Electronic spectroscopy of FUV-irradiated diamondoids: A combined experimental and theoretical study

TL;DR

This study combines experimental UV spectroscopy and theoretical calculations to investigate the electronic properties of diamondoids under UV irradiation, revealing their potential for astrophysical detection and similarities to meteoritic nanodiamonds.

Contribution

It provides new experimental data and theoretical insights into the UV absorption and ionization behavior of diamondoids, highlighting their stability and detectability in space.

Findings

Ionization leads to stable dehydrogenated cations.

UV absorption features shift with molecular size.

Spectral similarities to meteoritic nanodiamonds.

Abstract

Irradiation with high energy photons (10.2 - 11.8 eV) was applied to small diamondoids isolated in solid rare gas matrices at low temperature. The photoproducts were traced via UV absorption spectroscopy. We found that upon ionization the smallest of these species lose a peripheral H atom to form a stable closed-shell cation. This process is also likely to occur under astrophysical conditions for gas phase diamondoids and it opens the possibility to detect diamond-like molecules using their rotational spectrum since the dehydrogenated cations possess strong permanent dipole moments. The lowest-energy electronic features of these species in the UV were found to be rather broad, shifting to longer wavelengths with increasing molecular size. Calculations using time-dependent density functional theory support our experimental findings and extend the absorption curves further into the vacuum ultraviolet. The complete sigma - sigma* spectrum displays surprisingly strong similarities to meteoritic nanodiamonds containing 50 times more C atoms.