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Carbon monoxide (CO) is one of the most abundant molecular species in comets. Its photodissociation by the solar radiation in the vacuum ultraviolet (VUV) region produces excited atomic fragments C( 1 D), C( 1 S), and O( 1 D), which radiate at characteristic wavelengths when they decay to lower states. The fractional rate constants for generating these fragments from CO photodissociation under the entire range of the solar radiation field are key input values in modelling the observed atomic emission intensities from comets. In this study, the branching ratios of the four lowest dissociation channels C( 3 P)+O( 3 P), C( 1 D)+O( 3 P), C( 3 P)+O( 1 D), and C( 1 S)+O( 3 P) of the  12 C 16 O photodissociation are measured in the VUV energy range between the threshold of producing the C( 1 S)+O( 3 P) channel (~110 500 cm −1 ) and the ionisation energy (IE) of  12 C 16 O (~113 045 cm −1 ). We measured these ratios using the VUV time-slice velocity-map ion imaging apparatus. We observe a number of high Rydberg states in the aforementioned energy range, with most of them mainly producing ground C( 3 P) and O( 3 P) atomic fragments, and only a few of them producing a significant amount of excited C( 1 D) or O( 1 D) fragments. We also observe the excited C( 1 S) fragment from CO photodissociation and measured its branching ratio for the first time. Based on the photodissociation branching ratios measured in the current and previous studies, we are able to estimate the relative percentages of the excited atomic fragments C( 1 D), C( 1 S), and O( 1 D) from the solar photolysis of  12 C 16 O below its IE. We discuss the implications for the photochemical modelling of the CO-dominated comet C/2016 R2 (Pan-STARRS).

astronomy and astrophysics

Photodissociation branching ratios of12C16O from 110 500 to 113 045 cm−1: first observation of the C(1S) channel