Absolute Photoionization Cross Sections of Excited He States in the Near-Threshold Region

The absolute photoionization cross sections of the excited, short-lived ${\mathrm{He}}^{*}$ $1s2p$ ${}_{}{}^1{}_{}{}^{}P$ and $1s3p$ ${}_{}{}^1{}_{}{}^{}P$ states are determined experimentally in the region close to the ${\mathrm{He}}^{+}$ $1s$ ${}_{}{}^2{}_{}{}^{}S$ threshold (from 0 to 2 eV). The intermediate ${\mathrm{He}}^{*}$ states are prepared by photoabsorption of a high-order harmonic of an intense picosecond tunable laser and subsequently ionized by absorption of photons of several fixed frequencies, ranging from the near infrared to the ultraviolet. Our experimental results quantitatively confirm earlier theoretical work.