VUV Spectroscopy of Atoms, Molecules and Surfaces

132 Chapter 6. Two-colour pump-probe experiments on He ...
of the dye solution. The output wavelength of the DFDL oscillator, λDFDL,
can thus be tuned by adjusting θ (by turning the mirrors) or—on a finer
scale—by tuning the refractive index. In order to obtain tunability around
the ∼759 nm required for the present experiment while keeping the mirrors
parallel, the angle of incidence θ was increased from that of the +1st and
−1st order diffractions by means of a prism inserted in front of the dye cell.
The refractive index was tuned by tuning the temperature of the dye solution,
resulting in a wavelength change of −0.217 nm/ ◦C. In the experiment,
the absolute wavelength could be measured to an accuracy of ∼0.01 nm with
a commercial spectrometer.
Only a fraction of the ∼25 mJ, 532 nm, 70 ps output from the Nd:YAlG
laser was used to pump the DFDL oscillator, the rest being distributed to two
dye-cell amplifiers for amplification of the DFDL output to ∼1.5 mJ/pulse.
In addition, a few mJ were split off and mixed with the fundamental 1064nm
output from the Nd:YAlG laser in order to generate ∼0.5 mJ, 355 nm probe
pulses. Further amplification of the DFDL output to ∼35 mJ/pulse was
obtained by five passages through a Ti:Sapphire butterfly amplifier pumped
with a total of ∼400 mJ/pulse from a 532 nm, 10 Hz, 10 ns Nd:YAlG laser.
The DFDL pulses are expected to be approximately Fourier-transform limited
(time-bandwidth product 0.41) with a pulse duration similar to that of the
70 ps pump laser, implying a bandwidth of ∼0.05 nm [50].
The resulting DFDL laser beam, consisting of 35 mJ, ∼759 nm, ∼70 ps
pulses, was focused to an intensity of ∼5×1014 W/cm2 in a Kr gas jet with
a lens of 12 cm focal length. Harmonic orders were generated up to the 15th
with the 13th of 21.22 eV (58.4nm) photon energy being selected by a 1200
lines/mm holographic grating. The harmonic beam was intersected by the
355 nm probe beam at a 45◦ angle in a vacuum chamber equipped with a
gas jet for He inlet and an Electron-Multiplier Tube (EMT) for detection of
the harmonic signal. A flight tube extending from the top of the vacuum
chamber and terminated with an EMT, was used for time-of-flight detection
of He + ions repelled from the interaction region by a 1 kV potential. The
probe beam was focused with a 30 cm lens to a ∼17 µm diameter focal spot
∼7 cm before the interaction region, corresponding to a maximum in the
detected He + signal. The corresponding beam diameter in the interaction
region was ∼1 mm. With a probe energy of ∼350 µJ in the chamber this
corresponds to an intensity of ∼5×108 W/cm2 in the interaction region—
very close to the intensity which can be estimated to saturate the transition
(using the cross-section value stated in [36]). The probe beam passed through
a delay line on its way to the interaction region, and a halfwave plate was
positioned in front of the 30 cm lens for rotation of the polarization vector of
the (initially) vertically polarized probe beam with respect to the (vertically