Millimeter-wave spectroscopy of vdW bands of He–HCN near the dissociation limit and the determination of the intermolecular potential

Transitions to the internal rotation second excited state (j = 2) and the intermolecular stretching first excited state (vs = 1) of He–HCN, located close to the dissociation limit (by 0.2633 and 0.1759 cm−1 lower than the limit, respectively), were observed by millimeter-wave spectroscopy using the supersonic jet technique. The observed transition frequencies were fitted, together with those of the internal rotation fundamental band (j = 1 − 0), to the effective Hamiltonian with the intermolecular potential V(R, θ) expanded with the asymptotic long-range potential coefficients cn,l. The potential energy surface has a global minimum at the linear configuration (He⋯H–C–N) with a depth (De) of 30.150 cm−1 and a saddle point at the anti-linear configuration (He⋯N–C–H) located 9.014 cm−1 higher than the global minimum. The predissociation lifetimes of the quasi-bound states (τ = 0.16–0.29 μs) were derived from the line broadening of the observed transitions and agreed well with the calculated results with the radial probability density outside the centrifugal barrier.