Quantum‐mechanical derivation of the Bloch equations: Beyond the weak‐coupling limit

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Issue Date
1991-01-01Author
Laird, Brian Bostian
Budimir, Jane
Skinner, James L.
Publisher
American Institute of Physics
Type
Article
Article Version
Scholarly/refereed, publisher version
Metadata
Show full item recordAbstract
Two nondegenerate quantum levels coupled off‐diagonally and linearly to a bath of quantum‐mechanical harmonic oscillators are considered. In the weak‐coupling limit one finds that the equations of motion for the reduced density‐matrix elements separate naturally into two uncoupled pairs of linear equations for the diagonal and off‐diagonal elements, which are known as the Bloch equations. The equations for the populations form the simplest two‐component master equation, and the rate constant for the relaxation of nonequilibrium population distributions is 1/T 1, defined as the sum of the ‘‘up’’ and ‘‘down’’ rate constants in the master equation. Detailed balance is satisfied for this master equation in that the ratio of these rate constants is equal to the ratio of the equilibrium populations. The relaxation rate constant for the off‐diagonal density‐matrix elements is known as 1/T 2. One finds that this satisfies the well‐known relation 1/T 2=1/2T 1. In this paper the weak‐coupling limit is transcended by deriving the Bloch equations to fourth order in the coupling. The equations have the same form as in the weak‐coupling limit, but the rate constants are calculated to fourth order. For the population‐relaxation rate constants this results in an extension to fourth order of Fermi’s golden rule. We find that these higher‐order rate constants do indeed satisfy detailed balance. Comparing the dephasing and population‐relaxation rate constants, we find that in fourth order 1/T 2≠1/2T 1.
Description
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/94/6/10.1063/1.460626
ISSN
0021-9606Collections
Citation
Laird, Brian Bostian; Budimir, Jane; Skinner, James L. (1991). "Quantum‐mechanical derivation of the Bloch equations: Beyond the weak‐coupling limit." The Journal of Chemical Physics, 94(6):4391-4404. http://dx.doi.org/10.1063/1.460626
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