Optical Spectrum of the Diamantane Radical Cation

Abstract

Abstract Similarities exist between the infrared spectra of diamondoids and unidentified infrared emission bands seen in the spectra of young stars with circumstellar disks. Due to their low ionization energy and absorption in the visible range, the radical cations of these highly stable molecules are also suggested to contribute to the well-known but largely unassigned diffuse interstellar bands. However, thus far only the optical spectrum of the adamantane cation (Ada+) has been measured in the laboratory, which is required for astronomical identification. Herein, we present the optical spectrum of the diamantane radical cation (C14H20 +, Dia+, D 3d) between 400 and 1000 nm in the gas phase. The spectra are obtained by electronic photodissociation (EPD) of mass-selected ions generated by electron ionization and trapping in a cryogenic trap at 5 K. The most intense fragmentation channels are the losses of H and C4H9. The optical spectrum reveals two broad and unresolved bands centered near 760 and 450 nm that are assigned to the D 2(2 E u) ← D 0(2 A 1g) and D 5(2 A 2u) ← D 0(2 A 1g) transitions using density functional theory. Despite a vibrational temperature below 20 K, no vibrational structure is resolved. Franck–Condon simulations of the D 5 ← D 0 transition predict intense vibronic progressions that become indiscernible from the band contour at spectral widths above 350 cm−1. Thus, the lack of resolved spectral features is attributed to lifetime broadening, Franck–Condon congestion arising from geometric changes, and possibly vibronic coupling. In addition to the EPD spectra, we characterize the ground state of Dia+ by analysis of a remeasured photoelectron spectrum and a predicted infrared spectrum.