Ionization of Dichloromethane by a Vacuum Ultraviolet Krypton Lamp: Competition Between Photoinduced Ion-Pair and Photodissociation-Assisted Photoionization.

The photodissociation and photoionization behaviors of haloalkanes in the VUV regime are important to fully understand the mechanism of ozone depletion in the stratosphere. The ionization of dichloromethane (CH 2 Cl 2 ) under the irradiation of 10.0 and 10.6 eV light was investigated. CH 2 Cl + was observed at 10 Pa, while both CH 2 Cl + and CHCl 2 + were observed at higher pressure. The production efficiency of CH 2 Cl + decreased with the increasing number density of CH 2 Cl 2 , while that of CHCl 2 + increased. A kinetic model was successfully derived to quantitatively describe the variation trends of CH 2 Cl + and CHCl 2 + , in which the competition between photoinduced ion-pair and photodissociation-assisted photoionization (PD-PI) were included. The ion-pair channel was quenched efficiently at higher pressure or concentration, which reduced its contribution. Our study proposed new insights into the complicated photoexcitation behaviors of CH 2 Cl 2 in the VUV regime and revealed the important role of photodissociation in photoionization at low photon flux.