Optical Spectrum of the Adamantane Radical Cation

Known for their stable structural and thermal properties, diamondoids and particularly their radical cations are viable candidates as carriers for diffuse interstellar bands. While previous research has mainly focused on neutral diamondoids and their derivatives, little is known about their radical cations, which may form in interstellar environments by ionizing radiation. We report the first experimental optical spectrum of the simplest diamondoid cation, the adamantane radical cation (${{\rm{C}}}_{10}{{{\rm{H}}}_{16}}^{+}$), obtained via electronic photodissociation spectroscopy of cryogenic ions in the 310–1000 nm range. The main fragmentation channels are H loss at low energies and C3H7 loss at higher energies. The optical spectrum reveals a broad band spanning the range of 420–850 nm, assigned to the D2(2E) ← D0(2A1) transition using time-dependent density functional theory calculations. Despite a vibrational temperature below 20 K, we observe no vibrational structure because of lifetime broadening and/or Franck–Condon congestion. A second band system originating at 345 nm that shows vibrational progressions is attributed to the overlapping D5(2A1)/D6(2E) ← D0(2A1) transitions split by the Jahn–Teller effect. The lifetime deduced from the widths of these vibronic bands is ∼30 fs. Comparison of the spectrum with known diffuse interstellar bands suggests that ${{\rm{C}}}_{10}{{{\rm{H}}}_{16}}^{+}$ is not likely to be a carrier. However, the strong absorption features in the UV to near-IR show promise in the investigation of higher-order diamondoids as potential candidates.