**3.1.1 Laser excitation system**

The pulsed single mode Nd:YAG laser (the fundamental wavelength is 1064 nm) with the set of nonlinear crystals for generating the 2nd (532 nm), the 3d (355 nm) and the 4th (266 nm) harmonics of the fundamental radiation was used. At a pulse repetition frequency of 10 Hz, the average pulse energy was: 5 mJ (532 nm), 2 mJ (355 nm) and 0.7 mJ (266 nm). The 2 mm-diameter Nd:YAG crystal with the diaphragm (the diameter is 1.5 mm) on the outlet face was used to obtain the light generation in a single mode regime (single transverse mode). Q-switching was made by the electro-optical shutter, working based on the Pockels effect. Between the Nd:YAG crystal and the shutter, the polarizer, transmitting light only with horizontal polarization, was placed. Thus, at the generator output, we had a single mode beam with horizontal polarization. After the generator, the light beam passed through the linear telescope, which also worked as a rotating prism. The main purpose of this component was to decrease the beam divergence in the horizontal plane, which is necessary for effective conversion of the fundamental frequency to its harmonics. Then, the laser beam was sent to the 90 degree polarization rotator, which changed the horizontal polarization to vertical one (required for reducing energy losses on rotating mirrors). Passed through the rotator and reflected from the rotating mirror beam was directed through the round-trip amplifier, which was composed of the Nd:YAG crystal (5 mm in diameter) and the rear mirror. Then the beam arrived at the polarizer, which reflects only vertical polarization. The reflected beam was directed through the quarter-wave plate for changing the vertical polarization to the circular one. That improves the efficiency of frequency-doubling in a KTP crystal. The KDP and BBO nonlinear crystals were used for generating the 3d and 4th harmonics. After the frequency conversion, the radiation of the 4th (or the 3d), 2nd and the fundamental harmonics was transmitted through the quartz prism for their spatial divergence in the horizontal plane. The continuous adjustment of the laser intensity during the saturation curves measurement was carried out by the Pockels cell, which was placed in the beam way right after the KTP crystal. The cell consists of the electro-optical component (DKDP crystal) and two Glan prisms. Changing the voltage on the DKDP, one can adjust the radiation output intensity at the outlet from the Pockels cell.
