Vacuum-UV photofragmentation of polyatomic ions using coincidence techniques

doi:10.1016/S0368-2048(98)00418-6

Journal of Electron Spectroscopy and Related Phenomena

Volumes 101–103, June 1999, Pages 93-99

https://doi.org/10.1016/S0368-2048(98)00418-6 Get rights and content

Abstract

We describe an apparatus for performing three types of coincidence experiment to study state-selected photofragmentation of polyatomic ions in the gas phase; threshold photoelectron–photoion coincidences (TPEPICO), photoion–fluorescence coincidences (PIFCO), and threshold photoelectron–fluorescence coincidences (TPEFCO). Photoionisation in the vacuum-UV, 10–25 eV, is obtained using tunable radiation from the Daresbury, UK Synchrotron Radiation Source. Examples are taken from published work in CF₄⁺, PCl₃⁺ and PBr₃⁺, and a range of perfluorocarbon cations C_xF_y⁺.

Introduction

Our group has been studying the electronic spectroscopy and unimolecular decay dynamics of cations in the gas phase containing 4–14 atoms, especially the halides of groups III–VI of the periodic table (e.g. CF₄⁺). Although small enough to be amenable to ab initio calculations, many of these ions are sufficiently large that statistical factors should determine their fragmentation pathways. One aim of this project is to understand the factors that determine non-statistical effects in excited valence states of such medium-sized polyatomic ions, whose size is intermediate between that of diatomic ions (where dynamical processes should dominate) and much larger cations (where statistical effects should dominate). In general, an excited state of such an ion, (AB_n⁺)*, created by photoionisation can decay radiatively by fluorescence or non-radiatively by fragmentation: $AB_{n} +tunable VUV radiation→(AB_{n}^{+})* +e^{−},$ $(AB_{n}^{+})*— k_{rad} →(AB_{n}^{+})+ hν$ or $(AB_{n}^{+})*— k_{non-rad} →fragment ions+neutrals$ where k is a first-order decay rate constant. Processes (2) and (3) are studied in two separate experiments at the Daresbury Synchrotron Radiation Source, UK by fluorescence excitation and coincidence spectroscopy, respectively. Non-statistical effects in polyatomic ions are manifest either by the observation of radiative decay (process 2), or by unexpectedly large releases of kinetic energy (KE) into translational motion of the fragment ion+neutral products (process 3). To date, coincidence experiments have been performed to study fluorescence and fragmentation processes in a range of polyatomic ions (e.g. CF₄⁺ and SiF₄⁺, CCl₄⁺, SiCl₄⁺ and GeCl₄⁺, CF₃Cl⁺ and CF₃Br⁺, BCl₃⁺ and SF₆⁺) [1]. We have established [1] that the most important criterion in determining whether an ion fragments in a statistical or non-statistical manner is the presence or absence of a bound, ground electronic state of the parent ion.

Our group has also performed pure spectroscopic experiments in the VUV both at Daresbury and BESSY 1 in Berlin, studying fluorescence processes both in molecular ions (process 2) and in neutral free radicals produced by photodissociation of Rydberg states of the neutral, parent molecule, i.e. $AB_{n} +tunable VUV radiation→AB_{n} *→AB_{n−1} *+B→AB_{n−1} +B+ hν$ Non-dispersed fluorescence excitation experiments are performed at Daresbury where, by using fluorescence in a fragment free radical as a probe of VUV absorption in the parent molecule, experiments yield VUV spectra of the parent molecule. Dispersion of the induced fluorescence is possible in experiments performed at BESSY, yielding low-resolution electronic spectra of the fragment radicals (e.g. BCl₂· from BCl₃, SiF₃· and SiF₂ from SiF₄, PF₂· and PF from PF₃) [2], [3], [4].

