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The absolute photoionization cross sections of the excited, short-lived He* $1s2p$ ¹P and $1s3p$ ¹P states are determined experimentally in the region close to the He⁺ $1s$ ²S threshold (from 0 to 2 eV). The intermediate 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.
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Gisselbrecht et al. (1999) studied this question.
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