Nonresonant photon absorption

Third-order nonlinear optical properties of CdTe QDs were examined by Z-scan and FWM experiments in the nonresonant wavelength region. We found that the two-photon absorption cross section, a, is as high as 10 GM, although this value decreases with decreasing size. In addition, the nonlinear response is comparable to the pulse width of a fs laser and the figures of merit (FOM = Re Xqd/ Xqd)... 

Even more elegantly, the local resolution is improved by irradiation with very intense focused femtosecond laser pulses outside the absorption range of the fluoro-phore (e.g., in the near-infrared). The very intense focus of the laser beam—and only this—will excite the fluorophore by nonresonant two-photon absorption. Artifacts by scattered primary radiation are ruled out and the local resolution is comparable to a confocal microscope. In addition, the damage of the sample by laser light absorption is reduced to a minimum. 

A secondary mechanism for the absorption Df nonresonant frequencies involves the concerted absorption of two photons by a single molecule, with virtual photon conveyance of the excess energy to the second absorber (see Fig. 3). As with conventional two-photon absorption, there is no need for... 

The inclusion of only two Bom-Oppenheimer states in our derivation is warranted if the one-photon absorption process is resonant with the electronic transition frequency. (Two-photon and multiphoton absorption processes are assumed to be nonresonant.) However, at the same time the pulse must be broad enough in frequency to include the spread of Franck-Condon-allowed vibrational levels. 

These properties were utilized in the study of energy transfer from NO to a variety of accepting molecules. Acceptors whose one-photon absorption spectrum strongly overlaps that of NO were investigated for the first time. Efficiency of resonant and nonresonant processes could thus be empirically compared. A unified mechanism, involving a charge transfer intermediate, was found to account reasonably well for all the observed rate constants. 

The nonlinear optical properties in solution of selected functionalized PDAs, described herein, have also been evaluated by means of the z-scan technique. Off resonance studies (at 705 nm) show that nonlinear refraction is only comparable to that of the solvent for these dilute solutions, but that nonlinear absorption, characterized by p values, varies significantly, with the nature of the side-chains. O It can be inferred that if bulk films of these PDAs possess suitable nonresonant nonlinear refractive properties for optical devices, modification of side-chain structure can reduce the magnitude of undesirable two photon absorption. 

D.H. Kobe, Question of gauge Nonresonant two-photon absorption, Phys. Rev. Lett. 40... 

Figure 6 shows the dependence of the energy transmission through the samples as a function of the incident intensity. If nonresonant two-photon absorption were the only mechanism giving rise to the transmission drop, we would expect the nanosecond and the... 

Fig. 2.30 Level schemes of different two-photon transitions (a) resonant two-photon absorption with a real intermediate level k) (b) nonresonant two-photon absorption with a virtual level u) (c) Raman transition (d) resonant anti-Stokes Raman scattering...

In addition to the nonresonant NLO properties, one- and two-photon spectra of several of the compounds in Fig. 5.3 were also computed in Ref. [56]. The averaged two-photon absorption cross section for isotropic media in the case of two parallel linearly polarized photons <8° > is... 

Two-photon absorption can be measured by several techniques. Two of them are two-photon excited fluorescence (TPEF) and nonlinear transmission (NLT). Pulsed lasers are most often used because TPA is a third-order nonlinear optical process and therefore is most efficient at very high intensities. In the nonresonant TPA, two photons combine to bridge an energy gap larger than the energies of each photon individually, and the transition occurs without the presence of the intermediate state. This can be viewed as being due to a virtual state created by the interaction of the photons with the molecule. 

Line 3 describes the resonant and nonresonant (self) absorption within the source for the recoilless y photons emitted at different depths (x) of the source while they proceed toward the front of the source (x = 0) on their way to the absorber. 

When the energy for ionization is provided by photons (i.e., from a laser), there is the possibility to use resonant or nonresonant (multi-)photon absorption [280]. The former uses real intermediate electronic states while the latter does not since virtual states are very short-lived, the ionization probability for a process that relies on several of those is rather low and thus requires high photon fiuxes up to 10 " W/cm to be efficient [275]. The resonant process yields a much higher probability, but usually only for one element, thus it often serves as a selective ionization process to resolve isobaric interferences [281]. Single photon ionization usually does cover most but not all of the elements, for example C, N, and O are not included [282] nearly all... 

In laser-induced gas-phase reactions the first step is absorption of a vibrational quantum which, at moderate pressures, would only heat the gas since V-T energy transfer occurs. If this process were dominant the laser would be no more than a fancy (and expensive) Bunsen burner. At low pressures conditions are more favorable for multiple-photon absorption. The photon flux is large enough so that more than one quantum can be absorbed before significant V-T transfer occurs. The only trick is to maintain resonance between the laser frequency and the various transitions to be excited. If only pure vibrational transitions were involved anharmonicity would ensure that a laser tuned to the fundamental frequency ( 0 1) would be nonresonant for other transitions. However, since there are also changes in rotational quantum number resonance may be reestablished. The resonance conditions are illustrated in Fig. 6.9. 

As mentioned in Sect. 5.1, in the neutral silver trimer there is no excited electronic state which could be reached with a wavelength of 400 nm, and therefore the ionization is due to a nonresonant two-photon absorption pro-... 

A moment analysis of the spectrum can yield interesting general information. For the sake of simplicity, we consider the spectral shape of a one-photon transition, but in the limits reported in Section 8.3.1.1, the conclusion will be valid also for nonresonant two-photon absorption and circular dichroism processes. From now on, we neglect the frequency-dependent prefactor... 

The sum of all nonresonant terms constitutes a weakly dispersive background. Some terms are nonresonant for a Raman transition but can give rise to two- or three-photon absorption for a different set of input frequencies. 

So far we have considered only the recoil-free fraction of photons emitted by the source. The other fraction (1 —/s), emitted with energy loss due to recoil, cannot be resonantly absorbed and contributes only as a nonresonant background to the transmitted radiation, which is attenuated by mass absorption in the absorber... 

Principles and Characteristics Atomic fluorescence spectrometry (AFS) is based on excitation of atoms by radiation of a suitable wavelength (absorption), and detection and measurement of the resultant de-excitation (fluorescence). The only process of analytical importance is resonance fluorescence, in which the excitation and fluorescence lines have the same wavelength. Nonresonance transitions are not particularly analytically useful, and involve absorption and fluorescence photons of different energies (wavelength). 

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