High-laser fluence excitation

Fig. 8. Time-of-flight spectnim detected at m/e = 40 and 10 after high-laser fluence excitation. The scattered points are the experimental data and the solid line is a fit to them. The component of the fit indicated by the shorter dashed line is the heavier product of Reaction 1 (primary NOj loss) which fragments upon ionization to yield an ion at m/e = 40. The longer dashed line represents the m/e = 40 ion of the heavier product of Reaction 2 (secondary NOj loss). The dot-dashed line represents the C3H4 formed in Reaction 3,...

Quantum dynamical calculations on the IRMPE/D of 614, 615 O3. Quantum interference effects and discussion of the possibility of mode-selective excitation and reaction Confirmation that OCS does not undergo IRMPD at 616 high laser fluences (ca. 250 J cm ). Laser-induced dielectric breakdown in OCS, OCS-He, and OCS-Ar does lead to dissociation, giving CO + S Ab initio study of SO2 IRMPE using the most proba- 617 ble path approximation to select the most important paths within the semiclassical Floquet matrix. Conclude that collisionless MPD of SO2 will not occur at laser field strengths <20 GW cm ... 

Moreover, using high laser fluences one can saturate the i —> f transition in a manner that depends on the excited-state lifetime. Measuring the energy saturation threshold will relate directly to the decay time of this accessed reactive excited state a long decay will correspond to a low saturation threshold, and the converse. 

At high laser fluence the probability of directly exciting overtones is enhanced, complicating the interpretation of the IR-PD spectrum [11]. 

For high laser fluences, optically thick layers of homogeneous excited species are produced and one has to use the Kubelka-Munk function to perform the decay analysis Eq. (16) becomes now... 

In sharp contrast with the steady-state behavior described so far, pulsed laser excitation of the same samples produces quite different results for concentrated samples and for high laser fluences (above about 10 mJ per pulse and per square centimeter) a second emission appears in the case of TCC, as Figures 29 and 30 show. This emission is sharp and appears for lower energies relative to the monomer. 

In addition to direct one photon excitation in the Soret band, the Sj state of metalloporphyrin can also be excited by either coherent or sequential two-photon absorption in the visible. The early work of Stelmakh and Tsvirko showed that the S2 state can be populated sequentially either directly via prompt two-photon absorption at high laser fluence + hv S, + hv Sj Sj Sq + /tv... 

The fluorescence spectra measured just upon ablation are given in Figure 2A as a function of laser fluence. The contribution below 370 nm was suppressed, as a Hoya L37 filter was used in order to cut off the laser pulse. Fluorescence spectra of this polymer film consist of sandwich (max. 420 nm, lifetime 35 ns) and partial overlap (max. 370 nm, lifetime 16 ns) excimers (20). The latter excimer is produced from the initially excited monomer state, while the sandwich excimer from the partial overlap excimer and the monomer excited states. Since these processes compete with efficient interactions between identical and different excimers (Si - Si annihilation) (12), the sandwich excimer is quenched to a greater extent compared to the partial overlap one under a high excitation. Actually the fluence-dependent spectral change around the threshold can be interpreted in terms of Si - Si annihilation. 

Figure 1. Diagram of the intensity / (W/cm2) vs. duration of laser pulse tp(s) with various regimes of interaction of the laser pulse with a condensed medium being indicated very qualitatively. At high-intensity and high-energy fluence 4> = rpI optical damage of the medium occurs. Coherent interaction takes place for subpicosecond pulses with tp < Ti, tivr. For low-eneigy fluence (4> < 0.001 J/cm2) the efficiency of laser excitation of molecules is very low (linear interaction range). As a result the experimental window for coherent control occupies the restricted area of this approximate diagram with flexible border lines.

The OL efficiency of the fullerene-hybrid system can be enhanced, in principle, applying the bottleneck principle (Fig. 23-5) (Miles, 1994) to design a multilayer structure. The device optimizes the attenuation capabihty of a RSA optical limiter (Miles, 1994). To reach an efficient population of the excited state the laser fluence must be kept constant over the beam. The high flexibility of the FULP-hybrid nanocomposites allows to fabricate multilayer systems where the thickness of each layer can be easily adjusted, a multilayer system is formed, in fact, by FULP doped and undoped layers of controlled thickness (limocenzi, 1999). 

When the bleaching of the IR absorption band of many vibrational-rotational transitions was observed simultaneously, it was inferred that a great number of initial rotational states became depleted under the action of an intense IR pulse (Alimpiev et al. 1977). This experiment demonstrated that the fraction of molecules involved in the MPE process, g( ), at = 0.01-1 J/cm is much greater than the fraction / of molecules interacting linearly with the IR field, that is, q ) f = q 0). Thus, under typical conditions, the MPE process produces two molecular ensembles. The fraction q of molecules involved in MPE forms an ensemble of vibrationally hot, that is, highly excited, molecules. The rest of the molecules, 1 — q, remain in the lower vibrational levels and form an ensemble of vibrationally cold molecules (Fig. 11.8(a)). Figure 11.8(b) shows the dependence on the laser fluence of the fraction q of molecules excited into the vibrational QC for three different molecules. The conclusion as to the depletion of many rotational levels was later confirmed by probing the populations of individual rotational levels of the SFe molecule with a tunable diode laser (Apatin et al. 1983). 

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