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Home-Made Time-of-Flight Mass Spectrometer Helps Atmosphere Studies

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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).

The study was published in The Journal of Physical Chemistry Letters on Jan. 31.

Dichloromethane (CH2Cl2) is widely used as industrial solvent, reaction medium in 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.

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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.

Reference: Yu Y, Jiang J, Hua L, et al. Ionization of dichloromethane by a vacuum ultraviolet krypton lamp: Competition between photoinduced ion-pair and photodissociation-assisted photoionization. J Phys Chem Lett. 2023;14(5):1265-1271. doi: 10.1021/acs.jpclett.2c03572

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