Superposition states fluorescence intensity

Similar to the fluorescence intensity distribution, the visibility can provide us an information about the internal state of the system. When the system is prepared in the antisymmetric state or in a superposition of the antisymmetric and the ground states, p55 = pee = 0, and then the visibility has the optimum negative value V = — 1. On the other hand, when the system is prepared in the symmetric state or in a linear superposition of the symmetric and ground states, the visibility has the maximum positive value "V - 1. 

Due to the overlap of the monomer fluorescence emission and absorption from the aggregates, a quenching process may occur by a resonance interaction. However, in other cases [85, 86] where we determine the extinction coefficients for ground-state monomers and dimers, by simply taking into account the fraction of the excitation radiation absorbed by the monomers (which are emissive species) and aggregates (from which we could not detect any fluorescence emission), a good superposition between the calculated and experimental fluorescence intensity curve was obtained. 

FIGURE 19 The intensity of fluorescence, as a function of time, from SO2 created in a superposition state composed of two levels. [From Ivanco, M., Hager, J., Sharfin, W., and Wallace, S. C. (1983). J. Chem. Phys. 78, 6531.]... 

Figure 2.11 Vibrational and rotational motion of the I2 molecule. Left potential energy curves of the ground-state X Eg, the excited intermediate Ballou state reached by the pump laser and the final exdted (ion-pair) state fOg of I2 populated by the probe laser. The vibrational (and rotational) motion of the superposition of levels / of I2 in the Ballou state is monitored by the probe laser-induced fluorescence. Right laser-induced fluorescence intensity as a function of the delay time between probe pulse and pump pulse, showing the oscillation of the wave packet for the vibrational (top panel) and rotational (bottom panel) motion. For further details see text. Data adapted from Gruebele et al, Chem. Phys. Lett, 1990, 166 459, with permission of Elsevier...

A short pump pulse excites coherently different upper levels. The time evolution of the superposition of states following the coherent excitation causes time-dependent changes of the complex susceptibility x of the sample. Similar to the quantum beats in the fluorescence intensity the susceptibility x(t) is found to contain oscillating nonisotropic contributions which can be readily detected by placing the sample between crossed polarizers and transmitting a probe pulse with variable delay (see also Sect.10.3 on polarization spectroscopy). Even a cw broadband dye laser can be used for probing if the probe intensity transmitted by the polarizer is monitored with sufficient time resolution. 

One example of an application is the measurement of the gravitational acceleration g on earth with an accuracy of 3 x 10 g with a light-pulse atomic interferometer [1291]. Laser-cooled wave packets of sodium atoms in an atomic fountain (Sect. 9.1.9) are irradiated by a sequence of three light pulses with properly chosen intensities. The first pulse is chosen as 7r/2-pulse, which creates a superposition of two atomic states 1) and 2) and results in a splitting of the atomic fountain beam at position 1 in Fig. 9.71 into two beams because of photon recoil. The second pulse is a tt-pulse, which deflects the two partial beams into opposite directions the third pulse finally is again a tt/2-pulse, which recombines the two partial beams and causes the wave packets to interfere. This interference can, for example, be detected by the fluorescence of atoms in the upper state 2). 

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