REASSIGNMENT OF THE $11537 cm^{-1}$ BAND OF HYDROGEN FLORIDE DIMER AND OBSERVATION OF THE INTERMOLECULAR COMBINATION MODE $3\nu_{1} + \nu_{4}$

Author Institution: Department of Chemistry, Harvard University; Institute of Atomic and Molecular Sciences, Academica SinicaWe report a high resolution re-examination of K=1 subband of $3\nu_{2} + \nu_{4}$ and the $3\nu_{1}+\nu_{1}(K=0)$ combination mode of $(HF)_{2}$ by intracavity laser induced fluorescence. The fluorescence is monitored with a germanium detector which collects only first overtone emission $(2\rightarrow 0)$ of HF fragments. The band centered at $11537 cm^{-1}$ is shown to be K=1 of $3\nu 2 + \nu_{6}$ [previously assigned to K=1 of $\nu_{1} + 2\nu_{2}$, {J. Chem. Phys}. 100, 1 (1994)]. Hydrogen interchange tunneling is almost completely quenched ($\Delta \nu_{t}=0.6$ GHz). Vibrational predissociation rate is rapid (4.5 GHz) but half that of 10GHz observed in $3\nu_{2}$. The combination mode $3\nu_{1} + \nu_{4}$ is recorded with band origins of 11402.889(4) and $11402.868(8) cm^{-1}$ and rotational constants of 0.21639(17) and $0.21704(15) cm^{-1}$ for the two tunneling components $A^{+}$ and $B^{+}$, respectively. The frequency of this intermolecular vibration, $\nu_{4}$, upon $3\nu_{1}$ excitation is $129.36 cm^{-1}$, quite similar to that at the fundamental, suggesting only a minor dependence of the hydrogen bond vibration on the free-HF bond length. The predissociation linewidth for $3\nu_{1} + \nu_{4}$ mode is 2.5(2) GHz, which is one order of magnitude larger than the 0.24(2) GHz of the $3\nu_{1}$ state. The coupling of this state to the dark state of $3\nu_{2} + \nu_{4} + \nu_{5}$ is suggested for the origin of the observed linewidth increase