Resonant soft X-ray photofragmentation of propane

A comparison was made between the mass spectra of propane (CH3CH2CH3) for resonant soft X-ray photofragmentation and for electron-impact ionization. The soft X-ray photon energy was tuned to 287.7 eV to promote Auger relaxations from the C-H bonds (1s→C-H σ* transition). It was hypothesized that this would lead to proton loss without C-C chain fragmentation. Compared to traditional 70 eV electron-impact ionization, photoionization does bias the mass spectrum toward proton loss; however, residual vibrational energy in the room temperature propane leads to a non-trivial amount of C-C bond scission. The resonant soft X-ray photofragmentation is dominated by three-carbon ions between 38 and 40 u created by the loss of hydrogen while the electron-impact ionization is dominated by a major peak at 29 u corresponding to the two-carbon ion C2H5+. Within each envelope of three-, two-, or single-carbon ions the core-level photoionization spectrum showed a bias toward enhanced multiple hydrogen removal compared to the electron-stimulated spectrum