Multi-channel photodissociation dynamics of 14N2 in its b′ 1Σ +u(ν = 20) state

b′ 1Σ+u(ν = 20) is the first vibronic state above the dissociation limit N(2D3/2,5/2) + N(2D3/2,5/2) of 14N2 that has been observed in the absorption spectrum. It provides a unique opportunity for studying the multi-channel photodissociation dynamics of 14N2, particularly the competition between the spin-forbidden and spin-allowed photodissociation channels. Here, photofragment excitation (PHOFEX) and (1VUV + 1′UV) photoionization spectra of 14N2 in the b′ 1Σ+u(ν = 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(4S) + N(2D3/2,5/2) and N(4S) + N(2P1/2,3/2) are competitive with the spin-allowed channel N(2D3/2,5/2) + N(2D3/2,5/2) at low rotational levels, while quickly become undetectable as the rotational quantum number J increases. At high rotational levels, only the spin-allowed channel N(2D3/2,5/2) + N(2D3/2,5/2) 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(4S) + N(2D3/2,5/2) and N(4S) + N(2P1/2,3/2) are almost independent of J, while those of N(2D3/2,5/2) + N(2D3/2,5/2) show complicated rotational dependence. Possible coupling schemes between b′ 1Σ+u(ν = 20) and the high lying 1Πu and 3Πu states are analyzed, which provides deep insight into the multi-channel photodissociation dynamics of 14N2 in a high energy range.