Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

A New Full-Dimensional Ab Initio Intermolecular Potential Energy Surface and Rovibrational Energies of the H2O-H2 Complex

H2O-H2 is a prototypical five-atom van der Waals system, and the interaction between H2O and H2 plays an important role in many physical and chemical environments. However, previous full-dimensional intermolecular potential energy surfaces (IPESs) cannot accurately describe the H2O-H2 interaction in the repulsive or van der Waals minimum region. In this work, we constructed a full-dimensional IPES for the title system with a small root-mean-square error of 0.252 cm-1 by using the permutation invariant polynomial neural network method. The ab initio calculations were performed by employing the explicitly corrected coupled cluster [CCSD(T)-F12a] method with the augmented correlation-consistent polarized valence quintuple-ζ basis set. Based on the newly developed IPES, the bound states of the H2O-H2 complex were calculated within the rigid-rotor approximation. The transition frequencies and band origins agreed well with the experimental values [Weida, M. J.; Nesbitt, D. J. J. Chem. Phys. 1999, 110, 156-167] with errors less than 0.1 cm-1 for most transitions. Those results demonstrate the high accuracy of our new IPES, which would build a solid foundation for the collisional dynamics of H2O-H2 at low temperatures.

An intramolecular vibrationally excited intermolecular potential energy surface and predicted 2OH overtone spectroscopy of H2O-Kr

A new five-dimensional potential energy surface (PES) for H2O-Kr which explicitly includes the intramolecular 2OH overtone state of the H2O monomer is presented. The intermolecular potential energies were evaluated using explicitly correlated coupled cluster theory [CCSD(T)-F12] with a large basis set. Four vibrationally averaged analytical intermolecular PESs for H2O-Kr with H2O molecules in its |00+〉, |02+〉, |02-〉, and |11+〉 states are obtained by fitting to the multi-dimensional Morse/Long-Range potential function form. Each vibrationally averaged PES fitted to 578 points has root-mean-square (RMS) deviations smaller than 0.14 cm-1 and requires only 58 parameters. The combined radial discrete variable representation/angular finite basis representation method and the Lanczos algorithm were employed to calculate the rovibrational energy levels for |00+〉, |02+〉, |02-〉, and |11+〉 states of the H2O-Kr complexes. The calculated |02-〉Πf/e(101) ← |00+〉Σe(000) and |02+〉Πf/e(110) ← |00+〉Σe(101) infrared transitions are in excellent agreement with the experimental values with RMS discrepancies being only 0.007 and 0.016 cm-1, respectively. These analytical PESs can be used to provide reliable theoretical guidance for future infrared overtone spectroscopy of H2O-Kr.

Near-Threshold and Resonance Effects in Rotationally Inelastic Scattering of D2O with Normal-H2

We present a combined experimental and theoretical study on the rotationally inelastic scattering of heavy water, D2O, with normal-H2. Crossed-molecular beam measurements are performed in the collision energy range between 10 and 100 cm-1, corresponding to the near-threshold regime in which scattering resonances are most pronounced. State-to-state excitation cross-sections are obtained by probing three low-lying rotational levels of D2O using the REMPI technique. These measurements are complemented by quantum close-coupling scattering calculations based on a high-accuracy D2O-H2 interaction potential. The agreement between experiment and theory is within the experimental error bars at 95% confidence intervals, leading to a relative difference of less than 7%: the near-threshold rise and the overall shape of the cross-sections, including small undulations due to resonances, are nicely reproduced by the calculations. Isotopic effects (D2O versus H2O) are also discussed by comparing the shape and magnitude of the respective cross-sections.

The He–H3+ complex. I. Vibration-rotation-tunneling states and transition probabilities

