Spectroscopy of molecules confined in solid para-hydrogen

Solid para-hydrogen (p-H₂) has emerged as a novel low-temperature matrix host with unique characteristics associated with a quantum solid. In this chapter, we introduce p-H₂ matrix-isolation spectroscopy, presenting instrumentation and methods, the properties of solid p-H₂, and varied spectral applications. For stable molecules, infrared spectra of methane at high resolution, propene, CH₃OH and CH₃F with internal rotation, molecular complexes with ortho-H₂, and ultraviolet spectra of NO are discussed. The diminished cage effect allows the preparation of free radicals from photolysis in situ, either directly or via bimolecular reactions, such as Cl + unsaturated hydrocarbons to form radical products. Another novel method is the production of protonated and hydrogenated species on electron bombardment during deposition at 3.2 K. We have applied this method to small molecules (e.g., OCS), proton-bound dimers (e.g., N₂-H⁺-N₂) and polycyclic aromatic hydrocarbons that might be the carriers of unidentified infrared bands in astronomy. Hydrogen addition and abstraction reactions are also studied with photolytically produced H atoms from Cl + H₂ (v = 1); several reactions with nitrogen-containing molecules (e.g., HONO and formamide) revealed reaction networks that are important for chemical evolution in space. Finally we present a future perspective on research using p-H₂ matrix-isolation spectroscopy.