Laser and Detection Systems

All S FG spectra were obtained by averaging 50 400 pulses per data point and were normalized against the intensity of the visible and infrared inputs, which were simultaneously monitored by a power meter (Oriel instruments. Model 70833 and 70811) and Si photodiode, respectively. 

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Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

After 35 years, nLFPs has become a standard tool in photochemistry and physical organic chemistry. The choice of the right tools in terms of laser and detection systems, and a proper understanding to the kinetic methodologies available make it useful whether or not the intermediates of interest have a useful absorption in the spectral region accessible. 

It seems inevitable that LIFS will start to be used by more and more researchers. Combined with a technique such as coherent Anti-Stokes Raman Scattering (Eckbreth, et al., 16). which is best suited for measuring major species concentrations, a common laser and detection system provide a wide range of measurement possibilities. 

H. Wille, M. Rodriguez, J. Kasparian, D. Mondelain, J. Yu, A. Mysyrow-icz, R. Sauerbrey, J. P. Wolf, L. Woeste, TeramobUe a mobile femtosecond-terawatt laser and detection system, European Physical J. Appbed Physics 20, 183 (2002)... 

In this chapter, several examples of photochemistry of short-lived species by multibeam irradiation are introduced. In many cases, the properties of excited intermediates have been investigated by using nanosecond lasers. Since the lifetime of excited intermediates is usually quite short, investigations employing ultrashort laser pulses are intrinsically important. Various properties estimated by the direct manner will appear in the near future. Recent progress of ultrashort pulse lasers and detection systems will make this possible. On the other hand, utilization of multiple excitations is not limited to the basic study of excited intermediates. These are applicable to biological and environmental fields. Further fruitful results are expected for these explorations. 

LIBS has been also successfully applied to the analysis of geochemical samples [1485]. The most accurate information can be obtained for the relative concentrations of different elements in a sample. This is important, for example, for the classification of minerals on earth or in meteroites when it is not clear whether two different samples come from the same source. Also for archeological samples the precise knowledge of elemental composition is very helpful for the exact dating and assignment. The applications of LIBS has benefitted from the use of fiber optics which allows remote sensing, where the laser and detection systems are far away... 

A laser-based detection system for NO2 gas (which is an industrial hazard and common environmental pollutant) was developed by Koybayashi et al.4. This was achieved by splitting light, from an Ar-ion multi-line laser, into two paths, one passing through a measurement gas, and the other being transmitted directly to the measurement unit as a reference signal. The detection unit contained two filters to separate the two chosen laser lines, and these were then detected on separate optical receivers. One of these chosen laser lines coincided with a strong absorption line in the NO2... 

In frequency-domain FLIM, the optics and detection system (MCP image intensifier and slow scan CCD camera) are similar to that of time-domain FLIM, except for the light source, which consists of a CW laser and an acousto-optical modulator instead of a pulsed laser. The principle of lifetime measurement is the same as that described in Chapter 6 (Section 6.2.3.1). The phase shift and modulation depth are measured relative to a known fluorescence standard or to scattering of the excitation light. There are two possible modes of detection heterodyne and homodyne detection. 

Northrop Grumman Systems Corporation Electronic Sensors and Systems PO Box 17319/MS A-255 Baltimore, MD, 21203 Ph 410.993.6848 321.726.7526 (Jim Stratford) www.es.northropgrumman.com Automatic Mine Detection System, AN/AQS-14 Post Mission Analysis System and Airborne Laser Mine Detection System (ALMDS). 

It is clearly seen that negative lines are much stronger then the noise. Besides that, the negative lines are always situated at the same places. The invariabihty of the spectral positions provides the evidence that they are not connected with fluctuations of the laser pulses and detection system. Thus it may be concluded that we a deaUng with a reabsorption mechanism. The optical absorption spectra of natural apatites in the range 600-900 nm contain several lines and bands connected with Nd ", Pr ", Mn ", SOj (Gorobets 1975 ... 

