ИСТИНА

Laser-induced fluorescence (LIF) is a versatile tool with numerous applications in the fields of spectroscopy and plasma diagnostics. One of its primary uses is in the study of molecular rovibronic states, where LIF can provide valuable information on the structure and dynamics of these states. Additionally, LIF can be employed to observe fluorescence decay, which is useful for studying collision processes or measuring the natural lifetime of a given energy level. Another important application of this method is the study of species spatial distribution in laser-induced plasma based on their selective excitation by a tunable laser for better understanding of the processes and equilibrium in plasma. However, implementing spatially-resolved measurements requires the development of fluorescence schemes containing information about the electronic states and the combination of wavelengths for excitation and observation of fluorescence. Constructing fluorescence schemes can be challenging as the necessary data may not always be available in literature. Therefore, the primary purposes of this study are to design fluorescence schemes for Ca and Fe atoms as well as for their corresponding monoxides (CaO and FeO), and to apply these schemes to investigate species distribution within laser-induced plasma. To perform the experiment, we used pulsed Q-switched Nd:YAG (532 nm) and tunable Ti:Sapphire (1st and 2nd harmonics) lasers. We used pellets made from ultrapure iron oxide (II, III) (Fe3O4) and calcium carbonate (CaCO3). The second harmonic of the Nd:YAG was focused on the target surface to produce laser plasma. A Ti:Sapphire laser beam was directed parallel to the surface of the sample to excite the fluorescence of the studied particles. A system of linear stages provides the positioning of the probing beam in the plasma with an accuracy of 0.2 - 0.4 mm. The experiments were carried out in a vacuum chamber at pressures of 100 and 10 Torr and interpulse delays of 10 and 15 µs, respectively. For atomic calcium, we proposed and implemented a scheme of nonresonant fluorescence based on transitions between the 4s4p (3P°)–4p2 (3P) states [1]. The wavelength of Ti:Saphire laser was set at 428.301 nm, allowing us to observe fluorescence at 430.523 nm. The scheme for CaO molecular fluorescence involves transitions between B1Π and X1Σ+ states [2]. In the fluorescence spectra, we observed CaO molecular bands at 408.43 (0, 1 band) and 421 nm. We assigned the transition at 421 nm to the (0, 3) vibrational transition based on calculations of vibrational level energies using molecular constants since there is no assignment in literature. It should also be noted that by slightly changing the wavelength of the exciting laser, we observed selective excitation of rotational states. Fe atomic fluorescence scheme employs excitation at 396.926 nm and fluorescence at 406.359 and 413.206 nm between 3d7(4F)4s (a3F) and 3d7(4F)4p (y3F°) electronic states. The close proximity of the excitation wavelengths for CaO and Fe enabled the simultaneous observation of fluorescence from both species in a meteorite sample. In the case of FeO, we developed an excitation-emission scheme using near-infrared bands (823 and 835.5 nm), but their assignment to electronic states remains impossible. Using these LIF schemes in our experiments, we were able to investigate spatial distribution trends in the plasma core for all four particles fluorescence intensity. Our experimental approach provided us an opportunity to investigate molecule formation pathways in laser-induced plasma, including dissociation of ablated material, recombination into diatomic molecules, or reactions with atmospheric oxygen.