VUV Spectroscopy of Atoms, Molecules and Surfaces

6.3 Results and discussion 135
Ion signal (arb. units)
10
1
0.1
6x10 -5 mB
1x10 -4 mB
3x10 -4 mB
6x10 -4 mB
0.0 0.5 1.0 1.5 2.0
Delay (ns)
Figure 6.3: Semilogarithmic plot of the He + ion signal as a function of pump-probe delay
for four different He pressures.
four different pressures. For all of the measurements presented, a probepulse
energy of ∼350 µJ/pulse was used. The point of zero delay has been
(arbitrarily) chosen as the delay corresponding to a maximum in the ion
signal. From figure 6.3 the spike is seen to develop when the pressure is
increased from 6×10 −5 mB to 6×10 −4 mB, the latter being the maximum
pressure applied considering the presence of the EMT’s. The development
of the spike is seen to be accompanied by an almost one order-of-magnitude
decrease in the ion signal at long delays, implying a much reduced statistics
in the tails of the decay curves. Abstracting from the consequent scatter in
the data points it is evident that the slopes of the curves are constant, and
”only” the uncertainty on the measured lifetime is affected by the presence
of the spike. The influence of the pressure is further illustrated in figure
6.4where the magnitude of the ion signal at zero (black) and 0.23 ns (grey)
delays, respectively, is depicted as a function of pressure. The off-spike ion
signal increases up to ∼2×10 −5 mB and then decreases, while the on-spike
signal increases up to ∼6×10 −4 mB and then saturates. The presence of
the spike is also signified in the harmonic spectral profile which is broadened
by almost a factor of two when the pump- and probe pulses overlap in time.
This is illustrated in figure 6.5, showing the apparent harmonic profile at zero
(black) and 0.23 ns (grey) delays, respectively, for a pressure of 6×10 −4 mB.
Subsequent fits of the profiles to a Gaussian function yielded widths (FWHM)
of 0.0057 and 0.0035 nm for the on- and off-spike conditions, respectively, and
corresponding mean wavelengths of 58.4292 and 58.4295 nm.