Mode-Selective Vibrational-Tunneling Dynamics in the N=2 Triad of the Hydrogen-Bonded (HF)2 Cluster

Rovibrationally resolved spectra of the N_j=2₂, K_a=0←1 transition and of the N_j=2₃, K_a=0←0 and K_a=1←0 transitions of the hydrogen-bonded (HF)₂ have been measured in the near infrared range near 1.3 μm by cw-diode laser cavity ring-down spectroscopy in a pulsed supersonic slit jet expansion. The spectroscopic assignment and analysis provided an insight into the dynamics of these highly-excited vibrational states, in particular concerning the predissociation of the hydrogen bond and the tunneling process of the hydrogen bond switching. Together with our previously analyzed spectra of the N_j=2₁ and N_j=2₂ components, the mode-specific dynamics in all three components of this triad can now be compared. In the N=2 triad, the HF-stretching vibration is excited by two quanta with similar excitation energy, but the quanta are distributed in three different ways, which has a distinct influence on the dynamics. The observed band centers and tunneling splittings are in agreement with our recent calculations on the (HF)₂ potential energy hypersurface SO-3, resolving the long-standing discussion about the symmetry ordering of polyad levels in this overtone region. The results are also discussed in relation to the general questions of non-statistical reaction dynamics of polyatomic molecules and clusters and in relation to quasi-adiabatic channel above barrier tunneling.