Vacuum-UV negative photoion spectroscopy of CH4

作者：Rogers N J; Simpson M J; Tuckett R P^*; Dunn K F; Latimer C J

来源：Molecular Physics, 2010, 108(7-9): 895-904.

DOI：10.1080/00268970903535483

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

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 CH2- are recorded, the spectra of CH- and CH2- 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 CH4+ (A) over tilde (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 nu in the nu(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.

• 出版日期2010