The Laser Source

The laser source is especially useful for sampling small areas, where the amount of sample vaporized is very small and the bulk of the sample is not destroyed. It can be used on living tissue without destruction of the tissue. Thus the laser source is a valuable addition to the list of spectroscopic sources with special applications in the microsample area. 

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Figure 5.13 shows a typical experimental arrangement for obtaining the Raman spectmm of a gaseous sample. Radiation from the laser source is focused by the lens Lj into a cell containing the sample gas. The mirror Mj reflects this radiation back into the cell to increase... 

An FT-Raman spectrometer is often simply an FTIR spectrometer adapted to accommodate the laser source, filters to remove the laser radiation and a variety of infrared detectors. 

If the microstmcture becomes ever finer by improved deposition technology, the domain irregularities should diminish. The SNR is limited by the shot noise of the laser source and is equal to i . In this region a high 9 is of great value. 

The chapter is organized as follows in Section 8.2 a brief overview of ultrafast optical dynamics in polymers is given in Section 8.3 we present m-LPPP and give a summary of optical properties in Section 8.4 the laser source and the measuring techniques are described in Section 8.5 we discuss the fundamental photoexcitations of m-LPPP Section 8.6 is dedicated to radiative recombination under several excitation conditions and describes in some detail amplified spontaneous emission (ASE) Section 8.7 discusses the charge generation process and the photoexcitation dynamics in the presence of an external electric field conclusions are reported in the last section. 

The layout of the experimental set-up is shown in Figure 8-3. The laser source was a Ti sapphire laser system with chirped pulse amplification, which provided 140 fs pulses at 780 nm and 700 pJ energy at a repetition rate of 1 kHz. The excitation pulses at 390 nm were generated by the second harmonic of the fundamental beam in a 1-nun-thick LiB305 crystal. The pump beam was focused to a spot size of 80 pm and the excitation energy density was between 0.3 and 12 ntJ/crn2 per pulse. Pump-... 

Figure 24. The acquisition camera image of the Keck LGS with the segmented mirror "unstacked." The brightening on the left is the Rayleigh scatter of light from the laser. The 36 spots show an elongation increasing with distance from the laser source, which is left of the camera.

Fig. 3 a UV-Vis DRS spectra of dehydrated TS-1 catalyst reporting the typical 208 nm (48000cm i) LMCT hand, see Fig. 2h also reported are the four excitation laser lines used in this Raman study near-lR (dotted), visible (full), near-UV (dashed) and far-UV (dot-dashed), b Raman spectra of dehydrated TS-1 obtained with four different lasers emitting at 7 = 1064,422,325, and 244 nm (dotted, full, dashed, and dot-dashed lines, respectively). Raman spectra have been vertically shifted for clarity. Although the intensity of each spectrum depends upon different factors, the evolution of the 7(1125)//(960) ratio by changing the laser source is remarkable. The inset reports the Raman spectrum collected with the 244 nm laser in its full scale, in order to appreciate the intensity of the 1125 cm enhanced mode. Adapted from [48] with permission. Copyright (2003) by The Owner Societies 2003... 

The laser source is a Nd YAG pulsed laser, operating at 1064 nm, which delivers about 10 mJ on the sample surface, in 8 ns. The spatial lateral resolution of the LIBS measurements corresponds to the dimensions of the micro-crater left by the laser on the sample surface. The same dimensions are also a measurement of the damage induced on the pigment. A computer-enhanced enlargement of a typical laser crater is shown in Figure 2 its diameter does not exceed 10 pm, which is practically invisible at naked eye. The reduced size of the crater also allows for a high spatial resolution of the LIBS analysis. 

The beams are backreflected by the cube corner prisms which are fixed, respectively, on the sample and on the sample holder. Since the cube corner prisms are able to make reflected beam exactly parallel to incident beam, this interferometer is tilt independent. The reflected beams get back to the beam splitter through the same path, but shifted by about 2 mm in the vertical direction. The beam splitter lets a part of the two beams go towards the photodiode sensor and lets the other part of beams reach the laser source (off axis, therefore giving no feedback effect). 

The second method uses pulsed lasers and the laser-induced fluorescence is detected by telescope. If the telescope and the laser source have a definite base distance, the crossing of laser beam and the acceptance angle of the telescope define the height of the atmospheric layer at which fluorescence is detected. There is also the technique of delayed coincidence, where the time interval between laser pulse and detected fluorescence pulse determines the distance of the observed molecules from the observer (Lidar)... 

Eigure 5.8 shows a schematic of the proof-of-principle CARS endoscope system. The laser source consisted of a passively mode-locked 10 W Nd Y VO4 laser (High-Q Laser GmbH) operating at 1064 mn, which delivered transform-limited 7 ps pnlses... 

For the second task second harmonic generation by quartz has been proposed. The first procedure is to determine the relative intensity of SH compared with etalon, where the ratio of SH intensities is used for sorting. In the second procedure the laser source is working with a very high repetition rate and the number of pulses with SH intensity above a certain level is used as the separation criterion. Sorting using non-linear optics may be very effective, because... 

Sample handling is simplified as glass can be used for windows, lenses, and any other optical components. In addition, the laser source is easily focused on small sample area. Very small samples can be investigated without time-consuming preparation. It is also possible for the source radiation to be transmitted through optical fibers. The fiber-optic probe can be in contact with the sample or immersed in it. The probe consists of input fibers surrounded by several collection fibers that transport the scattered radiation to the monochromator. This makes it possible to collect spectra directly under relatively adverse conditions. 

The laser source in our spectrometer is an amplified femtosecond dye laser with a much larger repetition rate than many of the existing amplified laser systems used for femtosecond spectroscopy. The amplification is necessary to improve the signal intensity which actually depends on roughly the third power of the laser intensity. The large repetition rate helps average over pulse-to-pulse fluctuations of the laser. 

Setting the laser source at 10% power prevents the dyes from burning and the samples from melting. 

These measurements were made on an Auto EL-II Ellipsometer (Rudolph Research, Flanders, NJ). The laser source was a 1 mW continuous wave helium/ neon laser, with a wavelength of 6328 A. The angle of incidence was 70° and the spot size 2-3 mm. A refractive index of 1.5 was utilized for all the silane layers. The data were analysed on a Hewlett-Packard 85 computer using film 85 software package, version 30, program 13, and the film thickness was calculated using the McCrackin program. 

The refractometer with its present dimensions can be easily installed into any existing laser light-scattering spectrometer together with the laser source, the thermostat and the computer, as exemplified in Fig. 10. The optical glass plate... 

At suitably high laser irradiances at the solid surface, a plasma is formed as mentioned above. With properly chosen irradiation parameters, this plasma shows features, comparable to those of HF-spark sources, commonly used in mass spectrometry of inorganic solid samples. From results, reported in the literature (1, 2, 3), it can be deduced that the laser source may offer some advantages over classical spark sources with respect to sample preparation (nonconductors), reproducibility, collection efficiency and uniformity of elemental sensitivity factors. The laser in addition... 

The optical source is a diode laser and fiber-emitter and fiber-detector coupling is accomplished using standard optical fiber connectors. The detector is a PIN photodiode connected to a transimpedance preamplifier and the signal is amplified and filtered using a lock-in amplifier that also tunes the modulation frequency of the laser source. The analytical signal is collected and treated by a PC. 

Ion sources are also an area where it is planed to make improvements, once the new separator is working and manpower is liberated. New ideas for on-line sources include the laser source (which will probably not make new elements available to Isolde but will certainly improve the selectivity), and ECR sources (where the problem will be to adapt existing ECR technology to the high-temperature, high radiation environment of the Isolde separator). 

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