Reassignment of the 11 537 cm<SUP>-1</SUP> Band of Hydrogen Fluoride Dimer and Observation of the Intermolecular Combination Mode 3ν<INF>1</INF> + ν<INF>4</INF>

We reexamine the N = 3 valence excitations of (HF)<INF>2</INF> and their combinations with intermolecular vibrations using a high-sensitivity germanium detector which collects the first overtone emission of fragment HF. We use the specific vibrational product state production to assign the quantum numbers within a vibrational polyad. The band previously assigned to K = 1 of ν<INF>1</INF> + 2ν<INF>2</INF> (J. Chem. Phys. <BO>1994</BO>, 100, 1) is shown to originate from K = 1 of 3ν<INF>2</INF> + ν<INF>6</INF>. This assignment, based on photofragment HF vibrational state, is supported by the observation of quenched hydrogen interchange tunneling (Δν<INF>t</INF> = −0.6 GHz) and rapid vibrational predissociation [Δν<INF>pd</INF> = 3.5(10) GHz] of this state. The K = 1 band origin of the lower A^- level is 11 537.047(6) cm^-1. The rotational constants for the two tunneling components are the same within experimental error, (&Bmacr;) = 0.2182(2) cm^-1. The out-of-plane vibration frequency, 3ν<INF>2</INF> + ν<INF>6</INF> − 3ν<INF>2</INF> = 493.96(3) cm^-1, is increased 25% from the ground state. The predissociation rate of this combination state is a factor of three slower than that observed at 3ν<INF>2</INF>. The combination mode 3ν<INF>1</INF> + ν<INF>4</INF> has band origins of ν<INF>0</INF> = 11 402.889(4) and 11 402.867(8) cm^-1 and rotational constants of &Bmacr; = 0.216 39(17) and 0.217 04(15) cm^-1 for the two tunneling components A⁺ and B⁺, respectively. The tunnel splitting Δν<INF>t</INF> = ν<INF>0</INF>(B⁺) − ν<INF>0</INF>(A⁺) = −0.021(8) cm^-1. The frequency of ν<INF>4</INF> the intermolecular or hydrogen bond stretching vibration, 3ν<INF>1</INF> + ν<INF>4</INF> − 3ν<INF>1</INF> = 129.36 cm^-1, is quite similar to that at ν<INF>1</INF>, suggesting only a minor dependence of the hydrogen bond vibration on the free-HF bond length. The 3ν<INF>1</INF> + ν<INF>4</INF> band has a predissociation linewidth of 2.5(2) GHz, one order of magnitude larger than the 0.24(2) GHz of the pure overtone 3ν<INF>1</INF> state. The coupling of this level to the dark state 3ν<INF>2</INF> + ν<INF>4</INF> + ν<INF>5</INF> is suggested as the origin of the observed linewidth increase.