Using a home-built time-of-flight mass spectrometer (TOFMS), a research team led by Prof. Haiyang Li from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) has discovered a new competition mechanism in vacuum ultraviolet photoionization of dichloromethane that is useful for studying the stratospheric ozone depletion mechanism and photodegradation of harmful haloalkanes.

On January 31^st, 2023, the study was published in The Journal of Physical Chemistry Letters.

Dichloromethane (CH₂Cl₂) is often employed as a feedstock for the synthesis of other chemicals, a reaction medium in the pharmaceutical sector, and an industrial solvent. Due to its low boiling point and high volatility, CH₂Cl₂ can be hazardous to the environment and human health.

The generation of ozone-depleting Cl atom can be stimulated by the strong vacuum ultraviolet (VUV) radiation present in the solar emission spectrum; as a result, the photochemistry of CH₂Cl₂ is essential to stratospheric ozone chemistry.

In this investigation, the researchers have demonstrated how CH₂Cl₂ gets photoionized when exposed to 10.0 and 10.6 eV light from a VUV krypton (Kr) lamp.

They showed that photoinduced ion pair and photodissociation-assisted photoionization were two competing pathways that created CH₂Cl⁺ (PD-PI). At high CH₂Cl₂ number density, the ion-pair channel was effectively quenched, lowering its contribution.

Additionally, they demonstrated that CH₂Cl₂’s predominant photodissociation pathway was CH₂Cl₂ + hv → CH₂Cl + Cl and that the Cl radical that resulted from this reaction could then interact with another CH₂Cl₂ molecule to produce the CHCl₂ radical.

The photoionization of CHCl₂⁺ then produced CHCl₂⁺. For the quantitative description of the production efficiencies of CH₂Cl⁺ and CHCl₂⁺, they finally derived kinetic equations.

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.

Haiyang Li, Professor, Dalian Institute of Chemical Physics, Chinese Academy of Sciences

Li added, “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.”

The National Natural Science Foundation of China, the Scientific Instrument Developing Project of CAS, and DICP provided funding for this study.

Journal Reference:

Yu, Y., et al. (2023) Ionization of Dichloromethane by a Vacuum Ultraviolet Krypton Lamp: Competition Between Photoinduced Ion-Pair and Photodissociation-Assisted Photoionization. The Journal of Physical Chemistry Letters. doi:10.1021/acs.jpclett.2c03572.

Source: http://english.dicp.cas.cn/