VUV Spectroscopy of Atoms, Molecules and Surfaces

136 Chapter 6. Two-colour pump-probe experiments on He ...
Ion signal (arb. units)
1.0
0.5
1.0
0.5
0.5 1.0 1.5
0 2 4 6 8 10
Pressure (10 -4 mB)
Figure 6.4: The He + ion signal as a function of presure on- (black) and off-spike (grey),
respectively
The observed pressure-dependent characteristics of the decay curves and
the harmonic spectral profile can be explained by an enhanced absorption of
the harmonic by the He gas, combined with an AC Stark broadening [52] of
the 1s2p state at zero time delay where the probe is present. The absorption
arises from the propagation of the harmonic through at least 15 cm of He gas,
determined by the radius of the vacuum chamber. Only the spectral region
corresponding to the unbroadened profile is affected by the absorption, and
a ”hole” is burned in the harmonic spectral profile. As a consequence, the
ion signal is enhanced when the pump- and probe pulses overlap in time and
exhibits a spike as a function of time delay. Assuming an ionization cross
section at 355 nm of 16.6 Mb [36] and a probe intensity of 5×10 8 W/cm 2 ,the
Stark contribution to the ionization width can be estimated to be 0.0056 nm,
in very good agreement with the experimental observations. The probe pulse
is expected to give rise to an AC Stark shift of the 1s2p level also, since the
atomic ionization potential is known to be shifted up by the ponderomotive
energy (cf. chapter 1), equivalent to 0.0005 nm in the present experiment.
The higher-lying Rydberg levels shift upwards by approximately the same
amount whereas the ground state is slightly downshifted [53]. With the He
1s2p level being intermediate between these limiting cases, the experimentally
observed shift of 0.0003 nm may likely be in agreement with theory. The
conditions necessary for the observation of the spike are thus: (i) a probe
intensity sufficiently high to induce a significant broadening of the 1s2p level