3. Experimental setup 35
3.2. Lasers
In this work, the pulsed molecular beam and nanosecond lasers used for the preparation and characterization of molecules in the incoming and scattered beam are operated at 10 Hz repetition rate. The different laser systems used in the scattering experiments are described briefly below. For noncommercial and nonstandard laser systems references providing a detailed description are given in the respective section.
3.2.1. Dye lasers
Precision Scan
The "blue" dye laser (Sirah, Precision Scan, PRSCDA-24) is used to produce radiation in the range from 420 nm to 560 nm with a bandwidth of 0.06 cm−1(90 mm grating, 2400 l/mm). It is pumped by the third harmonic generation of a Nd:YAG laser (Spectra Physics, Quanta Ray PRO-270-10). The output of the dye laser can be doubled in a BBO crystal to obtain radiation in the UV.
Cobra-Stretch SL
The Nd:YAG (Continuum, Powerlite 7010) pumped "red" dye laser (Sirah, Cobra-Stretch SL) supplies radiation from 550 nm bis 800 nm with a bandwidth of 0.06 cm−1 (90 mm grating, 1800 l/mm). The output is doubled in a BBO crystal.
3.2.2. Sunlite Ex OPO with FX 1 UV frequency extension
The commercially available solid state laser Sunlite Ex OPO with FX 1 UV frequency extension (Continuum) is used to generate UV radiation in the range between 230 nm and 450 nm with a pulse energy of≈ 4 mJ/pulse and a bandwidth of 0.075 cm−1in the visible (the UV output is expected to have a bandwidth of 0.15 cm−1).
3.2.3. Homebuilt optical parametric oscillators
Two homebuilt injection seeded optical parametric oscillators[100] (OPOs) are used to generate narrow-bandwidth radiation in the near IR. Both OPOs are pumped by an injection seeded Nd:YAG Laser (Spectra Physics Lab 170-10, 532 nm, FWHM 8-10 ns). The ring cavity contains two rotatable KTP crystals (KTP, KTiOPO4, Altechna Co. Ltd., 67.4◦) for frequency conversion to the desired signal and idler wavelength. In order to obtain a
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3.2. Lasers narrow bandwidth, the OPO is seeded by a continuous wave external cavity diode laser at the desired wavelength (ECDL, Toptica Photonics, DL Pro 100, 875-940 nm). In addition, the cavity is actively stabilized by adjusting the voltage applied to a piezoelectric transducer (PZT, Piezomechanik GmbH, HPSt 150/200/12 VS35) to obtain the optimal position of the movable cavity mirror. In this way, the cavity length can be matched with the seed laser wavelength by monitoring the continuous wave output power of the cavity with a photodiode (PD, Thorlabs). The minimum output power corresponds to the optimal cavity length. Stabilization of the cavity is achieved using a Labview program which controls a data acquisition card.
The output of the OPOs is then used for sum frequency generation together with the second or fourth harmonic generation of the seeded Nd:YAG Laser, which yields narrow-bandwidth (≈ 0.007 cm−1) radiation in the UV. In this work the OPOs are used for stimulated emission pumping of NO via the A2Σ(v = 2,J = 0.5) state to the X2Π1
2(v =11,J =0.5)state. The laser radiation required for the pump step (dump step) is generated by setting the OPO to 887.146 nm (912.081 nm) and adding the fourth harmonic generation (second harmonic generation) of the Nd:YAG in a BBO crystal, Altechna Co.
Ltd., θ = 58.3 ◦ (Castech, θ = 23.6 ◦). Note that the OPOs are tunable over a narrow wavelength range by changing the seed frequency. This allows the compensation of ther-mal drifts in the Nd:YAG laser frequency, such that the radiation for the pump and dump step can be kept at a constant frequency.
3.2.4. Fluorine laser
The molecular fluorine laser (EX350 EXCIMER LASER, GAM Laser) supplies pulsed VUV radiation (FWHM 20-26 ns) on two laser lines at 157.52 nm and 157.63 nm each with a bandwidth of 8 cm−1and a pulse energy of typically 10 mJ/pulse. The optical path of the VUV radiation has to be evacuated. Thus, a custom beamline is used that is kept at a pressure of 2·10−2Torr. The beamline can be connected to adapters at the windows on both sides of the surface scattering chamber. See Figure 3.4 for a drawing of the VUV beamline connected to the molecular beam surface scattering apparatus. The laser beam is passed through a small chamber which is equipped with a feedthrough for a detector that can be used to measure the pulse energy of the laser. In most cases a CaF2 lens (f = 775 mm at 157 nm) is used to focus the laser beam into the detection region of the surface scattering chamber. The lens is positioned such that the actual focus lies behind the detection region and mild focusing to an area of 5.5 mm2 is achieved. Two mounts equipped with dichroic mirrors (157 nm, 45◦, Qioptics) are used to guide the laser beam
3. Experimental setup
Figure 3.4.: Setup of the VUV beam line at the ultra-high vacuum (UHV) chamber.
After a pass through a small chamber (not shown) used for pulse energy measurements of the incoming laser beam the VUV laser beam (propagation direction represented by the red arrow) is reflected at the dichroic mirror for radiation with a wavelength of 157 nm (Qioptics, 45◦) in mirror mount A. After a second reflection at mirror mount B the beam is sent into the UHV chamber passing through a CaF2window located in the adapters D and D’. Just upstream of this adapter an iris is placed (not drawn). The VUV beam leaves the UHV chamber via an analogous adapter on the opposite side of the chamber. A third reflection at mirror mount E steers the beam onto a power detector F. The mirror mounts B and E are equipped with anti-reflection coated windows suitable for passing UV laser light at around 350 nm. The mounts for the dichroic mirrors are designed such that the UV laser light can pass from the entrance window G to the exit window C.
into the surface scattering chamber. In addition, an iris is located directly in front of the viewport. A third mirror mount at the rear of the chamber reflects the VUV light passed through the scattering chamber onto a power detector. The mirror mounts are equipped with windows such that additional laser beams can be collimated with the VUV laser beam and sent through the scattering chamber.
3.2.5. Narrow-bandwidth IR-laser
A detailed description of the narrow-bandwidth IR-laser can be found in reference [101].
Briefly, the output of a cw Nd:YLF laser pumped ring-dye laser (Sirah Matisse DR, 20 MHz bandwidth) is pulse amplified in the PulsAmp 5X (Sirah) which is pumped by an injection-seeded Nd:YAG laser (Spectra Physics Quanta-Ray Pro-230). The pulsed light is then used together with the fundamental of the Nd:YAG laser in a difference frequency mixing process. The difference frequency is amplified in an OPA process which is pumped by the fundamental of the Nd:YAG laser. In this process signal and idler laser beams with
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3.2. Lasers a nearly Fourier-transform limited bandwidth are obtained. In this thesis the idler beam at around 2900 cm−1(≈4 mJ/pulse) is used.
In order to avoid reflection of UV/visible laser sources into the IR laser source when collimating the IR beam with counter propagating UV/visible laser beams the IR radiation can be sent through a germanium plate at the Brewster angle.