Vacuum Ultraviolet Photodissociation Branching Ratios of ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O from 100500 to 102320 cm^–1

Abstract

The C + ion photofragment spectra and photodissociation branching ratios into the two energetically available channels, C( 1 D) + O( 3 P) and C( 3 P) + O( 3 P), have been obtained for the three CO isotopologues, 12 C 16 O, 13 C 16 O, and 12 C 18 O, in the vacuum ultraviolet range 100500−102320 cm −1 . The two vibronic states of 1 Σ + symmetry, F(3dσ) 1 Σ + (υ′ = 1) and J(4sσ) 1 Σ + (υ′ = 0), predominantly dissociate into the lowest channel C( 3 P) + O( 3 P) through interactions with the repulsive D′ 1 Σ + state. All three vibronic states of 1Π symmetry, E′ 1 Π(υ′ = 1, 2) and G(3dπ) 1 Π(υ′ = 0), dissociate into both of the channels above. The photodissociation branching ratios into the channel C( 1 D) + O( 3 P) for E′ 1 Π(υ′ = 1, 2) are found to be independent of both the rotational quantum number and e/f parity, while those for G(3dπ) 1 Π(υ′ = 0) strongly depend on the rotational quantum number, indicating very different predissociation pathways between the valence states E′ 1 Π(υ′ = 1, 2) and the Rydberg state G(3dπ) 1 Π(υ′ = 0). The potential energy curves of CO in the aforementioned energy range and below have recently been well constructed due to a series of interplays between high-resolution spectroscopic studies and theoretical calculations; the photodissociation branching ratios measured in this study can provide further benchmarks for future theoretical investigations which aim to understand the detailed predissociation dynamics of CO.

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

“…Due to the relatively low spectroscopic resolution, the integrated photodissociation cross-sections, as measured in Letzelter et al (1987), are equally distributed to each of the resolved absorption bands by high resolution studies in the specific energy ranges. The photodissociation branching ratios for producing C( 1 D), C( 1 S), and O( 1 D) are from both earlier studies (Chi et al 2021;Gao et al 2013Gao et al , 2012Gao et al , 2011Guan et al 2020;Shi et al 2018) and the current study (as presented in the previous section). To compare the impact of the different photoionisation cross-section data of the C atom used in the calculation, we also recalculated all the branching ratios from our previous studies by using the photoionisation cross-section data from the TOPbase project (see footnote 1) and we present a comparison of the results in Table A.2.…”

“…See Appendix A for details. ratios into the three lowest channels C( 3 P)+O( 3 P), C( 1 D)+O( 3 P), and C( 3 P)+O( 1 D) have been systematically measured in the VUV wavelength range of 91.10-99.50 nm (Chi et al 2021;Gao et al 2013Gao et al , 2012Gao et al , 2011Guan et al 2020;Shi et al 2018), while the threshold of the channel for generating the excited C( 1 S) fragment is at ∼13.79eV, which has never been reached before. In this study, we collected the C + ion photofragment spectra and measured the photodissociation branching ratios into the above four channels in the energy range above 13.79eV and below the IE of 12 C 16 O.…”

“…A&A 647, A127 (2021) Photodissociation branching ratios % (a) C( 1 D) max (b) C( 1 D) min (b) O( 1 D) max (b) O( 1 D) min (b) C( 1 S) max (b) C( 1 S) min (b) C( 1 D) max (c) C( 1 D) min (c) O( 1 D) max (c) O( 1 D) min (c) C( 1 S) max (c) C( 1 S) min (c) Notes. (a) The maximum and minimum photodissociation branching ratios for all the 12 C 16 O states are adopted from Chi et al (2021); Gao et al (2013Gao et al ( , 2012Gao et al ( , 2011; Guan et al (2020); Jiang et al (2019a); Shi et al (2018). (b) Photoionisation cross-sections of C( 3 P), C( 1 D) and C( 1 S) are adopted from Burke & Taylor (1979); Cantu et al (1981).…”

“…Thus far, we have performed a series of experimental measurements on the photodissociation branching ratios of CO, including the current study. The photodissociation branching ratios of 12 C 16 O from the lowest dissociation threshold at ∼111.80 nm to its IE at ∼88.48 nm have been obtained (Chi et al 2021;Gao et al 2013Gao et al , 2012Gao et al , 2011Guan et al 2020;Jiang et al 2019a;Shi et al 2018). The most important finding is that photodissociation into the spin-forbidden channels C( 1 D)+O( 3 P) and C( 3 P)+O( 1 D), which were previously thought not to occur at all, can be as important as the spin-allowed channel C( 3 P)+O( 3 P) at many strong absorption bands.…”

“…Due to the great deal of potential applications for simulating the chemical properties of the interstellar medium, the photodissociation branching fractions of 12 C 16 O for producing excited O( 1 D) and C( 1 D) have been measured by Jackson, Ng, Gao, and their colleagues in the VUV wavelength range of 91.10-99.50 nm (Chi et al 2021;Gao et al 2013Gao et al , 2012Gao et al , 2011Guan et al 2020;Shi et al 2018). Because of the saturated absorption by H atoms, the photochemical properties of molecules at VUV wavelengths that are shorter than 91.17 nm (the IE of H) are usually considered to be irrelevant with regard to the chemical properties of the interstellar medium.…”