Two-photon ionisation of caesium

Abstract

We study the time dependence of the two-photon ionisation rate for atomic caesium calculated using a sudden approximation. We show that for pulse lengths similar to the ones used in the measurements by Granneman and Van der Wiel second-order perturbation theory gives results different from those obtained with our model. Finally we utilise molecular data to resolve the discrepancy still outstanding between theory and experiment. Three years ago, Granneman and Van der Wiel(l975) reported a measurement of cross sections for two-photon ionisation in atomic Cs involving a near one-photon resonance with the 7p levels. Their experimental results were shown to be quite different from the predictions of Bebb (1966). This discrepancy has prompted a recalculation of the two-photon ionisation cross section in Cs by Teague et a1 (1976) and independently by Flank et a1 (1976) who considered the effects of quadrupole contributions. The results of Teague et a1 (1976) are very similar to those of Bebb (1966). Finally, Klewer et a1 (1977) repeated the experiment of Granneman and Van der Wiel (1975) with an improved experimental arrangement only to verify the older results. All theoretical works utilise ordinary second-order perturbation theory to evaluate cross sections; thus the probability of ionisation per unit time is proportional to (h = 1) where /g) is the initial (ground) state, If) the final continuum state, and the sum runs over all intermediate states of the atom. The theory predicts a deep minimum of 2.6 eV photon energy arising from a cancellation produced primarily by the interference between the 6p and 7p intermediate-state contributions in equation (1). The experi- mental data show no sign of such a minimum and large discrepancies, up to four orders of magnitude, were found between the two results. However, as pointed out by Beers and Armstrong (1975), under certain conditions the type of perturbation calculation which leads to equation (1) may not be valid. Beers