Infrared Study of Electron-bombarded Phenanthrene (C₁₄H₁₀)/<i>Para</i>-H₂ Matrices: Isomers of Protonated Phenanthrene (1-, 3-, 4-, and 9-H⁺C₁₄H₁₀)

Protonated polycyclic aromatic hydrocarbons are potential carriers of the unidentified infrared (UIR) emission bands observed in interstellar space. Laboratory-generated infrared (IR) spectra of these protonated species can aid in identifying the molecular origins of these bands. In this study, a mixture of phenanthrene (C₁₄H₁₀) and <i>p</i>-H₂ was subjected to electron bombardment during deposition at 3.2 K, yielding new IR absorption features. These features decrease over time during maintenance of the matrix in darkness, consistent with the behavior of protonated species. Based on their photochemical responses to secondary laser irradiation at 619, 544, 524, and 463 nm, the features were categorized and assigned to four previously unreported isomers of protonated phenanthrene, 1-, 3-, 4-, and 9-H⁺C₁₄H₁₀. Spectral assignments were supported by comparisons with scaled harmonic vibrational wavenumbers and IR intensities of possible candidates predicted with the B3LYP/6-311++G(d,p) method. The IR spectra of these isomers show prominent absorption in the 6-9 μm range but lack significant features near 11.3 μm, indicating that protonated phenanthrene is unlikely to be a major contributor to the UIR bands.