Pressure Shifts and Electron Scattering in Atomic and Molecular Gases

Abstract

In this work, we focus on one aspect of Rydberg electron scattering, namely number density effects in molecular gases. Our recent study of Rydberg states of CH3I and C6H6 perturbed by H2 is the first attempt to investigate number density effects of a molecular perturber on Rydberg electrons1,2. Highly excited Rydberg states, because of their “large orbital” nature, are very sensitive to the surrounding medium. Photoabsorption or photoionization spectra of CH3I have also been measured3 as a function of perturber pressure in 11 different binary gas mixtures consisting of CH3I and each one of eleven different gaseous perturbers. Five of the perturbers were rare gases (He, Ne, Ar, Kr, Xe) and six were non-dipolar molecules H2, CT4, N2, C2H6, C3H8. The goal of this work is to underline similarities and differences between atomic and molecular perturbers. We first list some results of the molecular study: (1) The energy shifts of CH3I Rydberg states become independent of n, the principal quantum number, for n≥10. (2) The energy shifts for n≥10 vary in a linear fashion with perturber number density. (3) A particular advantage of molecular absorbers (relative, that is, to alkali metal absorbers) is that they may be studied at room temperature. Indeed, the lower temperature limit, being dependent only on the freezing point of the particular absorber/perturber system, can lie considerably below room temperature. That is, molecular perturbers can be studied well below their vibrational thermal excitation thresholds in a molecular absorber system whereas such is not possible with the common alkali metal absorber systems. (4) Since molecules can rotate and vibrate, the density of states in them is usually much larger than in atoms. Consequently, the band widths of molecular absorbers are always considerably broader than those of atoms. There is considerable disadvantage in this because the minimal shift size which can be measured for a molecule is larger than that for an atom, the minimal shift sizes being roughly in the ratio of the Rydberg excitation bandwidths. There is also some advantage in this: asymptoticity of energy may occur in molecular absorbers at principal quantum number values as low as n=10; and large perturber densities, certainly as large as p= 1021cm−3, may be investigated before the effects of pressure on band width and shape begin to confuse the measurements of band shift. The “signatures” of molecular rovibronic spectra are not totally insensitive to number density effects, and investigations on this matter are in progress in our Laboratory. (5) The measurement of the pressure shifts of absorption spectra has been restricted to perturbers that do not absorb in the region of Rydberg excitation of the absorber. This restriction may be removed by doing photoionization experiments3 in which synchrotron radiation excites the autoionization region of the designated absorber in a narrow sheath in the immediate vicinity of the surface of optical incidence. These radiations, however, must be insufficiently energetic to ionize the designated perturber. Since the vast majority of potential perturber systems absorb in the VUV region, this technique broadens the scope of the pressure shift technique. (LSU has a new synchrotron facility and the equipment for such investigations.)