Unique Applications of para-Hydrogen Matrix Isolation to Spectroscopy and Astrochemistry

Abstract

Cryogenic solid para-hydrogen (p-H₂) exhibits pronounced quantum effects, enabling unique experiments that are typically not possible in noble-gas matrices. The diminished cage effect facilitates the production of free radicals via in situ photolysis or photoinduced reactions. Electron bombardment during deposition readily produces protonated and hydrogenated species, such as polycyclic aromatic hydrocarbons, that are important in astrochemistry. In addition, quantum diffusion delocalizes hydrogen atoms in solid p-H₂, allowing efficient H atom reactions with astrochemical species and introducing new concepts in astrochemical models. Some H atom reactions display anomalous temperature behaviors, highlighting the rich chemistry in p-H₂. The investigation on quantum diffusion of heavier atoms and molecules is also important for our understanding of the chemistry in interstellar ice. Additionally, matrix shifts of electronic transitions of polycyclic aromatic hydrocarbons in p-H₂ are less divergent than those in solid Ne such that systematic measurements in p-H₂ might help in the assignment of diffuse interstellar bands.

Figure 1

Bevel-gear-type motion of •C₆H₆Br and its infrared spectrum. The benzene ring undergoes a bevel-gear-type motion with respect to Br in η₁-^•C₆H₆Br. η₂-^•C₆H₆Br is a transition state with a barrier of ∼1 kJ mol^–1. Spectral lines indicated with B (green) are due to η₁-^•C₆H₆Br. In the green box, two lines of out-of-plane CH bending modes were predicted for η₁-^•C₆H₆Br, but only one broad feature was observed. Partially reproduced from ref (27). Copyright 2023 American Chemical Society.

Figure 2

H atom tunneling reaction in solid p-H₂. (a) Potential energy scheme of Cl + H₂. The reaction is endothermic by ∼4 kJ mol^–1 (370 cm^–1) with a barrier of ∼21 kJ mol^–1 (1720 cm^–1). The reaction proceeds upon IR irradiation. (b) H atom tunneling in the solid p-H₂. The pair of blue balls indicates H₂, and the orange, green, and brown balls indicate H atom, reactant, and product, respectively. The red box indicates the formation of a new bond. The H atom does not have to diffuse physically through the matrix to approach the reactant.

Figure 3

Kinetic plots for the 193 nm photolysis (10 min, 18 mW cm^–2) induced reaction study conducted on a HC(O)OH/p-H₂ sample as a function of temperature. The top graph shows the temperature of sample T_B, and the bottom graph shows the measured HOCO (blue circles) and HCO (red circles) concentrations. This experiment shows that for a sample photolyzed at 4.3 K, the reaction that produces HOCO after photolysis only starts the first time the temperature is lowered below ∼2.7 K. The kinetics of this reaction again qualitatively changes at later times when the temperature is raised above 3.6 K. Partially reproduced from ref (70). Copyright 2014 American Chemical Society.

Figure 4

Ozone growth kinetics for an annealed O₂/p-H₂ sample recorded at 1.71(1) K after a 0.5 min 193 nm photolysis exposure. The O₃ concentration is plotted as black circles, and the red line is the result of a least-squares fit of the data to a first-order kinetics equation. Partially reproduced from ref (74). Copyright 2023 American Chemical Society.

Figure 5

Matrix shifts for species in solid p-H₂ and Ne as a function of mass m. Solid symbols: species in solid p-H₂; open symbols: species in solid Ne. Symbols: circles: PAH (black for planar PAH and blue for nonplanar PAH in solid p-H₂); square: hydrogenated PAH; triangles: protonated PAH. The average matrix shift of planar PAH in p-H₂ (black solid circles) is 93 ± 9 cm^–1 and that of all species in p-H₂ (solid symbols) is 70 ± 28 cm^–1, as indicated with dashed lines and gray regions representing listed uncertainties as one standard deviation in fitting.