ИСТИНА

Laser-induced fluorescence (LIF) is a versatile technique widely used in spectroscopy and plasma diagnostics. Beyond exploring molecular rovibronic states and measuring the natural lifetime of specific energy levels, LIF is used for mapping species’ spatial distribution within plasma source. Such measurement significantly enhances our understanding of plasma evolution, the processes occurring within it and the estimation of equilibrium conditions. However, spatially-resolved measurements require designing fluorescence schemes that incorporate knowledge about electronic states, saturation levels and optimal excitation and observation wavelengths, a task often complicated by limited data availability. Observation of two fluorescence schemes originating from different electronic state of the same species allows the calculation of excitation temperature, somewhat similarly to the two-line method. Our study aimed to develop two excitation-emission fluorescence scheme for Ti atoms applicable for the study of spatial distribution of particles and temperature in laser-induced plasma. All experiments were conducted in a vacuum chamber at a reduced pressure of 100 Torr, enabling measurements with high spatial resolution due to the sufficiently large plasma. A metallic titanium target was ablated by a Q-switched Nd:YAG laser (532 nm), while a pulsed tunable Ti:Sapphire laser was provided the selective excitation. After investigation of over 10 fluorescence schemes of Ti atoms, we found two suitable for temperature calculation under given experimental conditions: transitions between a3P2 → x3P°1 → a3P1 and z5G°5 → e5G6 → z5F°5 states. We also provide a detailed description of other schemes that either lacked signal amplification or saturation. Teoretical and experimental analyses of fluorescence intensity, saturation curves and typical lifetimes of the states under pulsed excitation demonstrate that saturation was achieved for both schemes across the entire plasma. A detailed examination of the kinetics of the transitions employed in these schemes led to an equation for temperature calculation. Spatially resolved studies of fluorescence intensity and temperature distribution show that a region of higher temperature close to the plasma edge (5 to 8 mm from the plasma axis), likely related to the propargation of particles in a higher excited state by the shock wave. The results of local temperature determination were used for the fine selection of plasma regions for determination of Stark parameters of Ti II 340.24, 340.72, 340.98 and 341.70 nm lines. The work was supported by the Russian Science Foundation (grant 23-22-00146).