Lasers detection

In this approach one uses narrow-band continuous wave (cw) lasers for continuous spectroscopic detection of reactant and product species with high time and frequency resolution. Figure B2.5.11 shows an experimental scheme using detection lasers with a 1 MFIz bandwidth. Thus, one can measure the energy spectrum of reaction products with very high energy resolution. In practice, today one can achieve an uncertainty-limited resolution given by... 

According to the distance from probe to the sample, three operation modes can be classified for the AFM. The first and foremost mode of operation is referred to as contact mode or repulsive mode. The instrument lightly touches the sample with the tip at the end of the cantilever and the detected laser deflection measures the weak repulsion forces between the tip and the surface. Because the tip is in hard contact with the surface, the stiffness of the lever needs to be less than the effective spring constant holding atoms together, which is on the order of 1 — 10 nN/nm. Most contact mode levers have a spring constant of <1 N/m. The defection of the lever can be measured to within 0.02 nm, so for a typical lever force constant at 1 N/m, a force as low as 0.02 nN could be detected [50]. 

A number of other laser spectroscopic techniques are of interest but space does not permit their discussion. A few specialized methods of detecting laser absorption worthy of mention include multiphoton ionization/mass spectrometry (28), which is extremely sensitive as well as mass selective for gas-phase systems optically detected magnetic resonance (29) laser intracavity absorption, which can be extremely sensitive and is applicable to gases or solutions (30) thermal blooming, which is also applicable to very weak absorbances in gases or liquids (31) and... 

Luminescence of Eu and Eu has been found in the X-ray excited luminescence spectra of synthetic leucophane activated by Eu (Prokofiev et al. 1982 Eig. 5.17), while in natural samples only Eu " " was detected. Laser-induced time resolved luminescence under 532 nm excitation enables us to detect clear lines of Eu " " with the strongest Dq- Fq electron transition at 573 nm (Eig. 4.25). 

Figure 19.1. Schematic diagram of a general pump-probe-detect laser spectrometer suitable for picosecond electronic absorption, infrared (IR) absorption, Raman, optical calorimetry, and dichroism measurements. For picosecond fluorescence—a pump-detect method, no probe pulse needs to be generated.

Figure 6.10 Laser-induced fluorescence detection of a hemolysate of human erythrocytes. Conditions capillary, 110 cm X 20 jum I.D. fused silica capillary voltage, -29 kV detection, laser-induced fluorescence (LIF), argon ion laser operating at 275.4 nm. Peaks A, carbonic anhydrase B, methemoglobin C, hemoglobin A 1, 2, and 3, unknown. (Reprinted from Ref. 49 with permission.)...

Fig. 10.16. Gradient electrochromatogram of derivatized neutral steroids. Macroporous monolithic column 350 (250) mm x 100 pm i.d. gradient mobile phase, acetonitrile-water-240 mM ammonium formate buffer, pH 3 (30 60 5-65 30 5 v/v/v) field strength, 600 V/cm injection, 100 V/cm for 10s detection, laser-induced fluorescence (excitation at 325 nm, emission at >495 nm). Reproduced with permission from Que et al. [76],...

Third, in selecting a particle sizing technique, one must consider the size of the material itself. Most techniques have effective upper and lower limits of detection. Laser diffraction, for example, can measure particles from approximately 0.01 pm to several millimeters. With optical microscopy and image analysis, it becomes very difficult to resolve features that are smaller than a 0.3 pm because of the wavelength of light used in conventional optical microscopes.1... 

The extracavity absorption coefficient of 12 at the laser wavelength is measured by detecting laser power with a thermopile before and after an I2 cell. 

Fig. 7. Setup for the degenerate four wave mixing experiments. The input beam is split in three beams. The beam splitter BS3 deflects a part of one of the pump beams to a power meter, which detects laser power fluctuations. The delay line with the retro reflector R adjusts the temporal overlap of the two pump beams coming from the front side on the sample. The long delay line with retro reflector R2 is moved to probe the temporal behavior of the nonlinearity in the sample. The phase conjugated signal beam propagates from the sample back to BSj and is then deflected through a stack of attenuation filters on a second power meter. An iris in front of the power meter increases the signal to noise ratio by removing scattered light...

