Perturbations of molecular extravalence excitations by rare-gas fluids

In this paper we report the results of an experimental study of the vacuum ultraviolet absorption spectra of molecular impurity states of methyl iodide in Ar (density range ? = 0–1.4 g cm^?3) and in Kr (? = 0–2.3 g cm^?3), of carbon disulphide in Ar (? = 0–1.4 g cm^?3) and of formaldehyde in Ar (? = 0–1.25 g cm^?3). The experimental results provide new information regarding medium perturbations of intravalenc transitions, of the lowest extravalence transitions and of transitions to mixed valence—Rydberg configurations, which serve as a diagnostic tool to distinguish between different types of electronic excitations. All the lowest extravalence molecular excitations exhibit appreciable blue spectral shifts at moderate and at high fluid densities, intravalence transitions are practically insensitive to medium effects, while excitations to mixed valence—Rydberg configurations are characterized by a moderate blue spectral shift. New information has been obtained concerning the energetics of molecular ionization processes in a dense fluid. The high n = 2–5 Rydberg states of CH₃l exhibit a large red shift at moderate (? = 0–0.5 cm^?3) Ar densities. The ionization potential E_g and the effective Rydberg constant G for CH₃I in Ar was found to decrease from G = 13.6 eV and E_g = 9.55 eV at ? = 0 and E_g = 9.08 eV and constant G for CH₃l in Ar was found to decrease from G = 13.6 eV and E_g = 9.55eV at ? = 0 and E_g = 9.08 eV and G ≈ 7.15 eV at ? = 0.5 g cm^?3. Experimental evidence was obtained for the identification of n = 2 molecular Wannier impurity states of CH₃I and of CH₂O in liquid Ar. These spectroscopic data result in E_g ≈ 8.6 eV for CH₃I in liquid Ar and E_g ≈ 10.2 eV for CH₂O in liquid Ar.