Five posters are presented at VUV12 illustrating these techniques for a range of molecules. For this special issue of J. Electron Spectrosc. Relat. Phenom., we present this work as two back-to-back papers. In this first paper on coincidence spectroscopy, data on CF₄⁺ are presented [5] which illustrate the different forms of coincidence experiments which can monitor radiative and non-radiative decay in excited states of cations. Then, more recent TPEPICO data on PCl₃⁺ and PBr₃⁺ [6] and a range of saturated and unsaturated perfluorocarbons C_xF_y⁺ [7] are presented.

Section snippets

Experimental

Studies of single-ionisation processes are performed using coincidence techniques in which threshold photoelectrons, ions and fluorescence photons are collected, and any pair correlated in time. Three forms of coincidence spectroscopy are currently used at Daresbury (Fig. 1):

1.
Threshold PhotoElectron–PhotoIon COincidences (TPEPICO). The fragment ions are separated by time-of-flight mass spectrometry (TOF-MS). A threshold electron analyser defines the energy of the photoelectron to be between zero

CF₄⁺

We have studied in detail fragmentation of the C̃²T₂ third excited valence state of CF₄⁺ using all three kinds of coincidence spectroscopy [5], [9]. This state exhibits vibrational structure in its photoelectron spectrum, is metastable with a lifetime of several ns, and exhibits both radiative and non-radiative decay pathways. Low vibrational levels of the ν₁ (a₁) manifold of CF₄⁺ C̃²T₂ decay with unity fluorescence quantum yield to the lower-lying Ã²T₂ and X̃²T₁ states. These states are

PCl₃⁺ and PBr₃⁺

We have studied the fragmentation of all the valence states of these two cations by TPEPICO spectroscopy [6]. Although fluorescence experiments have suggested that the Ẽ²E state of these parent cations does decay radiatively [10], to date PIFCO and TPEFCO fluorescence coincidence experiments have been unsuccessful. The ground and first excited states of both PCl₃⁺ and PBr₃⁺ are stable with respect to dissociation to PX₂⁺ (X=Cl, Br), whereas the B̃²E, C̃²E, D̃²A₁ and Ẽ²E states dissociate

The saturated and unsaturated perfluorocarbon cations, C_xF_y⁺

We have recently studied a range of saturated and unsaturated PFC molecules (e.g. C₂F₆, C₃F₈, n-C₄F₁₀, C₂F₄, C₃F₆, 2-C₄F₈) by TPEPICO spectroscopy [7], for two overriding reasons. Firstly, from a pure science viewpoint, it has been known for some time that C₂F₆⁺ exhibits non-statistical effects [18], including direct dissociation from excited states of the cation with large KE releases into fragment ions. Such effects should become less common as the size of the cation and hence the density of

Acknowledgements

We thank NERC, EPSRC and the Daresbury Laboratory for a Research Grant, Research Studentships (GKJ, KJB) and a CASE award (KJB). We also thank Dr. I. Powis (Nottingham University) for the use of his kinetic energy analysis programme, and Dr. P.A. Hatherly (Reading University) for technical advice on the apparatus.

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¹: Present address: Department of Chemistry, MS 6-2100, Lawrence Berkeley Laboratory, University of California Berkeley, CA 94720, USA.

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Journal of Electron Spectroscopy and Related Phenomena

Vacuum-UV photofragmentation of polyatomic ions using coincidence techniques

Abstract

Introduction

Section snippets

Experimental

CF4+

PCl3+ and PBr3+

The saturated and unsaturated perfluorocarbon cations, CxFy+

Acknowledgements

Chem. Phys.

Chem. Phys. Lett.

Int. J. Mass Spec. Ion Phys.

Chem. Phys. Lett.

Zeit. Phys. Chem.

J. Chem. Soc. Faraday Trans.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Phys.

J. Chem. Soc. Faraday Trans.

CF₄⁺

PCl₃⁺ and PBr₃⁺

The saturated and unsaturated perfluorocarbon cations, C_xF_y⁺