Using photochemistry to explain the formation and observation of C2 in comets

Laboratory data taken from the laser-induced fluorescence (LIF) spectrum of C2(X1Σg+) radicals produced during the 193 nm laser photolysis of C2H2 has been used to explain the band profiles of the Hubble Space Telescope (HST) on nucleus spectra of the C2 Mulliken system (X1Σg+ ← D1Σu+) in comet Hyakutake (C/1996 B2). The authors propose, using laboratory data and ab initio calculations, that photolysis of C2H2 in the laboratory and in comets occurs via a sequential mechanism, first producing C2H and then C2. The ab initio calculations were used to identify the two excited electronic states (22Σ+ and 22Π) of C2H through which photodissociation in the second step proceeds. In both the laboratory and cometary studies, the photodissociation of C2H appears to proceed via the same electronic surfaces. The C2 Mulliken bands obtained in the laboratory and the HST observations of these bands are used to explain the internal energy distribution and origin of C2(X1Σg+) in comet Hyakutake. The off nucleus HST spectrum at 16 arcsec shows that the C2 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.