Optical spectrum of the cyanoadamantane radical cation

Diamondoids form a molecular class of cage-like structures and serve as a link between small, cyclic hydrocarbons and bulk nanodiamonds. Comparisons have been made between the infrared spectra of diamondoids and unidentified infrared emission bands seen in the spectra of certain stars with circumstellar disks. It is also suggested that their radical cations may contribute to features in the largely unassigned diffuse interstellar bands due to their low ionisation energy and absorption in the visible range. However, the optical spectra of these cations have only recently begun to be measured experimentally, which is required for astronomical identification. Herein, we present the first optical spectrum of the 1-cyanoadamantane radical cation (C11H15N, AdaCN(+)) between 275 and 1200 nm. The spectrum is obtained by electronic photodissociation (EPD) of mass-selected ions generated by electron ionisation in a tandem mass spectrometer coupled to a cryogenic trap held at 5 K. Experimental results are interpreted by (time-dependent) density functional theory calculations. The dominant fragmentation channels are HCN loss at low energies and C4H6N loss at energies above 2.9 eV. The EPD spectrum reveals broad bands that are rationalised by lifetime broadening and Franck-Condon congestion arising from geometric changes upon excitation. The astrophysical implications are discussed.