Threshold laser irradiance

The laser induces instantaneous vaporization of a microvolume (called a plume), and a mixture of ionized matrix and analyte molecules is released into the vacuum of the ion source. The relationship between the laser irradiance, I ascn and the number of molecules formed, Gma di, is most peculiar. There exists a threshold irradiance, peculiar to each matrix, below which ionization is not observed. Above this level, the ion production increases in a very strong, nonlinear, manner (often Gma di grows as Ilaser is raised to the eighth power). 

Desorbed NO molecules due to laser irradiation cannot be detected by the REMPI method from this surface, except for a very small amount of the initial desorption. From RAIRS observations, on the other hand, the N-O stretching peak observed at 1717 cm-1 for saturated NO on clean Pt(l 1 1) is red-shifted by 3 cm-1, when NO is saturated on the surface after laser irradiation (shown in Fig. 13c), while the peak for NO coadsorbed with O atoms reveals a blue shift of 5 cm-1. These results show that the fee hollow species is desorbed as an O atom and the N atom remains as an adsorbate on the surface [57]. At X = 248 nm a similar result to that at X = 193 nm is observed by RAIRS, but the desorption cross section is much smaller, and at X = 354 nm the intensity reduction is not observed. Thus, the threshold energy for the O atom desorption from the fee hollow species is <5.0 eY [58]. 

The sudden increase in crater depth observed during high irradiance (> 10 W/cm ) laser ablation of silicon [17], which has been ascribed to phase explosion, can be used to establish a new threshold the threshold irradiance for phase explosion. This threshold depends on two laser parameters, viz. beam spot size and wavelength. The larger the beam size and the longer the incident wavelength are, the higher is the laser irradiance required to cause phase explosion. 

The rapid increase in crater depth above the threshold irradiance for phase explosion correlates with a significant increase in signal intensity. The ratio of crater volume to signal intensity, which represents the entrainment efficiency, remains the lowest at laser irradiances slightly above the phase explosion threshold. Such a ratio, however, increases at irradiances well above the threshold (> 10" W/cm ). 

Generally, peak pressure amplitudes range from a few MPa at the ablation threshold up to several hundred MPa at high laser fluences [104-108]. During propagation through the substrate, these high-amplitude waves may induce structural modifications at areas away from the ablation spot. Thus, in contrast to the photochemical effects which are confined to the laser-irradiated area, the photomechanical effects of UV ablation can be much more delocalized. 

For irradiation with fluences above the threshold of ablation a totally different behavior is detected. For 248-nm (KrF laser) irradiation only the O/N increases, whereas the O/C and N/C ratios decrease (Fig. 19). In particular, the decrease of the O/C ratio shows that no surface oxidation takes place but... 

It was demonstrated with surface analysis techniques that the polymer surface is modified selectively with different laser irradiation wavelengths. The two laser energy regimes, above and below the threshold for laser ablation, reveal pronounced differences. For both irradiation wavelengths (248 and 308 nm) the polymer surface modification is solely chemical after treatment with fluences below the threshold. Each irradiation wavelength leads to a surface oxidation, as shown with the contact angle and XPS measurements. The oxidation is a result of the radical pathway of photodecomposition of the triazene chromophore. 

Laser irradiation with a fluence equal to the threshold fluence of ablation results in a slow decomposition of the polymer. The changes in the peak areas of several selected bands suggest a decomposition pathway as follows. 

At the same time Kaldor, et al. (25) reported the observation of infrared multiphoton photodissociation of UFg using a one color CF4 laser irradiation source (16 pm) coincident with V3. A dissociation threshold and yield were estimated to be in the range of those found for SFg. The preliminary report was recently followed by a more extensive paper from the Exxon group (26) in which infrared multiphoton excitation and dissociation in the V3 mode was further described. In addition, two color excitation (16 ym) and dissociation (10.6 ym) experiments similar to those described by Wittig were described. 

Laser annealing of Ge/Si heterostructures with Ge quantum dots (QDs) embedded in the depth of 0.15 and 0.3 pm has been studied. The samples were irradiated by 80-nanosecond ruby laser pulses. The irradiation energy density was near the melting threshold of Si surface. The nanocluster structure was analyzed by Raman spectroscopy. Changes in the composition of QDs are observed for both types of samples. The decrease in dispersion of nanocluster sizes after laser irradiation is obtained for samples with QDs embedded in 0.3 pm depth. The numerical simulation shows that the maximum temperature in the depth of QDs bedding differs by -100 K. This difference is likely to lead to different effects of laser annealing of heterostructures with QDs. 

A ruby laser pulsed irradiation of Ge/Si heterostructures with Ge nanoclusters (quantum dots) at the irradiation energy density near the melting threshold of Si surface has been studied by means of Raman spectroscopy and by numerical simulation of the laser-induced processes. Two types of the structures have been tested. They differ mainly in the depth of nanoclusters occurence (0.15 and 0.3 pm). From the RS analysis one may conclude that laser irradiation results in different changes of QD properties. The decrease of QD size dispersion is observed in the samples with QDs at 0.3 pm, this effect is not observed in the samples with QDs at 0.15 pm. The numerical simulation of laser heating shows that the QDs are in a molten state for the same time, but the induced temperatures of nanoclusters for the two depths differ by -100 K. This result indicates that qualitatively different effects of the laser irradiation may be connected with different temperatures of QDs during laser irradiation. 

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