Electronic spectroscopy of open-chain and aromatic hydrocarbon cations in neon matrices

Considerable scientific interest has been devoted to the so-called <i>diffuse interstellar bands</i> (DIBs)—hundreds of absorption features of different strength and width, located in the visible and near infrared, and arising from the interstellar medium, whose origin remains mysterious. It is presumed that these bands are associated with electronic transitions of families of gaseous, carbon-containing molecules rather than dust grains or ices. This hypothesis has solid observational foundations and may give rise even to origins-of-life speculations. However, an unambiguous assignment of a DIB to a certain species can be made <i>only</i> upon measurement of its spectrum in the laboratory and a careful comparison with those detected along sight lines toward a variety of stars.<p>
Advances in experimental techniques over the last two to three decades have enabled the recording of electronic spectra for a number of such clusters in the gas phase. These employ discharge/ablation ion sources, supersonic expansions and sensible, laser-based detection schemes. The main problem is, however, to <i>locate</i> the region of absorption first, because even state-of-the-art computational approaches fail to predict reliable excitation energies.<p>
<i>Matrix isolation</i> is a suitable method to do this. With it, transient species can be embedded into rare-gas matrices at low temperatures and investigated comfortably by (a set of) spectroscopic means such as direct absorption, fluorescence emission or infrared spectroscopy.<p>
In the course of this work, a matrix setup that draws on another important experimental tool, <i>mass selection</i>, has been re-built and further developed. Ions are produced in appropriate sources and trapped selectively in detectable amounts in solid neon at 6 K. Scanning over broad spectral ranges with the help of a dispersion spectrograph provides then the basis for high-resolution surveys in the gas phase.<p>
With this apparatus, a number of reactive species, charged and neutral, have been investigated of relevance for astrophysics, as well as from fundamental chemistry aspects in view of the role they may play in combustion environments, flames or early Earth-like planetary atmospheres. These include unsaturated carbon chains and polycyclic aromatic hydrocarbon derivatives. Specifically, linear HC_2<i>n</i>+1H⁺, classical Hückel arenes such as benzylium, tropylium, benzotropylium, naphthylmethylium and indene-related structures, planar C₆H₄⁺ isomers, as well as some more exotic species were studied and are discussed herein. In most cases their vibrationally resolved electronic spectra were obtained for the first time and various chemical processes detected. Simple models such as the particle-in-a-box and the Hückel molecular orbital method, as well as (time-dependent) density functional calculations were used to describe these and provide an assignment for the observed spectroscopic features.<p>
The effectiveness of the approach was also illustrated on the example of H₂CCC, the first molecule in the nearly a century long history of DIB research for which convincing correlation with astronomical data could be shown. The negligible intermolecular interaction in the condensed phase allowed for excellent prediction of gas-phase line positions.