Three photon process

Equation (51) has a clear physical interpretation. Recalling the lineshape for a single excitation route, where fragmentation takes place both directly and via an isolated resonance [68], p oc (e + q)2/( 1 + e2), we have that 8j3 is maximized at the energy where interference of the direct and resonance-mediated routes is most constructive, e = (q I c(S )j2. In the limit of a symmetric resonance, where q —> oo, Eq. (51) vanishes, in accord with Eq. (53) and indeed with physical intuition. The numerator of Eq. (51) ensures that 8]3 has the correct antisymmetry with respect to interchange of 1 and 3 and that it vanishes in the case that both direct and resonance-mediated amplitudes are equal for the one-and three-photon processes. At large detunings, e —> oo, and 8j3 of Eq. (51) approaches zero. 

Bond cleavage from excited radicals has also been observed in a laser jet study of the fc/s-ether (219). Thus, the excited 9-anthrylmethyl radical is suggested to undergo loss of phenoxy radical to yield the 9,10-bis-anthrylmethyl biradical (Scheme 13). This is a three-photon process, with two photons required to produce the monoradical and an additional photon needed to generate the biradical. The authors note that the biradical itself may have photochemistry, forming the 9,10-bis-methylether (220) upon further photolysis in methanol [132]. 

The first panel of Figure 5.12 shows the bichromatic control scenario. The sec panel shows the simplest path to the continuum, consisting of one-photon absorpt of CO]. The subsequent panels show the three-photon process to the contir (absorption of a> followed by stimulated emission and reabsorption of coj, ctc ... 

Figure 5.12 Interfering pathways from Et) to the continuum associated with the scenario in Figure 5.11. The frequency and phase of the lasers are co, and (a) Bichromatic control, (b) One-photon absorption, (c) Three-photon process in which initially unpopulated state Ej) is coupled to the continuum at energy E and interferes with one-photon absorption from state ] ,). (d) Same as in (c) but for a five-photon process. Notice that in processes depicted in (c) and (d) the phase

The fluence dependence of the CO and CO+ desorption yield at X = 193 nm reveals that CO and CO+ are desorbed as single-photon and three-photon processes, respectively. From (2 + 1)REMPI spectra of neutral CO, the effective rotational temperature can be estimated to be 130 K for v = 0. The population ratio in the v = 1 state to the v = 0 state is estimated from the relative intensity of the Q band head to... 

Recently, Wong and coworkers reported an emissive Tb(III) complex [Tb(L73)(N03)3] (Tb-73, Figure 13.34) based on a ligand of 7V-[2-(bis 2-[(3-methoxybenzoyl)amino]-ethyl amino)ethyl]-3-methoxybenzamide) (L73) as a three-photon luminescence probe with low cytotoxicity [109]. The three-photon process of Tb-73 under excitation at a femtosecond 800 nm laser was confirmed by a power dependence experiment. The three-photon absorption cross section of Tb-73 is around 1.9 GM. Furthermore, the interaction of three selected cells with Tb-73 over different durations of time (from 0 to 60 min and 24 h) was investigated. As shown in Figure 13.34, intracellular luminescence increased as the exposure time increased. At an exposure time of 60 min, more than 95% of the cells under 800 nm excitation exhibited green luminescence, as was observed in the cytoplasmic foci around the cell nucleus. 

Figure 1C. Periodic table depicting utility of RIMS, a. Elements for which resonance ionization feasibility has been demonstrated at NBS using thermal atomization b> elements for which ionization feasibility has been demonstrated in other laboratories using resonance ionization mass spectrometry (38-Ul> UU-32) c, elements for which isotope dilution RIMS have been achieved at NBS and d, potentially applicable for resonance ionization via two and three photon processes (Schemes 1, 2, and 5 of Figure U), using the BIMS system in Figure 6.

