Photoionization of CH3OH, CD3OH, and CH3OD: Dissociative ionization mechanisms and ionic structures

The molecules CH3OH, CD3OH, and CH3OD have been investigated by photoionization mass spectrometry from the onset of dissociative ionization to 20 eV. The process forming methyl ion and hydroxyl radical is shown to be direct, rather than statistical. Near threshold, the process involving molecular hydrogen elimination is shown to lead to the structure HCOH+, rather than H2CO+. Removal of an H atom leads predominantly to CH2OH+, although some evidence exists for a CH3O+ structure at higher energy. Similarly, the structure HCO+ is seen to be much more prevalent than COH+, but the latter may be formed with increasing probability at higher energy. Some astrophysical implications regarding the formation of CH2O in dense interstellar clouds are discussed.