Nonhyphen;Markoffian theory of quantum beats in molecular fluorescence

摘要

We develop a theory of quantum beats in molecular fluorescence taking into account collision effects (such as pure dephasing due to elastic collision) without invoking the Markoff approximation. Model calculations of the time dependent fluorescence from molecules with two excited levels are performed in order to see how the nonhyphen;Markoffian character in the collision process exerts influence upon the quantum beat pattern. It is suggested that if the pure dephasing between the excited states is a Markoffian lsquo;lsquo;the degree of modulation of the beatsrsquo;rsquo; decreases exponentially with time; if it is in the static limit the degree of modulation of the beats decreases as a Gaussian function (the nonhyphen;Markoffian behavior in the quantum beats). A given case can be classified as either Markoffian or nonhyphen;Markoffian, depending on the relative magnitudes of the collision duration time tgr;c, the beat frequency ohgr;, and the coupling strengthvwhich is characteristic of the pure dephasing process. The Markoff approximation holds only if the condition ohgr;v1/tgr;cis satisfied; the nonhyphen;Markoffian behavior appears in the quantum beat pattern when ohgr;v, even ifv2tgr;2c1 .