Large-amplitude motion in highly quantum clusters: high-resolution infrared absorption studies of jet-cooled H-2-HCl and H-2-DCl

High-resolution infrared spectroscopy is used to interrogate a series of in ter and intramolecular vibrational quantum states in jet-cooled ortho and p ara H-2-HCl and H-2-DCl complexes, which as a result of weak binding and la rge zero-point effects provide a novel dynamical window into large-amplitud e motion in highly quantum mechanical clusters. The fundamental v(HCl) = 1 <-- 0 stretch bands of H-2-HCl/DCl are observed and elucidate dramatic diff erences in the vibrationally averaged intermolecular alignment of the H-2 s ubunit, i.e., T-shaped vs. more nearly isotropic for the lowest ortho (Pi) and para (Sigma) nuclear spin states, respectively. The two internal-rotor states correlating with H-2(j = 1) in the o-H-2-HCl complex are observed vi a fundamental and combination band excitation built on v(HCl) = 1 <-- 0. Th e 8.5 cm(-1) internal-rotor splitting between the ground (Pi) and excited ( Sigma) H-2 alignments confirms the T-shaped minimum energy configuration fo r the intermolecular potential, with the HCl proton donating into the H-2 s ubunit. At even higher energies for o-H-2-HCl, a rich but highly perturbed spectrum of combination band transitions is observed due to the strongly Co riolis coupled manifold ((2j(H2) + 1)(2j(HCl) + 1)= 9) of levels correlatin g with H-2(j = 1) and HCl(j = 1) subunits. Rotational predissociation broad ening accompanied by an abrupt cut off in these combination band spectra is observed and used to estimate a dissociation energy window of D-o = 45 +/- 2 and 47 +/- 2 cm(-1) for the v(HCl) = 0 and 1 intramolecular HCl stretchi ng states, respectively, of the o-H-2-HCl complex. (C) 1998 Published by El sevier Science B.V. All rights reserved.