Observation of the hemibond formation in (H ₂ O–Ar _<i>n</i> ) ⁺ radical cation clusters by electronic spectroscopy and ion imaging technique

The hemibond is a non-classical covalent bond formed by the overlap of non-bonding orbitals of a radical (cation) and a closed-shell molecule. For (H₂O-Ar_<i>n</i>)⁺ radical cation clusters, competition between the hemibonded type and hydrogen-bonded (H-bonded) type isomers has been discussed on the basis of infrared spectroscopy and theoretical computations. It has been commonly recognized that the H-bonded type is predominant, while the coexistence of the hemibonded type remains a topic of debate. Hemibonded species are known to exhibit very strong electronic transitions in the ultraviolet and/or visible (UV-vis) region, which are marker bands for hemibond formation. In this study, we performed electronic spectroscopy and photofragment ion imaging experiments on (H₂O-Ar_<i>n</i>)⁺ to observe the hemibond formation between H₂O⁺ and Ar. The observed spectra of (H₂O-Ar_<i>n</i>)⁺ (<i>n</i> = 1-3) exhibit absorption in the UV and visible regions. A comparison with quantum chemical calculations suggests the coexistence of the hemibonded type in (H₂O-Ar_<i>n</i>)⁺ (<i>n</i> = 1 and 2). In addition, the photofragment ion imaging experiment on (H₂O-Ar)⁺ showed an angular distribution attributed to the absorption of the hemibonded type, providing firm experimental evidence of the coexistence of the hemibonded type.