ИСТИНА

Clusters have recently attracted attention as a unique laser-plasma media. Compared with gaseous and solid targets, a cluster target formed by high-pressure gas nozzle has several significant properties, such as local solid electron density, but low density on average. The laser energy absorption in cluster media is higher than in the solid target. Clustering gas has been demonstrated as an efficient medium for self-guided propagation of laser pulses. In recent years, the generation of hard (above 2 keV) characteristic X-ray radiation arising from the interaction of high intensity (I > 1015 W cm2) femtosecond laser radiation with cluster targets has been the subject of active investigations. Characteristic X-ray radiation contains information about the processes occurring in cluster nanoplasma and can be used for monitoring its physical properties. Besides it can be used for radiography of micro and nano-objects with high spatial and temporal resolution. All this applications require bright characteristic X-rays with high-contrast (the ratio of the K-line intensity to the continuum intensity) produced with high efficiency. It should be stressed that in cluster nanoplasma the X-rays can be generated with higher contrast as compared with X-ray emission from a solid-state plasma. One of the most important parameters determining the efficiency of X-ray generation, is the density of electrons produced by the initial laser ionization of the cluster. One can expected that clusters formed from polyatomic molecules will have higher electron density after ionization in comparison to the noble gas clusters. Furthermore, using molecular clusters for X-ray generation has an additional advantage, since one could obtain X-ray lines in a wide range. In our experiments we firstly observed an efficient generation (109 photons/J with energy conversion efficiency about 10−6) of hard (2−4 keV) characteristic X-rays under the interaction of intense (I ~ 1016 W/cm2, E = 5 mJ) laser radiation with large molecular clusters (SF6, CF3I, CF2Cl2). Maximum of X-ray yield corresponds to the positively chirped laser pulse with duration of about 300 fs and laser beam focusing in the front edge of the cluster jet. It was obtained that at optimal experimental conditions the efficient X-ray generation is accompanied with the multifocal plasma filament formation. The multifocal structure of generated plasma filament can be used as a qualitative criterion for the optimization of control parameters. We observed that as a result of laser-cluster interaction, the amplitude of the trailing spectral components of the laser pulse drops. This fact can be used for the evaluation of the absorbed laser energy. We have firstly observed generation of two X-ray lines corresponding to chlorine and argon (efficiency 3∙10-7 and 7∙10-8 correspondingly) as a result of adding to the mixture CF2Cl2-Ar (1:32) light carrier gas He. Adding of He to the mixture CF2Cl2-Ar leads to more efficient cooling of the mixture components that results in forming mixed clusters (containing from molecules CF2Cl2 and Ar atoms). The presence of mixed clusters is confirmed by the generation of X-ray lines that correspond to the both molecular and atomic gas. Using the ternary mixtures is a promising way to create the source of multiline X-ray radiation with tunable line intensities, which can be controlled by changing the mixture concentration. The dual-energy X-ray source can be used for nanoscale material discrimination with temporal resolution. The dynamic of relative intensities of lines can provide information about the stability of mixed cluster formation. In the presentation we will discuss the peculiarities of mixed clusters formation in the binary and ternary mixtures of molecules with carrier noble gases and hard X-ray lines (energy range 2-5 keV) generation under femtosecond laser excitation of these clusters.