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

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    LairdB_JCP_1991(94)4391.pdf (1021.Kb)
    Issue Date
    1991-01-01
    Author
    Laird, Brian Bostian
    Budimir, Jane
    Skinner, James L.
    Publisher
    American Institute of Physics
    Type
    Article
    Article Version
    Scholarly/refereed, publisher version
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    Abstract
    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
    URI
    http://hdl.handle.net/1808/16159
    DOI
    https://doi.org/10.1063/1.460626
    ISSN
    0021-9606
    Collections
    • Chemistry Scholarly Works [610]
    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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    Contact KU ScholarWorks
    785-864-8983
    KU Libraries
    1425 Jayhawk Blvd
    Lawrence, KS 66045
    785-864-8983

    KU Libraries
    1425 Jayhawk Blvd
    Lawrence, KS 66045
    Image Credits
     

     

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