Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)



First Advisor

John D. Scott


The use of atomic and molecular Rydberg states to investigate dopant/perturber interactions is a well established research area. In this Dissertation, photoionization spectroscopy is used to probe the interactions between various dopants (D = CH3I, C2H5I and C6H6) and perturbers (P = Ar, N2, CO2, CF4, c-C4F8 and SF6) in a static gas-phase system. One overall goal is a better understanding of the nature of dopant/perturber interactions in the gas phase which, in turn, can lead to a better understanding of solvation in the condensed phase. Two separate perturber effects are investigated in this work: the perturber-induced energy shift of high-n dopant Rydberg states, and the appearance of photoionization structure below the first dopant ionization threshold (i.e., subthreshold photoionization) in some dopant/perturber systems. From the perturber-induced energy shifts, the zero-kinetic-energy electron scattering lengths and cross-sections of Ar, CF4, c-C4F8 and SF6 are evaluated. (To the best of our knowledge, this represents the first experimental determination of the zero-kinetic-energy electron scattering lengths of CF 4 and c-C4F8, and the first determination of the zero-kinetic-energy electron scattering length of SF6 using the shift of dopant high-n Rydberg states.). The appearance in various dopant/perturber systems of subthreshold photoionization structure, which tracks the absorption of discrete dopant Rydberg states in the same energy region, is reported and analyzed. This subthreshold photoionization structure is explained by invoking two separate pathways, both of which involve the excited-state processes of electron attachment and associative ionization. From the measurements reported here, subthreshold photoionization in C 2H5I/SF6 and C6H6/SF 6 is shown to proceed through electron attachment to SF6, and through the formation of heteromolecular dimers. Subthreshold photoionization in CH3I/P is shown to proceed through electron attachment to CH 3I and to the perturber, and through the formation of both homomolecular and heteromolecular dimers. Finally, the effective rate constants for these processes are determined from the variation in the subthreshold photocurrent as a function of dopant number density rhoD and perturber number density rho p. These constants are then analyzed with respect to the properties of the excited dopant Rydberg state. For the dopant CH3I, the variation in the effective rate constants is discussed in terms of ground-state perturber properties (such as electron affinity and polarizability).