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Analytical expressions for spontaneous and induced by blackbody radiation energy shifts and widths of atomic circular states are derived. The features of asymptotic behavior of the total width and energy shifts for highly excited Rydberg circular states in the blackbody radiation field are described. Approximate equations are proposed for simple evaluations of energy characteristics and an absence of temperature-independent spontaneous contribution to the total energy width in a range of high temperatures and principal quantum numbers is demonstrated. [ABSTRACT FROM AUTHOR]

Quadratic Stark corrections to the wave functions, matrix elements, and probabilities of transitions between the singlet states 1 S 0 and 1 P 1 of helium atoms are calculated. The coefficients of the polynomials that depend on the effective principal quantum number of the upper level v f and that approximate the numerical values of the polarizabilities, the quadratic corrections to the wave functions, and the probabilities of transitions to highly excited Rydberg states with large v f are determined. The results of calculations testify that the probabilities of all σ transitions n i 1 S 0 → n f 1 P 1 and π transitions to the states with n f > n i /2 are decreased with increasing electric field strength, except for the transition 21 S 0 → 21 P 1, whose probability increases both for σ and for π transitions. [ABSTRACT FROM AUTHOR]