ISSN 1063-7761, Journal of Experimental and Theoretical Physics, 2008, Vol. 107, No. 1, pp. 20–27. © Pleiades Publishing, Inc., 2008. Original Russian Text © I.I. Beterov, I.I. Ryabtsev, D.B. Tretyakov, N.N. Bezuglov, A. Ékers, 2008, published in Zhurnal Éksperimental’noœ i Teoreticheskoœ Fiziki, 2008, Vol. 134, No. 1, pp. 29–38. 20 1. INTRODUCTION Equilibrium thermal blackbody radiation, or, sim- ply, blackbody radiation, which is always present in space, can appreciably affect the dynamics of popula- tion of highly excited (Rydberg) states [1] of any atoms. Interactions of atoms in Rydberg states with blackbody radiation have been theoretically and experimentally studied in a number of works, in which the greatest attention was concentrated on transitions in the discrete spectrum and on the decrease in the lifetime of Rydberg states under the action of blackbody radiation [2–4]. Detailed theoretical investigations of the depopulation of Rydberg levels were performed in [5]. Also, cascade transitions in the discrete spectrum were theoretically and experimentally studied [6]. Recent growing interest in the interaction of Ryd- berg atoms with blackbody radiation was stimulated by investigations of ultracold plasma. In first experiments, conversion of a cloud of cold Rydberg atoms in a mag- netooptical trap into ultracold plasma was observed [7, 8]. It was assumed that this occurs due to avalanche ion- ization of Rydberg atoms in the cloud, which is initi- ated by photoionization of Rydberg atoms in the black- body radiation field. To construct a model of formation of ultracold plasma, calculation data on the total rates of ionization of different Rydberg states by blackbody radiation are necessary. However, analysis of the literature available shows that only a few works were devoted to transitions to the discrete spectrum, which are responsible for pho- toionization of atoms. In [9], the temperature depen- dence of the blackbody photoionization rate of the 17D state of sodium was theoretically calculated and exper- imentally measured. A simple analytical formula for estimating the ionization rate proposed in that study is still in use [7], although it takes into account only the ionization potential of the Rydberg state, while the orbital momentum L is ignored. This makes this for- mula inapplicable in calculations of photoionization rates of non-hydrogen-like states with small L. The rates of direct photoionization of hydrogen and sodium atoms by blackbody radiation were numerically calcu- lated in [10] to a high accuracy using the model poten- tial method. Our recent results for sodium obtained in [11] agree well with these calculations. In [11], we also calculated the total ionization rates for rubidium atoms. Theoretical calculations of direct photoionization by blackbody radiation were recently calculated for helium atoms as well [12]. We also note that radial matrix elements for transitions between coupled and ATOMS, MOLECULES, OPTICS Ionization of nS, nP, and nD Lithium, Potassium, and Cesium Rydberg Atoms by Blackbody Radiation I. I. Beterov a, *, I. I. Ryabtsev a , D. B. Tretyakov a , N. N. Bezuglov b, c , and A. Ékers d, e a Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia b St. Petersburg State University, St. Petersburg, 199034 Russia c Fock Research Institute of Physics, St. Petersburg, 198904 Russia d University of Latvia, LV-1586 Riga, Latvia e Institute of Atomic Physics and Spectroscopy, LV-1585 Riga, Latvia *e-mail: beterov@isp.nsc.ru Received October 11, 2007 Abstract—The results of theoretical calculations of the blackbody ionization rates of lithium, potassium, and cesium atoms residing in Rydberg states are presented. The calculations are performed for nS, nP , and nD states in a wide range of principal quantum numbers, n = 8–65, for blackbody radiation temperatures T = 77, 300, and 600 K. The calculations are performed using the known quasi-classical formulas for the photoionization cross sections and for the radial matrix elements of transitions in the discrete spectrum. The effect of the blackbody- radiation-induced population redistribution between Rydberg states on the blackbody ionization rates measured under laboratory conditions is quantitatively analyzed. Simple analytical formulas that approximate the numer- ical results and that can be used to estimate the blackbody ionization rates of Rydberg atoms are presented. For the S series of lithium, the rate of population of high-lying Rydberg levels by blackbody radiation is found to anomalously behave as a function of n. This anomaly is similar to the occurrence of the Cooper minimum in the discrete spectrum. PACS numbers: 32.80.Fb, 32.80.Rm, 32.70.Cs DOI: 10.1134/S1063776108070029