Ionization of $\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}$ near Threshold

An experimental study of the threshold of ionization of H ($1s$) has been made. The electron-impact spectrometer used has an electron-energy resolution ≲0.06 eV. For atomic hydrogen, it has been demonstrated that for ∼0.4 eV above threshold, the cross section for ionization is a nonlinear and complicated function of the electron energy. The form of the cross section approaches a ${(E_e-IP)}^{1.13\pm 0.03}$ power law, although within 0.05 eV of threshold a higher power law would be consistent with our data. Above 0.4 eV and for somewhat less than 3 eV, a linear law more accurately describes our result. The slope of the linear portion of the cross section is in good agreement with other experiments, but in disagreement with theory. The intercept of the linear extrapolation of the straight-line portion with the energy axis is 0.032±0.005 eV above threshold.