2023ApJ...952...13H

Efforts over 40 yr still leave the source of astronomical infrared emission bands largely unidentified. Here, we report the first laboratory infrared (6-25 μm) spectra of gas-phase fullerene-metal complexes, [C₆₀-Metal]⁺ (Metal = Fe, V) and show with density functional theory calculations that complexes of C₆₀ 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 [C₆₀-Metal]⁺ coincide with four bands attributed earlier to neutral C₆₀ bands and in addition also with several bands unexplained to date. Abundance and collision theory estimates indicate that [C₆₀-Metal]⁺ could plausibly form and survive in astrophysical environments. Hence, [C₆₀-Metal]⁺ are proposed as promising carriers, in supplement to C₆₀, of observational bands, potentially representing the largest molecular species in space other than C₆₀, C₆₀⁺, and C₇₀.