tokens: data presented here from the ion trap measurements are not of the bare species itself because of the slight perturbation induced by the weakly bound helium atom; however, the shift is expected to be small. Laboratory data on the band at and the weak features near for are shown in Figure 2 and indicate that the helium causes a shift on the electronic transition of of around . Considering the width of the absorption band profiles, for comparison with astronomical data, the spectrum of (Figure 1 ) can be considered to be that of the bare cation. In Table 1 , the band maxima of the absorptions shown in Figure 1 are listed. Figure 2. Electronic spectra of below 10 K. Data are shown for two of the bands presented in Figure 1 . The similarities between the spectra indicate that the shift on the electronic transition of due to the presence of the helium is small. Therefore, the spectrum of may be regarded as that of for comparison with DIB data. A linear offset has been applied to the vertical axis of the data shown. Table 1 Absorption Maxima of the Bands Shown in Figure 1 and Compared with the Gas-phase Prediction Reported by Steglich et al. ( 2011 ) Note. a Weak absorptions with intensities lower than 20% of the band at are not listed. Steglich et al. ( 2011 ) predicted the gas-phase transition energies of based on their matrix data using a model (Gredel et al. 2011 ) to account for the polarizability of the different rare gases used. The present work enables a direct evaluation of this model and their results are also given in Table 1 . The predicted values are found to be in accord with the gas phase; however, the large error bars () indicate that the extrapolation is not quite good enough for an unambiguous comparison with DIB data, especially if the latter are weak (). For this, gas-phase measurements as done here on are required. In the context of astronomical detectability it is important to determine the oscillator strength f of the individual transitions, in particular for the strongest band in the spectrum. This is because the theoretically calculated values are not expected to be very accurate because of the nature of the method that had to be used (TD-DFT). Furthermore, these f values correspond to the the whole electronic absorption system(s). To obtain the oscillator strength for the origin band at , its Franck–Condon factor relative to all the other transitions has to be estimated, as carried out by Steglich et al. ( 2011 ). Thus, it is more reliable to measure absolute absorption cross sections directly in the ion trap, as previously demonstrated by measurements on the fullerene cations (Campbell et al. 2016a , 2017 ). Absorption cross-section measurements were carried out at the wavelength of the band maxima of the two strongest absorptions in the spectrum. Plots showing typical exponential fits to the data are shown in Figure 3 . The characteristic fluence, , was found to be 0.8 and at 8281 and , respectively. These values correspond
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