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Physical Review A

Test of Neoclassical Radiation Theory: Incoherent Resonance Fluorescence from a Coherently Excited State

Hyatt M. Gibbs

Published Jul 1, 1973• 14 citations
Publisher

Abstract

The resonance-fluorescence experiment described in the preceeding paper and found to be consistent with quantum electrodynamics is analyzed using the neoclassical theory (NCT) of radiation, i.e., the electromagnetic field is not quantized. We have not found a value of the dynamic shift for which the NCT computer simulation is consistent with the data. NCT predicts an increased radiative lifetime as appreciable excitation is achieved; the observed decay curves were simple exponentials with characteristic times independent of the fractional excitation. The observed oscillatory dependence of the integrated fluorescence upon input pulse area has not been reproduced by a NCT simulation either. Objections raised against this disproof of NCT are answered.

DOI
10.1103/physreva.8.456

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•Jan 1, 1976•0 citations
The resonance-fluorescence experiment described in the preceeding paper and found to be consistent with quantum electrodynamics is analyzed using the neoclassical theory (NCT) of radiation, i.e., the electromagnetic field is not quantized. We have not found a value of the dynamic shift for which the NCT computer simulation is consistent with the data. NCT predicts an increased radiative lifetime as appreciable excitation is achieved; the observed decay curves were simple exponentials with characteristic times independent of the fractional excitation. The observed oscillatory dependence of the integrated fluorescence upon input pulse area has not been reproduced by a NCT simulation either. Objections raised against this disproof of NCT are answered.
The neoclassical or semiclassical theory (NCT) of the radiative decay of an isolated atom predicts that the decay rate of an excited state is proportional to the atoms's population in the state to which the atom is decaying. For weak excitation of a two-level system, this prediction is consistent with observations and with the Weisskopf-Wigner theory of decay, which assumes a quantized electromagnetic field as in quantum electrodynamics (QED). But for weak excitation of a system with three or more levels, there can be gross disagreements between NCT and QED. NCT predicts that the lifetime of an excited state depends on the nuclear spin which determines the number of ground states between which the atom's unity population density is divided. Also NCT predicts no fluorescence to a third level not initially populated, even though the transition is highly allowed. Observations in Pb vapor are inconsistent with NCT and consistent with QED. The oscillator strengths of the 2833-, 3640-, and 4058-A transitions were determined to be 0.167(20), 0.061(04), and 0.146(10), respectively, by measuring the ratios of oscillator strengths and using the lifetime measurements of Happer and Saloman.
We propose and analyze the use of quantum-beat phenomena to test neoclassical radiation theory (NCT) and quantum electrodynamics (QED). For a beam-foil type of experiment with atoms having one upper level and two closely spaced lower levels, all coherently excited, NCT predicts the presence of quantum beats in the emitted radiation; beats are not expected in QED. QED predicts beats when many atoms are present, in agreement with recent photon-echo experiments. An experiment to test NCT and QED is suggested.
The neoclassical theory of radiation predicts that the decay rate of an excited atomic state depends on the population density of the lower state. Experimental evidence is presented here which shows that in the case of 39K^{39}\mathrm{K}39K the decay rate is in agreement with the predictions of quantum electrodynamics and definitely in disagreement with the neoclassical theory.
In the past several years theoretical methods have been developed which allow the treatment of the excitation process of excited state molecules as well as the decay process. The studies have shown that the excitation process in many cases can play a crucial role in determining the type of decay curve which is observed. This is of relevance to the experimentalist using laser excitation where the distinction between coherent and incoherent light sources is essential to many experiments. The theory described here will first describe some simple aspects of the theory of the radiation field with respect to coherence and Incoherence, then discuss its application to various types of excitation-decay experiments. In particular non-exponential fluorescence decay curves will be discussed with the emphasis on why they should appear and what they mean.