2018ApJ...858...36C

Laboratory measurements are reported that enable a more accurate determination of the characteristics of the near-infrared absorptions of C₆₀⁺ below 10 K. These data were obtained by photofragmentation of C₆₀⁺–He complexes in a cryogenic trap. Asymmetry in the profiles of the observed 9577 and 9632 Å absorption bands of C₆₀⁺–He is caused by the attachment of the weakly bound helium atom to hexagonal or pentagonal faces of C₆₀⁺. The implication is that the FWHM of the bands in the electronic spectrum of C₆₀⁺ below 10 K is 1.4 Å. The effect of ¹³C isotopes on the C₆₀⁺ electronic spectrum is experimentally evaluated by measurement of ¹²C₆₀⁺–He, ¹³C₁¹²C₅₉⁺–He, and ¹³C₂¹²C₅₈⁺–He. Data on the 9365 Å absorption band indicate a wavelength shift of about 0.3 Å between the former and latter. This result is consistent with models used to interpret the vibrational isotope effect in the Raman spectrum of neutral C₆₀. The influence of ¹³C isotopes on the 9348, 9365, 9428, 9577, and 9632 Å diffuse interstellar bands is expected to be minor considering other broadening factors that affect astronomical observations. The presented data also provide more accurate relative intensities of the five interstellar bands attributed to C₆₀⁺.