ing procedure was observed to remain clean according to Auger electron spectroscopy and vibrational relaxation probability for weeks. The daily sputtering procedure was nonetheless maintained.
In the current setup it is not possible to characterize the surface structure, e.g. by low–energy electron diffraction (LEED). However, it is well known that Au(111) sur-faces form a 22×√
3 “herringbone“ reconstructed surface (see Ref. (93) and references therein). While the surface is mostly characterized by an face centered cubic (fcc) top–
layer alignment, there are smaller fractions with hexagonal–close–packed (hcp) surface sites. These regions are connected via straight ridges visible with a scanning tunnelling microscope. Existing computational work on the NO/Au(111) (see Section 2.4.3) does not take this reconstruction into account.
3.3 Laser systems
The vibrational relaxation experiments described in this work were performed using several laser systems for the optical preparation of NO molecules in high vibrational states and the detection of ro–vibrational state distributions after the collision with the Au surface. In particular, molecules were excited to the vibrational state vi = 3 with a Fourier–transform limited infrared source (subsection 3.3.1). The preparation of high vibrational states (vi = 11 and vi = 16) was achieved with two home–built, injection seeded optical parametric oscillators (OPOs) (subsection 3.3.1). The quantum state purity in the excited high vibrational states was improved with the output of a commercial dye laser (subsection 3.3.1) in the sweep step of the pump–dump–sweep approach. Finally, the scattered ro–vibrational state distributions were probed with a commercial solid–state OPO laser system (Continuum, Sunlite Ex OPO with FX–1 UV frequency extension) that is widely tunable in frequency. These laser systems are briefly described in this section and references for further reading are provided. All laser systems have in common, that they are pumped by the second or third harmonic of a Nd:YAG laser at a repetition rate of 10 Hz.
3.3.1 Fourier–transform limited IR source
For vibrational overtone pumping of NO molecules from the vibrational ground state tov = 3, I used a high power ns infrared laser system with nearly Fourier–transform
3. Experimental
limited bandwidth. This complex laser system consists of
1. a continuous wave (cw) Nd:YLF laser (Coherent, Verdi–10) 2. a cw ring dye laser (Sirah, Matisse DR)
3. a five stage pulsed amplifier (Sirah, PulsAmp 5X), which is pumped by the second harmonic of an injection–seeded, pulsed Nd:YAG laser (Spectra Physics, Quanta–
Ray Pro–230)
4. a combined difference frequency mixing and parametric amplification unit (Sirah, DFM–2400-T and Sirah, DFA–2400–T)
The laser system actually belongs to a different experimental setup in the same laboratory (the Beamer 1 machine) and has been extensively described in the PhD thesis from Kai Golibrzuch(10). For the purpose of understanding my Thesis, it is suf-ficient to know that it is capable of producing intense infrared laser pulses (≈20 mJ at 1.8µm with 0.006 cm−1 linewidth). This resolution and power is sufficient to populate either the (e) or the (f) parity state in v = 3 by saturating either of the transitions NO X2Π1/2(v = 0, J = 0.5, e) → X2Π1/2(v = 3, J = 0.5, f) (the Q11(0.5)e line) or NO X2Π1/2(v = 0, J = 0.5, f) → X2Π1/2(v = 3, J = 0.5, e) (the Q11(0.5)f line), see Fig. 6.1 for an energy diagram and Fig. 6.2 for examples of scans of the infrared laser system.
3.3.2 Home–built narrow–bandwidth optical parametric oscillators For the optical pumping of high vibrational states, we used two very similar home–
built optical parametric oscillators (OPO) laser systems generating tunable, narrow–
bandwidth (<0.01 cm−1 at 206 nm), nanosecond, laser pulses with high output power.
Following the work of Velarde et al.(94) and Mahnke et al. (95), I built one of these OPOs from scratch for my masters thesis(96). The optical design of the OPOs is
3.3 Laser systems
Figure 3.3: Optical layout of the injection seeded OPO – EDCL = external cavity diode laser, DAQ = data acquisition card, PZT = piezoelectric transducer, BD = beam dump, OI = optical isolator, PD = photo diode, SMF = single–mode fiber, LV Amp = low voltage amplifer. Figure from my maters thesis (96).
3. Experimental
which is also referred to as ramp, hold and fire technique. The OPOs produce Fourier transform limited laser pulses in the infrared (OPO signal), tunable in the range of 875–
940 nm with the current laser diode. This infrared radiation can be used in frequency mixing. In this work, frequency doubling of the IR signal as well as sum–frequency generation with the second or fourth harmonic if the Nd:YAG laser was performed.
Both OPOs were pumped by the second harmonic of the same Nd:YAG laser and it was thus possible to perform the pump as well as the dump step in the optical preparation of high vibrational states (section 5) with the same Nd:YAG radiation source.
3.3.3 Dye laser
A Nd:YAG (PRO–270, Spectra Physics) pumped dye laser (Sirah, Precision scan, PRSC–DA–24) was used for the sweep step in the optical preparation of high vibra-tional states. The bandwidth of≈0.1 cm−1 is achieved via a holographic grating (2400 lines/mm, 1st order) at grazing incidence. The laser was typically operated with a solution of C–450 (Exciton, coumarin dye) in methanol to generate radiation in the range between 445 nm and 455 nm.
3.3.4 Sunlite Ex OPO with FX–1 UV frequency extension
In order to be able to perform scans over very broad wavelength ranges to probe many scattered vibrational states of the NO molecule via REMPI spectroscopy, an OPO laser system (Continuum, Sunlite Ex) pumped by the third harmonic of a Nd:YAG laser (Continuum, Powerlite DLS 9010) was purchased in 2012. Similar to the dye laser, this all solid–state laser uses a holographic grating to achieve its linewidth and thus achieves approximately the same bandwidth (0.1 cm−1). The OPO signal (445