Interpretation of temperature-dependent photoionization mass spectra of n-hexane, cyclohexane, and diethyl ether

The mass spectra of vapor-phase n-hexane, cyclohexane, and diethyl ether are measured as a function of temperature by photoionization mass spectrometry. Three fixed wavelengths are used, the Kr I, Ar I, and Ne I resonance lines. The results are interpreted on the basis of a simplified version of the quasi-equilibrium theory. In this model it is assumed that the density of states of a transition state can be described by the density of states of the neutral molecule multiplied by a phase space scaling factor. The phase space scaling factors are fitted for an optimum reconstruction of the photon and temperature-dependent mass spectra. The knowledge obtained about the fragmentation reaction rates of n-hexane is applied to field ionization mass spectra, which results in an estimate of the average energy deposition in the molecular ion of 0.77 {plus minus} 0.1 eV.