Vibrational spectroscopy and predissociation dynamics of the electronically excited HgAr₂ triatomic cluster

Vibronic structures of the Ã-X̃ and B̃-X̃ bands of the HgAr₂ triatomic cluster were investigated by laser-induced fluorescence (LIF) spectroscopy. The LIF spectra of the Ã-X̃ and B̃-X̃ bands of HgAr₂ were observed free from hindrance of strong A-X and B-X transitions of HgAr by monitoring the fluorescence in the visible region. The analysis of the vibronic structures led to the fundamental frequencies of ω₁=26.3(3), ω₂=12.0(5), and ω₃=28.1(4) cm^-1 for the à state and ω₁=9.2(3) and ω₂=23.8(6) cm^-1 for the B̃ state, where subscripts "1," "2," and "3," respectively, denote symmetric stretching, bend, and asymmetric stretching modes. A number in parentheses represents an estimated limit of error (σ). It was found that HgAr₂ in the B̃ state predissociates to produce HgAr(A)+Ar. The vibrational and rotational distributions of the HgAr fragment in the A state were derived by observing LIF spectra of the E-A band. Based on these distributions, level energies of HgAr₂ in the B̃ state were estimated with respect to those of HgAr in the A state, and then, the dissociation energies (D_e) of HgAr₂ to Hg+Ar+Ar were derived to be 374(2), 540(2), and 249(2) cm^-1 for the X̃, Ã, and B̃ states, respectively. The vibronic structure of HgAr₂ in the lowest Rydberg (Ẽ) state associated with Hg (7 ³S₁) was investigated by the optical-optical double resonance spectroscopy via the à state. The geometrical structure, in which an outer Ar atom is loosely bound to the Rydberg-type HgAr core, that is close to HgAr⁺, is proposed for the Ẽ state, and a presence of large amplitude motion of the outer Ar atom is deduced from the obtained fundamental frequencies of ω₁=102. 1(3) and ω₂=5.4(2) cm^-1, where subscripts "1" and "2" denote a stretching mode of the HgAr core and the Ar-Hg-Ar bending mode.