Jiang Pan, Lu Liya, Liu Min, Gao Hong
Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Phys Chem Chem Phys. 2022 May 18;24(19):11544-11551. doi: 10.1039/d2cp01148d.
b' Σ+u( = 20) is the first vibronic state above the dissociation limit N(D) + N(D) of N that has been observed in the absorption spectrum. It provides a unique opportunity for studying the multi-channel photodissociation dynamics of N, particularly the competition between the spin-forbidden and spin-allowed photodissociation channels. Here, photofragment excitation (PHOFEX) and (1VUV + 1'UV) photoionization spectra of N in the b' Σ+u( = 20) state and the time-slice velocity-map ion (TS-VMI) images at each individual rotational levels are collected by using a vacuum ultraviolet (VUV) pump-VUV probe scheme. It is found that the spin-forbidden channels N(S) + N(D) and N(S) + N(P) are competitive with the spin-allowed channel N(D) + N(D) at low rotational levels, while quickly become undetectable as the rotational quantum number increases. At high rotational levels, only the spin-allowed channel N(D) + N(D) can be observed, supporting previous theoretical modeling. Channel-resolved partial predissociation rate constants (PPRCs) are calculated by combining branching ratios in this study and total predissociation rate constants (TPRCs) from previous absorption spectroscopic measurements. PPRCs for dissociation into channels N(S) + N(D) and N(S) + N(P) are almost independent of , while those of N(D) + N(D) show complicated rotational dependence. Possible coupling schemes between b' Σ+u( = 20) and the high lying Π and Π states are analyzed, which provides deep insight into the multi-channel photodissociation dynamics of N in a high energy range.
b'Σ⁺u(ν = 20)是在N的吸收光谱中观测到的高于N的解离极限N(D) + N(D)的首个振转态。它为研究N的多通道光解离动力学，特别是自旋禁阻和自旋允许光解离通道之间的竞争，提供了独特的机会。在此，利用真空紫外（VUV）泵浦 - VUV探测方案，收集了处于b'Σ⁺u(ν = 20)态的N的光碎片激发（PHOFEX）和(1VUV + 1'UV)光电离光谱以及各个转动能级的时间切片速度成像（TS - VMI）图像。研究发现，自旋禁阻通道N(S) + N(D)和N(S) + N(P)在低转动能级时与自旋允许通道N(D) + N(D)具有竞争力，但随着转动量子数增加迅速变得不可探测。在高转动能级时，仅能观测到自旋允许通道N(D) + N(D)，这支持了先前的理论模型。通过结合本研究中的分支比和先前吸收光谱测量得到 的总预解离速率常数（TPRCs），计算了通道分辨的部分预解离速率常数（PPRCs）。解离成通道N(S) + N(D)和N(S) + N(P)的PPRCs几乎与ν无关，而N(D) + N(D)的PPRCs则显示出复杂的转动依赖性。分析了b'Σ⁺u(ν = 20)与高激发态Π和Π态之间可能的耦合方案，这为深入了解高能范围内N的多通道光解离动力学提供了依据。
