Multiphoton effects

Ionization of molecules of neat liquids or of solutions can also be achieved by multiphoton absorption. Generally, the sum of the photon energies should exceed the ionization energy, 1 or Isoiute(liq)/ respectively. Two cases have to be distinguished  

Only a few studies on the photoconductivity of pure liquids induced by multiphoton absorption have been reported. Biphotonic ionization of liquid tetrameth- 

Multiphoton-induced conductivity changes of neat benzene were investigated by Scott et al. (1982). They found that the ionization process was due to a three- 

Most other studies on multiphoton ionization were carried out on solutions. Early work is due to Pilloff and Albrecht (1968), who studied the biphotonic ionization of TMPD (N,N,N N tetramethyl-p-phenylene diamine) in 3-methylpentane. They found a quadratic dependence on the light intensity indicating that only virtual states were involved. Siomos et al. (Siomos and Christophorou, 1980 1982 Siomos et al., 1981) investigated the two-photon-induced ionization threshold of several aromatic molecules in n-pentane. The values obtained were lower than the corresponding thresholds measured in single photon ionization experiments. In a later paper Faidas and Christophorou (1987) attributed these lower values to three-photonic processes near the threshold which were not identified in the earlier work. For fluoranthene in tetramethylsilane they report a threshold of 5.7 eV. In another more complete study two-photon ionization onsets were reported for several aromatic molecules in various hydrocarbons (Faidas and Christophorou, 1988). Some of their results are compiled in Table 7. Within the variation of the relative dielectric constant and the limits of error, the polarization P+ energy is constant. 

Data from Faidas, H. and Christophorou, L.G., Radial. Phys. Chem., 32,433,1988. 

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Multiphase copolymers, Ziegler-Natta catalysts for, 26 535, 537-540 Multiphase laminar flow patterning, in microfluidics, 26 961 Multiphase reactions, in microbial transformations, 16 412-414 Multi-phase reactors, 21 333-335 Multiphoton effects, in photochemical technology, 19 109... 

In the calorimeter depicted in figure 13.6 there are two independent systems to measure the sample transmittance. Both have advantages and disadvantages, and both have been adopted in other photoacoustic calorimeters described in the literature. When used simultaneously, they provide a straightforward way to test whether the assumptions of equation 13.1 are being met (e.g., testing for multiphoton effects) and generally allow for more confidence in the experimental results. 

The procedure described, involving the variation of the laser energy, has some advantages relative to the alternative method of using several solutions with different transmittances. First, it provides a check for multiphoton effects simply by analyzing the quality of the linear correlations obtained. It should be stressed that the excellent correlations in figure 13.7 are typical, that is, correlation factors are usually better than 0.9995. Second, the method requires considerably less sample (only one solution is needed). Third, the analysis of experimental data is also conceptually simpler, because no normalization is required. 

Further complications may arise when more concentrated solutions are used, even before reaching the limiting conditions that invalidate equations 13.4 or 13.9. The most noticeable problem is the appearance of multiphoton effects, for which the relation between photoacoustic signal and energy absorbed is no longer linear. As mentioned, this phenomenon is easily detected by a deviation... 

Nanosecond Absorption Spectroscopy Absorption apparatus, 226, 131 apparatus, 226, 152 detectors, 226, 126 detector systems, 226, 125 excitation source, 226, 121 global analysis, 226, 146, 155 heme proteins, 226, 142 kinetic applications, 226, 134 monochromators/spectrographs, 226, 125 multiphoton effects, 226, 141 nanosecond time-resolved recombination, 226, 141 overview, 226, 119, 147 probe source, 226, 124 quantum yields, 226, 139 rhodopsin, 226, 158 sample holders, 226, 133 singular value decomposition, 226, 146, 155 spectral dynamics, 226, 136 time delay generators, 226, 130. 

Multiphoton effects in molecules are, as one might expect, similar to those in atoms but even richer. As for atoms, more than the minimum number of photons required to produce ionisation can be absorbed, so that ATI is observed [499, 501]. Many other effects are possible, for example, Coulomb explosions [502] which arise when two charged fragments fly apart. 

Baumert T, Engel V, Meier Ch and Gerber G 1992 High laser field effects in multiphoton ionization of Na2 -experiment and quantum calculations Chem. Rhys. Lett. 200 488... 

Wight C A and Armentrout P B 1993 Laser photoionization probes of ligand-binding effects in multiphoton dissociation of gas-phase transition-metal complexes ACS Symposium Series 530 61-74... 

AFID = alkali-flame ionization detection FID = flame ionization detection FPD = flame photometric detection GC = gas chromatography IGEFET = interdigitated gate electrode field-effect transistor ITMS = ion trap mass spectrometry MIMS = multiphoton ionization mass spectrometry MS = mass spectrometry... 

Direct visualization of femtosecond filamentation is crucial to understanding the phenomenon. As the energy of a single infrared photon is much too small to effect an electronic transition, one has to take recourse to multiphoton absorption induced fluorescence to come up with a scheme to directly visualize filamentation in condensed media. One such scheme that has been successfully implemented involves the use of a crystal of barium fluoride, a material that is known to be very good scintillator [38]. 

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