PRESSURE SHIFTS AND ELECTRON-SCATTERING LENGTHS IN ATOMIC AND MOLECULAR GASES

Photoabsorption or photoionization spectra of CH3I are discussed as a function of perturber pressure for 11 different binary gas mixtures co nsisting of CH3I and each one of 11 different gaseous perturbers. Five of the perturbers were rare gases and six were nondipolar molecules. The energy shifts of CH3I Rydberg states become independent of n, the principal quantum number, for n greater-than-or-equal-to 10. The energ y shifts for n greater-than-or-equal-to 10 vary in a linear fashion wi th perturber number density. The electron scattering lengths for the p erturbers are extracted from the shifts using Fermi theory in which th e polarization term is that of Alekseev and Sobel'man. These scatterin g lengths are compared with those from swarm and time-of-flight experi ments. It is found that the uncorrected shift scattering lengths corre spond to the zero energy or near-zero energy scattering lengths obtain ed from extrapolated swarm and time-of-flight data. It is found that p lots of scattering length vs polarizability alpha (alphaBAR for molecu les) define two linearities, one for the rare gases and one for molecu les, CO2 being an exception to the latter linearity (presumably becaus e of its large quadrupole moment). For a given polarizability, it is a lso found that molecules exhibit a larger scattering length than the r are gases. These results are discussed and consequences for scattering cross sections are elaborated.