After decades of searching, astronomers have recently identified specific Polycyclic Aromatic Hydrocarbons (PAHs) in space. Remarkably, the observed abundance of cyanonaphthalene (CNN, C10H7CN) in the Taurus Molecular Cloud (TMC-1) is six orders of magnitude higher than expected from astrophysical modeling. Here, we report unimolecular dissociation and radiative cooling rate coefficients of the 1-CNN isomer in its cationic form. These results are based on measurements of the time-dependent neutral product emission rate and kinetic energy release distributions produced from an ensemble of internally excited 1-CNN+ studied in an environment similar to that in interstellar clouds. We find that Recurrent Fluorescence – radiative relaxation via thermally populated electronic excited states – efficiently stabilizes 1-CNN+, owing to a large enhancement of the electronic transition probability by vibronic coupling. Our results help explain the anomalous abundance of CNN in TMC-1 and challenge the widely accepted picture of rapid destruction of small PAHs in space.

The DΣ-XΣ electronic absorption spectrum of the astrophysically relevant yttrium oxide (YO) molecule has been recorded for the first time in the 400-440 nm region using laser induced fluorescence. YO molecules are produced by corona discharge of oxygen between the tips of two yttrium needles in a supersonic jet expansion. An unambiguous spectroscopic identification of the DΣ-XΣ transition becomes possible from a combined analysis of the moderate-resolution laser excitation spectrum and dispersed fluorescence spectrum. We have also performed multi-state complete active space second order perturbation theory calculations on the first six doublets of YO, and the results support our assignment of the DΣ state. Accurate spectroscopic constants for DΣν' = 0 and 1 levels have been determined from a rotational analysis of the high resolution spectra that are recorded with a resolution of ∼0.018 cm. Severe perturbations are observed in the experimental spectra and are considered to originate from interactions with at least one nearby Π electronic state, e.g., the undetected CΠ state. We have also measured the radiative lifetimes of BΣν' = 0, and DΣν' = 0 and 1 states, based on which the BΣ-XΣ (0, 0) and DΣ-XΣ (0/1, 0) band oscillator strengths have been determined.

Rotationally resolved spectra of the H3Σu ––X3Σg – electronic transition bands of Si2 have been experimentally studied using laser-induced fluorescence in the 380–520 nm range. Si2 molecules are produced in a supersonically expanding planar plasma by discharging a silane/argon gas mixture. In total, 44 bands belonging to the H3Σu ––X3Σg – electronic transition system of the most abundant isotopologue 28Si2 are experimentally recorded. With a spectral resolution of ∼0.04 cm–1, the triplet spin-splitting structures in individual rotational transition lines are fully resolved. Detailed analyses on the high-resolution spectra have yielded an accurate determination of spectroscopic constants for both X3Σg – and H3Σu – states. The spin–spin interaction constants for the two triplet states are found to be comparable (λ ≈1.5 cm–1), which may originate from the 3p atomic orbital interaction in the triplet Si2 molecule. The measured isotopologue spectra of 29Si28Si and 30Si28Si indicate that the H3Σu ––X3Σg – transition system of 29S28S and 30S28S can be reasonably reproduced by the isotope mass-scaling rule. Spectroscopic parameters, including the Franck–Condon factors, the Einstein coefficients, and the oscillator strengths, are also determined from the experimental results and the Rydberg–Klein–Rees (RKR) calculations. The agreement between the experimentally measured and calculated dispersed fluorescence spectra indicates that the RKR calculations with the molecular constants determined in this work can accurately reproduce the diatomic potentials of both states. These molecular data provide a benchmark in high-level theoretical studies on Si2 and likely other small silicon clusters.

We present a pulsed single longitudinal mode optical parametric oscillator that was recently constructed for sub-Doppler spectroscopic studies of transient species in a supersonic slit jet expansion environment. The system consists of a Littman-type grazing-incidence-grating resonator and a KTP crystal and is pumped at 532 nm. By spatially filtering the pump laser beam and employing an active cavity-length-stabilization scheme, a frequency down-conversion efficiency up to 18% and generation of Fourier-transform limited pulses with a typical pulse duration of ∼5.5 ns and a bandwidth less than 120 MHz have been achieved. In combination with a slit jet expansion, a sub-Doppler spectrum of SiC has been recorded at ∼498 nm, showing a spectral resolution of Δν/ν ≈ 6.2 × 10.