Atomic and Molecular Low-n Rydberg States in Near Critical Point Fluids*

The structure of low-n Rydberg states doped into supercritical fluids represents an important probe to investigate solvation effects, especially near the solvent (or perturber) critical point. We have investigated the solvation of dopant low-n Rydberg states in various perturbing fluids. This systematic study was performed from low perturber number densities to the density of the triple point liquid, at both noncritical temperatures and on an isotherm near the critical isotherm. The absorption spectra of these states, which were measured using vacuum ultraviolet spectroscopy, were then simulated using a semi-classical statistical line shape function. With accurate line shape simulations, the perturber induced energy shift of the primary transition was obtained using a standard moment analysis. The moment analysis indicated that the dopant low-n Rydberg state energy blue shifts as a function of perturber number density without a significant temperature effect (except near the perturber critical point). A significant critical point effect was observed in all dopant/perturber systems investigated here. This critical point effect is caused by a large increase in the dopant/perturber radial distribution function near the critical temperature of the perturber. Since the first perturber solvent shell shields the cationic core, the binding energy of the optical electron decreases. This acts to increase the dopant low-n Rydberg state excitation energy. However, the overall blue shift and critical point effect varies from atomic to molecular perturber systems due to the structure of the perturber. These differences are also discussed in more detail in this work. *This work was originally submitted in part by Luxi Li to the faculty of the Graduate Center of the City University of New York in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry.