Natural homogeneous linewidth

The inability to "burn" a narrow hole in the polyethylene spectrum is an indication that the "natural homogeneous linewidth"... 

The single-mode laser naturally gives less output power than a multimode laser with the same active volume since its induced emission is concentrated into a smaller frequency range. This loss in intensity, however, is much less than one would expect from the ratio of linewidths or from the reduction in oscillating mode number 3i. 32,41) jbis is due to the fact, that not only atoms with the exact transition frequency can contribute to the induced emission, but also those inside the homogeneous linewidth which is determined by collision processes in the case of gas lasers or by crystal line broadening in solid lasers... 

The homogeneous component includes all mechanisms that involve every atom in both the upper and lower laser levels of the gain medium in identically the same way. These mechanisms include the natural radiative linewidth due to spontaneous emission, broadening due to collisions with other particles such as free electrons, protons, neutrons, or other atoms or ions, or power broadening in which a high-intensity laser beam rapidly cycles an atom between the upper and lower levels at a rate faster than the normal lifetime of the state. In each of these effects, the rate at which the process occurs determines the number of frequency components or the bandwidth (linewidth) required to describe the process, with faster processes corresponding to broader linewidths. The natural radiative linewidth is associated with the radiative decay of individual atoms. It is related to the radiative decay rates of both the upper and lower energy levels involved in the laser transition. 

In the cascade configuration shown in Fig.4, coherent processes dominate. On exact resonance, again given by Eqs.(2) and (3), a resonantly enhanced two-photon absorption n- takes place. This absorption is Doppler free to first order and is, because of its coherent nature, not dependent on the first-order broadening due to the transverse velocity distribution. Thus a resolution, only limited by the homogeneous linewidth Y is easily obtained. 

The peak cross section is determined by the oscillator strength and the linewidth of the transition. The linewidth may be (1) the natural or homogeneous width, governed by... 

It is well known, however, that the width of a spectral line, at least in principle, yields information on the dephasing dynamics of the optical transition. Spectral lineshapes of purely electronic transitions in solids unfortunately are seldom determined by dynamic interactions, but, at least at low temperature, quite often by the effects of strain. The observed, named inhomogeneous linewidth is therefore of little interest. In case of vibronic transitions, however, the effect of vibrational relaxation on the lineshape may exceed the inhomogeneous linebroadening. Even so, classical spectroscopy quite often fails to elucidate the nature and strength of the perturbing forces on the optical (homogeneous) lineshape. 

In this case, the levels with m = l are populated. As long as the Zeeman splitting is smaller than the homogeneous width of the Zeeman levels (e.g., the natural linewidth Aco = 1 /r), both components are excited coherently (even with monochromatic light ). The wave function of the excited state is represented by a linear combination xj/ = axjfa + bxj/b of the two wavefunc-tions of the Zeeman sublevels m = l. The fluorescence is nonisotropic. 

In the case of inhomogeneous laser transitions, the whole line profile can be divided into homogeneously broadened subsections with the spectral width (for example, the natural linewidth or the pressure- or... 

The most important effect of the spin-spin relaxation time T2 is that it determines the natural width of the lines in the spectrum. However, for most H and C spectra the observed linewidth is determined by the homogeneity of the applied magnetic field. 

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