Laser multi-photon absorption

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

Finally, we note that 2 PA, and more generally simultaneous multi-photon absorption processes, has been used as a mechanism for pumping organic lasers using light available from semiconductor lasers [197-203]. 

Laser micromachining is automatic, fast, and cheaper than hot embossing [29], Because it creates no or very little thermal effects, it allows a very clean cut surface [24], The direct writing of subsurface features with multi-photon absorption is unique among available nano and micro fabrication methods. 

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 experimental studies of two- and multi-photon absorption processes, the multiphoton transition strength, a function of aU the frequencies of aU photons absorbed, is analyzed. For two laser sources of circular frequency a>i and a>2 with associated wavelengths Ai and A2, the two-photon transition strength S wi, W2) for the transition between states 0) and n) in isotropic samples is given by, see e.g., Craig and Thirunamachandran (1984) ... 

To understand why laser radiation of the visible wavelength range is also used nowadays for absorption by transparent glass, non-linear optical effects and multi photon absorption need to be considered. 

The photo-induced microcrystallisation in photosensitive glasses was also investigated [304]. The crystallisation is induced by multi-photon absorption of a fs-laser beam operating in the non-resonant wavelength range. 

Figure 5.9 From left to right structure of water soluble tris-dipicolinate complexes (Ln = Tb or Eu), three photon excited (lex = S00nm) luminescence, variation of the luminescence intensity with the incident laser power for a two (right, top) or three (right, bottom) photon excitation (Zex = 532 or 800 nm respectively) In linear or log scale, respectively (between parenthesis is indicated the experimental determination of the multi-photon absorption)...

The NIR femtosecond laser microscope realized higher order multi photon excitation for aromatic compounds interferometric autocorrelation detection of the fluorescence from the microcrystals of the aromatic molecules confirmed that their excited states were produced not via stepwise multiphoton absorption but by simultaneous absorption of several photons. The microscope enabled us to obtain three-dimensional multiphoton fluorescence images with higher spatial resolution than that limited by the diffraction theory for one-photon excitation. 

Photoionization, where electrons are released by molecules following the absorption of energy from photons, has long been viewed as a non-radioactive means to ionize explosives in the vapor phase [39]. In recent years, two teams have sought to employ laser ionization with IMS for explosive determinations. A team at Implant Sciences Corporation has utilized a laser (or flash lamp) for sampling surfaces and for ionization of sample vapors in an IMS analyzer [40, 41]. In their approach, the sample is removed from a surface with an increased temperature from laser exposure. Gases (and presumably particulate matter) from over the surface are drawn into an IMS drift tube using a wall-free inlet vida supra). In the IMS drift tube, resonance multi-photon ionization by a laser is used to produce ions from the explosives. Their... 

Resonant multi-photon ionization (REMPI). This is a variant of MPI described above, in which one or more photons promote a molecule to an electronically excited state and then additional photons generate ions from the excited state. The power of this method in the study of chemical reactions is its selectivity. In a chemically reacting system, individual reactants and products can be chosen by tuning the frequency of the laser generating the radiation to the electronic absorption band of specific molecules. 

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