it is araiysed the change oi' refractive coeri'icient ot active gas medium iTn pulsed chain chemical HF-laser due to temporal variations of chemical composit ion and populat ions or vibrational states HF(v) and H.(v)-mnolecuies. These changes is related with diri'erent polarizability o components ol chemical laser medium and dependence or polarizability on vibrational state population ror atomic and electronic constituents or polarizability. The atomic polarizability is characterized by sizable Irequency dispersion. The change o rei'lective ooeicient due to change o1 vibIationa] excitation is nonline eUect., since the vibrational excitation dependented rrom intenity o1 laser beam.
The variational methods of Kohn and HulthCn are employed to calculate the scattering lengths for elastic collisions between positrons and hydrogen and helium atoms. Allowance is made for the dipole polarizability of the target atom by using a trial function which includes a term corresponding to the first-order perturbation of the atom due to the electric field of the positron.The effect of allowing for the dipole polarizability of the atom is to change the sign of the scattering lengths for both hydrogen and helium in agreement with the variational calculations of Spruch and Rosenberg for hydrogen. I n the case of hydrogen the total elastic cross section at zero impact energy is increased when allowance is made for polarization, but in the case of helium it is decreased from 0*72na," to 0.10.irao2. The experimental cross section derived by Teutsch and Hughes for 18 ev positrons in helium is still smaller, being only 0.023 & 0 . 0 0 6~~~~.
ALwtracL We analyse relativistic effects in transverse momentum using quantum molecular dynamics and its covariant extension, relativistic quantum molecular dynamics. The strength of the relativistic effects is found to increase with the bombarding energy and with an averaged impact parameter. The variation in the intensity of the relativistic effects with variation in the m m of the colliding nuclei is not systematic. Furihennore, the hard equation of state is affected drastically by the relativistic effects, whereas the soft equation of state is affected less. Our analysis shows that up to the bombarding energy of I GeVlnucl.. the influence of relativistic effecLs is small. whereas at higher energies relativistic e&& become naturally very important.
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