Pathway to the identification of C60+ in diffuse interstellar clouds

Abstract

The origin of the attenuation of starlight in diffuse clouds in interstellar space at specific wavelengths ranging from the visible to the near-infrared has been unknown since the first astronomical observations around a century ago. The absorption features, termed the diffuse interstellar bands, have subsequently been the subject of much research. Earlier this year four of these interstellar bands were shown to be due to the absorption by cold, gas phase [Formula: see text] molecules. This discovery provides the first answer to the problem of the diffuse interstellar bands and leads naturally to fascinating questions regarding the role of fullerenes and derivatives in interstellar chemistry. Here, we review the identification process placing special emphasis on the laboratory studies which have enabled spectroscopic measurement of large cations cooled to temperatures prevailing in the interstellar medium.This article is part of the themed issue 'Fullerenes: past, present and future, celebrating the 30th anniversary of Buckminster Fullerene'.

Keywords: C+60; diffuse interstellar bands; interstellar.

Figure 1.

Schematic of the experimental set-up used for absorption measurements in a neon matrix. The ion beam is mass-selected using a quadrupole mass spectrometer and co-deposited with neon ontoa substrate plate held at 5 K. Over a period of several hours, a matrix of 100 μm thickness is grown. A waveguide approach (inset) is used to pass light from a Xe or Hg lamp through the matrix.

Figure 2.

Electronic absorption spectrum of recorded in a 5 K neon matrix, showing the four strongest features of the lowest energy transition, assigned as in D_5d symmetry. The band maxima are at 9645, 9583, 9431 and 9375 Å.

Figure 3.

Electronic spectrum of recorded in a neon matrix (blue) compared to astronomical observations (black) [24]. The laboratory spectrum shows the two strongest features of the lowest energy transition, assigned as in D_5d symmetry, located at 9645 and 9583 Å . The narrower astronomical spectrum from observations towards HD 183143 has band maxima at 9632 and 9577 Å . The astronomical spectrum shown from [24] is of higher signal-to-noise ratio than the original observations from [23].

Figure 4.

Schematic of the experimental set-up used for gas-phase spectroscopy. Ions are produced by electron bombardment of the neutral gas, transmitted through a quadrupole mass filter and injected into a cryogenic 22-pole radiofrequency ion trap. Here, they interact with high number density helium buffer gas and laser radiation for a certain time period after which the trap contents are extracted and mass analysed.

Figure 5.

Gas-phase electronic spectrum of the diacetylene cation, HC₄H⁺. The absence of hot bands around the origin band indicates that the vibrational degrees of freedom have been relaxed. The 20 K rotational temperature is obtained from simulations of the band profile (inset). The spectrum was measured using the ‘action’ spectroscopy scheme shown on the right. The red arrow indicates the electronic transition of interest, while the blue arrow represents the UV radiation used to fragment the ion following electronic excitation. The spectrum was recorded by monitoring the C₄H⁺ mass channel.

Figure 6.

The gas-phase electronic spectrum recorded by fragmentation of –He. The circles are the experimental data and the solid black lines are the results of Gaussian fits. The resulting data are listed in table 1. The intensities have been scaled by the relative absorption cross sections. The perturbation due to the presence of the helium is small such that the spectrum can be considered as that of for comparison with astronomical data. The solid red lines are the wavelengths of the DIBs, their width represents the uncertainty [26].

Figure 7.

Astronomical data from observations towards HD 183143 (a) and the archive spectrum of HD 169454 (b) in the regions of the laboratory absorptions after correction for the radial velocity of one of the two interstellar clouds. Telluric water vapour lines have been removed after division by the spectra of unreddend stars [39]. The red lines are Gaussian fits to the observed DIBs. The vertical lines show the wavelengths of the laboratory –He absorptions, their width represents the uncertainty. The wavelengths and widths of the DIBs are given in table 1.