ENERGY TRANSFER DYNAMICS OF HIGH ENERGY MOLECULES

Abstract

This dissertation investigates the energy transfer dynamics of high-energy molecules excited electronically, vibrationally, and rotationally. The molecules studied in this thesis were excited electronically and vibrationally with UV photon absorption and rotationally with an optical centrifuge. The excited molecules are relaxed either by collisions or a chemical process. The products of the relaxation process are measured using high-resolution transient IR absorption spectroscopy to determine the state-resolved products and the energy partitioning. In the first study the collisional relaxation of highly vibrationally excited collidine with bath CO2 is investigated. The collidine was excited to an E_vib=38,552 cm-1 after absorption of a λ=266 nm photon and the full state-resolved distribution of the scattered CO2 is reported. The results are compared to previous studies done on methylated pyridines. The translational energy and rotational energy gain of the scattered CO2 is similar for the methylated pyridines, however, the integrated appearance rate constant for collidine-CO2 collisions is higher then the other methylated pyridine molecules. The effect that the donor complexity has on collisional relaxation is explored. For the second study, SO2 is electronically excited to the predissociative metastable C̃ electronic state. The SO product quantum yields, rotational distributions, and product energy partitioning show that translational energy is preferred by a 4:1 ratio over rotational energy for the photoproducts. The preference for translational energy is evidence that a linear transition state could be involved in the dissociation process. Theoretical calculations of the SO2 potential energy surfaces for the ground state and the excited state show that for SO2 photodissociation to occur near the dissociation threshold the SO2 in the C̃ state becomes linear and that coupling to the repulsive triplet state lowers the height of the energy barrier so dissociation can proceed. The third study used a tunable optical centrifuge to rotationally excite N2O into extreme rotational states. The optical centrifuge was tuned to selectively populate rotational states between J=100-200. N2O IR transitions for the (0001-0000) band are known up to J=100 for the R-branch. Line center profiles were collected over each IR transition between J=100-200 to identify the IR transition frequencies. The newly identified N2O transitions and the tunable optical centrifuge were used to maximize the population in N2O J=140 and J=165, using two different optical traps, to determine the relaxation dynamics in this region. Doppler-broadened line profiles show that rotational states below the maximized population have higher translational temperatures than rotational states near the peak of the distribution. From the near nascent distributions, the relaxation rate of the N2O was measured to be 65 % of the gas kinetic collision rate for both optical traps. This result is compared with a previous study on CO relaxation dynamics.