Cesium Ionization Cross Section from Threshold to 50 eV

The Cs ionization cross section $\sigma$, which was recently measured by Brink at 50 eV and above, has been measured down to threshold in a Tate- and Smith-type apparatus designed for absolute measurements. The density in the collision space was determined by a surface ionization detector which permitted an over-all accuracy on the cross section of 10%. The results are: (1) $\sigma =5.3\mathrm{}$ ${\mathrm{\AA }}^2$ at 50 eV. This is smaller by a factor of 1.8 than the results of the above experiment and the theoretical values of the Gryziński method—the discrepancy has not been explained. (2) The cross section near threshold is linear over a range of 0.8 eV with a slope of 2.2 Å e${\mathrm{V}}^{-1\mathrm{}}$. Above 0.8 eV, it rises less than linearly. The initial slope is smaller by a factor of 3 than the earlier measurements of Nottingham. (3) Above 12 eV, excitation to the autoionizing states is seen, and above 18 eV, an additional contribution to the total cross section begins. The latter is due to ionization to an excited state of the ion. Cross sections for these two processes have been related to spectroscopic measurements of Beutler et al. and have been separated from the basic ionizing channel. The curve for autoionization is resonant in nature with a maximum of 0.8 ${\mathrm{\AA }}^2$. The ionization to the excited ion has an estimated maximum cross section of 1.5 ${\mathrm{\AA }}^2$ at 60 eV.