Multiphoton absorption process

Examples of studies on multiphoton absorption processes and nonlinear second-and third-harmonic generation processes will be discussed along with some possible radiative and nonradiative processes. The selection rules for multiphoton absorption will be mentioned in Section 7.3, and molecular examples will be shown along with their correlating photophysical properties in Section 7.4. The effect of some parameters relating to second-order activity along the lanthanide... 

The structure-property relationships are affected by three main structural elements-coordination and hence packing, which determines some of the nonlinear harmonics the degree of conjugation along the backbone and the effects of the substituents-all of which have previously been reported to affect two-photon absorption processes thus it is proposed that the same applies for other higher-order systems. However, it is rare for reports of these high-order multiphoton absorption processes displayed by organic metal complexes to be accompanied with detailed molecular structure to show the correlation between the photoproperties. 

Figure 7. Schematic energy level diagram showing the principle of the ionization method for detecting electron transfer in gas-phase adducts. Naphthalene cation (the hole donor) is formed by resonance-enhanced two-photon ionization of the neutral. A hole acceptor, whose ionization potential is lower than that of naphthalene, is not ionized, since its S level is not resonant with the UV photons used (vi). The vibrational levels of the ionic form of the acceptor are resonant with the naphthalene cation, and accept the hole easily. Detection is by photodissociation, using photons of different frequency (V2) that dissociate the naphthalene cation in a resonance-enhanced multiphoton absorption process. Charge transfer is detected by the diminution of the product ion signal in the presence of a suitable acceptor. Adapted from Ref. [32].

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. 

A different model describes one specific aspect of laser ablation, i.e., the thermalization of the laser energy in doped polymers. This model is based on spectroscopic data (time-resolved absorption/emission measurements [93, 94] and TOF-MS data [95]), but is mainly valid for irradiation with wavelengths >248 nm, and for polymers which contain polyaromatic compounds as dopants. The mechanism involves a cyclic multiphotonic absorption process with up to ten photons [96]. From the highly excited polyaromatic dopant molecules, the photon energy is transferred to the polymer matrix via rapid internal conversion. The associated temperature increase results in the thermal decomposition of the polymer. From the time-depen-dent absorption studies it was suggested that, in view of their longer lifetimes, excited triplet states should play a key role in this process. 

Among the possibilities available to postionize sputter-ejected neutral surface particles, electron impact ionization has been employed in a variety of experimental approaches. More recently, photoionization by resonant or nonresonant multiphoton absorption processes has been established as another very effective technique in SNMS. Other processes as Penning ionization or charge exchange play only a minor role in postionization for SNMS. 

The laser-induced breakdown is not necessarily related to the multiphoton absorption process [145] while photopolymerization depends on it strictly [24, 25]. Therefore, measures could be taken to increase the quantum yield of polymerization, and therefore the dynamic power range. A simple and effective method is choosing laser wavelength so that radicals are produced more efficiently. It was experimentally observed [127] that the two-... 

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