Superhydrogenation of indene at low temperatures

TL;DR

This study investigates the low-temperature hydrogenation process of indene, a polycyclic aromatic hydrocarbon, highlighting the role of tunneling and providing kinetic parameters for astrophysical modeling.

Contribution

It presents the hydrogenation sequence of indene, confirms rules with DFT barriers, and introduces a Monte Carlo method for optimizing kinetic fit parameters for astrophysical applications.

Findings

Hydrogenation of indene follows rules similar to other PAHs.

Tunneling significantly promotes hydrogenation at 30-75 K.

Rate limiting step identified as the third hydrogenation step.

Abstract

The hydrogenation of polycyclic aromatic hydrocarbons (PAHs) is crucial to understanding molecular hydrogenation formation in the interstellar medium. This process also helps to elucidate the weakening of the aromatic bonds in PAHs, which may function as a carbon reservoir. Tunneling can significantly promote the hydrogenation process in a low to moderate temperature range. We present the hydrogenation sequence of the newly observed PAH molecule, indene, and clarify the tunneling rule at temperature in photodissociation region (PDR) and dark molecular cloud conditions. In addition, we report fit parameters to be utilized in astronomical modeling. The hydrogenation sequence was studied using simple hydrogenation rules and confirmed by barriers from density functional theory (DFT). To make our kinetic studies useful to modelers, we implemented a Monte Carlo method based program to generate and optimize random initial fit parameters (alpha, beta, gamma, and T0) to achieve the statistically best fit. We find that indene hydrogenation follows rules similar to those of other PAHs, such as pentacene, coronene, and corannulene, with binding energies for odd numbered hydrogenation steps ranging from 0.5 to 2 eV and barriers around 0.13 eV for the first, fifth, and seventh hydrogenation steps. The third hydrogenation step is the rate limiting step, similar to what is found for other PAHs. Even numbered hydrogenation steps have lower barriers and lead to more stable intermediates as a result of radical recombinations. The hydrogenation sequence follows a scheme that strongly depends on the PAH's shape, the number of aromatic rings, and the presence of five membered rings. Furthermore, we observe that tunneling plays an important role in the hydrogenation of indene at temperatures between 30 and 75 K, which corresponds to the temperatures of dust in PDRs.