Low-Energy Water-Hydrogen Inelastic Collisions

Scattering resonances in the rotational excitation of HDO by Ne and normal-H2: theory and experiment

The rotational excitation of a singly deuterated water molecule (HDO) by a heavy atom (Ne) and a light diatomic molecule (H2) is investigated theoretically and experimentally in the near-threshold regime. Crossed-molecular-beam measurements with a variable crossing angle are compared to close-coupling calculations based on high-accuracy potential energy surfaces. The two lowest rotational transitions, 000 → 101 and 000 → 111, are probed in detail and a good agreement between theory and experiment is observed for both transitions in the case of HDO + Ne, where scattering resonances are however blurred out experimentally. In the case of HDO + H2, the predicted theoretical overlapping resonances are faithfully reproduced by experiment for the 000 → 111 transition, while the calculated strong signal for the 000 → 101 transition is not detected. Future work is needed to reconcile this discrepancy.

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.

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.

Towards chemistry at absolute zero

The prospect of cooling matter down to temperatures that are close to absolute zero raises intriguing questions about how chemical reactivity changes under these extreme conditions. Although some types of chemical reaction still occur at 1 μK, they can no longer adhere to the conventional picture of reactants passing over an activation energy barrier to become products. Indeed, at ultracold temperatures, the system enters a fully quantum regime, and quantum mechanics replaces the classical picture of colliding particles. In this Review, we discuss recent experimental and theoretical developments that allow us to explore chemical reactions at temperatures that range from 100 K to 500 nK. Although the field is still in its infancy, exceptional control has already been demonstrated over reactivity at low temperatures.

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.

Cold and controlled chemical reaction dynamics

State-to-state chemical reaction dynamics, with complete control over the reaction parameters, offers unparalleled insight into fundamental reactivity.

State-to-state inelastic rotational cross sections in five-atom systems with the multiconfiguration time dependent Hartree method

We present a MultiConfiguration Time Dependent Hartree (MCTDH) method as an attractive alternative approach to the usual quantum close-coupling method that approaches some computational limits in the calculation of rotational excitation (and de-excitation) between polyatomic molecules (here collisions between triatomic and diatomic rigid molecules). We have performed a computational investigation of the rotational (de-)excitation of the benchmark rigid rotor H₂O-H₂ system on a recently developed Potential Energy Surface of the complex using the MCTDH method. We focus here on excitations and de-excitations from the 0₀₀, 1₁₁, and 1₁₀ states of H₂O with H₂ in its ground rotational state, looking at all the potential transitions in the energy range 1-200 cm^-1. This work follows a recently completed study on the H₂O-H₂ cluster where we characterized its spectroscopy and more generally serves a broader goal to describe inelastic collision processes of high dimensional systems using the MCTDH method. We find that the cross sections obtained from the MCTDH calculations are in excellent agreement with time independent calculations from previous studies but does become challenging for the lower kinetic energy range of the de-excitation process: that is, below approximately 20 cm^-1 of collision energy, calculations with a relative modest basis become unreliable. The MCTDH method therefore appears to be a useful complement to standard approaches to study inelastic collision for various collision partners, even at low energy, though performing better for rotational excitation than for de-excitation.