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

Pan Jiang,Min Cheng,Hong Gao,Xiaoping Chi,Qihe Zhu

doi:10.1038/s41467-019-11086-z

Abstract

Rare isotope (13C, 17O and 18O) 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 12C16O and 13C16O 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 13C16O are dramatically different from those of 12C16O, 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.

Highlights

Rare isotope (13C, 17O and 18O) 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
To test the self-shielding models, Thiemens and coworkers performed an experiment at the advanced light source in Berkeley, strong and wavelength selective mass-independent oxygen isotope fractionation processes have been observed, from which they concluded that the self-shielding effect is not the major reason for the anomalously enriched atomic oxygen reservoirs observed in the Solar System[13]
The present photodissociation branching ratio measurements were performed on a time-slice velocity-map ion imaging apparatus (TSVMI) (Fig. 1c) which equips with a high-resolution tunable VUV laser radiation source

Summary

Introduction

Rare isotope (13C, 17O and 18O) 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. It has long been known that rare isotope (13C, 17O, and 18O) substitutions can substantially change photoabsorption line positions, oscillator strengths and photodissociation rates of CO5–8, and these are key input parameters for the self-shielding models[9,10,11] These models have been proposed for explaining the anomalous oxygen isotopic distributions observed among various early Solar System objects which contain both 16O-rich and 16Opoor reservoirs[12]. The photodissociation branching ratios of 12C16O have been measured recently[28,29,30,31,32,33], which showed that photodissociation of 12C16O generates C and O atoms in the ground state, and produces significant amount of C and O atoms in the excited states, and the ratios between them strongly depend on the specific rovibronic state of 12C16O being excited

Methods

Results

Conclusion