High resolution ultraviolet spectroscopy has been used to investigate the rotationally resolved excitation spectrum of the first singlet-singlet transition in the benzoic acid dimer. The measured spectrum consists of two overlapping components. The corresponding lines in the two components are shown to originate in different levels of the ground state potential separated by a tunneling splitting produced by concerted proton exchange between the two subunits forming the dimer. The frequency separation between the two components is equal to the difference between the tunneling splittings in the ground and the excited electronic state. This frequency separation is found to be 1107Ϯ7 MHz. From the analysis, it is estimated that the barrier for proton tunneling changes by about 20% upon electronic excitation. The structure of the dimer in the ground state is determined to be linear, while in the excited S 1 state it is slightly bent (3.4°Ϯ1.7°).

The lowest frequency parallel fundamental band ν5 of CH3SiH3 near 700 cm−1 has been measured at a resolution of 0.004 cm−1 with Fourier transform spectroscopy to investigate vibration–torsion–rotation interactions in symmetric tops. The torsional splittings in the spectrum are increased from ∼0.005 cm−1 to ∼1 cm−1 by Fermi-type vibration–torsion interactions between the torsional stack (v6=0,1,2,…) in the ground vibrational state and the corresponding stack for v5=1. Resonant interactions were observed between the states (v5=1,v6=0) and (v5=0,v6=5) for the rotational series with (k=±1,σ=∓1), where σ labels the torsional sublevels. In this resonance, the two unperturbed states are near opposite limits for torsional motion: (v5=0,v6=5) involves nearly free rotation, while (v5=1,v6=0) involves small amplitude torsional oscillation. For the (k=±1,σ=∓1) rotational series, perturbation-allowed transitions in the high overtone (v6=5←0) were observed. Over 750 frequencies measured here have been analyzed together with more than 2500 measurements involved in the recent analysis of the lowest-lying degenerate fundamental band ν12 given by Moazzen-Ahmadi et al. [J. Mol. Spectrosc. 175, 54 (1996)]. A fit to within experimental error was achieved using 41 parameters, an increase of only 4 when the new band is added. The analysis shows that the inclusion of the Fermi-type interactions leads to a considerable simplification of the Hamiltonian for the ground vibrational state. For example, both the second and third terms (V0,6,V0,9) in the Fourier expansion of the hindering potential as well as the torsional flexing term (F0,m) vanish in the ground state. The changes in the leading terms in the torsional Hamiltonian have been quantitatively explained by a contact transformation. The large perturbations produced by the interaction matrix elements off-diagonal by 5 units in v6 have serious implications for vibrational relaxation in molecules undergoing internal rotation.

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