Buckyball-metal Complexes as Potential Carriers of Astronomical Unidentified Infrared Emission Bands

Efforts over 40 yr still leave the source of astronomical infrared emission bands largely unidentified. Here, we report the first laboratory infrared (6-25 & mu;m) spectra of gas-phase fullerene-metal complexes, [C-60-Metal](+) (Metal = Fe, V) and show with density functional theory calculations that complexes of C-60 with cosmically abundant metals, including Li, Na, K, Mg, Ca, Al, V, and Fe, all have similar spectral patterns. Comparison with observational infrared spectra from several fullerene-rich planetary nebulae demonstrates a strong positive linear cross-correlation. The infrared features of [C-60-Metal](+) coincide with four bands attributed earlier to neutral C-60 bands and in addition also with several bands unexplained to date. Abundance and collision theory estimates indicate that [C-60-Metal](+) could plausibly form and survive in astrophysical environments. Hence, [C-60-Metal](+) are proposed as promising carriers, in supplement to C-60, of observational bands, potentially representing the largest molecular species in space other than C-60, C-60 (+), and C-70.