Blackbody-induced decay, excitation and ionization rates for Rydberg states in hydrogen and helium atoms

New features of the blackbody-induced radiation processes on Rydberg atoms were discovered on the basis of numerical data for the blackbody-induced decay P-nl(d)(T), excitation P-nl(e)(T) and ionization P-nl(ion)(T) rates of nS, nP and nD Rydberg states calculated together with the spontaneous decay rates P-nl(sp) in neutral hydrogen, and singlet and triplet helium atoms for some values of the principal quantum number n from 10 to 500 at temperatures from T = 100 K to 2000 K. The fractional rates R-nl(d(e, ion))(T) = P-nl(d(e, ion))(T)/P-nl(sp) equal to the ratio of the induced decay (excitation, ionization) rates to the rate of spontaneous decay were determined as functions of T and n in every series of states with a given angular momentum l = 0, 1, 2. The calculated data reveal an essential difference between the asymptotic dependence of the ionization rate P-nl(ion)(T) and the rates of decay and excitation P-nl(d(e))(T) proportional to T/n(2). The departures appear in each Rydberg series for n > 100 and introduce appreciable corrections to the formula of Cooke and Gallagher. Two different approximation formulae are proposed on the basis of the numerical data, one for R-nl(d(e))(T) and another one for R-nl(ion)(T), which reproduce the calculated values in wide ranges of principal quantum number from n = 10 to 1000 and temperatures between T = 100 K and T = 2000 K with an accuracy of 2% or better. Modified Fues' model potential approach was used for calculating matrix elements of bound-bound and bound-free radiation transitions in helium.