Even before this discovery, charge-transfer complexes consisting of C60 and astronomically abundant metal ions were proposed as potential carriers for the UIEs and for a second set of spectral bands, now in the visible wavelength range and observed in absorption (diffuse interstellar bands, DIBs). Such conjectures were impossible to test for lack of laboratory spectra of C60-metal ion complexes.
Researchers from HFML-FELIX, KU Leuven (Belgium), and Xi'an Jiaotong University (China) used the FELICE free-electron laser to record the first laboratory infrared (6–25 μm) spectra of gas-phase C60-Metal+ (Metal = Fe, V) complexes. The complexation with the metal ion breaks the icosahedral symmetry leading to the appearance of a wealth of infrared active bands. The team further shows with quantum-chemical calculations that complexes of C60 with cosmically abundant metals, including Li, Na, K, Mg, Ca, Al, V, Fe, all have similar spectral patterns. They compared the laboratory and calculated spectra with the Spitzer observational infrared spectra from several fullerene-rich planetary nebulae and found a strong positive linear cross-correlation. The infrared bands of C60-Metal+ coincide with four bands attributed earlier to neutral C60, but also with several to date unexplained bands.