UC Davis

Photodissociation by ultraviolet (UV) radiation is a key destruction pathway for small molecules in regions where the interstellar UV radiation can penetrate, such as the diffuse molecular clouds, protoplanetary disks, and photon-dominated regions. Wavelength-dependent photodissociation data is a critical input for accurately modeling the physical and chemical evolution of these astronomical environments. Carbon monosulfide (CS) and dicarbon (C2) are two transient molecules widely detected in space and they play important roles in carbon chemistry and/or sulfur chemistry. However, due to the lack of high-resolution laboratory studies and high-level quantum calculations focusing on photodissociation through their highly excited states, their photodissociation cross sections in modern astrochemical models have large uncertainties. Based on previous studies, the C 1Σ+ state of CS and the F 1Πu state of C2 are predissociative Rydberg states and are considered to be important for their photodissociation. In this dissertation, a combination of laboratory vacuum UV (VUV) spectroscopy and high-level ab initio quantum chemical calculations are performed to provide a better set of photodissociation data for CS and C2.

For the theoretical calculation part, potential energy curves of CS and C2 electronic states are calculated at the SA-CASSCF/MRCI+Q level using Dunning quintuple-zeta basis sets with additional diffuse functions. By including several additional σ (CS) and σg (C2) molecular orbitals beyond valence orbitals into the active space, the Rydberg nature of these states are successfully obtained. Coupled-channel models involving the C 1Σ+ state of CS and the F 1Πu state of C2 are built by combining potential energy curves and related transition dipole moments, nonadiabatic couplings, and spin-orbit couplings. The photodissociation cross sections obtained by solving the coupled-channel model are used to calculate the photodissociation rate of CS and C2 in standard interstellar radiation fields and other astronomical environments.

Experimentally, a state-selective photodissociation study using the vacuum ultraviolet laser pump-probe velocity-map imaging (VUV-VUV-VMI) technique was initiated. CS and C2 are successfully generated by photolysis of CS2 and laser ablation of a graphite rod, respectively. A tentative signal is observed in the region where the C − X transition of CS is expected, however, owing to the high vibrational temperature of CS in the experiment, further work is needed. The future plan is to build an electrical discharge source to obtain CS with a cooler vibrational distribution. Besides the main project, the direct C + S2 channel in CS2 photodissociation has been observed, and a new highly accurate photoionization energy for C3 as 11.8341±0.0025 eV has been derived. Despite the delay of the experimental progress, the photodissociation cross sections obtained in the theoretical part of this dissertation provide much needed improvements to the astrochemical models used to simulate astrochemistry in the diffuse interstellar medium.