一氧化碳在真空紫外波段解离通道分支比的同位素效应研究

Carbon monoxide (CO) is the second most abundant molecules in the interstellar medium after H2. Knowing the photoabsorption and photodissociation properties of CO and its isotopologues in the vacuum ultraviolet (VUV) region is the key to understand the very large oxygen isotopic heterogeneities among different objects in the solar system that has been observed recently. Many experiments have been performed to measure the accurate photoabsorption cross sections, including the peak positions and peak widths, aiming to obtain the photodissociation cross sections of the six CO isotopologues (12C16O，12C17O，12C18O，13C16O，13C17O，13C18O). While these experiments can provide accurate photoabsorption and photodissociation cross sections for CO, they are still missing one of the most important aspects of the photodissociation process, the branching ratio among different dissociation channels. CO can dissociate into three possible channels in the VUV region: C(3P)+O(3P)、C(1D)+O(3P) and C(3P)+O(1D). The current project we are proposing here will aim to use the high-resolution tunable VUV laser radiation generated by four-wave mixing, and the newly built time-slice velocity-map ion imaging setup in our lab to measure the branching ratio among different dissociation channels after 12C17O and 12C18O are excited to different electronic, vibrational and rotational states, and especially its dependence on the isotopic substitution. This measurement will provide important information for understanding the origin of the oxygen isotopic heterogeneities in the solar system, and thus help to build a consistent model of describing the formation and evolution history of the solar system. The measurement will also help understand the complex photodissociation dynamics of CO.

一氧化碳(CO)是宇宙空间中丰度仅次于H2的重要小分子。了解CO各同位素分子在真空紫外(VUV)波段的光吸收和光解离性质对理解最近观察到的太阳系中氧同位素的不均匀分布有重要意义。测量CO各同位素分子精确的吸收谱峰的位置、强度和峰宽，并由此得到其光解离截面成了很多实验工作的重要内容。然而这些实验无法测量CO解离的一个重要参数，即不同解离通道的分支比。CO在VUV波段有三个可能的解离通道，即C(3P)+O(3P)、C(1D)+O(3P)和C(3P)+O(1D)。本项目拟采用四波混频原理产生可调频的高分辨VUV激光，并结合本实验室新近研制的新型高分辨时间切片式离子速度成像装置，初步测量12C17O和12C18O分子被激发到不同电子振转态后各个解离通道的分支比，为理解太阳系中氧同位素的不均匀分布，建立准确的太阳系形成和演化模型提供重要的实验数据，同时也为理解CO的解离机理提供重要的实验依据。

一氧化碳（CO）是宇宙空间中丰度仅次于H2的分子物质，是C、O原子的主要存在形式之一。测量CO各同位素分子精确的吸收谱峰的位置、强度、峰宽和解离通道分支比对理解最近观察到的太阳系中氧同位素的不均匀分布，进而研究太阳系的形成和演化历史有重要意义。同时，CO也是研究双原子分子非直接预解离的模型体系，测量上述参数对深入理解CO复杂的光解动力学过程有重要作用。本项目利用自主搭建的基于四波混频的可调频真空紫外激光系统和切片离子速度成像装置，测量了CO多个同位素分子在真空紫外波段的光解离碎片激发谱和解离通道分支比，取得了如下三个方面的重要进展：（1）首次发现CO光解离通道分支比具有强烈的同位素效应。该发现对于了解目前天文观测中发现的太阳系中不同天体的氧同位素丰度呈现不均匀分布的分子机理，进而研究太阳系的形成和演化历史有重要的价值。（2）首次观测到CO光解离产生高激发态C(1S)通道，证实了彗星C/2016 R2上观测到的激发态C(1S)的发射光谱是来源于太阳辐射引起的CO光解离过程的猜测。（3）首次获得CO每单个通道的解离速率及其与转动量子数的依赖关系，对深入研究CO每个通道的光解离机理具有重要的作用。

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