Electronic spectroscopy of ions of interest to astrochemistry
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Date
28/02/2023Item status
Restricted AccessEmbargo end date
28/02/2024Author
Reedy, Elliott S.
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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.