The research group led by Prof. ZHANG Weijun at Anhui Institute of Optics and Fine Mechanics, Hefei Institutes of Physical Science has made new progress on the vacuum ultraviolet photodynamics of the methyl peroxy radical (CH₃O₂).

Peroxy radicals (RO₂) are important reaction intermediates in the atmosphere. Their reactions can lead to the formation of secondary pollutants such as ozone and fine particles, taking a large influence on the atmospheric environment and human health. The structure, thermochemistry and kinetics of RO2 are of considerable importance in order to fully understand their chemistry in the atmosphere.

TANG Xiaofeng, one staff of the group, worked in international collaboration with the scientists of Synchrotron SOLEIL, France, to investigate the vacuum ultraviolet (VUV) photoionization of the methyl peroxy radical, CH₃O₂, the most abundant peroxy radical in the atmosphere, by using the state-of-the-art method of double imaging photoelectron photoion coincidence (i2PEPICO). In addition, the unimolecular dissociation of internal energy selected CH₃O₂⁺ cations was also been studied in detail.

In their work, a microwave discharge flow tube was employed as reactor to produce CH₃O₂ via the reaction of methyl radicals (CH₃) with oxygen gas. The high resolution slow photoelectron spectrum (SPES) of CH₃O₂ was obtained exhibiting two broad bands superimposed with a complex vibrational structure.

The first band of the SPES was attributed to the X³A^" and a¹A^’ overlapped electronic states of CH₃O₂⁺ and the second was assigned to the b¹A^’ electronic state with the help of theoretical calculations.

The adiabatic ionization energy (AIE) of CH₃O₂ was derived as 10.215 ± 0.015 eV, in good agreement with high-accuracy theoretical data in the literature. The vertical ionization energy of the b1A' electronic state was measured to be 11.5 eV and this state fully dissociates into CH₃⁺ and O₂ fragments. The 0 K adiabatic appearance energy (AE_0K) of the CH₃⁺ fragment ion was determined to be 11.15 ± 0.02 eV.

This work was financially supported by the National Natural Science Foundation of China (Nos. 21773249, 91961123, 91544228) and the International Partnership Program of Chinese Academy of Sciences (No. 116134KYSB20170048).

Link to the paper: Vacuum Ultraviolet Photodynamics of the Methyl Peroxy Radical Studied by Double Imaging Photoelectron Photoion Coincidences

(a) Mass-selected slow photoelectron spectra (SPES) corresponding to m/z = 47 and 15 ions; (b) SPES of CH₃O₂ (black) and its partial Franck–Condon fitting (purple). (Image by LIN Xiaoxiao)

Contact:

ZHOU Shu

Hefei Institutes of Physical Science (http://english.hf.cas.cn/)

Email: zhous@hfcas.ac.cn