Using photochemistry to explain the formation and observation of C ₂ in comets

Abstract

Laboratory data taken from the laser-induced fluorescence (LIF) spectrum of C ₂(X ¹Σ_g⁺) radicals produced during the 193 nm laser photolysis of C ₂H ₂ has been used to explain the band profiles of the Hubble Space Telescope (HST) on nucleus spectra of the C ₂ Mulliken system (X ¹Σ_g⁺ ← D ¹Σ_u⁺) in comet Hyakutake (C/1996 B2). The authors propose, using laboratory data and ab initio calculations, that photolysis of C ₂H ₂ in the laboratory and in comets occurs via a sequential mechanism, first producing C ₂H and then C ₂. The ab initio calculations were used to identify the two excited electronic states (2 ²Σ⁺ and 2 ²Π) of C ₂H through which photodissociation in the second step proceeds. In both the laboratory and cometary studies, the photodissociation of C ₂H appears to proceed via the same electronic surfaces. The C ₂ Mulliken bands obtained in the laboratory and the HST observations of these bands are used to explain the internal energy distribution and origin of C ₂(X ¹Σ_g⁺) in comet Hyakutake. The off nucleus HST spectrum at 16 arcsec shows that the C ₂ radicals are much colder vibrationally than they are on the nucleus. This vibrational cooling is inconsistent with literature reports and is discussed in the text.