  We report the results of experimental studies related to implantation of
thorium ions into thin silicon dioxide by pulsed plasma fluxes expansion.
Thorium ions were generated by laser ablation from a metal target, and the
ionic component of the laser plasma was accelerated in an electric field
created by the potential difference (5, 10 and 15 kV) between the ablated
target and SiO2/Si(001) sample. Laser ablation system installed inside the
vacuum chamber of the electron spectrometer was equipped with YAG:Nd3+ laser
having the pulse energy of 100 mJ and time duration of 15 ns in the Q-switched
regime. Depth profile of thorium atoms implanted into the 10 nm thick
subsurface areas together with their chemical state as well as the band gap of
the modified silicon oxide at different conditions of implantation processes
were studied by means of X-ray photoelectron spectroscopy (XPS) and Reflected
Electron Energy Loss Spectroscopy (REELS) methods. Analysis of chemical
composition showed that the modified silicon oxide film contains complex
thorium silicates. Depending on local concentration of thorium atoms, the
experimentally established band gaps were located in the range of 6.0 - 9.0 eV.
Theoretical studies of optical properties of the SiO2 and ThO2 crystalline
systems have been performed by ab initio calculations within hybrid functional.
Optical properties of the SiO2/ThO2 composite were interpreted on the basis of
Bruggeman effective medium approximation. A quantitative assessment of the
yield of isomeric nuclei in "hot" laser plasma at the early stages of expansion
has been performed. The estimates made with experimental results demonstrated
that the laser implantation of thorium ions into the SiO2 matrix can be useful
for further research of low-lying isomeric transitions in 229Th isotope with
energy of 7.8(0.5) eV.
