VUV Spectroscopy of Atoms, Molecules and Surfaces

6.3 Results and discussion 139
Ion signal (arb. units)
8
7
6
5
0 60 120 180 240 300 360
Polarization angle (degrees)
Figure 6.7: The He + ion signal as a function of angle of polarization between the pump
and the probe for a pressure of 6×10 −5 mB. The theoretically predicted cross section is
indicated by the solid curve.
to occur at somewhat lower pressure (∼10 −7 mB) than found experimentally.
The model may need some fine adjustment—for example, the Stark shift of
the 1s2p level has been assumed to be equal to the ponderomotive energy—
but it is evidently capable of predicting the essential features in the decay.
It should be noted that without taking the absorption, or hole-burning, into
account the spike cannot be reproduced unless an artificially high probe intensity
(several times saturation) is used in the model. Finally, it remains
to be checked how closely the pressure dependence of the ion signal measured
on- and off spike, as given in figure 6.4, is reproduced by the model.
The influence of radiation trapping and pressure broadening [25, 26] can be
considered insignificant in the present experiment due to the low harmonic
photon flux and given the pressure-independent slopes of the decay curves in
figure 6.3.
6.3.2 Photoionization cross-section measurements
Concurrently with the spike investigations discussed above, experiments were
performed to show that relative partial photoionization cross sections of the
1s2p state could be determined by varying the angle between the pump- and
probe polarization vectors. The measurements presented below were obtained
with the time delay fixed at 0.23 ns and at a pressure of 6×10 −5 mB in order
to minimize an eventual influence from the spike. By conservation of angular