Two-photon association

The significance of resonantly enhanced two-photon association stems from the visibility of using it to form ultracold molecules, a topic of considerable interest. fSr cooling schemes that work for atoms [338-340] tend to fail for molecules, ily due to the presence of many near-resonance lines and the presence of other ges, of freedom, in addition to translation (rotations, vibrations, etc.), that must... 

Figure 11.6 Potentials and vibrational wave functions used in the simulatioi two-photon association. (Taken from Fig. 2, Ref. [345].)...

A typical application is the use of the (2 + 1) REMPI scheme for measuring the (v,./) distribution of H2 produced in associative desorption from a surface. When the laser is tuned to a spectroscopic transition between individual quantum states in the X -> E electronic band, resonant two-photon absorption populates the E state and this is subsequently ionized by absorption of another photon. The ion current is proportional to the number in the specific (v,./) quantum state in the ground electronic state that is involved in the spectroscopic transition. Tuning the laser to another spectroscopic feature probes another (v, J) state. Therefore, recording the ion current as the laser is scanned over the electronic band maps out the population distribution of H2(v, J) produced in the associative desorption. Ef of the (v, J) state can also often be simultaneously measured using field - free ion TOF or laser pump - probe TOF detection techniques. The (2 +1) REMPI scheme for detecting H2 is almost independent of the rotational alignment and orientation f(M) of molecules so that only relative populations of the internal states... 

One of these is the superheavy Crowell boson [42], associated with index (3) in the ((1),(2),(3)) basis, and the other two are massive photons associated with indices (1) and (2). The superheavy Crowell boson comes from electroweak theory with an SU(2) electromagnetic sector and may have been observed in a LEP collaboration at CERN [44,56],... 

Whether the components of the gene carriers actually remain associated during import into the nucleus or enter individually cannot be answered by optical methods as their resolution is limited. A possible technique to study the complexation of DNA within cells is fluorescence correlation spectroscopy (FCS). Clamme et al. studied the intracellular fate of PEI after transfection with polyplexes by two-photon fluorescence FCS [54]. They showed that PEI binds to the inner membrane of endosomes and lysosomes and shows free diffusion in the cytosol as well as the nucleus. However, they did not detect any PEI/DNA complexes inside the nucleus. 

As with [I and y, few-state models are useful for understanding structure-property relationships for the two-photon absorption (2PA) cross-section, d. In dipolar molecules, the peak cross-sections associated with 2 PA into an excited state e from the ground state, g, will have a two-level contribution according to... 

Frequently, values of P for wavelengths where experimental data do not exist are estimated by extrapolation using a two-level model description of the resonance enhancement of P (see Appendix). Levine and co-workers [170] have also shown how to estimate the wavelength (frequency) dispersion of two-photon contributions to p. Because of the potential of significant errors associated with each measurement method, it is important to compare results from different measurement techniques. Perhaps the ultimate test of the characterization of the product of pP is the slope of electro-optic coefficient versus chromophore number density at low chromophore loading. It is, after all, optimization of the electro-optic coefficient of the macroscopic material that is our ultimate objective. 

Figure 16 shows M(B0) curves for several pulse lengths, measured at v = 14 GHz. Two symmetrical dips are clearly visible at Bv = 0.491 T. They result from resonant absorptions of photons associated with ms =1/2 to -1/2 spin transitions, as indicated in the inset of Fig. 17. Typical measurements at other frequencies are also shown in Fig. 17.

If fset of equations used in Section 11.1 for two-photon dissociation problems can ilfqsed to address another significant problem—that of resonantly enhanced two-t mpfon association, depicted schematically in Figure 10.1c. 

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