VUV Spectroscopy of Atoms, Molecules and Surfaces

130 Chapter 6. Two-colour pump-probe experiments on He ...
Figure 6.1: Energy level diagram of He indicating the atomic states and laser excitation
schemes involved in the photoionization cross-section measurements of Gisselbrecht et al.
in 1999 [36].
determined to an accuracy of 5 %. The state was resonantly excited with the
13th harmonic of the ∼50 ps, ∼759 nm pulsed output from a tunable distributed
feedback dye laser (DFDL) and subsequently ionized with ∼50 ps,
355 nm pulses from a Nd:YAlG laser. The ∼50 ps pulse duration was sufficiently
short that the lifetime could be measured and sufficiently long that
the mis-match between the laser bandwidth and the atomic linewidth was
not too large. This technique was later used for measurements of unknown
lifetimes of excited electronic states of CO [32] and N2 [38], respectively.
By keeping the time delay fixed at a certain value and scanning the pulse
energy of the probe while monitoring the ion signal, the technique can, in
addition, be used to determine absolute, total, excited-state photoionization
cross sections as demonstrated in 1999 by Gisselbrecht et al. [36]. Absolute
cross sections were obtained for the 1s2p 1 P o and 1s3p 1 P o states of He for
three different photon energies of the probe (cf. figure 6.1) and found to be
in good agreement with theory. This method [39, 40] and a variant hereof
[41, 42], measuring the fluorescence signal from the excited state decay, have
previously been applied to excited states of alkali- and alkaline- earth atoms.
Other recent absolute excited-state photoionization cross-section measurements
have concentrated on the heavier noble-gas elements Ne [43], Ar, Kr
[44, 45] and Xe [46, 47] with reference to their similarity to the theoretically
interesting alkali atoms and their relevance in connection with excimer
lasers and discharges. All of the noble-gas cross sections were determined