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Physical Review A

Simplified solution for the energy eigenvalues of a two-state atom in a sinusoidal field

Wilhelmus M. Ruyten

Published Jul 1, 1997• 1 citation
Publisher

Abstract

In a famous 1965 paper [Phys. Rev. 138, B979], J. H. Shirley calculates the time-averaged transition probability for an atom in a periodic field in terms of the eigenvectors and eigenvalues of the time-independent Floquet Hamiltonian. It is shown in the present paper that, for the specific case considered by Shirley, a two-state atom in a sinusoidal field, Shirley's solution can be simplified considerably. Also, Shirley's results are generalized to include, for an arbitrary atom-field system, the time-varying components of the transition probability as well.

DOI
10.1103/physreva.56.737

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In a famous 1965 paper [Phys. Rev. 138, B979], J. H. Shirley calculates the time-averaged transition probability for an atom in a periodic field in terms of the eigenvectors and eigenvalues of the time-independent Floquet Hamiltonian. It is shown in the present paper that, for the specific case considered by Shirley, a two-state atom in a sinusoidal field, Shirley's solution can be simplified considerably. Also, Shirley's results are generalized to include, for an arbitrary atom-field system, the time-varying components of the transition probability as well.
A previous exact formal and practical solution for the time-dependent Schrödinger equation for a two-level non-degenerate system in an arbitrarily strong sinusoidal field is transformed for an N-level system to a Floquet form. Using the Floquet form of the solution computationally convenient expressions are obtained for the phase and the phase-long time-averaged induced transition probabilities. The method exploits the time periodicity of the Schrödinger equation and the solution over the initial period of the hamiltonian provides all the information needed to obtain both the unaveraged and the averaged transition probabilities for the system. The method is illustrated by specific examples which are used to discuss some of the temporal features of the multi-photon spectra of two-level systems.
An exact method of solution to the time-dependent wave equation for a system interacting with a sinusoidal field, which formally treats time and the phase of the field on an equal footing, is used to discuss (1) the importance of averaging the properties of a system over the phase of the applied field and (2) the nature and behavior of the characteristic exponents derived from the Floquet solution (with particular emphasis on the frequency-sweep experiment). The results for the Zeeman tuning experiment are used to resolve recent discrepancies in the literature which relate to the validity of Shirley's important result for the two-level average induced transition probability involving the derivative of the characteristic exponents with respect to the Zeeman splitting parameter ${\ensuremath{\omega}}_{0}$. The explicit calculations included in this work are for the single-photon two-level problem. Important implications can be inferred from them with respect to both the importance of phase averaging and the usefulness of the characteristic exponents as a quantitative means of obtaining resonance frequency shifts and half-widths for multiphoton and multilevel problems.
The problem of the interaction of a quantum system having discrete states, with a classical oscillating field, is reexamined as a problem in the solution of the time-dependent Schrodinger equation with a periodic Hamiltonian. A method is presented for approximating the time-dependent Hamiltonian with a time-independent one in the weak oscillating field case. With the aid of Floquet's theorem the problem is exactly converted to one with a time-independent Hamiltonian represented by an infinite matrix. The approximation of only two states then permits finding the resonance line shape by perturbation theory for both single and multiple quantum transitions with equal ease. The simple case of only two states connected by an off diagonal sinusoidal perturbation is studied in detail, and a complete description of the average transition probability is found for the strong oscillating field case. A few more complex cases are discussed. A deeper understanding of the analysis is obtained by examining the theory with the oscillating field quantized. Experimental verification of the theory could best be obtained by the methods of atomic beam spectroscopy at radio frequencies.
Rabi oscillations and Floquet states are likely the most familiar concepts associated with a periodically time-varying Hamiltonian. Here, we present an exactly solvable model of a two-level system coupled to both a continuum and a classical field that varies sinusoidally with time, which sheds light on the relationship between the two problems. For a field of the rotating-wave-approximation form, results show that the dynamics of the two-level system can be mapped exactly onto that for a static field, if one shifts the energy separation between the two levels by an amount equal to ℏω, where ω is the frequency of the field and ℏ is Planck's constant. This correspondence allows one to view Rabi oscillations and Floquet states from the simpler perspective of their time-independent-problem equivalents. The comparison between the rigorous results and those from perturbation theory helps clarify some of the difficulties underlying textbook proofs of Fermi's golden rule, and the discussions on quantum decay and linear response theory.
It is shown that, in modulated magnetic resonance on spin-1/2 systems, the time-averaged absorption signal of the optical pump beam can be obtained directly from the dressed-atom energy eigenvalues of the Floquet Hamiltonian for arbitrary orientations of the optical pump beam, namely, by a straightforward generalization of Shirley's well-known result [Phys. Rev. 138, B979 (1965)]. Analytical and numerical results derived from this generalized diagonalization technique are compared with the vanishing damping solution of the Bloch equations from the preceding paper [W. M. Ruyten, Phys. Rev. A 42, 4226 (1990)] and are shown to be in complete agreement with the latter.
The phase and the rotational averaging of single and multi-photon transition probabilities for the interaction of an atom or molecule with an applied sinusoidal field is discussed. It is shown that while the correct interval for carrying out the phase average is 2π in general, a phase interval of π is satisfactory under certain specified conditions. Illustrative two-level examples indicate that if these conditions are not satisfied, the two choices of phase interval can lead to significant differences in the phase averaged temporal transition probabilities which are subsequently essentially removed upon long-time averaging. A numerical scheme for evaluating rotational averaged transition probabilities is developed and illustrated for model calculations involving two-level molecular systems. The examples are chosen to illustrate the effects of rotational averaging, both when a molecule possesses non-zero diagonal dipole matrix elements (‘permanent dipole moments’) and when it does not, on single and multi-photon spectra. Often resonance profiles are sharpened, Bloch Siegert shifts and resonance maxima reduced, and the structure due to diagonal dipole matrix elemnts eliminated, upon rotational averaging. A recently developed rotating wave or Rabi type approximation is used to explain the effects of diagonal dipole matrix elements for both the fixed orientation and rotationally averaged spectra.