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Title:Electronic spectroscopy of organic cations in gas-phase at 6 k:identification of C60 + in the interstellar medium
Author(s):Maier, John P.
Abstract:After the discovery of C$_{60}$,\footnote{H.~W.~Kroto, J.~R.~Heath, S.~C.~O'Brian, R.~E.~Curl \& R.~E.~Smalley, Nature, 318, 162, 1985} the question of its relevance to the diffuse interstellar bands was raised. In 1987 H.~W.~Kroto wrote: ``The present observations indicate that C$_{60}$ might survive in the general interstellar medium (probably as the ion C$_{60}\/^+$)''.\footnote{H.~W.~Kroto in ``Polycyclic aromatic hydrocarbons and astrophysics'', eds. A.~Leger, L.~B.~d'Hendecourt \& N.~Boccara, Reidel, Dordrecht, 1987, p.197} In 1994 two diffuse interstellar bands (DIBs) at 9632 and 9577\,\AA\/ were detected and proposed to be the absorption features of C$_{60}\/^+$.\footnote{B.~H.~Foing \& P.~Ehrenfreund, Nature, 369, L296, 1994} This was based on the proximity of these wavelengths to the two prominent absorption bands of C$_{60}\/^+$ measured by us in a neon matrix in 1993.\footnote{J.~Fulara, M.~Jakobi \& J.~P.~Maier, Chem. Phys. Lett., 211, 227, 1993} Confirmation of the assignment required the gas phase spectrum of C$_{60}\/^+$ and has taken 20 years. The approach which succeeded confines C$_{60}\/^+$ ions in a radiofrequency trap, cools them by collisions with high density helium allowing formation of the weakly bound C$_{60}\/^+$--He complexes below 10\,K. The photofragmentation spectrum of this mass-selected complex is then recorded using a cw laser. In order to infer the position of the absorption features of the bare C$_{60}\/^+$ ion, measurements on C$_{60}\/^+$--He$_2$ were also made. The spectra show that the presence of a helium atom shifts the absorptions by less than 0.2\,\AA, much less than the accuracy of the astronomical measurements. The two absorption features in the laboratory have band maxima at 9632.7(1) and 9577.5(1)\,\AA, exactly the DIB wavelengths, and the widths and relative intensities agree. This leads to the first definite identification of now five bands among the five hundred or so DIBs known and proves the presence of gaseous C$_{60}\/^+$ in the interstellar medium.\footnote{E.~K.~Campbell, M.~Holz, D.~Gerlich \& J.~P.~Maier, Nature, 523, 323, 2015}~\footnote{G.~A.~H.~Walker, D.~A.~Bohlender, J.~P.~Maier \& E.~Campbell, Astrophys. J. Lett., 812, L8, 2015} The absorption of cold C$_{70}\/^+$ has also been obtained by this approach. In addition the electronic spectra of a number of cations of astrophysical interest ranging from those of carbon chains including oxygen to larger polycyclic aromatic hydrocarbon could be measured in the gas phase at around 10\,K in the ion trap but using an excitation-dissociation approach. The implications of these laboratory spectra in relation to the diffuse interstellar band absorptions can be discussed.
Issue Date:2016-06-20
Publisher:International Symposium on Molecular Spectroscopy
Genre:Conference Paper / Presentation
Rights Information:Copyright 2016 by the authors
Date Available in IDEALS:2017-01-26

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