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Physical Review A

Resonance fluorescence in radiative collisions: Theoretical study of the spectral line shapes

Roberto Buffa, M. Matera

Published Nov 1, 1991• 3 citations
Publisher

Abstract

We describe the spectral properties of the resonance fluorescence emitted in radiative collisions. The line shapes of the anelastic components of the spectrum are found to be strongly asymmetric, with extended wings related to the van der Waals shift of atomic levels. The specific case of europium and strontium atoms is considered for a combined collisional-radiative interaction following adiabatic dynamics.

DOI
10.1103/physreva.44.5833

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Annual Meeting Optical Society of America•Jan 1, 1992•0 citations
We describe the spectral properties of the resonance fluorescence emitted in radiative collisions. The line shapes of the anelastic components of the spectrum are found to be strongly asymmetric, with extended wings related to the van der Waals shift of atomic levels. The specific case of europium and strontium atoms is considered for a combined collisional-radiative interaction following adiabatic dynamics.
A theoretical study of the resonance fluorescence emitted in a radiative collision is presented. A treatment of the spontaneous emission based on the short-pulse approximation is shown to be suitable for an analysis of the spectral distribution of the scattered radiation. Following an adiabatic dressed-state picture, a seven-peak spectral structure is predicted, in agreement with the three-level nature of the problem. Some of the spectral lines are found to be due to emission processes taking place only during the collision, allowing a characterization of the process as resonance fluorescence of a transient molecule.
The effect of collisions on the resonance fluorescence spectrum of three-level atoms with a V-configuration of levels when excited by two monochromatic resonance fields has been theoretically investigated. The analysis has been carried out for the systems with a small Doppler broadening compared to the collision frequency (high buffer gas pressures) and for the general case of an arbitrary change (from complete failure to complete preservation) of the phase memory at any of the three transitions in the V-system. It has been shown that in the three-level V-type atoms excited by two monochromatic fields, the resonant fluorescence spectrum (the number of spectral lines and their widths) at one of the transitions strongly depends on the presence of resonant radiation at the adjacent transition. Under certain conditions, the line widths will be completely determined by the relaxation characteristics of the adjacent transition and, in particular, can be repeatedly narrowed with an increase in the radiation intensity at the adjacent transition. It has been shown that despite the homogeneous nature of the broadening of the absorption lines, the resonance fluorescence spectrum has a pronounced anisotropy with respect to the mutual orientation of the wave vectors of spontaneous and exciting radiation. It has been established that the line widths in the resonance fluorescence spectrum are proportional to the diffusion coefficient of the atoms interacting with radiation under certain conditions. This fact can serve as the basis for a spectroscopic method for measuring the transport frequencies of the collisions of the radiation-absorbing particles with the buffer ones. It has been shown that the effects of phase memory in the collisions lead to a strong qualitative and quantitative modification of the resonance fluorescence spectrum.
The effect of collisions on the resonance fluorescence spectrum of two-level atoms excited by monochromatic resonance radiation is studied. Analysis is performed for systems where the Doppler broadening is small compared to the collision frequency (high buffer gas pressures) and for the general case where collisions arbitrarily change the phase of the radiation-induced dipole moment (from completely interrupted to completely unaffected phase of the dipole moment). Both at a relatively low and at a high excitation radiation intensity, the resonance fluorescence spectrum is shown to depend on whether the two-level system is closed or open. This is especially true for the narrow unshifted Rayleigh scattering line. It is shown that, although the absorption line is homogeneously broadened, the resonance fluorescence spectrum exhibits a clearly pronounced anisotropy. In a direction close to the direction of propagation of the excitation radiation, the Rayleigh scattering line is maximally narrowed. Under certain conditions (that can easily be created in experiments), the width of this line is proportional to the diffusion coefficient of atoms interacting with the radiation.
The authors present an approach that allows the study of the influence of velocity-changing collisions (VCC) on the spectrum of resonance fluorescence. They give a theoretical framework yielding general, though quite complicated results. Working in the dressed-atom basis and describing the VCC by a strong-collision model, they focus their attention on the particular case of well separated spectral lines, which leads to relatively simple expressions. The results allow the authors to compare the effects of Doppler shifts and VCC on the profiles of the central line and on the spectra sidebands. They predict that VCC cause a narrowing of the spectral lines, which they attribute to Dicke narrowing. This prediction is also confirmed by numerical results. The adopted method of dealing with VCC in terms of collision operators can be applied to a larger class of collision models than just the strong-collision model used here.
The effects of different laser line shapes on the intensity and spectrum of resonance fluorescence from a two-level atom are investigated. The laser model considered is that of an ideal single-mode laser operating high above threshold, with constant amplitude and undergoing phase-frequency fluctuations analogous to Brownian motion. As the correlation time of the frequency fluctuations increases from zero to infinity, the laser line shape changes from Lorentzian to Gaussian in a continuous way. It is shown that for intermediate and strong fields, the average intensity of fluorescence in the case of a resonant broadband Lorentzian line shape is higher than that in the case of a Gaussian line shape with the same bandwidth and total power. This is in contrast to the weak-field case where the higher peak power of the Gaussian line shape makes it more effective than the Lorentzian line shape. It is also shown that in the case of a nonzero frequency correlation time (non-Lorentzian line shape) the intensity of fluorescence undergoes non-Markovian fluctuations. In relation to the spectrum of resonance fluorescence, it is shown that as the line shape is varied from Lorentzian to Gaussian the following changes take place: In the case of off-resonance excitation, the asymmetry of the spectrum decreases. In the case of resonant excitation, the center-peak to side-peak height ratio for the triplet structure increases. Moreover, the recently predicted center-line dip, which develops in the spectrum in the case of broadband excitation when the Rabi frequency and the bandwidth are nearly equal, becomes increasingly deeper.
The spectral lineshapes of the Sr (51P1–51P0) resonance transition perturbed by rare gases have been examined with a high-resolution laser-induced fluorescence method. In He and Ne, the collisional broadened spectral profiles are found to be symmetrical over the entire studied pressure range (0–600 Torr). The resulting line shapes are accurately represented by a simple Voigt profile expression. The Ar, Kr, and Xe perturbing gases, however, produce anomalous asymmetrical line broadening. To our knowledge, this represents the first measurement of asymmetrical broadening in the near wings of the emission profiles of the Sr-rare gas systems. Quantitative comparisons of the measured and theoretical line asymmetries suggest that atomic interactions in the Sr-rare gas systems are not entirely due to van der Waals (R-6)-type interactions. The line asymmetries could be caused by the correlations between thermal motions of the Sr atoms and collisional interactions.