MALDI-TOF-MS facilitates the analysis of carotenoids and other natural products with detection limits that are lower than most other techniques. For example, subpicomole quantities can be detected (Wingerath et al., 1999). The enhanced sensitivity is the result of the efficiency of the pulsed ionization and detection system in which a complete mass spectrum is recorded with each laser flash. Like FAB and LSIMS, molecular ions are the most abundant sample ions, although some protonated molecules and [M-H]+ ions may be formed as well. Abundant molecular ions of carotenoid esters have been observed using MALDI-TOF-MS (Kaufmann et al., 1996 Wingerath et al., 1996),... 

Some researchers see a bright future for dendrimers in many different industrial, medical, research, and consumer applications. One company that produces dendrimers lists applications in drug delivery systems, gene transfection, biotechnology, sensors for diagnostics and detection systems, carbon fiber coatings, microcontact printing, adhesion, molecular batteries, catalysis, separation systems, lasers, composites, and ultrathin films used in optics. 

FIGURE 7.2 Four-color optical excitation and detection system for LIF. Excitation was provided by a laser emitting at 488 nm and 514 nm. (a) Dichroic mirrors (b) emission band-pass filters. Detection was carried out at 4 wavelengths (525, 555, 580 and 605 nm) [543]. Reprinted with permission from Wiley-VCH Verlag. 

Figure 33F-1 On-column laser fluorescence detection system for capillary array electrophoresis. A laser is focused as a line onto the array of capillaries at a 45° angle. The fluorescence is filtered and detected by a CCD camera through a wide-angle lens. (Reprinted with permission from K. Ueno and E. S. Yeung, Anal. Chem., 1994,66. 1424. Copyright 1994 American Chemical Society.)...

FIGURE 4 - Schematic diagram of the optics and detection system employed in picosecond diffuse reflectance laser flash photolysis... 

The crossed-beam arrangement (Fig. 2) used in the MPD experiments consisted of nozzle beam system, multimode pulsed TEA-COj laser, pulsed tunable probe laser, fluorescence detection system. The COj laser beam of about rectangular cross-section, 2x3 cm, was focused onto the molecular beam with a lens of 25 cm focal length. Most of the laser output ( 70%) of 1 J/pulse was contained in a 0.2-jas pulse, the rest in a 2-/is long tail. The repetition rate of 10 Hz was compatible with the repetition of the pulsed probe laser. The experiments with CjHjCN have been carried out at the 10.591 fim P(20), (00°1-10°0) and with CH3NH2 at the 9.586 jam P(24), (00°l-02°0) laser line. 

Dichroic beam splitters (or filters) use the same principle as interference filters. They are placed in a collimated beam, typically at a 45-degree angle. There are long-pass, short-pass and bandpass versions. They are extremely suitable for building high-efficiency optical systems. Figure 7.17, left, shows how a laser is coupled into a high-NA fluorescence excitation and detection system. 

Y. pestis in experimentally infected fleas and monkey blood and oropharyngeal swabs. The signal is detected and interpreted by the ABl 7700 Sequence Detector (Applied Biosystems, Foster City, Calif.), a combination of thermal cycler, laser, and detection per software system [60]. (c) PCR-based assay, with FRET registration, was used to detect ciprofloxacin-resistant (CpO mutants of plague. It was possible to distinguish the wild type from antibiotic-resistant type, and detect approximately 10 pg of DNA (or 4 CFU, colony forming units) of wild-type F. pestis KIM 5 or Cp mutants in crude lysates [61]. 

The eombination in a compact system of an infrared sensor and a laser as excitation source is called a photothermal camera. The surface heating is aehieved by the absorption of the focused beam of a laser. This localisation of the heating permits a three-dimensional heat diffusion in the sample to be examined. The infrared (IR) emission of the surface in the neighbourhood of the heating spot is measured by an infrared detector. A full surface inspection is possible through a video scanning of the excitation and detection spots on the piece to test (figure 1). 

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