**1. Introduction**

Since electronic excited states are sensitive to the local fluid environment, dopant electronic transitions are an appropriate probe to study the structure of near critical point fluids (i.e., perturbers). In comparison to valence states, Rydberg states are more sensitive to their environment [1]. However, high-*n* Rydberg states are usually too sensitive to perturber density fluctuations, which makes these individual dopant states impossible to investigate. (Nevertheless, under the assumption that high-*n* Rydberg state energies behave similarly to the ionization threshold of the dopant, dopant high-*n* Rydberg state behavior in supercritical fluids can be probed indirectly by studying the energy of the quasi-free electron, through photoinjection [2–11] and field ionization [12–19].) Low-*n* Rydberg states, on the other hand, are excellent spectroscopic probes to investigate excited state/fluid interactions.

The study of low-*n* Rydberg states in dense fluids began with the photoabsorption of alkali metals in rare gas fluids [20, 21]. Later researchers expanded the investigation into rare gas dopants in supercritical rare gas fluids [22–26], and molecular dopants in atomic and molecular perturbers [27–36]. However, none of these previous groups studied dilute solutions near the critical point of the solvent. (The single theoretical study of a low-*n* Rydberg state in a near critical point fluid was performed by Larrégaray, *et al.* [35]; this investigation predicted a change in the line shape and in the perturber induced shift of the Rydberg transition.) These results from previous experimental and theoretical investigations of low-*n* Rydberg states in dense fluids are reviewed in Section 2.

In this Chapter, we present a systematic investigation of the photoabsorption of atomic and molecular dopant low-*n* Rydberg transitions in near critical point atomic fluids [37–40]. The individual systems probed allowed us to study (dopant/perturber) atomic/atomic interactions (i.e., Xe/Ar) and molecular/atomic interactions (i.e., CH3I/Ar, CH3I/Kr, CH3I/Xe) near the perturber critical point. The experimental techniques and theoretical

<sup>\*</sup>This work is adapted from that originally submitted 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.

©2012 Evans et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0),which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

methodology for this extended study of dopant/perturber interactions are discussed in Section 3. Section 4 presents a review of our results for low-*n* atomic and molecular Rydberg states in atomic supercritical fluids. The accuracy of a semi-classical statistical line shape analysis is demonstrated, and the results are then used to obtain the perturber-induced energy shifts of the primary low-*n* Rydberg transitions. A striking critical point effect in this energy shift was observed for all of the dopant/perturber systems presented here. A discussion of the ways in which the dopant/perturber interactions influence the perturber-induced energy shift is also presented in Section 4. We conclude with an explanation of the importance of the inclusion of three-body interactions in the line-shape analysis, and with a discussion of how this model changes when confronted with non-spherical perturbers and polar fluids.
