**Acknowledgements**

26 Will-be-set-by-IN-TECH

the formation of a perturber solvent shell around the dopant core, thereby inducing local perturber density inhomogeneities. This solvent shell begins to shield the optical electron from the cationic core. Therefore, the dense perturber fluid increases the dopant excitation energy, resulting in a blue shift of the abosrption band, which is observed experimentally. The local density of the first perturber solvent shell is almost proportional to the perturber bulk density at non-critical temperatures. However, near the critical isotherm and critical density of the perturber, the dopant/perturber interactions strengthen due to the increased perturber/perturber correlation length. This increased order yields a corresponding increase of the local density in the solvent shell that, in turn, leads to a stronger shielding of the optical electron from the cationic core. Thus, increased blue shifts of the low-*n* absorption bands are observed in all dopant/perturber systems near the critical point of the perturber. The area of this critical effect is demarcated by the turning points that bound the saddle point in the

For fluids with similar compressibilities, the structures of low-*n* dopant Rydberg states in the perturbing fluid show systematic behaviors. At non-critical temperatures, Δ(*ρ*P) is determined by the polarizability and size of the perturbing fluid. The larger the polarizability and, therefore, the larger the size, the smaller the perturber-induced energy shift of the dopant absorption bands. This is caused by the number of atoms that can exist between the optical electron and the dopant cationic core, coupled with the strength of the shielding. The large overall energy shift observed in the dopant low-*n* Rydberg states perturbed by CF4 [39, 40], on the other hand, was caused by the larger compressibility of CF4 in comparison to the other gases in this study [37, 38, 40]. This larger compressibility implies that CF4 is closer together on average at high perturber number densities than are the other perturbers studied, which increases the local density of CF4 and, therefore, increases the blue shift in this perturber.

The critical point effect, on the other hand, is dominated by the similarity of the perturber/ perturber interaction with the dopant/perturber ground state and dopant/perturber excited state interactions, coupled with the overall local density of the system. In krypton, the well depth of the ground state perturber/perturber intermolecular potential and the dopant/ perturber intermolecular potential shows greater similarity in comparison to that in Ar and Xe. Moreover, the excited state CH3I/Kr interaction is slightly stronger than the ground state Kr/Kr interaction. These facts dictate that the largest critical point effect for CH3I in atomic perturbers is in Kr. Similarly, the largest overall critical effect was observed in CH3I/CH4 [39, 40]. This large critical effect is caused by both the ground state and excited state CH3I/CH4 interactions having strengths comparable to the CH4/CH4 interaction. Although the excited state CH3I/CF4 interactions are comparable in strength to the CF4/CF4 interactions, the ground state CH3I/CF4 interactions are not close to those of CF4/CF4. Similarly, the Xe/CF4 ground state interactions are comparable to the ground state CF4/CF4 interactions, but the excited state Xe/ground state CF4 interactions are weaker. Moreover, the bulk critical density in CF4 is small in comparison to the rest of the perturbers investigated here. This results in the CF4 critical effect on Δ(*ρ*P) being the smallest one observed [39, 40]. These data sets also allowed us to generate a consistent set of intermolecular potential parameters for various dopant/perturber systems, which are summarized in Appendix A. Several general trends in these parameters can be observed. For atomic perturbers, the steepness of the exponential-6 intermolecular potential (i.e., *γ*) used to model the

thermodynamic phase diagram of the critical isotherm.

All experimental measurements were made at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588), with support from the Petroleum Research Fund (PRF#45728-B6), the Professional Staff Congress - City University of New York, the Louisiana Board of Regents Support Fund (LEQSF(2006-09)-RD-A-33), and the National Science Foundation (NSF CHE-0956719).
