Photodissociation branching ratios of 12C16O from 108000 cm−1 to 113200 cm-1 measured by two-color VUV-VUV laser pump-probe time-slice velocity-map ion imaging method: Observation of channels for producing O(1D)

A New Band System of the Dicarbon Molecule in the Vacuum Ultraviolet Region

As one of the most abundant molecules in the universe, the long history of spectroscopic studies of the dicarbon molecule, C₂, reaches back two centuries. While many electronic band systems with upper states below the lowest dissociation threshold have been well characterized, much less is known about transitions to higher-lying states. Here, we report the observation of a new band system of C₂ from the lowest triplet state a³Π_u through a resonance-enhanced multiphoton ionization scheme. The upper state is identified as 1³Σ_g⁺, which is determined to be 61539.0 cm^-1 (7.630 eV) above ground state X¹Σ_g⁺. The spectroscopic parameters determined for the 1³Σ_g⁺ state are in excellent agreement with those predicted by the high-level ab initio calculations. This study paves the way for systematic investigations of the photoabsorption and photodissociation of C₂ in the vacuum ultraviolet region, which has important applications in the field of astrochemistry.

Vacuum ultraviolet photoexcitation and photofragment spectroscopic studies of 14N15N between 109000 and 117500 cm-1

In this study, we employed a newly built time-slice velocity-map ion imaging setup, equipped with two tunable vacuum ultraviolet (VUV) laser sources, to obtain the first comprehensive high-resolution photoexcitation and photofragment excitation spectra of ¹⁴N¹⁵N in the VUV photon energy range 109 000-117 500 cm^-1. The spectroscopic simulation program PGOPHER was used to analyze the rotationally resolved spectra. Band origins, rotational constants, and isotope shifts compared with those of ¹⁴N₂ have been obtained for 31 electric-dipole-allowed vibrational states of ¹⁴N¹⁵N in the aforementioned energy range. These spectroscopic parameters are found to depend on the vibrational quantum number irregularly. Systematic perturbations of the rotational transition energies and predissociation rates within individual absorption bands have also been observed. These are proved to be caused by the strong homogeneous interactions between the valence b'¹Σ_u ⁺ state and the Rydberg c_n ^{' 1}Σ_u ⁺ states, and between the valence b¹Π_u states and the Rydberg o₃ ¹Π_u states. Heterogeneous interactions between the Rydberg c_n ¹Π_u states and c_n ^{' 1}Σ_u ⁺ states also play an important role.

Vacuum Ultraviolet Photodissociation Branching Ratios of 12C16O, 13C16O, and 12C18O from 100500 to 102320 cm-1

The C⁺ ion photofragment spectra and photodissociation branching ratios into the two energetically available channels, C(¹D) + O(³P) and C(³P) + O(³P), have been obtained for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, in the vacuum ultraviolet range 100500-102320 cm^-1. The two vibronic states of ¹Σ⁺ symmetry, F(3dσ) ¹Σ⁺(υ' = 1) and J(4sσ) ¹Σ⁺(υ' = 0), predominantly dissociate into the lowest channel C(³P) + O(³P) through interactions with the repulsive D'¹Σ⁺ state. All three vibronic states of ¹Π symmetry, E'¹Π(υ' = 1, 2) and G(3dπ) ¹Π(υ' = 0), dissociate into both of the channels above. The photodissociation branching ratios into the channel C(¹D) + O(³P) for E'¹Π(υ' = 1, 2) are found to be independent of both the rotational quantum number and e/f parity, while those for G(3dπ) ¹Π(υ' = 0) strongly depend on the rotational quantum number, indicating very different predissociation pathways between the valence states E'¹Π(υ' = 1, 2) and the Rydberg state G(3dπ) ¹Π(υ' = 0). The potential energy curves of CO in the aforementioned energy range and below have recently been well constructed due to a series of interplays between high-resolution spectroscopic studies and theoretical calculations; the photodissociation branching ratios measured in this study can provide further benchmarks for future theoretical investigations which aim to understand the detailed predissociation dynamics of CO.

Reinvestigation of the Rydberg W1Π(ν = 1) level of 12C16O, 13C16O, and 12C18O through rotationally dependent photodissociation branching ratio measurements

A recent high resolution photoabsorption study revealed that the Rydberg W¹Π(ν = 1) level of carbon monoxide (CO) is perturbed by the valence E″¹Π(ν = 0) level, and the predissociation linewidth shows drastic variation at the crossing point due to the interference effect [Heays et al., J. Chem. Phys. 141(14), 144311 (2014)]. Here, we reinvestigate the Rydberg W¹Π(ν = 1) level for the three CO isotopologues, ¹²C¹⁶O, ¹³C¹⁶O, and ¹²C¹⁸O, by measuring the rotationally dependent photodissociation branching ratios. The C⁺ ion photofragment spectra obtained here reproduce the recent high resolution photoabsorption spectra very well, including the presence of the valence E″¹Π(ν = 0) level. The photodissociation branching ratios into the spin-forbidden channel C(¹D) + O(³P) show sudden increases at the crossing point between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, which is in perfect accordance with the drastic variation of the linewidth observed in the recent spectroscopic study. Further analysis reveals that the partial predissociation rate into the lowest channel C(³P) + O(³P) shows a much more prominent decrease at the crossing point, which is caused by the interference effect between the W¹Π(ν = 1) and E″¹Π(ν = 0) levels, than that into the spin-forbidden channel C(¹D) + O(³P), and this is the reason of the sudden increase as observed in the photodissociation branching ratio measurements. We hope that the current experimental investigation will stimulate further theoretical studies, which could thoroughly address all the experimental observations in a quantitative way.