# Vacuum-UV negative photoion spectroscopy of CH4

Nicola Rogers, Matthew Simpson, Richard Tuckett, KF Dunn, CJ Latimer

Research output: Contribution to journalArticlepeer-review

4 Citations (Scopus)

## Abstract

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.
Original language English 895-904 10 Molecular Physics 108 7-9 https://doi.org/10.1080/00268970903535483 Published - 10 Apr 2010

## Keywords

• absolute cross sections
• methane
• ion-pair formation
• vacuum-UV
• quantum yields

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