Electronic spectroscopy of ions of interest to astrochemistry

Abstract

The Diffuse Interstellar Bands (DIBs) are a well known conundrum amongst astronomers and those interested in the constituents of astronomical environments, such as astrochemists. These absorption features, first discovered 100 years ago by Mary Lea Heger, are observed in the spectra of starlight passing through diffuse material in the interstellar medium. Believed to be molecular in origin, and now numbering >500 features, the DIBs have been the focus of efforts for decades by many research groups seeking to attribute these absorption bands to so-called carriers. In 2015 the first and so far only molecule was assigned to two DIBs, this was the cationic form of Buckminsterfullerene, C⁺₆₀. As such, attention has been focused on recording the electronic spectra of related carbon species. The experimental requirements to produce results that are comparable to astronomical data are challenging and therefore the catalogue of sufficient laboratory spectra for these molecules is limited.

Using a modular, home-built cryogenic ion trapping instrument, it was possible to produce spectroscopic results suitable for astrochemical consideration. The instrument was operated in two configurations, with an electron impact source or a laser vaporisation source to produce ions. Experiments were conducted through two action spectroscopy approaches. The messenger tagging approach used cold, dense helium buffer gas to form weakly bound ion-helium complexes in the trap. These complexes are fragmented upon electronic excitation of the ion and the absorption was observed by monitoring the number of complexes as a function of excitation laser wavelength. The second approach used a two colour experiment with one laser responsible for excitation of the ion and a second, fixed frequency laser to induce fragmentation. The number of primary ions is depleted upon excitation and this change is recorded by mass spectrometry. It was also possible to monitor the increase in the number of product ions from this process. Using these approaches, it was possible to record the action spectra of various ions of interest to astrochemistry.

The first of these astrochemically relevant ions studied was the endohedral fullerene He@C⁺₆₀. A solid sample was sublimed before electron impact ionisation. He@C⁺₆₀ − He complexes were formed in the ion trap and photofragmented. These results were compared to the electronic spectrum of C⁺₆₀ − He in the range 9300 − 9650 ˚A and showed a blue shift of 2 − 3 ̊Å for the endohedral when compared to the empty cage. Increasing the number of external helium atoms coordinated to the endohedral fullerene showed a similar ∼ 0.7 ˚A red shift per additional helium, as seen for C⁺₆₀. Finally, these results were considered in the context of astronomical observations.

Smaller carbon cation clusters are also of interest. Below 10 atoms, linear chains are the most prevalent structure according to ion mobility experiments. One such chain, C⁺₅, was produced using the laser vaporisation source. The ²Π₉ ← X ²Σ⁺ᵤ electronic spectrum was recorded around 5137 ˚A through two action spectroscopy approaches, helium tagging and two colour fragmentation. The results from these two approaches were compared and found to be in close agreement.

Another collection of pure carbon structures, monocyclic rings, are considered the dominant structure for carbon cations with 10 − 20 atoms. Produced with the laser vaporisation source, the electronic spectra of a series of C⁺ ₂ₙ monocycles; C⁺₁₂, C⁺₁₄, C⁺₁₈, and C⁺₂₂, were recorded through helium tagging. Photofragmentation spectra of the helium complexes show a linear dependence of the origin band wavelength with increasing ring size. Complexes with two helium atoms were recorded to indicate the shift for the bare ion, to inform astronomical searches. C⁺₁₂ and C⁺₁₄ results were followed by two colour fragmentation experiments.

The conclusions are summarised and an outlook for this research is considered. Future work to record the electronic spectra of endohedral fullerenes is discussed and the inclusion of the laser vaporisation source opens avenues to investigate other targets, incorporating elements of astronomical relevance. The pure carbon cation clusters can also be investigated, particularly in the IR, unveiling structural information based on vibrational spectoscopy.