Field ionization and photoionization of CH$_3$I perturbed by diatomic molecules: Electron scattering in H$_2$, HD, D$_2$, O$_2$ and CO*

Photoionization and field ionization studies of CH$_3$I doped into the diatomics H$_2$, HD, D$_2$, O$_2$ and CO (up to a density of $\rho = 1.0 \times 10^{21}$ cm$^{-3}$) are presented. These data are used to extract the zero-kinetic-energy electron scattering length of each diatomic molecule from the density-dependent shift of the CH$_3$I ionization energy. Scattering lengths obtained from fits of the photoionization spectra are compared to those determined from field ionization measurements.