Sub-Doppler electronic spectrum of the benzene-D₂ complex

Excitation spectrum of the benzene-D₂ van der Waals complex in the vicinity of the S ₁ ← S ₀ 6₀ ¹ vibronic transition of the monomer was recorded with sub-Doppler resolution by utilizing mass-selective two-color resonance-enhanced two-photon ionization. Contrary to the previous report on the benzene-H₂ complex [M. Hayashi and Y. Ohshima, J. Phys. Chem. A 117, 9819 (2013)], both spin isomers correlating to para and ortho D₂ (with rotational angular momentum j = 1 and 0, respectively) are identified by using a gas sample of normal D₂. Three and two vibronic bands involving vdW-mode excitation were observed for the para and ortho species, respectively, in addition to their origin bands. Comparison of the results for the two spin isomers has allowed us to make unambiguous band assignments, and vibrational frequencies of all the three vdW modes have been determined for benzene-H₂ and -D₂. Among the three modes, the two-dimensional vdW twist is correlated to the hindered internal rotation of H₂/D₂ and the barrier for the internal rotation has been evaluated: 72 and 66 cm^-1 for benzene-H₂ and -D₂, respectively. Vibronic-state dependence of the intermolecular distance between benzene and H₂/D₂ is discussed on the basis of precisely determined rotational constants. Homogenous line broadening has been identified for all the observed vibronic bands, and the corresponding upper-state lifetimes are determined to be in the range of 0.3-0.7 ns.