Fragmentation of the CH₂Cl₂ molecule by proton impact and VUV photons

A comparative study for the fragmentation of a CH₂Cl₂ molecule has been performed for collisions with 0.2–2.0 MeV H⁺ beam and 12.0–90.0 eV photons using the time-of-flight coincidence technique. Branching ratios for fragmentation products have been determined as a function of the energy of the projectiles. The present results show that the more the proton energy increases, the more the fragmentation pattern resembles the corresponding photon impact spectra at lower energies. For instance, at 2.0 MeV proton impact, the fragmentation pattern closely resembles the corresponding photon impact pattern at hν = 60 eV. From the analysis of the peak shapes, the fragmentation products were found to be formed with low kinetic energies (<1 eV). The main observed fragments, in the proton impact case and for photons above 30 eV, were associated with the release of a chlorine atom. Combining the information from the molecular orbital energies, one estimates the relative contributions of the molecular orbitals to the total ionization of CH₂Cl₂ by proton impact. It is also shown for the first time that the fragmentation pattern for charged products in the proton impact spectra can be directly compared to the corresponding fragmentation pattern for photon impact, through the transferred momentum, which depends on the projectile velocity as v⁻¹. This is a clear indication that the main dynamical variable behind the branching ratios is the momentum transfer at high velocities.