Taking into account all available data of frequency differences obtained during the course of the matrix measurements, and correcting for different iodine pressures, different iodine cells and different HFS-separations, we derive a combined frequency reproducibility of the two laser systems in the experiment of better than 1.5 0.7 kHz. This is a notable result, given the fundamental differences between the two iodine spectrometers as far as saturated absorption signal detection, laser frequency stabilization and laser set-ups are concerned. 

In order to obtain two-dimensional information, i.e. the angular distribution of desorbed neutral molecules, the probe laser beam is widened to a sheet of 20-25 mm with the aid of a telescope. The ions are recorded perpendicular to the surface normal and the detecting laser beam via a system consisting of a repeller (—3 kY), a drift tube, a microchannel plate and a phosphor screen, as depicted in Fig. 6 [33]. 

To develop a four-color detectable laser-induced fluorescence (LIF) detection system for capillary elec-... 

LEI has been applied successfully to the trace determination of T1 [674] for certification purposes, and for combinations with laser evaporation and all other atomization techniques represents a powerful approach to detection. Laser photoionization and galvanic detection have been applied to hollow cathode dark space diagnostics [675]. Photoionization is produced to measure the dark space widths of linear field distributions directly. A theoretical model has been developed and its predictions verified with experimental findings for a uranium hollow cathode discharge operated in neon or xenon. Variations in the ground-state densities of sputtered neutrals have also been measured. 

Most double resonance experiments exploit one of the four energy level schemes shown in Fig. 1.17. The variety of detection schemes is enormous, but most schemes may be divided into those which result in a signal on an essentially dark background vs. those which result in a dip in an essentially constant background level. By definition, the frequency of the PROBE (or DUMP) laser is scanned and that of the PUMP or DETECT laser is held fixed while a double resonance spectrum is being recorded. 

Figure 6.8 AlphaScreen detection. Laser excitation of the donor bead causes it to convert ambient oxygen into singlet oxygen. If appended molecules A and B bind (top), the chemiluminescent acceptor bead is brought close enough to emit light. If a molecule C that does not bind A is attached to the acceptor bead (bottom) or if a small molecule interferes with the binding of A and B, then the donor and acceptor will be too far apart for light to be emitted. (Courtesy of PerkinElmer.)...

The electronic ground state of this non-polar species is still of interest because classical rotationally resolving methods are difficult to apply. Bermejo et al. [02Ber] report on high-resolution rovibrational Raman spectra of Cl2, Cl2, and Cl Cl, and their spectroscopic constants for the vibrational states v = 0,1,2. Wang et al. [98Wan] have measured rotationally resolved vacuum ultraviolet REMPI detected laser spectra of the Cl2 1 <— X transition, and obtained rotational constants of levels loealized in the double-well potential of... 

An improved Doppler method has been used recently. In this method the probe laser is delayed relative to the dissociation laser so that only the products that fly parallel or antiparallel to the detection laser are excited. This yields much better state resolution and a deconvolution of complicated Doppler line shapes is no longer necessary. 

In atomic laser spectroscopy, the laser radiation, which is tuned to a strong dipole transition of the atoms under investigation, penetrates the volume of species evaporated from the sample. The presence of analyte atoms can be measmed by means of the specific interaction between atoms and laser photons, such as by absorption techniques (laser atomic absorption spectrometry, LAAS), by fluorescence detection (laser-induced fluorescence spectroscopy, LIFS), or by means of ionization products (electrons or ions) of the selectively excited analyte atoms after an appropriate ionization process (Figures lA and IB). Ionization can be achieved in different ways (1) by interaction with an additional photon of the exciting laser or of a second laser (resonance ionization spectroscopy, RIS, or resonance ionization mass spectrometry, RIMS, respectively, if combined with a mass detection system) (2) by an electric field applied to the atomization volume (field-ionization laser spectroscopy, FILS) or (3) by collisional ionization by surrounding atoms (laser-enhanced ionization spectroscopy, LEIS). 

Fig. 1 Separation of DNA sequencing fragments. Conditions capillary, 30 cm (45 cm total length) x 75 JLm I.D., coated with poly(vinyl alcohol) background electrolyte, 2% linear polyacrylamide (LPA), 9 mDa and 0.5% LPA, 50 kDa in 7 M urea and 500 mM Tris/500 mMTAPS/ 20 mM EDTA buffer, 50 mM Tris/50 mM TAPS/2 mM EDTA with urea in catholyte injection, 25 V/cm for 10 sec field strength, 200 V/cm temperature, 60° C detection, laser-induced fluorescence.

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