Laser shots

By using a laser with less power and the beam spread over a larger area, it is possible to sample a surface. In this approach, after each laser shot, the laser is directed onto a new area of surface, a technique known as surface profiling (Figure 2.4c). At the low power used, only the top few nanometers of surface are removed, and the method is suited to investigate surface contamination. The normal surface yields characteristic ions but, where there are impurities on the surface, additional ions appear. 

Yes, repeated laser shots sample progressively deeper layers depth resolution 50-100 nm... 

The potential of LA-based techniques for depth profiling of coated and multilayer samples have been exemplified in recent publications. The depth profiling of the zinc-coated steels by LIBS has been demonstrated [4.242]. An XeCl excimer laser with 28 ns pulse duration and variable pulse energy was used for ablation. The emission of the laser plume was monitored by use of a Czerny-Turner grating spectrometer with a CCD two-dimensional detector. The dependence of the intensities of the Zn and Fe lines on the number of laser shots applied to the same spot was measured and the depth profile of Zn coating was constructed by using the estimated ablation rate per laser shot. To obtain the true Zn-Fe profile the measured intensities of both analytes were normalized to the sum of the line intensities. The LIBS profile thus obtained correlated very well with the GD-OES profile of the same sample. Both profiles are shown in Fig. 4.40. The ablation rate of approximately 8 nm shot ... 

Fig. 4.45. SEM photograph of TiN-TiAIN-Fe sample after 100 laser shots (laser fluence 0.35 ) cm ) [4.245].

For symmetric PS-fo-P4VP (20 000 19 000) diblock copolymer films with the wormlike phase separation structures, the TCPP-doped films were irradiated using one laser shot with a fluence of 150 mJ cm in air. The ablation phenomenon is observed for this irradiation fluence (Figure 12.5c and f), but it is difficult to conclude that this is a selective ablation of the doped-P4VP parts. We cannot deny the possibility that the decomposition of the P4VP parts affects the PS parts because of the existence of large interfaces between the two symmetric blocks in wormlike structures. Thus, for the site-selective ablation of diblock copolymer films, the surface morphology of the phase separation structures is one of the most important parameters. 

Laser desorption methods (such as LD-ITMS) are indicated as cost-saving real-time techniques for the near future. In a single laser shot, the LDI technique coupled with Fourier-transform mass spectrometry (FTMS) can provide detailed chemical information on the polymeric molecular structure, and is a tool for direct determination of additives and contaminants in polymers. This offers new analytical capabilities to solve problems in research, development, engineering, production, technical support, competitor product analysis, and defect analysis. Laser desorption techniques are limited to surface analysis and do not allow quantitation, but exhibit superior analyte selectivity. 

Figure 8.3 Positive ion LD TOF mass spectrum of blood from a P. vivax infected human patient (only asexual parasites have been observed by microscopy estimated parasitemia approximately 72 parasites/pl). Protocol C is used for sample preparation estimated number of parasites deposited per well is approximately 90. A commercial TOF system is used laser wavelength 337 nm. All one hundred single laser shot spectra, obtained from hnear scanning of an individual well, are averaged (no data smoothing). The characteristic fingerprint ions of detected heme are denoted.

The LIBS spectral signal is detected using an Echelle spectrometer + iCCD camera, which provides the whole time-resolved NUV-NIR spectrum in a single laser shot. 

Fig. 4.6. The integrated signals of harmonics from an H2 gas (lO.OTorr), a D2 gas (lO.OTorr), and a gas of their mixture (lO.OTorr) measured as a function of the harmonic order q. The signals are normalized to those from the H2 gas and vertical errors represent SEM (standard error of the mean) for 800 laser shots...

Fig. 4.8. (a) The measured (squares) and predicted (thick lines) intensity ratio r(q) between harmonics from H2 and that from D2 molecules as functions of harmonic order q. tqm and rcM are the predictions with and without quantum effect for nuclear dynamics see text, (b) The measured (squares) and predicted (thick line) relative phases between harmonics from H2 and those from D2 molecules as functions of harmonic order q. Measuring the relative phases directly corresponds to observing the nuclear motions, i.e., the nuclear displacement of H2 Aii1 2 (right axis) as a function of excursion time r (superior axis). For (a) and (b), vertical and horizontal errors represent SEM for 800 laser shots and those from quantum mechanical uncertainty [27]... 

Although phenomenological and simplistic, these simulations show that the efficiency of laser triggering and guiding may be improved by orders of magnitude, provided the laser shots are triggered by electric activity in thunderclouds and kept oriented towards the most active regions of the clouds. 

Fig. 7.4. Spectrum of X-ray emission from a 12 pm thick Ti foil irradiated at an intensity of 5 x 1017 Wcm-2. The spectrum was obtained from accumulation of 30 laser shots (see text for details of the irradiation conditions)...

Fig. 7.7. Spectrum of the X-ray radiation detected from the front side and from the rear side of a three-layer Cr-Ni-Fe target irradiated at an intensity of 5 x 1019 W cm 2. The spectrum was obtained adding up all identified photon from the analysis of the low flux images obtained from 350 laser shots. The target was irradiated on the Cr side. Note that the relative intensities of the two spectra are not to scale...

Three dimensional particle-in-cell simulation performed with the numerical code CALDER [77] reveals that the unprecedented efficiency of this accelerator was due to the achievement of a physical regime in which multiple electron bunches are accelerated in the gas-jet plasma during the action of each laser shot. This effect is shown in Fig. 8.7 by a snap-shot from the simulation sequence. 

Schwoerer et al. used tantalum for both accelerator and radiator but in two separate targets and the Jena 15 TW laser system at an intensity of 102°Wcm-2. A 5() pm first foil employed for the accelerator and a second slab of 1mm that acted as radiator [89]. As a result, 104 fission events per Joule of laser energy of 232Th and 238U, placed behind the second tantalum layer, were obtained with a reaction rate of the order of 1 event per laser shot. 

Concerning the Ka yield, the experimental works published in the last years showed a conversion efficiency of laser energy to Ka photons of 10 4-10-5, leading to a total number of 109-1010 Ka photons per joule of laser energy for each driving laser shot. As an example, Fig. 9.5 from [66] shows... 

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