[Paper Review] Quantum nature of molecular vibrational quenching: Water - molecular hydrogen collisions

L. Wiesenfeld|arXiv (Cornell University)|Jul 18, 2021

Advanced Chemical Physics Studies50 references14 citations

TL;DR

This study presents the first fully quantum ab initio computation of ro-vibrational quenching in H2O–H2 collisions using a converged coupled-channel approach on a full-dimensional 9D potential energy surface. It reveals that excitation of the H2 rotor (j₂ = 1 → 3) dominates vibrational quenching, increasing rates by orders of magnitude compared to classical or non-rotating projectile models, highlighting the essential role of quantum dynamics even at high energies and large vibrational energy transfer.

ABSTRACT

Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allows 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 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 principle computation, potential and dynamics, converging it at all stages, in a full coupled channel formalisms. We present here rates for the quenching of the first bendingmode of ortho-H2O by ortho H2, up to 500K, in a fully converged coupled channels formalism.

Motivation & Objective

• To compute accurate ro-vibrational quenching rates for H2O–H2 collisions relevant to warm astrophysical gases and exoplanet atmospheres.
• To investigate whether quantum effects persist in vibrational quenching despite large energy transfers and high quantum state populations.
• To assess the impact of including rotational states of the projectile (H2) on quenching cross sections and rates.
• To challenge the classical approximation in collisional energy transfer by demonstrating strong quantum dependence on H2 rotational levels.

Proposed method

• Employed a full-dimensional 9D potential energy surface (val08) for H2O–H2, including 5 intermolecular and 4 intramolecular coordinates.
• Used time-independent quantum scattering with a converged coupled-channel formalism, solving the S-matrix for coupled vibrational-rotational states.
• Constructed block-diagonal potential matrices (W₀₀, W₁₁, V₀₁) by averaging the 9D potential over vibrational states and coupling via spherical harmonics.
• Treated the H2 projectile quantum mechanically with rotational quantum numbers j₂ = 0, 1, 3, and included nuclear spin symmetry for ortho-H2.
• Performed computations in a basis of total angular momentum J and inversion symmetry, using a coarse energy grid to avoid resonance details.
• Validated results via comparison with a toy model using N2 as a heavier projectile, showing similar quantum effects persist.

Experimental results

Research questions

• RQ1 Does quantum dynamics significantly influence vibrational quenching rates in H2O–H2 collisions, even at high collision energies and large vibrational energy transfer?
• RQ2 How does including rotational excitation of the H2 projectile affect the magnitude and energy dependence of vibrational quenching cross sections?
• RQ3 To what extent do avoided crossings and adiabatic potential curves depend on the rotational state of the H2 projectile, and how do they influence coupling efficiency?
• RQ4 Can classical or semi-classical approximations reliably predict ro-vibrational quenching rates in this system, or are quantum effects dominant across the energy range studied?
• RQ5 How do the quantum numbers of both H2O and H2 influence the dominant quenching pathways, particularly in the context of rotational energy transfer?

Key findings

• The dominant quenching pathway is excitation of the H2 rotor from j₂ = 1 to j₂ = 3, which accounts for the majority of vibrational energy transfer.
• Including H2 rotational states increases quenching rates by at least a factor of 10 compared to models neglecting projectile rotation.
• The quenching cross sections exhibit strong dependence on the quantum state of the H2 projectile, indicating that quantum effects do not diminish with increasing energy.
• Avoided crossings in the adiabatic potential curves become significantly more numerous and complex when j₂ ≥ 1, enhancing coupling between vibrational and rotational channels.
• A toy model with N2 as the projectile confirms that the same quantum effects persist when the projectile has a smaller rotational constant, suggesting generality across light diatomics.
• The results imply that classical or statistical approximations may severely underestimate or misrepresent vibrational quenching rates in systems involving light, rotating projectiles.