Lasers three pulsed

Pastirk I, Brown E J, Grimberg B I, Lozovoy V V and Dantus M 1999 Sequences for controlling laser excitation with femtosecond three-pulse four-wave mixing Faraday Discuss. 113 401... 

From photolysis of methylene blue by ruby-laser giant pulses, Danzinger et al. found that a 0.5 Joule, 30 nsec laser pulse causes almost total conversion of the original molecules into transients, but that the photochemical change is completely reversible. The lifetimes of three transients have been measured as 2, 30 and 140 jusec resp. at a 5.5 x 10 M dye solution. 

The first NeNePo experiments dealt with silver clusters, Ag3, Ags, Ag7, and Ag9, particularly with the first of these. The photodetachment and photoionization were done with a single titanium-sapphire laser producing pulses of approximately 60 fs duration. Doubled in frequency, these could be tuned over a wavelength span from above 420 to below 390 nm. As with the dimer, photodetachment was a one-photon process and photoionization a two-pho-ton process. (The clusters of odd numbers of atoms could be studied this way the even-numbered clusters require at least three photons in the available energy range for photoionization). The interval between pulses could be varied from zero (simultaneous pulses) to 100 ps the two pulses were made to differ in intensity by about a factor of 2, and either could be the leading pulse. 

By means of the sub-microsecond time resolution achieved by employing a pulsed laser photolysis/pulsed laser induced fluorescence technique, Hynes and wine are able to obtain k14a, k.14a and k and obtain an overall expression for kj4 as a function of temperature and [O2]- Their result in 700 torr of air is again substantially lower than the result from three competitive studies (Table IV)... 

In order to interpret the results of our experiments, optimal-control calculations were performed where a GA controlled 40 independent degrees of freedom in the laser pulses that were used in a molecular dynamics simulation of the laser-cluster interactions for Xejv clusters with sizes ranging from 108 to 5056 atoms/cluster. These calculations, which are reported in detail elsewhere [67], showed optimization of the laser-cluster interactions by a sequence of as many as three laser pulses. Detailed inspection of the simulations revealed that the first pulse in this sequence initiates the cluster ionization and starts the expansion of the cluster, while the second and third pulse optimize two mechanisms that are directly related to the behaviour of the electrons in the cluster. We consistently observe that the second pulse in the three-pulse sequence arrives a time delay where the conditions for enhanced ionization are met. In other words, the second pulse arrives at a time where the ionization of atoms is assisted by the proximity of surrounding ions. The third peak is consistently observed at a delay where the collective oscillation of the quasi-free electrons in the cluster is 7t/2 out of phase with respect to the driving laser field. For a driven and damped oscillator this phase-delay represents an optimum for the energy transfer from the driving force to the oscillator. 

Figure 13.9 Aspects of optimum laser pulses in adaptive feedback control of products pi laser excitation of CpFe(CO)2Cl, Autocorrelation G2(t) is shown for three different cases described in text. Pulse shape differences in these three pulses is evident. (Taken from Fig. 4 Ref. [43].)...

The laser excitation prepares the atoms in the lowest energy Stark state with m =2 in a chosen n-manifold (n=24 for example) (see Fig. 1-b). This excitation is itself a stepwise process involving three pulsed dye laser beams in resonance with the 2S-2P, 2P-3D and 3D-n, m =2, nx=0 transitions... 

Reductions in spin adduct yield in the three pulse experiments for the photoreduction of (a) anthraquinone and (b) benzophenone in SDS micellar solutions. The concentration of spin trap was 1.0 mM. Laser irradiation was made for 20 s at 10 mJ (A = 335 nm). 

Due to its relevance to the next section, we observed and analyzed the fluorescent emission of Tryptophan in water solution excited by one, two, and three-photon absorption. For that, three different light sources were used a UV (180-375 nm) lamp, the second harmonic of a Q-switched Nd YAG laser (with 8 ns pulse duration at 532 nm) and a Ti-Sapphire laser delivering pulses at 76 MHz, with 150 fs pulse duration and 500 mW average power at 800 nm. 

An example for an incubation process is displayed in Fig. 16. The SEM pictures show results of a treatment of barium borosilicate glass at F0=2.5 Jem-2 with a dye laser (r=300 fs, 1=620 nm) for different numbers of pulses per spot N [46]. Three pulses generated only darkening at the surface (not shown here). Five pulses caused slight ablation below the depth resolution of the optical microscope (Fig. 16a). For a higher number of pulses per spot (AT=20), the crater edges are heavily splintered off (Fig. 16b). Above 60 pulses, circular craters with a satisfying quality are formed (Fig. 16d). 

A three-pulse technique using a synchronously pumped mode-locked dye laser together with a modified Michelson interferometer has been described. By this technique, ps fluorescence decay times may be evaluated without the disadvantages of up-conversion or Kerr cell methods. The suitability of the system for the analysis of low. optical quality samples was suggested. An injection mode-locked Nd-YAG ring laser was used as an excitation source for a zero-background fluorescence study of the time evolution of the emission from large hydrocarbons with 12 ps resolution. ... 

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