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American Journal of Physics

Effect of blackbody radiation on the hydrogen spectrum

Ellen D. Yorke

Published Jan 1, 1983• 2 citations
Publisher

Abstract

The partition function of a single hydrogen atom diverges. While the astrophysical solution to this dilemma is the introduction of a density dependent alteration of the partition function, the question of what happens in an arbitrarily rarefied gas remains open. It is argued that in this case blackbody radiation changes the atom’s spectrum. A unified derivation including these effects and the nonrelativistic Lamb shift and spontaneous emission is presented. Then recent numerical calculations performed by workers in the field of Rydberg atom spectroscopy are utilized to estimate the principal quantum number for which blackbody radiation significantly alters the spectrum.

DOI
10.1119/1.13428

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The partition function of a single hydrogen atom diverges. While the astrophysical solution to this dilemma is the introduction of a density dependent alteration of the partition function, the question of what happens in an arbitrarily rarefied gas remains open. It is argued that in this case blackbody radiation changes the atom’s spectrum. A unified derivation including these effects and the nonrelativistic Lamb shift and spontaneous emission is presented. Then recent numerical calculations performed by workers in the field of Rydberg atom spectroscopy are utilized to estimate the principal quantum number for which blackbody radiation significantly alters the spectrum.
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 d nl ( T ), excitation P e nl ( T ) and ionization P ion nl ( T ) rates of n S, n P and n D Rydberg states calculated together with the spontaneous decay rates P sp nl 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 d ( e , ion) nl ( T ) = P nl d ( e , ion) ( T )/ P sp nl 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 ion nl ( T ) and the rates of decay and excitation P d ( e ) nl ( T )∝ 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 d ( e ) nl ( T ) and another one for R ion nl ( 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.
A highly excited (Rydberg) atom bathed in blackbody radiation is perturbed in two ways. A dynamic Stark shift is induced by the off-resonant components of the blackbody radiation. Additionally, electric-dipole transitions to other atomic energy levels are induced by the resonant components of the blackbody radiation. This depopulation effect shortens the Rydberg-state lifetime, thereby broadening the energy level. Calculations of these two effects in many states of hydrogen, helium, and the alkali-metal atoms Li, Na, K, Rb, and Cs are presented for T=300T=300T=300 K. Contributions from the entire blackbody spectrum and from both discrete and continuous perturbing states are included. The accuracy is considerably greater than that of previous estimates.
We consider the Zeeman shift in Rydberg atoms induced by room-temperature blackbody radiation (BBR). BBR shifts to the Rydberg levels are dominated by the familiar BBR Stark shift. However, the BBR Stark shift and the BBR Zeeman shift exhibit different behaviors with respect to the principal quantum number of the Rydberg electron. Namely, the BBR Stark shift asymptotically approaches a constant value given by a universal expression, whereas the BBR Zeeman shift grows steeply with principal quantum number due to the diamagnetic contribution. We show that for transitions between Rydberg states, where only the differential shift between levels is of concern, the BBR Zeeman shift can surpass the BBR Stark shift. We exemplify this in the context of a proposed experiment targeting a precise determination of the Rydberg constant.
Second-order and fourth-order intensity-dependent corrections to the energy levels of the hydrogen atom are calculated. By comparing the results obtained for these two orders and for various values of the principal quantum number, it is seen that, at a given intensity, the second-order correction becomes more and more questionable as one approaches the ionization limit. It is shown how the divergences, which appear in calculating the matrix elements of the shift operator in a discrete basis of continuum functions, can be avoided by performing the computation in the velocity gauge. The relevant second-order numerical results indicate serious modifications of the continuum spectrum. Intrinsic limitations of the second-order calculations are discussed.
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 numerical 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.
Transitions between energetically close-lying Rydberg states are characterized by large electric dipole matrix elements and frequencies low enough that at room temperature the photon occupation number is \ensuremath{\simeq} 10. Consequently the absorption of blackbody radiation and the stimulated emission produced by it lead to an efficient redistribution of population among nearby levels. In some cases dramatic effects are observed, for example, radiative lifetimes that are shortened to a small fraction of their T=0T=0T=0 values. In addition, the 300-K blackbody field of \ensuremath{\sim}10 V/cm produces ac Stark shifts of the Rydberg levels. Calculations of the population redistribution effects and ac Stark shifts are presented. Examples are given to illustrate that care must be taken in experiments with highly excited atoms to ensure that only one state is being observed rather than a distribution.