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Journal of Physics B Atomic and Molecular Physics

Bloch-Siegert shifts and multi-quantum transitions in spin-one systems

J.A. Hermann, S. Swain

Published Aug 1, 1977• 17 citations
Publisher

Abstract

The Bloch-Siegert-type shifts and multiple-quantum resonance shifts of a spin-one system subjected to an oscillating radio-frequency applied at right angles to a DC field have been calculated to sixth order in V, the amplitude of the oscillating field, for the single-quantum resonance, and to fourth order in V for the higher resonances. The level crossing criterion appropriate to two-level systems cannot be used here, and a resonance condition based on a previously derived analogue of Shirley's equation for spin-one systems is used instead. The calculation uses degenerate continued-fraction perturbation theory. Numerical calculations were performed in order to obtain the shape of the resonances, and partially check the analytic results. The atomic levels were assumed to be completely stable; the corrections due to finite atomic lifetimes are expected to be small.

DOI
10.1088/0022-3700/10/11/013

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Journal of Experimental and Theoretical Physics Letters•Apr 1, 2008•25 citations
The Bloch-Siegert-type shifts and multiple-quantum resonance shifts of a spin-one system subjected to an oscillating radio-frequency applied at right angles to a DC field have been calculated to sixth order in V, the amplitude of the oscillating field, for the single-quantum resonance, and to fourth order in V for the higher resonances. The level crossing criterion appropriate to two-level systems cannot be used here, and a resonance condition based on a previously derived analogue of Shirley's equation for spin-one systems is used instead. The calculation uses degenerate continued-fraction perturbation theory. Numerical calculations were performed in order to obtain the shape of the resonances, and partially check the analytic results. The atomic levels were assumed to be completely stable; the corrections due to finite atomic lifetimes are expected to be small.
For a spin-1 atomic system subjected to an oscillating radiofrequency field applied at right angles to a DC magnetic field, the Block-Siegert shift has been calculated to sixth order in V, the amplitude of the oscillating field, and the three-quantum resonance has been calculated to eight order. The results are obtained for the three different polarisations- pi , sigma and t (unpolarised)-and expressions for the maxima are also obtained. For simplicity the atomic levels were assumed to have zero natural lifetime. It is shown that the level-crossing criterion may be used to calculate the positions of the p-quantum resonance minimum to the order of 2(p-1) in the oscillating field amplitude. Expressions are given for this quantity to order V 6 , and the separation of the maxima is calculated to order V p . The author's analytic results are compared with the numerical and experimental work of Jacobson (1980) and reasonable agreement is found.
Exact expressions for the transition probabilities of a three-level atom interacting with a rotating magnetic field are obtained, and used to obtain the double-quantum resonance shift of sixth order and the linewidths to fourth order of perturbation theory. The treatment is extended to deal with overlap effects, oscillating fields and atomic systems with more energy levels. Numerical calculations are performed to check the validity of the analytic expressions and to obtain results applicable at higher field strengths. Although the majority of the detailed work is concerned with the double-quantum resonance, the formalism is general, and the methods may be applied to single-quantum or higher multiple-quantum transitions.
Based on the vanishing-damping continued-fraction method which I developed previously for the analysis of the resonance behavior of a two-level atom in a two-frequency field with equal intensities in the two field modes [W. M. Ruyten, Phys. Rev. A 40, 1447 (1989)], I analyze here the related problem in which the two field intensities are unequal. Several analytical results are derived, namely, expressions for the lowest-order Bloch-Siegert shifts of all resonances, and an expression for the positions of the resonances in the limit of one strong and one weak field. It is shown that, in this limit, the resonances exhibit level crossings of the field-atom system.
A semi-classical approach developed by Pegg and Series (1970, 1973) is used to study higher order effects in magnetic resonance produced by an oscillating magnetic field perpendicular to a static field as the oscillating field strength is increased. It is shown that the well-known 'multiple quantum' transitions arise from a semi-classical theory, being transformed into 'single quantum' transitions induced by field harmonics in the most appropriate reference frame for treating the problem. Higher order terms in the Bloch-Siegert shift support those derived by Shirley (1965) by means of Floquet theory, and are in gross disagreement with those derived by Chang and Stehle (1971) from quantum electrodynamics, particularly at very high field intensities where the results are not even qualitatively similar. It should be possible to decide easily between the two approaches by experiment.
Abstract. We consider a spin-boson model in which a spin 1 system is coupled to an oscillator. A unitary transformation is applied which allows a separation of terms responsible for the Bloch-Siegert shift, and terms responsible for the level splittings at anticrossings associated with Bloch-Siegert resonances. When the oscillator is highly excited, the system can maintain resonance for sequential multiphoton transitions. At lower levels of excitation, resonance cannot be maintained because energy exchange with the oscillator changes the level shift. An estimate for the critical excitation level of the oscillator is developed. PACS numbers: 32.80.Bx,32.60.+i,32.80.Rm,32.80.WrMultiphoton Bloch-Siegert shifts and level-splittings in spin-one systems 2 1.
The dynamics of a two-level spin system dressed by bichromatic radiation is studied under the conditions of double resonance when the frequency of one (microwave) field is equal to the Larmor frequency of the spin system and the frequency of the other (radio-frequency) field ω rf is close to the Rabi frequency ω 1 in a microwave field. It is shown theoretically that Rabi oscillations between dressed-spin states with the frequency ɛ are accompanied by higher-frequency oscillations at frequencies n ω rf and n ω rf ± ɛ, where n = 1, 2,.... The most intense among these are the signals corresponding to n = 1. The counter-rotating (antiresonance) components of the RF field give rise to a shift of the dressed-state energy, i.e., to a frequency shift similar to the Bloch-Siegert shift. In particular, this shift is manifested as the dependence of the Rabi-oscillation frequency ɛ on the sign of the detuning ω 1 − ω rf from resonance. In the case of double resonance, the oscillation amplitude is asymmetric; i.e., the amplitude at the sum frequency ω rf + ɛ increases, while the amplitude at the difference frequency ω rf − ɛ decreases. The predicted effects are confirmed by observations of the nutation signals of the electron paramagnetic resonance (EPR) of E ′ 1 centers in quartz and should be taken into account to realize qubits with a low Rabi frequency in solids.