Vacuum-UV negative photoion spectroscopy of CH₄

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 are recorded, the spectra of CH⁻ and 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 SF₆ and CF₄. 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 → 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 CH₄, ionisation to ²A₁ 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 ν₁ (a₁) vibrational ladder. They are the first direct measurement of an upper value to the dissociation rate of these levels into fragment ions.

Keywords:

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

Acknowledgements

We thank Dr David Shaw (Manager of beamline 3.1) for help with recording the anion spectra at the Daresbury synchrotron source. We thank Drs Andras Bodi and Melanie Johnson (SLS) and Ms Jonelle Harvey (University of Birmingham) for help with recording the TPES of CH₄ on beamline X04DB of the Swiss Light Source. The collaboration between the groups in Birmingham and Belfast was partially funded by EPSRC Network Grant No. GR/N26234/01. The STFC and EU are thanked for the provision of beamtime at the Daresbury synchrotron and the Swiss Light Source, respectively.