Nuclear spin/parity dependent spectroscopy and predissociation dynamics in v OH = 2 ← 0 overtone excited Ne-H 2 O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2 O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1 ) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02- 〉 and |02+ 〉) out of the ortho (101 ) and para (000 ) internal rotor states of the H2 O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2 O angular momentum along the Ne-H2 O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2 O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2 O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2 O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2 O subunit (i.e., |02± 〉 → {|01± 〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2 ,3/2 ), and lambda doublet product distributions.