We propose that the anomalous (non-monotonic) behavior of physical properties of supercritical fluids (SCF) in the Widom delta is attributed to the formation of medium-sized clusters. This hypothesis is experimentally verified for carbon dioxide using both experimental methods and molecular dynamics simulations. From a microscopic point of view, the non-monotonic behavior of the nonlinear refractive index, speed of sound, and Raman scattering efficiency is caused by the formation of quasi- linear clusters of medium size (5-200 molecules per cluster). Within the clusters, the molecule concentration is close to that of the liquid phase, while outside the clusters, it resembles the gas phase, leading to experimentally observed high (similar to 15 %) density fluctuations. Isolated linear clusters exhibit high second-order hyperpolarizability, resulting in an increase in the molecular contribution to the nonlinear refractive index and the intensity of Raman scattering. The appearance of multiple Widom lines on the pressure-temperature (p-T) diagram, each associated with unique physical properties, arises from the combined effects of cluster-specific and density-related factors. This interplay results in the divergence of Widom lines and the formation of the characteristic feature known as the Widom delta.

The dynamics of femtosecond laser impact on water was experimentally studied and reconstructed using numerical modeling based on the classical molecular dynamics method in combination with the two-temperature model and dynamical rate equations. This process occurs in several stages. Initially, a femtosecond laser pulse interacts with the electron subsystem, generating plasma due to multiphoton, tunnel, and impact ionization. The energy transfer from plasma electrons to atoms, as shown using the two-temperature model, leads to ultrafast heating of the substance to a temperature of 10 000 K, and the pressures achieved in the irradiated area are 15 GPa, which leads to the generation of a shock wave. The temperatures and pressures exceeding the critical values, combined with high density fluctuations and clustering, indicate the transition of the substance to a supercritical state. The pressures and temperatures exceeding the critical values are achieved in a region slightly exceeding the cavitation zone, and this region experiences oscillations with a period close to the period of oscillations of the cavitation bubble. In the case of the femtosecond laser impact, the experimentally measured deposited energy density can be used as an initial condition under assumtion of unstantaneous heating of the medium, which significantly simplifies numerical modeling. Both the pressures achieved at the shock wave front and the dynamics of cavitation bubbles are successfully reconstructed within the framework of this approach.

The dynamics of submicron gold particle formation during laser ablation in supercritical and liquid CO2 are investigated using time-resolved Mie scattering absorption spectroscopy. It is found that a high density of CO2 of approximately 800 kg/m3, regardless of the phase state, leads to the formation of laser-induced medium fluctuations on time scales of 10 to 1000 μs, during which the defocusing of subsequent laser pulses may occur. Under conditions of the reduced fluid density in the vicinity of the Widom region (P = 8.5 МРa, T = 310 K), it is demonstrated that nanoparticles can leave the interaction zone unhindered as evidenced by the exponential increase in integral absorption. Data from the absorption spectra allow the observation of the nanoparticle formation dynamics with an average diameter of approximately 160 nm and a log-normal size distribution, the width of which is determined by the thermodynamic conditions of CO2.

An approach to simulating the Raman spectra of supercritical carbon dioxide was demonstrated. The approach is based on the Fourier analysis of the autocorrelation function of the dipole moment calculated by molecular dynamics. Based on these data, the Raman spectra of carbon dioxide were calculated along the isotherm at 310 K in the pressure range of 0.1–15 MPa. It was determined that, in the Widom delta, the spectral lines are broadened and the pressure dependences of the Raman shifts have extrema. The results obtained agree well with the known experimental data and indicate a significant effect of cluster formation on the macroscopic properties of carbon dioxide in the vicinity of the critical point.

We created a laser-plasma X-ray source based on the femtosecond fiber laser with high yield $\sim 2 \times 10^{9} \mathrm{phot} / \mathrm{s} / 2 \pi(3-12$ keV), and with a source size diameter of approximately 10 microns. The X-ray yield and the source size were optimized by using artificial intelligence, the He flow and nanosecond pre-pulse.

We studied the evolution of shock waves and cavitation bubbles generated by femto- and nanosecond laser pulses over a time scale ranging from femtoseconds to microseconds in various molecular environments, including supercritical states. Through time-resolved shadow photography techniques, we observed that when highly intense laser pulses are focused into a supercritical fluid like carbon dioxide, cavitation bubbles do not collapse as in liquid media. Instead, clusters ranging from 1 to 200 microns in size are formed, presenting a significant differnce from the typical dynamics of cavitation bubbles induced by laser in liquids which exhibit size fluctuations

Unusual time-delayed changes in the x-ray diffraction parameters of LiNbO3 and LiNbO3 : Fe crystals were observed under nanosecond laser impact. Subnanosecond time resolution in the registration of diffraction rocking curve (DRC) dynamics was achieved through the synchronization of a 4 ns laser pulse with the circulation phase of electron bunches within a synchrotron storage ring. The response of the crystals to optical impact resulted in a reversible center-of-mass shift and integral intensity decrease of the DRC recovering in similar to 35 ns. The dynamics of lattice deformation indicates the process of formation and subsequent decay of an electrical charged layer near the surface due to directed migration of photoelectrons as a result of the bulk photovoltaic effect. The drop in the integral intensity of the DRCs is apparently caused by a running wave generated by a sharp change in the deformation of the crystal lattice. In the case of the nominally undoped crystal, the time-delayed processes occur within the same time interval but with significantly smaller amplitudes.

Продемонстрирован подход к моделированию рамановских спектров сверхкритического диоксида углерода, основанный на Фурье-анализе автокорреляционной функции дипольного момента, рассчитанного методом молекулярной динамики. На его основе рассчитаны рамановские спектры диоксида углерода вдоль изотермы 310 К в диапазоне давлений 0,1—15 МПа. Установлено, что в дельте Видома наблюдается уширение спектральных линий, а также экстремумы в зависимостях рамановских сдвигов от давления. Полученные результаты хорошо согласуются с известными экспериментальными данными и свидетельствуют об существенном влиянии кластерообразования на макроскопические свойства диоксида углерода в окрестности критической точки An approach to modeling the Raman spectra of supercritical carbon dioxide based on Fourier analysis of the autocorrelation function of the dipole moment calculated by the molecular dynamics method is demonstrated. On its basis, Raman spectra of carbon dioxide were simulated along the 310 K isotherm in the pressure range 0.1–15 MPa. It has been established that in the Widom delta there is a broadening of spectral lines, as well as extrema in the dependences of Raman shifts on pressure. The results obtained are in good agreement with known experimental data and indicate a significant influence of cluster formation on the macroscopic properties of carbon dioxide in the vicinity of the critical point

The spectra of transmisson coefficients and absorption indices of single-domain and multidomain LaBGeO5 samples have been measured. It is shown that, to measure more exactly the optical rotation ρ, it is necessary to use the spectra of transmission coefficients not only for the cases of parallel and crossed polarizers but also at other angles between them. The obtained ρ values for both samples are described quite well by only dispersion using the Drude formula. This is in agreement with the fact that the ρ value should not change during transition to the single-domain state of the crystal at a given symmetry (P31 in the ferroelectric phase and P3121 in the paraelectric phase). It is shown that the Cherenkov-type second harmonic generation (SHG) is observed only in a polydomain sample, while the second-harmonic radiation is not polarized. The domain structure of the samples was observed by scanning electron microscopy and piezoelectric force microscopy. The presence of a labyrinth-like domain structure was shown for the multidomain sample, whereas for a single-domain sample no changes in contrast were observed within the scanned region.