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

Rovibrationally resolved spectra of the N-j=2(2), K-a=0 & LARR;1 transition and of the N-j=2(3), K-a=0 & LARR;0 and K-a=1 & LARR;0 transitions of the hydrogen-bonded (HF)(2) have been measured in the near infrared range near 1.3 & mu;m 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=2(1) and N-j=2(2) 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.