Near infrared overtone (v OH = 2 ← 0) spectroscopy of Ne-H 2 O clusters.

Vibrationally state selective overtone spectroscopy and dynamics of weakly bound Ne-H2 O complexes (D0 (para) = 31.67 cm-1 , D0 (ortho) = 34.66 cm-1 ) are reported for the first time, based on near infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02- ⟩←|00+ ⟩ and |02+ ⟩←|00+ ⟩) 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 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-H2 O internuclear axis. End-over-end tumbling of the ortho Ne-H2 O 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+ ⟩f Π(111 )←e Σ(000 ) para Ne-H2 O 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 (Trot ≈ 4(1) K), which are attributed to "evaporative cooling" of weakly bound Ne-H2 O clusters and provide support for similar cooling dynamics in rare gas-tagging studies.