Strong and selective isotope effect in the vacuum ultraviolet photodissociation branching ratios of carbon monoxide

Abstract

Rare isotope ( 13 C, 17 O and 18 O) substitutions can substantially change absorption line positions, oscillator strengths and photodissociation rates of carbon monoxide (CO) in the vacuum ultraviolet (VUV) region, which has been well accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system and molecular clouds. Here, we demonstrate a strong isotope effect associated with the VUV photodissociation of CO by measuring the branching ratios of 12 C 16 O and 13 C 16 O in the Rydberg 4p(2), 5p(0) and 5s(0) complex region. The measurements show that the quantum yields of electronically excited C atoms in the photodissociation of 13 C 16 O are dramatically different from those of 12 C 16 O, revealing strong isotope effect. This isotope effect strongly depends on specific quantum states of CO being excited, which implies that such effect must be considered in the photochemical models on a state by state basis.

Photodissociation branching ratios of 13C16O and 12C18O in the vacuum ultraviolet region from 107 800 to 109 700 cm−1

“…For 13 C 16 O, the relative ratios into the two spin-forbidden channels C( 1 D) + O( 3 P) and C( 3 P) + O( 1 D) are ∼12% and ∼50%, respectively; for 12 C 18 O, the percentages are ∼12% and ∼60% respectively. A similar strong isotope effect was also observed for the C 1 Σ + (v = 7) state recently (Jiang et al 2019b). The percentages slightly decrease with increasing J levels, which might be caused by partial overlapping with the nearby (5dσ) 1 Σ + (v = 0) state.…”

“…The photodissociation branching ratios are strongly quantum-state dependent. Recently, Gao and coworkers (Jiang et al 2019b(Jiang et al , 2020Chi et al 2020b) measured the photodissociation branching ratios of 13 C 16 O in the VUV region from 102 807 cm −1 (97.27 nm) to 107 685 cm −1 (92.87 nm), which are found to be dramatically different from those of 12 C 16 O. This strong and selective isotope effect on the photodissociation branching ratio is important for future quantitative photochemical modeling.…”

State-to-state dissociative photoionization of molecular nitrogen: the full story

“…For instance, the present findings may help to fully understand the isotopic effects observed recently in the ultrafast dynamics of dissociative photoionization of N 2 and the unexpected branching ratios measured recently during the dissociation dynamics of N 2 + induced by isolated attosecond XUV pulses in combination with few-optical-cycle near-infrared/visible (NIR/VIS) [30]. Similar approaches can be applied also to explain the isotopic effects in the VUV photodissociation of the N 2 and CO molecules [16,82] and that of larger molecular systems relevant for astrophysical, planetary, and atmospheric media and having environmental impacts such as HNCO [83].…”

Strong isotope effect in the VUV photodissociation of HOD: A possible origin of D/H isotope heterogeneity in the solar nebula

“…However, the D/H ratios returned by these measurements deviate significantly from Earth’s water D/H ratio, which brings back to the table the long-standing question whether or not water on Earth was delivered by comet impacts. Photochemistry has been accounted for in recent photochemical models for understanding the large isotopic fractionation effects that are apparent in carbon and oxygen in the solar system ( 58 ), which may also suit for understanding of the D/H isotope heterogeneity.…”