Multiphoton process

Some recent advances in stimulated desorption were made with the use of femtosecond lasers. For example, it was shown by using a femtosecond laser to initiate the desorption of CO from Cu while probing the surface with SHG, that the entire process is completed in less than 325 fs [90]. The mechanism for this kind of laser-induced desorption has been temied desorption induced by multiple electronic transitions (DIMET) [91]. Note that the mechanism must involve a multiphoton process, as a single photon at the laser frequency has insufScient energy to directly induce desorption. DIMET is a modification of the MGR mechanism in which each photon excites the adsorbate to a higher vibrational level, until a suflBcient amount of vibrational energy has been amassed so that the particle can escape the surface. 

The conmron flash-lamp photolysis and often also laser-flash photolysis are based on photochemical processes that are initiated by the absorption of a photon, hv. The intensity of laser pulses can reach GW cm or even TW cm, where multiphoton processes become important. Figure B2.5.13 simnnarizes the different mechanisms of multiphoton excitation [75, 76, 112], The direct multiphoton absorption of mechanism (i) requires an odd number of photons to reach an excited atomic or molecular level in the case of strict electric dipole and parity selection rules [117],... 

In contrast to the ionization of C q after vibrational excitation, typical multiphoton ionization proceeds via the excitation of higher electronic levels. In principle, multiphoton ionization can either be used to generate ions and to study their reactions, or as a sensitive detection technique for atoms, molecules, and radicals in reaction kinetics. The second application is more common. In most cases of excitation with visible or UV laser radiation, a few photons are enough to reach or exceed the ionization limit. A particularly important teclmique is resonantly enlianced multiphoton ionization (REMPI), which exploits the resonance of monocluomatic laser radiation with one or several intennediate levels (in one-photon or in multiphoton processes). The mechanisms are distinguished according to the number of photons leading to the resonant intennediate levels and to tire final level, as illustrated in figure B2.5.16. Several lasers of different frequencies may be combined. 

Laser ionization. Occurs when a sample is irradiated with a laser beam. In the irradiation of gaseous samples, ionization occurs via a single- or multiphoton process. In the case of solid samples, ionization occurs via a thermal process. 

Figure 9.27 Multiphoton processes (a) Raman scattering, (b) absorption of two identical photons, (c) absorption of two different photons and (d) absorption of three identical photons. V and V are virtual states...

Multiphoton processes are also undoubtedly involved in the photodegradation of polymers in intense laser fields, eg, using excimer lasers (13). Moreover, multiphoton excitation during pumping can become a significant loss factor in operation of dye lasers (26,27). The photochemically reactive species may or may not be capable of absorption of the individual photons which cooperate to produce multiphoton excitation, but must be capable of utilising a quantum of energy equal to that of the combined photons. Multiphoton excitation thus may be viewed as an exception to the Bunsen-Roscoe law. 

S. I. Chu, Advances in Multiphoton Processes and Spectroscopy, World Scientific, Singapore, 2986, vol. 2. 

Bhawalkar JD, He GS, Prasad PN (1996) Nonlinear multiphoton processes in organic and polymeric materials. Rep Prog Phys 59 1041-1070... 

Here we will consider photochemical processes initiated by multiphoton absorption. A theoretical treatment of multiphoton processes was developed as early as 1931 by Goeppert-Mayer i 9) but experimental verification was not possible until lasers were available to provide light of the necessary intensity. 

Multiphoton processes caused by the high peak power of ultrafast pulses significantly contribute to sample photodamage (Hopt and Neher 2001 Nan et al. 2006) these processes are reduced at longer excitation wavelengths. No sample photodamage was observed at excitation powers of 20-30 mW for the wavelengths provided by this OPO. 

L. Woste You showed that the above-threshold ionization process always ends at the bottom of the ionic state when exciting the system with femtosecond pulses. So, going to higher laser powers, you observe the consecutive onset of multiphotonic processes. What happens when you cross the double-ionization barrier Is the same true for doubly charged clusters ... 

In all cases a two-color multiphoton process is used to excite the molecules to the high-n Rydberg states. In the remainder of this text we use a primed notation to refer to the transitions from the ground state to the intermediate state (e.g., O branch, O branch, etc.), whereas an unprimed notation is used for transitions from the intermediate state J to the Rydberg states with core rotation quantum number for example, 5(2) implies the transition J = 2- N+ = 4, whereas S (2) implies J" = 2 — J = 4. 

Photodecomposition of a 4-nitrophenyl moiety is a multiphoton process absorption of one photon leads to the singlet, which relaxes immediately into a long-lived triplet state. Capturing another photon transforms the singlet through a dissociative decay process into reactive intermediates, probably radicals, which react with nucleophilic sites. In the absence of a reactive species, these reactive intermediates will relax into the initial ground state 23 ... 

Delone, N.B, and V.P. Krainov Multiphoton Processes in Atoms, 2nd Edition, Springer-Verlag, Inc., New York, NY, 2000. 

The discussion of microwave multiphoton processes given here is largely a Floquet description, i.e. a steady state description. We have implicitly assumed,... 

It is hardly surprising that, as the microwave power is raised, higher order multiphoton processes are observed. On the other hand, it may be surprising that for each m 0 the cross sections first increase then decrease with microwave power. For example, the m = cross section is clearly zero in the lowest trace. Similarly, the m = 0 cross section vanishes in the trace one above the lowest but reappears in the lowest trace. Such behavior, typical of the strong field regime, is not predicted by perturbation theory. Close inspection of Fig. 15.5 reveals that the positions of the collisional resonances shift to lower static field as the microwave power is raised. Finally, in contrast to the usual observation of broadening with increased power, the (0,0)m resonances, which are well isolated from other resonances, develop from broad asymmetric resonances to narrow symmetric ones as the microwave power is raised. 

S.-I. Chu, D.A. Telnov, Beyond the Floquet theorem Generalized Floquet formalisms and quasienergy methods for atomic and molecular multiphoton processes in intense laser fields, Phys. Rep. 390 (2004) 1. 

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