The laser jet system of irradiation allows the observation of multi-photon processes and is becoming more popular. Under these conditions, the keto ether (35) in carbon tetrachloride solution undergoes two processes to give 4-phenyl-benzaldehyde (one photon) and 4-phenylbenzyl chloride (two photons), while from ethanol solutions of (35), evidence is obtained to suggest that the formation of the ether (36) arises from a three-photon process (Adam and Schneider). 

Nonlinear optical properties have recently been observed for the polysilane (PhMeSi) 132, suggesting that polysilanes may eventually find use in optical technology. Irradiation of a thin film of the polysilane at 1064 nm gave rise to efficient third-harmonic generation, while irradiation at 1907 cm-1 produced no nonlinear signal. The third-harmonic resonance is believed to be due to a three-photon process, but its origin is uncertain. Farther research will undoubtedly be carried out on this new phenomenon. 

In the experiment of Deutsch and Brown, the positronium was deteoted through the annihilation y-rays. Radiofrequency resonance was indicated by an increase in the ratio of two-photon to three-photon processes. The value of ATT calculated by means of equation 12.7 was (2-032 0 003)x 106 Mc/s. More recent experiments [133], [90], give... 

For molecules or crystals having a symmetry center, the total eigenfunctions are classified into symmetric g states and antisymmetric u states. In the equation for a three-photon process in Table 1.5, if a symmetric molecule has a g state at i), m) will be the u state and n) will be the g state, so that the matrix elements have non-zero values. But this results in an u state for f )m, which is in contradiction with the fact that f) and i) should be in the same state for three-photon processes. When a molecule or crystal has no symmetry center, there is no distinction between g and u states. So, three-photon processes can proceed only in molecules and crystals without symmetry center. The intuitive correspondence between phenomena (schemes) and equations is emphasized in Table 1.5. 

Three-photon processes Four-photon process... 

This quantity plays an important role in other multi-photon processes, such as two-photon absorption, second harmonic generation and hyper-Raman scattering as three-photon processes, and coherent anti-Stokes Raman scattering (CARS), a four-photon process (Table 1.5). The two-photon absorption can be treated theoretically from Eq. (1.115) in the same way as the Raman scattering process discussed above. Thus, the transition rate for two-photon absorption is given by Eq. (1.161). 

This experiment involves a one-color, three-photon process. That is, the SH and CH3S radicals are first prepared by the single-photon dissociation of HjS, CH3SCH3, and CH3SSCH3 in the wavelength range of 235-240... 

HS from HjS at 240-235nm. The PIE spectrum for SH observed in the region 83,000-85,080 cm (2 x 41,500-2 x 42,540 cm is depicted in Figure 37 [61], Here, we note that SH ions are formed by N2P ionization instead of PFI, Since SH is produced by a three-photon process, the SH" " signals have been normalized by the cubic power of the laser energy. The N2P-PIE spectrum is roughly similar to the PIE spectrum for SH measured in a VUV-PIE experiment [195],... 

Thus, the cycloreversion of a biplanemer appeared to depend linearly on the laser intensity, whereas the formation of anthracene from its photo-dimer was proportional to the cubic of laser intensity (see Scheme 1 la). The unusual result with the biplanemer was explained as combination of a three-photon intramolecular cycloreversion and the conversion back to the reactant by a two-photon intramolecular cyclodimerization. The fact that multiphotonic processes of different order could occur within the same laser pulse was justified by the inhomogeneous spatial distribution of the laser intensity. At the center of the laser focus, the intensity is higher and a three photon process is favored, at the wings of the laser focus it is the two-photon process that predominates. ... 

If the saturation condition is met, the velocity-tuned three-photon processes bum holes in the velocity profile of molecules at one third the velocities Uq at which normal single-photon processes can create holes. Freund et aL studied these effects in CH3F. The setup was similar to an earlier experiment carried out by the same group (see above. Ref. 215), and made use of a CO2 laser with the absorption cell installed inside the cavity. Lamb dips were detected as sharp variations of the output power of the laser. 

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