Electronic absorptions of C5+ detected in the visible through action spectroscopy in a cryogenic trap

Electronic Transition of the l-C6+ Cation at 417 nm

A new electronic transition is reported for the linear C6+ cation with an origin at 416.8 nm. This spectrum can be compared to the matrix isolation spectra at lower energies reported previously by Fulara et al. (J. Chem. Phys. 123, 044305 (2005)), which assigned linear and cyclic isomers, and to the gas phase spectrum reported previously by Campbell and Dunk (Rev. Sci. Instrum. 90, 103101 (2019)), which detected the same cyclic-isomer spectrum reported by Fulara. Comparisons to electronically excited states and vibrations predicted by various forms of theory allow assignment of the spectrum to a new electronic state of linear C6+. The spectrum consists of a strong origin band, two vibronic progression members at higher energy and four hotbands at lower energies. The hotbands provide the first gas phase information on ground state vibrational frequencies. The vibronic structure of this excited state of C6+ provides a severe challenge to computational chemistry.

Photofragment Imaging of Carbon Cluster Cations: Explosive Ring Rupture

Carbon cluster cations (C_n⁺) produced by laser vaporization are mass selected and photodissociated at 355 nm. Multiphoton dissociation of smaller ions leads to the elimination of neutral C₃, as in previous work, whereas larger clusters exhibit more varied fragmentation channels. Photofragment velocity-map imaging detects significant kinetic energy release (KER) in the various n - 3 cation fragments. Small cations (n = 6 or 7) with linear structures produce moderate KER, whereas larger cations (n = 10, 11, 12, 15, or 20) having monocyclic ring structures produce much higher KER values. Such high KER values are unanticipated, as optical excitation should produce a wide distribution of internal energies. These carbon clusters have a surprising ability to absorb multiple photons of ultraviolet radiation, achieving a state of extreme excitation prior to dissociation. The remarkable nonstatistical distribution of energy is apparently influenced by the significant ring strain that can be released upon photodissociation.

Electronic Spectroscopy of Monocyclic Carbon Ring Cations for Astrochemical Consideration

Gas phase electronic spectra of pure carbon cations generated by laser vaporization of graphite in a supersonic jet and cooled to below 10 K and tagged with helium atoms in a cryogenic trap are presented. The measured C_2n⁺–He with n from 6 to 14, are believed to be monocyclic ring structures and possess an origin band wavelength that shifts linearly with the number of carbon atoms, as recently demonstrated through N₂ tagging by Buntine et al. (J. Chem. Phys.2021, 155, 21430234879679). The set of data presented here further constrains the spectral characteristics inferred for the bare C_2n⁺ ions to facilitate astronomical searches for them in diffuse clouds by absorption spectroscopy.