A research group led by Prof. LI Haiyang from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has revealed a new competition mechanism in vacuum ultraviolet photoionization of dichloromethane using a home-built time-of-flight mass spectrometer (TOFMS), which is helpful to study stratospheric ozone depletion mechanism and photodegradation of harmful haloalkanes.
The study was published in The Journal of Physical Chemistry Letters on Jan. 31.
Proposed mechanism for the generation of CH2Cl+ and CHCl2+ when the neutral CH2Cl2 molecule was irradiated by a VUV Kr lamp (Image by YU Yi)
Dichloromethane (CH2Cl2) is widely used as industrial solvent, reaction medium in the pharmaceutical industry and feedstock for producing other chemicals. CH2Cl2 can cause environmental harm and health hazards due to its low boiling point and high volatility.
Strong presence of vacuum ultraviolet (VUV) light in the solar emission spectrum can induce the production of ozone-depleting Cl atom, therefore, the photochemistry of CH2Cl2 is crucial to stratospheric ozone chemistry.
In this study, the researchers have revealed the photoionization mechanism of CH2Cl2 under the irradiation of 10.0 and 10.6 eV light from a VUV krypton (Kr) lamp.
They demonstrated that CH2Cl+ was produced by two competitive channels: photoinduced ion-pair and photodissociation-assisted photoionization (PD-PI). The ion-pair channel was quenched efficiently at high number density of CH2Cl2, which reduced its contribution.
Moreover, they indicated that the dominant photodissociation channel of CH2Cl2 was CH2Cl2 + hν → CH2Cl· + Cl·, and the formed Cl· radical could further react with the CH2Cl2 molecule to form CHCl2· radical. Then CHCl2+ was generated by the photoionization of CHCl2·. Finally, they derived kinetic equations for the quantitative description of the production efficiencies of CH2Cl+ and CHCl2+.
"Our study enhances the overall understanding of the complicated photoexcitation behaviors of CH2Cl2 in the VUV regime, which helps to study the atmospheric photochemical process of haloalkanes and provides guidance for the photodegradation of hazardous haloalkanes," said Prof. LI.
"Our study proposed new insights into the complicated photoexcitation behaviors of CH2Cl2 in the VUV regime and revealed the important role of photodissociation in VUV photoionization at low photon flux," Prof. LI added.
This work was supported by the National Natural Science Foundation of China, the Scientific Instrument Developing Project of CAS, and DICP.