Stimulated-emission contribution

Analogously to experiment, we thus obtain the nonlinear response of the molecular system resolved in the directions of emission, but summed up over all contributions in each direction. Assuming nonoverlapping laser fields within the RWA, it is moreover possible to separate the PP signals arising from the stimulated Raman and the stimulated emission contribution, respectively. This important feature, which is rather helpful in the interpretation of complex PP spectra (see below), stems from the fact that after the interaction with the pump pulse we may separately consider the electronic ground-state component ( ol (0) or excited-state component ( Ffl f(t)) of the total wave function. 

Lasing occurs whenever the gain arising from stimulated emission exceeds the cavity losses. Internal losses, a, result from absorption and scattering of light. The reflectivity, R, of the mirror facet must be <1 and this contributes a loss term of (1/L)ln(l/E), where Eis the cavity length. At threshold, the gain, is equal to losses and... 

Direct evidence for the competition of two counteracting contributions to the transient absorption changes stems from the temporal evolution of the transmission change at 560 nm. From Figure 10-3 it can be seen that the positive transmission change due to the stimulated emission decays very fast, on a time scale of picoseconds. On the other hand the typical lifetime of excitations in the 5, slate is in the order of several hundred picoseconds. Therefore, one has to conclude that the stimulated emission decay is not due to the decay of the. Sj-population (as is typically the case in dye solutions). The decay is instead attributed to the transiei.i build up of spatially separated charged excitations that absorb at this wavelength. 

The term in Eq. (49) which describes radiative decay is relatively small under conditions where stimulated emission does not occur. Thus, only when all other contributions to the over-all decay are accurately determined will this term be accessible. At present only time-resolved atomic emission studies provide the required precision.11 The mean radiative lifetime has been obtained by observing the variation in the first-order decay coefficient for this emission as a function of inert gas pressure (Fig. 11). The slope of the graph so obtained yields the diffusion coefficient, while the intercept represents the sum of... 

An additional piece of information can be obtained by studying a synthetic compound derived from the GFP chromophore (1-28) fluorescing at room temperature. In Fig. 3a we show the chemical structure of the compound that we studied in dioxan solution by pump-probe spectroscopy. If we look at the differential transmission spectra displayed in Fig. 3b, we observed two important features a stimulated emission centered at 508 nm and a huge and broad induced absorption band (580-700 nm). Both contributions appear within our temporal resolution and display a linear behavior as a function of the pump intensity in the low fluences limit (<1 mJ/cm2). We note that the stimulated emission red shifts with two characteristic time-scales (500 fs and 10 ps) as expected in the case of solvation dynamics. We conclude that in the absence of ESPT this chromophore has the same qualitative dynamical behavior that we attribute to the relaxed anionic form. 

The transition probability for the upward transition (absorption) is equal to that for the downward transition (stimulated emission). The contribution of spontaneous emission is neglible at radiofrequencies. Thus, if there were equal populations of nuclei in the a and f spin states, there would be zero net absorption by a macroscopic sample. The possibility of observable NMR absorption depends on the lower state having at least a slight excess in population. At thermal equlibrium, the ratio of populations follows a Boltzmann distribution... 

The development of lasers has revolutionized several branches of chemistry and spectroscopy, along with its well-known contributions to commerce and industry, not to mention CD players. LASER is an acronym for Light Amplification by Stimulated Emission of Radiation. ... 

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