ULTRAVIORET AND INFRARED PHOTODISSOCIATION SPECTROSCOPY OF HYDRATED ANILINIUM ION

To understand the temperature effect on the microscopic hydration, we have been carrying out the laser spectroscopy of temperature-controlled hydrated phenol cation clusters using our temperature-variable ion trap apparatusfootnote{R.~Yagi, Y.~Kasahara, H.~Ishikawa, the 70th International Symposium on Molecular Spectroscopy (2015).}$^,$footnote{H.~Ishikawa, T.~Nakano, T.~Eguchi, T.~Shibukawa, K.~Fuke textit{Chem. Phys. Lett.} textbf{514}, 234 (2011).}. In the present study, we have chosen an anilinium ion (AnH$^+$) as a solute. Since the phenol cation has $(pi)^{-1}$ configuration, the phenyl ring does not play as a proton-acceptor. On the contrary, the $pi$-orbitals in the AnH$^+$ are fulfilled and both the NH$_3^+$ and phenyl groups can behave as hydrogen-bonding sites. Thus, hydration structures around the AnH$^+$ are expected to be different from those of the phenol cation. Since there is no spectroscopic report on the hydrated AnH$^+$ clusters, we have carried out the UV and IR photodissociation spectroscopy of AnH$^+$(H$_2$O) clusters. par In the present study, the AnH$^+$(H$_2$O) is produced by an electrospray ionization method. As the first step, spectroscopic measurements are carried out without temperature control. In the UV photodissociation spectrum, the 0-0 band appears at 36351,cm$^{-1}$ which is red-shifted by 1863,cm$^{-1}$ from that of the AnH$^+$ monomerfootnote{G.~F'{e}raud, textit{et al.} textit{Phys. Chem. Chem. Phys.} textbf{16}, 5250 (2014).}. The band pattern is similar to that of the AnH$^+$ monomer. This indicates that the structure of the AnH$^+$ is not so affected by the single hydration. In the IR photodissociation spectrum, OH stretching band of the H$_2$O moiety and free NH stretching band of AnH$^+$ moiety are observed. Comparison with the results of the DFT calculation at M05-2X/6-31++G(d,p) level, we determined the structure of the AnH$^+$(H$_2$O) cluster