Sub-doppler Electronic Spectrum Of The Benzene–d2 Complex

Excitation spectrum of the benzene–D$_{2}$ van der Waals (vdW) complex in the vicinity of the S$_{1}$ $\leftarrow$ S$_{0}$ 6$_{0}^{1}$ vibronic transition of the monomer was recorded by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Extensive adiabatic cooling with the rotational temperature of \verb|<| 0.5 K was conducted by the high-pressure pulsed expansion, and sub-Doppler resolution yielding the line width of 250 MHz was realized in a collimated molecular beam by employing Fourier-transform-limited ultraviolet pulses for the excitation.\footnote{M. Hayashi and Y. Ohshima, \textit{Chem. Phys.} \textbf{419}, 131–137 (2013).} In contrast to our previous study on the benzene–H$_{2}$ complex,\footnote{M. Hayashi and Y. Ohshima, \textit{J. Phys. Chem. A} \textbf{117}, 9819–9830 (2013).} the weaker binding \textit{ortho} nuclear-spin isomer, correlating to the \textit{j} = 0 state of a freely rotating D$_{2}$, was observed in addition to the stronger binding \textit{para} isomer (with \textit{j} = 1), by using a gas sample of normal D$_{2}$. Three and two vibronic bands involving vdW-mode excitation were observed for the \textit{para} and \textit{ortho} isomers, respectively. By comparing the present results with those of the benzene–H$_{2}$ complex, we made unambiguous assignments on the vdW modes involved in each observed band, and obtained complete sets of vibrational frequencies of all the three vdW modes for the both H$_{2}$ and D$_{2}$ isotopomers in the S$_{1}$ 6$^{1}$ manifold. One of the vdW frequency correlates to the splitting between the \textit{m} = 0 and $\pm 1$ sublevels in the \textit{j} = 1 state of a freely rotating H$_{2}$/D$_{2}$ molecule, and the potential barrier for the hindered internal rotation has been evaluated to be ca. 60 cm$^{-1}$ from the values. Ratio of the vdW frequencies between the H$_{2}$ and D$_{2}$ species deviate significantly from the value for the harmonic vibration (i.e., $\sqrt{2}$ $\approx$ 1.4), indicating substantial anharmonic character of the vdW modes in the complex