Density effects on high-nmolecular Rydberg states:CH3I in He, Ne, Ar, and Kr

Absorption studies of the high-n Rydberg states of ${\mathrm{CH}}_3$I perturbed by varying number densities of He, Ne, Ar, and Kr (up to 23.0, 24.0, 11.3, and 6.6×${10}^{20}$ ${\mathrm{cm}}^{\mathrm{-}3}$, respectively) are reported. Energy shifts, which increase with increasing perturber number density, are observed and analyzed for both discrete [nd${(}^2$ $E_{3/2}$)] and autoionizing [nd’ ${(}^2$ $E_{1/2}$)] states. These shifts vary linearly with the perturber number density for principal quantum numbers n≥10. Moreover, depending upon the nature of the perturber, these shifts are to the blue region (He), slightly to the blue region (or nearly zero) (Ne), or the red region (Ar and Kr). We explain these results quantitatively on the basis of the electron scattering length in the various rare gases, as well as on the polarization of the medium by ${\mathrm{CH}}_3$ ${\mathrm{I}}^{+}$. For low perturber number densities, it is well known that the Fermi model of perturber effects on high-n Rydberg states is invalid. The present experimental results show, for the first time, that this model also fails at high number densities. On the other hand, the energy shifts can be reproduced quantitatively by extending a model developed by Alekseev and Sobel’man to high number densities.