A resonance fluorescence method is demonstrated whereby the density of an atomic beam can be determined reliably. This determination is based on the distribution of time intervals between detected photons, and is made without independent knowledge of excitation cross sections, laser beam intensity, or photon counting efficiency.
The method of atomic-resonance fluorescence has been applied to the detection of very small concentrations of radioactive 20Na atoms (<3 × 109 m−3). The 20Na isotope is produced by the reaction 20Ne(p, n)20Na by passing a 20 MeV proton beam through a neon gas target of density 3.5 × 1024 atoms/m3. A beam of a continuous-wave dye laser tuned to the D2 line of sodium is transmitted through the production region and the fluorescence light is detected by means of photon counting. A digital synchronous-detection technique has been applied to measure the time-dependent behavior of the 20Na atom density shortly after a proton irradiation. This behavior is determined both by radioactive decay and by diffusion of the atoms out of the production region. The absolute 20Na atom density has been estimated, using Rayleigh scattering of the laser beam on the neon gas for calibration of the optical system. The density of neutral 20Na atoms appeared to be an order of magnitude lower than the density produced. The detection method has also been used to measure the temperature dependence of saturated sodium-vapor density down to 3 × 1011 atoms/m3 (292 K).
A theory appropriate for investigating the angular spectra of atoms scattered out of an atomic beam by resonance fluorescence due to interactions with an orthogonal uniform light beam is presented. Particular emphasis is made on the scattering in the plane containing the two beams. The atomic distribution is shown to be intimately related to the angular distribution of the spontaneously emitted photons. Both distributions show marked variations with the detuning and field intensity parameters, leading to the possibility of maximum scattering of atoms to occur on either side of the atomic beam, while the accompanying photons are always emitted on one particular side of the light beam. Since the angular distributions reflect the momentum changes suffered by both the atoms and the electromagnetic field, this lends insight into the basic mechanisms involved in resonance fluorescence.
We have studied the statistics of fluorescent photons emitted by a two-state atom in a laser beam in the limit where either the detuning or the Rabi frequency is large. For this case, the spectrum of resonance fluorescence has three separated lines. We have obtained closed form expressions for the conditional probability density for the emission of the nth photon and for the probability for emission of n photons in a time interval [0,T]. Our solutions are complementary to the known solutions for the case of perfect resonance.