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

Rovibrationally resolved spectra of the Nj=22, Ka=0←1 transition and of the Nj=23, Ka=0←0 and Ka=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 Nj=21 and Nj=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.