either on their own or in conjunction with MPI photoelectron spectra. Ion-current spectra were obtained for H2 D2 NO O2, Xe2, C2H2, cyclic ketones, and other molecules, yielding information about the structure and dynamics of the neutral intermediate states. A theory has been presented to reduce 1+1 resonance enhanced multiphoton ionization spectra to accurate rovibrational state population distributions and alignment factors. The degree of alignment of desorbing NO molecules, which is a sensitive measure of desorption dynamics, was probed via 1+1 REMPI. MPI ion-current spectra in combination with PES have detected the UV multiphoton dissociation of iron complexes.’Other studies of large polyatomic molecules involved the autoionization in diazabicyclooctane (DABCO) and ionization thresholds of phenol-(NH3)0 clusters.

2.2.1. Mass-spectrometric Studies. – MPI-MS and unimolecular ion decay have been reviewed recently. Both experimental and theoretical aspects were covered in depth. The major breakthrough in this area has been the ability to produce state- and internal energy-selected molecular ions, which led to very significant contributions to the understanding of unimolecular fragmentations of polyatomic ions. Species selectivity, isomer specificity, soft ionization and hard fragmentations are now well understood on the basis of the unique excitation mechanism in multi-photon mass spectrometry. Several of these aspects such as soft ionization, isomer specificity, wavelength-dependent fragmentation’ and intensity dependent fragmentation were discussed in several recent publications. The high relative abundance of C+ in MPI-MS of benzene, particularly at higher laser intensities has attracted considerable attention over the years. Recent experiments employing photoelectron spectroscopy have unambiguously identified the presence of 1D carbon atoms. While the contribution to the C+ signal from fragmentation of larger ions is greater than from ionization of neutral carbon atoms, future theoretical interpretations of MPI-MS should take into account ionization of neutral fragments.

(Continues…)Excerpted from Mass Spectrometry Volume 10 by M. E. Rose. Copyright © 1989 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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