3. Experimental Setup
CCD camera
MCP + Phosphor screen
spectrometerVMI
Skimmers Even-Lavie
valve
Electrostatic deflector
Figure 3.1.:Sketch of general experimental setup with emphasis on the parts of the molecular beam machine.
The VMIS was used in both, spatial imaging and velocity map imaging mode, where spatial imaging was used to characterize the molecular beam and record time-of-flight spectra, and the velocity-map imaging mode was used to measure angular distributions of aligned molecules and photoelectron momentum distributions. The MCP can be gated to filter out different ionic fragments according to their arrival time, i. e., their mass to charge ratio. The gating is realized using a fast switch (e. g., Behlke HTS 31-03-GSM), which applies a rectangular gate of 1 kV within a short temporal window, with a time duration of typically 10 ns to 100 ns. A permanent voltage is applied on the MCP, chosen such that its sensitivity is low and no counts are recorded without the external gate.
When the gate is applied, the sensitivity is increased and only ions and electrons arriving within the gating time window are recorded.
36
3.2. Molecular Beam
In a supersonic expansion [187, 188] using a pulsed Even-Lavie valve [44, 46, 47], a high-pressure gas mixture is expanded through a small orifice into vacuum. The major part of the gas mixture consists of an inert atomic gas such as helium, neon or argon.
Throughout this thesis helium was used. These valves are specified to be operated at repetition rates up to 1 kHz, but often leakage occurs due to magnetization of the plunger and the beam suffers from time-dependent density and beam-profile modifications. We thus operated the valve typically at 250 Hz, or at most at 500 Hz. In Figure 3.2, the
-40 -20 0 20 40
Time (µs) 0
0.2 0.4 0.6 0.8 1.0 1.2
Signal(norm.arb.u.)
a
Temporal profile of molecular beam Gaussian Fit, σ= 26.2µs
-3 -2 -1 0 1 2
Lens position (mm) 0
0.2 0.4 0.6 0.8 1.0 1.2
Signal(norm.arb.u.)
b
Spatial profile of molecular beam
Figure 3.2.: Measured temporal and spatial profile of molecular beam with 500 ppm OCS seeded in helium, operated at a repetition rate of 250 Hz. a Temporal profile of the molecular beam with FWHM = 26.2 µs and bspatial profile of the molecular beam.
temporal and spatial profile of the molecular beam is shown, as measured with 500 ppm of OCS seeded in helium at a stagnation pressure of 90 bar. The valve was operated at repetition rate of 250 Hz and had an opening diameter of 100 µm, defined by the aperture of the front gasket in the valve. The spatial and temporal beam profiles, shown inFigure 3.2, were recorded with the experimental setup at the MBI. The gas pressure and opening time of the valve were chosen such that no cluster formation was observed, which would result in a higher beam temperature.
3.3. Electrostatic Deflector
Except for the experiment performed in Aarhus (seechapter 5), an electrostatic deflector was used to spatially disperse quantum states in the molecular beam according to their effective dipole moment and to separate the molecules from the helium seed gas. Both, the a-type deflector [1] and later its successor, the b-type deflector [61], were used.
The electrostatic fields of both are shown in Figure 3.3. The deflection profile shown in Figure 3.4 was recorded in the MBI setup with the b-type deflector at voltages of
3. Experimental Setup
350 300 250 200 150 100 50
Electric field strength (kV/cm)El ect ric fiel d st reng th ( )
50 100 150 200 250 300 350
x-axis (mm)
y-axis (mm)
x-axis (mm)
y-axis (mm)
0 2 4
2 -2 0
a
x-axis (mm)
y-axis (mm)
x-axis (mm)
y-axis (mm)
0
2 -2 0
2 4 b
Figure 3.3.: Inhomogeneous electric fields in a-type and b-type electrostatic deflectors. a electric field inside a-type deflector,b electric field inside b-type deflector. The figures are taken from [61].
±15 kV applied to the two electrodes. These measurements were performed on the same molecular beam as shown in Figure 3.2. The beam exhibits strong deflection and
−4 −3 −2 −1 0 1 2
Lens position (mm)
0.0 0.1 0.2 0.3 0.4 0.5
Yield (norm . arb . u . )
∆V = 0 kV (experiment)
∆V = 0 kV (simulation)
∆V = 30 kV (experiment)
∆V = 30 kV (simulation)
Figure 3.4.: Spatial molecular beam profiles for 500 ppm of OCS seeded in helium. The undeflected (0 kV) and deflected (±15 kV) spatial molecular beam profiles, measured after they have passed the b-type deflector, are shown together with corresponding simulations. The best fit of the undeflected beam profile was obtained for a rotational temperature ofTrot = 0.6 K.
experiments were performed in the deflected part at a lens position around −3 mm.
The best fit from the simulated beam profiles yielded a rotational temperature in the undeflected beam of Trot = 0.6 K. The choice of the spatial position in the molecular
38
beam that is probed represents a compromise between the most deflected and, hence, coldest part of the beam and still having high enough signal to perform the experiments with high signal-to-noise ratio.
3.4. Laser System
The laser system used at the MBI consists of a commercial Ti:Sapphire laser system (KMLabs, Wyvern30), delivering laser pulses with a duration of 38 fs (FWHM), a
rep-etition rate of 1 kHz, a central wavelength of 800 nm and pulse energies of 30 mJ. The beam was compressed in a grating compressor to account for chirps, acquired during its propagation through the optics and crystals. The beam was then further divided into two parts, whose time delay was controlled using a motorized delay stage. The weaker beam (3 mJ) was used as pump (alignment) laser for the molecules and the high-energy beam (20 mJ) was sent into a tunable optical parametric amplification system (HE-TOPAS, Light Conversion). A simplified sketch showing the most important components of the
Ti:Sapphire
LASER HE-TOPAS
BS MR
Delay line BS
Beam separator
MR MR
BS Telescope
MR SF11
stretching glass
MR MR
Beam block
MR MR
Beam recombiner
CaF2
focussing lens Iris
wave plateIR
mid-IR wave plate
VMI Delay line
BS - beamsplitter MR - optical mirror
Periscope
Figure 3.5.: Sketch of optical laser setup. The optical setup with the two-pulse alignment beamline is shown. This setup was used inchapter 4, chapter 7and chapter 8.
optical setup is shown inFigure 3.5. Through successive optical parametric amplification, output pulses with a total energy of 6.5 mJ (signal+idler) and a pulse duration of 60 fs were achieved. The central wavelength of the signal could be tuned in the range from 1.2 µm to 1.6 µm yielding wavelengths in the range from 1.6 µm to 2.4 µm for the idler, respectively. The IR pump pulse(s), used for alignment, and the mid-IR probe pulse were finally collinearly combined using a beam recombiner, with the IR in reflection and the mid-IR in transmission. Both beams were then focussed into the VMIS using a 25 cm focal length CaF2 lens.