Bond dissociation energy of N2 measured by state-to-state resolved threshold fragment yield spectra

The precise determination of the bond dissociation energy of N2 is crucial for thermochemistry database and theoretical calculations. However, there has been ongoing debate regarding its exact value. In this study, we used the velocity map imaging method combined with an extreme ultraviolet laser to measure the threshold fragment yield (TFY) spectra of N2 in the N(2D) + N(2D) photodissociation channels. By integrating the signals within a small circular area on the fragment velocity map images, we were able to obtain TFY spectra at nine different dissociation thresholds. These spectra are rotational state-resolved for the N2(J″) molecules and spin-orbit state-resolved for the dissociation channels involving N(2D) fragments. By employing the Wigner threshold law to simulate the TFY spectra and conducting statistical analysis on the comprehensive dataset, we determined the N2 bond dissociation energy to be 78 691.09 ± 0.15 cm-1. This work now places N2 among the few diatomic molecules with bond dissociation energies measured at sub-wavenumber precision.

Observation of Continuum State Dissociation Enables the Determination of N2 Bond Dissociation Energy to Spectroscopic Accuracy

Nitrogen (N) is one of the most fundamental elements of life. Precise determination of the bond dissociation energy (BDE) of the corresponding homonuclear diatomic molecule N2 is not only important for calculating the enthalpies of formation for any N-containing molecules but also provides the best benchmark for evaluating theoretical computational methods. Thus, it has attracted many experimental and theoretical studies, while controversies still exist. Here, we report the observation of continuum state dissociation of N2 into the channel N(2D5/2,3/2)+N(2D5/2,3/2) for the first time by using the vacuum ultraviolet (VUV)-pump-VUV-probe time-sliced velocity-mapped imaging setup. The quantum-state-resolved images enable the direct visualization of the dissociation onsets corresponding to each of the correlated spin-orbit fine-structure channels within a few tenths of wavenumber. The BDEs of 14N2 and 15N2 are directly determined to be 78691.8 ± 0.3 cm-1 and 78731.5 ± 0.3 cm-1, respectively, which should represent the most accurate BDE of N2 thus far.

Multireference configuration interaction study of the predissociation of C2 via its F 1Πu state

Photodissociation is one of the main destruction pathways for dicarbon (C2) in astronomical environments such as diffuse interstellar clouds, yet the accuracy of modern astrochemical models is limited by a lack of accurate photodissociation cross sections in the vacuum ultraviolet range. C2 features a strong predissociative F 1Πu-X 1Σg+ electronic transition near 130 nm originally measured in 1969; however, no experimental studies of this transition have been carried out since, and theoretical studies of the F 1Πu state are limited. In this work, potential energy curves of excited electronic states of C2 are calculated with the aim of describing the predissociative nature of the F 1Πu state and providing new ab initio photodissociation cross sections for astrochemical applications. Accurate electronic calculations of 56 singlet, triplet, and quintet states are carried out at the DW-SA-CASSCF/MRCI+Q level of theory with a CAS(8,12) active space and the aug-cc-pV5Z basis set augmented with additional diffuse functions. Photodissociation cross sections arising from the vibronic ground state to the F 1Πu state are calculated by a coupled-channel model. The total integrated cross section through the F 1Πu v=0 and v=1 bands is 1.198×10−13 cm2cm-1, giving rise to a photodissociation rate of 5.02×10−10 s−1 under the standard interstellar radiation field, much larger than the rate in the Leiden photodissociation database. In addition, we report a new 2 1Σu+ state that should be detectable via a strong 2 1Σu+-X 1Σg+ band around 116 nm.

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(²D_3/2,5/2) + N(²D_3/2,5/2) 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(²D_3/2,5/2) and N(⁴S) + N(²P_1/2,3/2) are competitive with the spin-allowed channel N(²D_3/2,5/2) + N(²D_3/2,5/2) 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(²D_3/2,5/2) + N(²D_3/2,5/2) 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(²D_3/2,5/2) and N(⁴S) + N(²P_1/2,3/2) are almost independent of J, while those of N(²D_3/2,5/2) + N(²D_3/2,5/2) show complicated rotational dependence. Possible coupling schemes between b' ¹Σ+u(ν = 20) and the high lying ¹Π_u and ³Π_u states are analyzed, which provides deep insight into the multi-channel photodissociation dynamics of ¹⁴N₂ in a high energy range.