In this paper we analyse the effect of coherent saturation in the laser-induced fluorescence (LIF) of molecules. We develop an earlier model introduced by Billy et al. in 1987 of this coherent effect and propose a simple formula predicting the line shape and the line intensity for a laser-molecule interaction time considerably longer than the excited-state radiative lifetime. The predictions of this formula are tested on a LIF experiment in which a Na2 molecular beam is excited with a continuous-wave single-frequency dye laser. Considerable spectral broadening, up to two hundred times the natural linewidth is observed, in good agreement with the model.
In this paper we analyse the effect of coherent saturation in the laser-induced fluorescence (LIF) of molecules. We develop an earlier model introduced by Billy et al. in 1987 of this coherent effect and propose a simple formula predicting the line shape and the line intensity for a laser-molecule interaction time considerably longer than the excited-state radiative lifetime. The predictions of this formula are tested on a LIF experiment in which a Na2 molecular beam is excited with a continuous-wave single-frequency dye laser. Considerable spectral broadening, up to two hundred times the natural linewidth is observed, in good agreement with the model.
The behaviour of an optically thick column of barium vapour resonantly 10 -2 illuminated at 553.5m by a 50 ns, 10 Wm pulse from a flashlamp pumped dye 19 21 -3 laser system was investigated for barium number densities in the range 10 -10 m in the presence of He buffer gas at 100 Torr.A variety of optical saturation effects associated with the principal resonance transition were studied.In marked contrast to earlier work, saturation of the intensity of the scattered light near 553.5 nm with increasing laser power was found for a suitable geometry of the collection optics but at a laser intensity typically two orders of magnitude higher than that predicted by simple theory, despite the fact that this theory was found to describe correctly transmission measurements along the length of the vapour column.The apparent contradiction between the fluorescence and transmission results, which indicated that the scattered radiation intensity continued to rise with incident laser power, despite the population of the upper resonance level having reached a limiting saturation value, could be understood in terms of a simple model which took into consideration the spectral distribution of the scattered light and its transfer through the unexcited vapour.However, in common with other similar experiments, saturation of the scattered radiation intensity with laser power could not be totally achieved when a more usual collection optics geometry was employed.This was interpreted as a consequence of laser intensity variation across the face of the laser beam.A number of subsidiary experiments were performed.These included spatially resolved intensity measurements of the scattered radiation and time resolved metastable level population measurements.In addition, evidence for laser induced ionization was sought using two different methods and this enabled an upper limit of 10% ionization to be placed on the experiments in the present work.
We calculate the effect of light-induced forces on the fluorescence line shape of a two-level atom crossing at right angles two counterpropagating light beams of parallel linear polarizations $(\mathrm{lin}\ensuremath{\Vert}\mathrm{lin})$ in a common configuration for ultrahigh-precision optical spectroscopy. For an incident atomic beam with a narrow spread of transverse velocities the dipole force induces a redshift of the fluorescence maximum, while in the reverse case of a wide spread of transverse velocities the radiation-pressure force induces a blueshift of the saturation dip minimum. We then use our theory to explain the blueshift of the saturation line-center dip occurring for the closed transition $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{2}$ of a 4He beam. The observed shift, which is in quite good agreement with the theory, can be of the order of 1/10 of the transition natural linewidth and hence quite important for ultrahigh-precision spectroscopy measurements.
For pt.I see ibid., vol.9, p.357 (1976). A theory of the optical pumping of high-J molecule levels with a broad-band CW laser has previously been developed. The linewidth of the Hanle effect is analysed in the present paper for two particular cases in which an exact integration is possible. These cases are: (i) gT (where g is the Lande factor and T the lifetime) is the same for the two levels; and (ii) one of the Lande factors vanishes. In general, the Hanle resonance is power-broadened for high laser intensities. However, in the case of a zero Lande factor of the ground state, the theory predicts a radiative narrowing of the Hanle effect. It is shown that this most unusual feature is connected to the fact that, in the latter case, the physical observables of the resonance level may get the ground-state lifetime, when the transition is saturated.
We show that it is possible to model accurately optical phenomena in intense laser fields by taking into account the intensity distribution over the laser beam. We present an extension of an earlier theoretical model that divides an intense laser beam into concentric regions, each with a Rabi frequency that corresponds to the intensity in that region, and solve a set of coupled optical Bloch equations for the density matrix in each region. Experimentally obtained magneto-optical resonance curves for the ${F}_{g}=2\ensuremath{\longrightarrow}{F}_{e}=1$ transition of the D1 line of 87Rb agree very well with the theoretical model up to a laser intensity of around 200 mW/cm2 for a transition whose saturation intensity is around 4.5 mW/cm2. We examine the spatial dependence of the fluorescence intensity in an intense laser beam experimentally and theoretically. We present and discuss the results of an experiment in which a broad, intense pump laser excites the ${F}_{g}=4\ensuremath{\longrightarrow}{F}_{e}=4$ transition of the D2 line of cesium while a narrow probe beam scans the atoms within the pump beam and excites the D1 line of cesium, whose fluorescence is recorded as a function of probe beam position. Experimentally obtained spatial profiles of the fluorescence intensity agree qualitatively with the predictions of the model.
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.
Various characteristics of molecules have been clarified by spectroscopic methods in which the interactions of molecules with the radiation field were investigated. A stationary response of a molecule excited by a radiation field appears as a spectral line. Information on molecules may be obtained from position (resonance frequency), intensity, and shape and width of the spectral line. So far the interaction of molecules with the radiation field has been studied primarily in a “frequency domain.” Although molecular spectroscopy is one of the research fields in modern science which has the longest history, a variety of important aspects have been demonstrated very recently in this field as radiation sources have been improved. When the radiation field is “weak,” the photon nature of the radiation field is dominant in the interaction with a molecule, such as in the cases of emission, absorption, and scattering of photons. When the radiation field is “strong,” in the sense that many photons exist in a proper mode of the radiation field, the wave nature of the field plays an important role in the interaction. In this case the radiation may be described in the form of the classical field, say, a sinusoidal function of the time and spatial coordinates, coherently interacting with the molecule, which is described quantum mechanically. Here nonlinear properties would appear in the interactions: saturation in absorption, induced scattering, multiphoton processes, coherent transients, etc.