Isomeric Broadening of C₆₀⁺ Electronic Excitation in Helium Droplets: Experiments Meet Theory

Abstract

Helium is considered an almost ideal tagging atom for cold messenger spectroscopy experiments. Although helium is bound very weakly to the ionic molecule of interest, helium tags can lead to shifts and broadenings that we recorded near 963.5 nm in the electronic excitation spectrum of C60+ solvated with up to 100 helium atoms. Dedicated quantum calculations indicate that the inhomogeneous broadening is due to different binding energies of helium to the pentagonal and hexagonal faces of C60+, their dependence on the electronic state, and the numerous isomeric structures that become available for intermediate coverage. Similar isomeric effects can be expected for optical spectra of most larger molecules surrounded by nonabsorbing weakly bound solvent molecules, a situation encountered in many messenger-tagging spectroscopy experiments.

Electronic Spectroscopy of for Astrochemical Consideration

“…The slope of the linear fit indicates a shift per helium atom of 0.7Å, in agreement with the results found previously for C + 60 in both ion trapping (Campbell et al 2016b) and helium droplet experiments (Kuhn et al 2016). The increasing FWHM of the bands with n is believed to be due to isomeric broadening, as described in Kaiser et al (2018).…”

Cold physics and chemistry: Collisions, ionization and reactions inside helium nanodroplets close to zero K

“…Complexes of this type have been proposed to form in dense interstellar clouds (where thermal or photo desorption is diminished) if they contain fullerenes, but not so in the higher temperature diffuse clouds that are the major sources of the diffuse interstellar bands. In the case of He attachment to C60 + , shifts and isomeric broadenings in the electronic excitation spectra have been observed for up to 100 helium atoms, despite the weak interaction of helium to the ionic molecule [622][623][624]. These line-shifts and linebroadenings observed even for helium tagging makes and extrapolation to bare spectra challenging.…”

Infrared spectroscopy of cold trapped molecular ions using He‐tagging

“…The He–H 3 + ion mentioned below is a “new” molecule showing IR spectra completely different from H 3 + . For the astrophysical applications of the C 60 + spectra, many recent papers discuss the shifts and broadening of the lines measured by He tagging . This raises the question of whether it is possible to record the LIICG spectra of C 60 + .…”