With a He–[Formula: see text] interaction potential obtained from advanced electronic structure calculations, we computed the vibration-rotation-tunneling (VRT) states of this complex for total angular momenta J from 0 to 9, both for the vibrational ground state and for the twofold degenerate v2 = 1 excited state of [Formula: see text]. The potential has three equivalent global minima with depth D e = 455.3 cm−1 for He in the plane of [Formula: see text], three equatorial saddle points that separate these minima with barriers of 159.5 cm−1, and two axial saddle points with energies of 243.1 cm−1 above the minima. The dissociation energies calculated for the complexes of He with ortho-[Formula: see text] (o[Formula: see text]) and para-[Formula: see text] (p[Formula: see text]) are D0 = 234.5 and 236.3 cm−1, respectively. Wave function plots of the VRT states show that they may be characterized as weakly hindered internal rotor states, delocalized over the three minima in the potential and with considerable amplitude at the barriers. Most of them are dominated by the j k = 10 and 11 rotational ground states of o[Formula: see text] and p[Formula: see text], with the intermolecular stretching mode excited up to v = 4 inclusive. However, we also found excited internal rotor states: 33 in He–o[Formula: see text], and 22 and 21 in He–p[Formula: see text]. The VRT levels and wave functions were used to calculate the frequencies and line strengths of all allowed v2 = 0 → 1 rovibrational transitions in the complex. Theoretical spectra generated with these results are compared with the experimental spectra in Paper II [Salomon et al., J. Chem. Phys. 156, 144308 (2022)] and are extremely helpful in assigning these spectra. This comparison shows that the theoretical energy levels and spectra agree very well with the measured ones, which confirms the high accuracy of our ab initio He–[Formula: see text] interaction potential and of the ensuing calculations of the VRT states.

Quantum nature of molecular vibrational quenching: Water-molecular hydrogen collisions

Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allow us to compute collisional line shapes and transport properties of gases. Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses, including exo-atmospheres. For a system of paramount importance in this context, the vibrational bending mode quenching of H₂O by H₂, we show here that the exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer. The excitation of the quantized rotor of the projectile is by far the most effective ro-vibrational quenching path of water. To do so, we use a fully quantum first-principles computation, potential and dynamics, converging it at all stages, in a full coupled channel formalism. We present here rates for the quenching of the first bending mode of ortho-H₂O by ortho-H₂, up to 500 K, in a fully converged coupled channel formalism.

Probing Low-Energy Resonances in Water-Hydrogen Inelastic Collisions

Molecular scattering at collisional energies of the order of 10-100  cm^{-1} (corresponding to kinetic temperatures in the 15-150 K range) provides insight into the details of the scattering process and, in particular, of the various resonances that appear in inelastic cross sections. In this Letter, we present a detailed experimental and theoretical study of the rotationally inelastic scattering of ground-state ortho-D_{2}O by ground-state para-H_{2} in the threshold region of the D_{2}O(0_{00}→2_{02}) transition at 35.9  cm^{-1}. The measurements were performed with a molecular crossed beam apparatus with variable collision angle, thence with variable collisional energy. Calculations were carried out with the coupled-channel method combined with a dedicated high-level D_{2}O-H_{2} intermolecular potential. Our theoretical cross section 0_{00}→2_{02} is found to display several resonance peaks in perfect agreement with the experimental work, in their absolute positions and relative intensities. We show that those peaks are mostly due to shape resonances, characterized here for the first time for a polyatomic molecule colliding with a diatom.

Low-Energy Water-Hydrogen Inelastic Collisions

New molecular beam scattering experiments are reported for the water-hydrogen system. Integral cross sections of the first rotational excitations of para- and ortho-H₂O by inelastic collisions with normal-H₂ were determined by crossing a beam of H₂O seeded in He with a beam of H₂. H₂O and H₂ were cooled in the supersonic expansion down to their lowest rotational levels. Crossed-beam scattering experiments were performed at collision energies from 15 cm^-1 (below the threshold for the excitation to the lowest excited rotational state of H₂O: 18.6 cm^-1) up to 105 cm^-1 by varying the beam crossing angle. The measured state-to-state cross-sections were compared to the theoretical cross-sections (close-coupling quantum scattering calculations): the good agreement found further validates both the employed potential energy surface describing the H₂O-H₂ van der Waals interaction and the state-to-state rate coefficients calculated with this potential in the very low temperature range needed for the modeling of interstellar media.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H₂O complexes (D_0(ortho) = 34.66 cm^-1 and D_0(para) = 31.67 cm^-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with v_OH = 2 ← 0 overtone transitions (|02^-〉 and |02⁺〉) out of the ortho (1₀₁) and para (0₀₀) internal rotor states of the H₂O 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 H₂O angular momentum along the Ne-H₂O 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 H₂O v_OH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τ_prediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H₂O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H₂O 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 H₂O 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 (Π_1/2,_3/2), and lambda doublet product distributions.