Laser pump-pulse irradiation

Figure 5.11 shotvs the temporal profile of the intensity change in the SFG signal at the peak of the Vco mode (2055 cm ) at OmV induced by visible pump pulse irradiation. The solid line is the least-squares fit using a convolution of a Gaussian function for the laser profile (FWFJ M = 20 ps) and a single exponential function for the recovery profile. The SFG signal fell to a minimum within about 100 ps and recovered... 

Andreoni et al. 49,83) investigated a selective photodamage of dye molecules bound to polynucleotides. The experiments were based on a time-delayed two-step photoionization the experimental set up is shown in Fig. 19. The two-step excitation was performed with a nitrogen laser (337 nm) and a dye laser (pumped by the nitrogen laser) at 430 nm. The selectivity arises from the fact that the lifetime of the first excited electronic state of the dye may be sensitive to the binding site (Chapter 4.2.2). Two complexes with different lifetimes Tj and t2 (tj < x2) are irradiated with two laser pulses (Fig. 20). The second pulse is delayed with rD against the first pulse. If xt < xD < t2 the second pulse (frequency v2) will find a smaller singlet-state... 

When an ultrashort laser pulse is used as the light source for the SHG spectroscopy, the signal contains dynamical information of the system. In time-resolved SHG spectroscopy, two ultrashort pulses with tunable relative time-delay irradiate the sample. The first pulse (pump pulse) excites the system, and the SHG intensity or its spectra induced by the second pulse (probe pulse) are measured as a function of the time delay. 

This possibility is indicated in the study of the ablation of PMMA and PS doped with haloaromatic compounds (iodonaphthalene (NapI), iodo-phenanthrene (PhenI)). Experimentally these dopants offer the advantage that they exhibit negligible fluorescence, whereas their products are moderate or strong emitters. Thus, product formation can be conveniently monitored via laser-induced fluorescence (LIF). It is noted that the study has focused on the characterization of the species/products that remain in the substrate upon ablation. Thus, the experiment is of the pump-probe type, in which pump pulses at a particular wavelength (2=308, 248 or 193 nm) are used to irradiate the films, and laser pulses of very low fluence are employed to induce photoproduct fluorescence (FLaser<3 mj cm-2, probing effected always at 2=248 nm). Detailed information on the experimental setup and procedures is provided elsewhere [83-85]. 

Fig. 11 Laser-induced fluorescence spectra recorded from Napl-doped PMMA samples (1.2 wt%) after their irradiation with a single pump pulse at 248 nm at laser fluences below a, and above b the corresponding ablation thresholds. For comparison purposes, the spectra have been scaled. The figure also illustrates the approximate deconvolution of the probe spectrum into the emission bands of the suggested species (the Nap2 spectrum is recorded in the photolysis of high-concentration NapI solution, while the NapH/PMMA spectrum is recorded from PMMA doped with 0.08 wt% NapH)...

Fig. 12 Laser-induced fluorescence spectra recorded from Napl-doped PMMA samples (1.2 wt%) after their irradiation with a single pump pulse a at 248 nm and b at 308 nm at laser fluences about 1.5 times the corresponding ablation thresholds...

We now show that the counter-intuitive pulse ordering leads to the desired complete population transfer. Consider irradiating the (A-type) three-state system depicted in Figure 3.13 (left) by two laser pulses, a pump pulse (P) of center frequency and electric field envelope p t) and a Stokes pulse (S)... 

The pump beam comes from a laser. The necessity of high light intensity in a short time demands this. Exceptions are possible for relatively unreactive intermediates a flash lamp was used in the first direct detection of a carbene (Closs and Rabinow, 1976), but the availability of modern high-power, pulsed uv-lasers has made this approach obsolete. One requirement then is that the precursor to be irradiated absorb at an available laser frequency. For aromatic carbenes, this is not a restrictive requirement. 

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