First results are reported on overtone (v OH = 2 ← 0) spectroscopy of weakly bound H 2 -H 2 O complexes in a slit supersonic jet, based on a novel combination of (i) vibrationally mediated predissociation of H 2 -H 2 O, followed by (ii) UV photodissociation of the resulting H 2 O, and (iii) UV laser induced fluorescence on the nascent OH radical. In addition, intermolecular dynamical calculations are performed in full 5D on the recent ab initio intermolecular potential of Valiron et al. [J. Chem. Phys. 129, 134306 (2008)] in order to further elucidate the identity of the infrared transitions detected. Excellent agreement is achieved between experimental and theoretical spectral predictions for the most strongly bound van der Waals complex consisting of ortho (I = 1) H 2 and ortho (I = 1) H 2 O (oH 2 -oH 2 O). Specifically, two distinct bands are seen in the oH 2 -oH 2 O spectrum, corresponding to internal rotor states in the upper vibrational manifold of and rotational character. However, none of the three other possible nuclear spin modifications (pH 2 -oH 2 O, pH 2 -pH 2 O, or oH 2 -pH 2 O) are observed above current signal to noise level, which for the pH 2 complexes is argued to arise from displacement by oH 2 in the expansion mixture to preferentially form the more strongly bound species. Direct measurement of oH 2 -oH 2 O vibrational predissociation in the time domain reveals lifetimes of 15(2) ns and <5(2) ns for the and states, respectively. Theoretical calculations permit the results to be interpreted in terms of near resonant energy levels and intermolecular alignment of the H 2 and H 2 O wavefunctions, providing insight into predissociation dynamical pathways from these metastable levels.

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-HO complexes (D = 31.67 cm, D = 34.66 cm) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with v = 2 ← 0 overtone transitions (|02⟩←|00⟩ and |02⟩←|00⟩) out of the ortho (1) and para (0) internal rotor states of the HO moiety. Quantum theoretical calculations for nuclear motion on a high level ab initio potential energy surface (CCSD(T)/VnZ-f12 (n = 3,4), corrected for basis set superposition error and extrapolated to the complete basis set limit) are employed for assignment of Σ←Σ,Π←Σ, and Σ←Π infrared bands in the overtone spectra, where Σ(K = 0) and Π (K = 1) represent approximate projections (K) of the body angular momentum along the Ne-HO internuclear axis. End-over-end tumbling of the ortho Ne-HO cluster is evident via rotational band contours observed, with band origins and rotational progressions in excellent agreement with ab initio frequency and intensity predictions. A clear Q branch in the corresponding |02⟩Π(1)←Σ(0) para Ne-HO spectrum provides evidence for a novel e/f parity-dependent metastability in these weakly bound clusters, in agreement with ab initio bound state calculations and attributable to the symmetry blocking of an energetically allowed channel for internal rotor predissociation. Finally, Boltzmann analysis of the rotational spectra reveals anomalously low jet temperatures (T ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-HO clusters and provide support for similar cooling dynamics in rare gas-tagging studies.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-HO complexes (D = 34.66 cm and D = 31.67 cm) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with v = 2 ← 0 overtone transitions (|02〉 and |02〉) out of the ortho (1) and para (0) internal rotor states of the HO 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 HO angular momentum along the Ne-HO 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 HO v = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τ = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-HO complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-HO 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 HO subunit (i.e., |02〉 → {|01〉; v = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π,), and lambda doublet product distributions.