Large-amplitude motion in highly quantum clusters: high-resolution infrared absorption studies of jet-cooled H2–HCl and H2–DCl Original Research Article

High-resolution infrared spectroscopy is used to interrogate a series of inter and intramolecular vibrational quantum states in jet-cooled ortho and para H2–HCl and H2–DCl complexes, which as a result of weak binding and large zero-point effects provide a novel dynamical window into large-amplitude motion in highly quantum mechanical clusters. The fundamental vHCl=1←0 stretch bands of H2–HCl/DCl are observed and elucidate dramatic differences in the vibrationally averaged intermolecular alignment of the H2 subunit, i.e., T-shaped vs. more nearly isotropic for the lowest ortho (Π) and para (Σ) nuclear spin states, respectively. The two internal-rotor states correlating with H2(j=1) in the o-H2–HCl complex are observed via fundamental and combination band excitation built on vHCl=1←0. The 8.5 cm−1 internal-rotor splitting between the ground (Π) and excited (Σ) H2 alignments confirms the T-shaped minimum energy configuration for the intermolecular potential, with the HCl proton donating into the H2 subunit. At even higher energies for o-H2–HCl, a rich but highly perturbed spectrum of combination band transitions is observed due to the strongly Coriolis coupled manifold ((2jH2+1)(2jHCl+1)=9) of levels correlating with H2(j=1) and HCl(j=1) subunits. Rotational predissociation broadening accompanied by an abrupt cut off in these combination band spectra is observed and used to estimate a dissociation energy window of D0=45±2 and 47±2 cm−1 for the vHCl=0 and 1 intramolecular HCl stretching states, respectively, of the o-H2–HCl complex.