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Journal of Mathematical Physics

Two-level radiative systems and perturbation theory

F. T. Hioe, Elliot W. Montroll

Published Jun 1, 1975• 13 citations
Publisher

Abstract

We obtain expressions for the radiative level shifts of a two-level system in terms of (i) recurrence relations, (ii) ratios of determinants, (iii) continued fractions, and (iv) a Lidstone expansion. These expressions are shown to be very useful for numerical computations. It is pointed out that perturbation series in powers of the coupling constant are not the most appropriate way of representing the solutions of the problem, but if they are to be used, different series should be employed depending on the relative value of the frequency of the field to the frequency of the two-level system. The significance of these results in the general theory of perturbation is discussed.

DOI
10.1063/1.522666

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We obtain expressions for the radiative level shifts of a two-level system in terms of (i) recurrence relations, (ii) ratios of determinants, (iii) continued fractions, and (iv) a Lidstone expansion. These expressions are shown to be very useful for numerical computations. It is pointed out that perturbation series in powers of the coupling constant are not the most appropriate way of representing the solutions of the problem, but if they are to be used, different series should be employed depending on the relative value of the frequency of the field to the frequency of the two-level system. The significance of these results in the general theory of perturbation is discussed.
A general perturbation procedure is described for writing down the contributions to any order for the probability of a quantum system described by a time-independent Hamiltonian making a transition from an arbitrary initial state to an arbitrary final state. The results are expressed in terms of certain propagators which involve continued fractions, so that some processes are summed to all orders. The general theory is then applied to a single multilevel atom interacting with any number of modes of the electromagnetic field in the electric dipole approximation. The fundamental theory is considered primarily but a detailed discussion is given of spontaneous and stimulated emission. Expressions for the lineshape are derived incorporating the generalized Lamb shifts and damping constants.
Abstract A general and exact formalism is developed that permits the calculation of the line shape in all line-breadth phenomena, in principle to any order of approximation. Expansions for the line breadth and the numerator occurring in the line-shape formula are given. Corrections to the classical line shape include: (i) radiative corrections to the line breadth, (ii) a level shift additional to the usual radiative shift for levels with finite transition probability, (iii) radiative corrections to the numerator and (iv) a slight frequency dependence of the numerator. All these corrections, with the exception of (iv), are exceedingly small. The case of radio-frequency transitions is discussed.
The spontaneous radiative decay to the ground state of an atomic system initially in the higher of two excited states, coupled in a radiationless fashion by an external field, is investigated by the method of multiple-time-scale perturbation theory. The coupled differential equations of motion for the probability amplitudes are solved to second order, while the usual secular behavior of conventional time-dependent perturbation theory is eliminated in terms of two natural time scales: a fast time scale corresponding to the reciprocal of the level transition frequency and a slower time scale associated with the inverse of the radiation linewidth. Expressions are obtained to this order for the energy shifts and decay characteristics of the excited states and the energy distribution of the final-state photons, quantities previously determined by Fourier analysis and contour integration and by the phenomenological approach of Weisskopf and Wigner. A detailed comparison of these various methods of solution includes conditions under which the present results agree to second order with the previous calculations.
The connection between the projection operator approach and the continued fraction approach to perturbation theory is investigated. A concise solution to the linear operator equation A mod x>= mod b> is found in terms of the level shift operator using projection operator techniques. The analogous process to the use of projection operators in the continued fraction method of solving the same problem is identified, and a parallel development performed. The connection between the two approaches is thereby established, and continued fraction expressions for the level shift operator obtained. The abstract equation is then specialised to deal with (i) the eigenvalue problem and (ii) the calculation of transition probabilities for quantum mechanical systems described by a time-independent Hamiltonian. Particular attention is paid to the problem of degeneracy and it is shown that the most convenient expressions are found by a hybrid of the two approaches.
Expressions for energy-level shifts and transition amplitudes of atoms interacting with an incoming radiation field are developed in a completely quantum theoretic way and the results are compared with others. The continued-fraction expressions reproduce both those of Swain and of Gontier et al., and the numerical results for the particular case of Bloch-Siegert shifts in mercury vapor agree with Stenholm's but go further in giving the shape of the resonances. These results are also obtained by a method of direct diagonalization of the Hamiltonian of the coupled system.
The summation of all the higher-order contributions to multiphoton absorption in atoms is presented. The nonperturbative expressions for the transition amplitudes are expressed in terms of continued fractions. They account for the whole perturbation originated by the radiation field on the atom but do not exhibit explicit expression for the level shift. In the case of radiation fields of high intensity the only reliable determination of the level shift involves a careful location of the poles of the resolvent operator. The theory has been tested in considering a two-level atom. An extremely rapid convergence of the continued fractions has been observed, and accurate results concerning high-order resonance shifts have been obtained. The expressions for the transition amplitudes being nonperturbative with regard to the field strength, they can be considered as analytic continuations of the perturbation series. This provides a useful approach to the problem of ultraintense fields.