Laser fluence

The strength of a photon—molecule interaction is deterrnined by the frequency-dependent cross section 0 (v), expressed in cm for absorption and related to a(y) in equation 1 or by the differential cross section (k5(y) jin units of cm /sr for scattering (14). The latter specifies the likelihood that active species scatter some portion of the incident laser fluence (photons /cm ) into a viewing soHd angle, AQ, measured in steradians (Fig. 1). The cross sections can be expressed as in equation 5 ... 

So far powerful lasers with picosecond to nanosecond pulse duration have usually been used for the ablation of material from a solid sample. The very first results from application of the lasers with femtosecond pulse duration were published only quite recently. The ablation thresholds vary within a pretty wide interval of laser fluences of 0.1-10 J cm , depending on the type of a sample, the wavelength of the laser, and the pulse duration. Different advanced laser systems have been tested for LA ... 

The depth profiling of the Cu-Ag-Si samples was performed with the laser beam focused on to the sample surface to the spot of approximately 30-40 pm. To obtain the best depth resolution the laser fluence was maintained near the 1 J cm level, close to the threshold of the LIBS detection scheme. The intensity profiles of Cu and Ag emission lines are shown in Fig. 4.43. The individual layers of Cu and Ag were defi-... 

Fig. 4.43. Depth profile of a triple Cu-Ag sandwich (the laser fluence is about 1 ) cm" 10 pulses accumulation. The dashed lines indicate different layers [4.245].

Fig. 4.44. Depth profile ofTiN-TiAIN-Fe sample normalization to the sum of all the major components is used (laser fluence 0.35 j cm ) [4.245],...

The "soft" ablation of the TiN-TiAlN samples by the low fluence laser beam was performed by use of LA-TOF-MS. Because of the greater sensitivity of this technique compared with direct LIBS the lower laser fluence of approximately 0.3-0.4 J cm was used. One of the depth profiles, obtained by use of femtosecond LA-TOF-MS, is shown in Fig. 4.44. Each 280-nm-thick layer was ablated by approximately 20-25 pulses, which result in an average ablation rate of 11-14 nm pulse . The ablation rate was low enough for resolution of all layers. 

The size distribution of the clusters produced in the cluster source is quite smooth, containing no information about the clusters except their composition. To obtain information about, for example, the relative stability of clusters, it is often useful to heat the clusters. Hot clusters will evaporate atoms and molecules, preferably until a more stable cluster composition is reached that resists further evaporation. This causes an increase in abundance of the particularly stable species (i.e., enhancing the corresponding peak in the mass spectrum, then commonly termed fragmentation spectrum ). Using sufficiently high laser fluences (=50 /iJ/mm ), the clusters can be heated and ionized simultaneously with one laser pulse. 

Photodecomposition. A greyish-white film of solid material slowly formed along the bottom of the sample cell in PuF6 cells irradiated at 1064 nm. A similar film formed considerably faster in sample cells irradiated at 532 nm with the film forming on the entrance face of the cells as well as along the walls of the cell. Film formation was evident after less than a minute of irradiation at a laser fluence of 5 J/cm2 at 532 nm. The exact chemical composition of the film has not yet been determined. It is known that it contains plutonium and... 

J/cm2. As previously reported (1), excitation at 532 nm resulted in a shorter lifetime (86 4 microseconds) even at the lowest fluence used (0.1 J/cm2), where, with extensive signal averaging, a final signal-to-noise ratio of about 20 was obtained. As noted above the observed fluorescence decays at 532 nm became increasing non-exponential with increasing laser fluence. 

The "add-to-memory" signal averaging method currently available to us distorts fluorescence intensity versus time plots when the fluorescence intensity is a non-linear function of incident laser energy and the laser energy varies from shot to shot. For this reason we have not attempted detailed kinetic modelling of the observed fluorescence intensity decay curves recorded at high 532 nm laser fluence. 

Figure 4. Energy diagram for 532 nm excitation of PuF g). The 5f electron states of PuF are shown at the left. The solid arrows indicate photon absorption or emission processes. The wavy arrows indicate nonradiative processes by which excited states of PuFg may be lost. The laser-fluence dependent fluorescence decay found at this excitation wavelength can be explained in terms of a bimolecular reaction between PuFg(g) in its 4550 cm l state and PuF (g) to form PuFj(g). It is assumed that PuF (g) is formed via dissociation of the initially populated PuF state.

Fig. 10. (a) G11,j (b) C2H3 signal intensities as a function of photolysis laser fluence. 

Figure 13 shows the images of masses m/e = 42, corresponding to C3D3, at various delay times. The image of m/e = 42 was disk-like and its width did not change with the delay time. Therefore, it was totally from the dissociation of hot benzene after the ionization. Photolysis laser fluence dependence study showed that it was from one-photon absorption. 

A photolysis laser fluence dependence measurement was carried out in the region of 0.7 to 10mJ/cm2 for 193nm and 2.2 to 33mJ/cm2 for 248nm. The shapes and the ratio of the components in the translational energy... 

For comparison purposes, two other MS systems have been used with in vitro grown parasite cultures. In the first case, a home-built miniaturized linear TOF multi-array analyzer1718 was interfaced to the frequency-tripled output at 355nm of a Q-switched Nd-YAG laser ( Polaris, New Wave Research, Fremont, CA). In this experiment the laser beam was not scanned, and the estimated laser fluence after attenuation was similar to that of the commer-... 

OL behavior is assessed simply by monitoring the transmission of a (usually solution) sample as a function of the incoming laser fluence measured in joules per square centimeter (rather than intensity in watts per square centimeter).22,23 Limiting thresholds Fth, defined as the incident fluence at which the actual transmittance falls to 50% of the corresponding linear transmittance, are then commonly quoted. Since excited-state absorption processes generally determine the OL properties of molecules, the excited-state structure and dynamics are often studied in detail. The laser pulse width is an important consideration in the study of OL effects. Compounds (1-5)58-62 are representative non-metal-containing compounds with especially large NLO and/or OL... 

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