Mass-Spectrometric Study of Ions at Near-Atmospheric Pressure. III. The Alpha Radiolysis of Methane

Abstract

The ion—molecule reactions occurring in the alpha-particle radiolysis of methane have been studied in a mass spectrometer under near-conventional radiolysis conditions (1–160 torr pressure and a few milliseconds ion reaction time). Comparison of the observed high intensities of CH5+ and C2H5+ at 1 torr with spectra obtained previously with conventional microsecond reaction times shows that the reactions of CH5+ and C2H5+ with methane are very slow (k<10−15 cc molecule−1·sec−1). At higher pressures CnH+2n−1, CnH+2n+1, and CnH+2n+3 are observed with n≦6. The addition of trace concentrations of normal and deuterated hydrocarbons helps to elucidate some of the reactions involved. The ions observed in the presence of traces of deuterated alkanes present a complete confirmation of reactions proposed earlier by Ausloos whose work was based on neutral product studies in α-radiolyzed methane and deuterated alkanes. The present work with additives shows that impurities at concentrations of 1 ppm affect the course of the ionic reactions in the radiolysis of methane at near-atmospheric pressures. The origin of the ions observed in the present work in ``pure'' methane at pressures above 10 torr is therefore uncertain. These ions are probably largely due to impurities.