Multiphotonic process, cyclic

Interestingly, variations of this cyclic multiphotonic scheme have been independently advanced in the study of the UV ablation of cryogenic films [65] as well as in the matrix-assisted laser desorption/ionization (MALDI) of biomolecules [66]. The reason for giving these results is to illustrate the emergence of common concepts/processes concerning UV ablation of molecular substrates. 

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. 

The multiphoton absorption cycle was confirmed by a comparison of the temporal profile of the fluorescence of anthracene-doped polystyrene films with computational results based on the cyclic process [97]. In the computational studies, the ground state, first excited singlet state, and lowest triplet state have been included. The calculated temperature rise during the laser pulse depends nonlinearly on the laser intensity. Rapid internal conversion within the triplet manifold is the most effective mechanism for depositing heat at the irradiated surface. 

Fig. 10.16 Isotope enrichment of Cp2 by a cyclic process following isotope-enhanced multiphoton dissociation of Freon CF2HCI [1433]...

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