Arrangement photon absorption

The class of compounds that has been most extensively investigated from the point of view of two-photon absorption is that of so-called quadrupolar chromophores. hi essence, these molecifles are linear conjugated chains with electron donating or withdrawing substituents arranged symmetrically with respect to the center of the molecifle (Fig. 8, classes I-IV). With the inversion center being preserved, the lowest order moment supported by these molecules is the quadrupole moment. 

Enhancement of two-photon cross-sections by two-dimensional and three-dimensional arrangements of monomers has been demonstrated with fluorene V-shapes and dendrimeric structures. Such multidimensional structures lead to lower two-photon absorptions than Hnear oUgomers, but they have better one-photon transparencies. An accurate calculation of large exitonic systems is obtained by diagonalizing the Hamiltonian operator on a reduced basis set. 

Figure 2.32 shows a possible experimental arrangement for the observation of Doppler-free two-photon absorption. The two oppositely traveling waves are formed by reflection of the output beam from a single-mode tunable dye laser. The Faraday rotator prevents feedback into the laser. The two-photon absorption is monitored by the fluorescence emitted from the final state Ef into other states Em - From (2.66) it follows that the probability of two-photon absorption is proportional to the square of the laser intensity. Therefore, the two beams are focused into the sample cell by the lens L and the spherical mirror M. 

The isotope shift between different stable lead isotopes shown in Fig. 2.36 has been determined by Doppler-free two-photon absorption of a cw dye laser beam at X = 450.4 nm on the transition 6p Po lp Po [250] using the experimental arrangement of Fig. 2.32. 

A possible experimental arrangement for Doppler-free three-photon absorption spectroscopy is depicted in Fig. 2.41. The three laser beams generated by beam splitting of a single dye laser beam cross each other under 120° in the absorbing sample. 

FIGURE 34 The use of two-photon absorption with counterpropagating beams for Doppler-free spectroscopy, (a) Experimental arrangement, (b) Level diagram of transitions used. 

Fig.9.48. Two-photon absorption spectroscopy signals for different laser beam arrangements...

Fig.9.50. Experimental arrangement and recorded signals for simultaneous saturation spectroscopy (2s-4p) and two-photon absorption spectroscopy (Is - 2s) in atomic hydrogen [9.201]... 

Extensive studies on the third-order NLO properties of arylated TEEs have been carried out, and one important result from these studies is that the arrangement of an anilino donor and />-nitrophenyl acceptor in linear conjugation (as in 41) provides a much higher value of the second hyperpolarizability than when placed in cross-conjugation (as in 40) [35]. Strong two-photon absorption properties of sublimable donor-substituted cyanoethylenes indicate good potential for optoelectronic applications [33]. 

F1q.l0.49. Experimental arrangement for the observation of Doppler-free two-photon absorption... 

Instead of SHG in nonlinear crystals, often two-photon induced fluorescence (see Sect.8.5) is used to monitor and to visualize the pulse profiles of picosecond pulses [11.25]. The experimental arrangement is shown in Fig. 11.21. The liquid dye solution has no absorption at the fundamental frequency 0) but shows fluorescence at high light intensity due to two-photon absorption. For two identical optical pulses moving into opposite direction through the dye solution the observed fluorescence intensity as a function of the delay time t is again [11.25] given by... 

Figure 20 Experimental arrangement for the transient two-photon absorption method.

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