The toluene-Ar complex: S0 and S1 van der Waals modes, changes to methyl rotation, and torsion-van der Waals vibration coupling

The methyl rotor and van der Waals vibrational levels in the S₁ and S₀ states of toluene-Ar have been investigated by the technique of two-dimensional laser induced fluorescence (2D-LIF). The S₀ van der Waals and methyl rotor levels are reported for the first time, while improved S₁ values are presented. The correlations seen in the 2D-LIF images between the S₀ and S₁ states lead to a reassignment of key features in the S₁ ← S₀ excitation spectrum. This reassignment reveals that there are significant changes in the methyl rotor levels in the complex compared with those in bare toluene, particularly at low m. The observed rotor energies are explained by the introduction of a three-fold, V₃, term in the torsion potential (this term is zero in toluene) and a reduction in the height of the six-fold, V₆, barriers in S₀ and S₁ from their values in bare toluene. The V₃ term is larger in magnitude than the V ₆ term in both S₀ and S₁. The constants determined are V₃(S₁) = 33.4 ± 1.0 cm^-1, V₃(S₀) = 20.0 ± 1.0 cm^-1, V ₆(S₁) = -10.7 ± 1.0 cm^-1, and V ₆(S₀) = -1.7 ± 1.0 cm^-1. The methyl rotor is also found to couple with van der Waals vibration; specifically, the m^″ = 2 rotor state couples with the combination level involving one quantum of the long axis bend and m^″ = 1. The coupling constant is determined to be 1.9 cm^-1, which is small compared with the values typically reported for torsion-vibration coupling involving ring modes.