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Vacuum-UV negative photoion spectroscopy of CH4

Rogers, N. J. and Simpson, M. J. and Tuckett, R. P. and Dunn, K. and Latimer, C. J. (2010) Vacuum-UV negative photoion spectroscopy of CH4. Molecular Physics, 108 (7-9). pp. 895-904. ISSN 0026-8976

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URL of Published Version: http://www.tandfonline.com/doi/abs/10.1080/00268970903535483

Identification Number/DOI: doi:10.1080/00268970903535483

Using synchrotron radiation in the range 12-35 eV, negative ions are detected by mass spectrometry following vacuum-UV photoexcitation of methane. Ion yields for H\(^-\), CH\(^-\) and CH\(_2^-\) are recorded, the spectra of CH\(^-\) and CH\(_2^-\) for the first time. All ions display a linear dependence of signal with pressure, showing that they arise from unimolecular ion-pair dissociation. Cross sections for ion-pair formation are put onto an absolute scale by calibrating the signal strengths with those of F\(^-\) from SF\(_6\) and CF\(_4\). Following normalisation to total vacuum-UV absorption cross sections, quantum yields for anion production are reported. There is a major discrepancy in the H\(^-\) cross section with an earlier measurement, which remains unresolved. The anions arise from both direct and indirect ion-pair mechanisms. For a generic polyatomic molecule AB, the former is defined as AB \(\rightarrow\) A\(^-\) + B\(^+\) (+ neutrals), the latter as the predissociative crossing of an initially-excited Rydberg state of AB by an ion-pair state. In a separate experiment, the threshold photoelectron spectrum of the second valence band of CH\(_4\), ionisation to CH\(_4^+\) A \(^2\)A\(_1\) at 22.4 eV, is recorded with an instrumental resolution of 0.004 eV; many of the Rydberg states observed in indirect ion-pair formation converge to this state. The widths of the peaks are lifetime limited, increasing with increasing \(v\) in the \(v_1\) (a\(_1\)) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions.

Type of Work:Article
Date:2010 (Publication)
School/Faculty:Colleges (2008 onwards) > College of Engineering & Physical Sciences
Department:School of Chemistry
Keywords:methane; ion-pair formation; vacuum-UV; absolute cross sections; quantum yields
Subjects:QD Chemistry
QC Physics
Institution:University of Birmingham
Copyright Holders:Taylor & Francis
ID Code:767
Refereed:YES
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