Laser beam excitation

In this technique, a laser beam of He-Ne focuses tangentially onto the surface of tubular membrane. A fraction of the laser light is absorbed by the deposited fouling layer and the signal reduction is monitored to translate the deposited thickness [72]. 

This technique was used to measure the fouling thickness deposited on a tubular membrane. Bentonite as a particulate model solution was filtered during fouling characterization experiments. The maximum CFV and solid concentration used for this analysis were 0.3 m s and 375 mg L , respectively [72]. The limitations of the technique are the inability to visualize the cake structure and the requirement of light adsorption into the lumen, reducing the visualization potential for MBR study. Extended information can be found in Chapters 11 and 15. 

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LIF Laser-induced fluorescence Incident laser beam excites Excited-state processes ... 

An efficient optical coupling to the WGMs is instrumental in order to harvest the full potential of the high-2 droplet resonators. In most reported experiments, the droplet resonators are probed by free-space excitation, where, e.g., a Gaussian laser beam excites resonator modes and scattered light or fluorescence is detected. This approach... 

The Laser Raman microprobe constitutes a physical method of microanalysis based on the vibration spectra characteristic of polyatomic structures. A focused laser beam excites the sample. The light diffused by the Raman effect is used for identification and localisation of the molecular constituents present in the sample. An optical microscope allows a survey of the interesting structures and the placing of the laser beam. The spectra obtained from fossil organic particles generally match well the corresponding IR-spectra, but the features in particular yield additional information, which will be discussed below with the given examples (Fig. 23, p. 36). 

The instrument uses an excitation source of one of the harmonics (266 nm/355 nm/532 nm, 7 ns FWHM) of a Nd YAG laser (Continuum Surelite I) or tunable visible light from an optical parametric oscillator (OPO) (Opotek). Other pulsed lasers can be employed, the criteria being that a short pulse of light (<20 ns) be produced, with sufficient per pulse energy to generate enough absorbance to rise above the noise inherent in the spectrometer. During the experiment, the laser beam excites the sam-... 

A UV double pulsed Nd YAG laser (Litron Lasers) illuminated the micro-fluidic device from a 45° angle with a 355 nm beam (volume illumination). The observation region was 30 mm away from the inlet. The laser beams excited the particles which emitted blue light within a band centred on 445 nm. Particle blue fluorescence was detected by a 12bit 4 MPixels 16 fjps CCD camera (TSI instmments), focused on the test section, after passing through a narrow pass band filter centred... 

Due to the very high intensity of the laser beams and their coherent nature they may be used in a variety of ways where controlled energy is required. Lasers are used commercially for excitation with a specific energy, e.g. in Raman spectroscopy or isotope separation. 

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). 

While a laser beam can be used for traditional absorption spectroscopy by measuring / and 7q, the strength of laser spectroscopy lies in more specialized experiments which often do not lend themselves to such measurements. Other techniques are connnonly used to detect the absorption of light from the laser beam. A coimnon one is to observe fluorescence excited by the laser. The total fluorescence produced is nonnally proportional to the amount of light absorbed. It can be used as a measurement of concentration to detect species present in extremely small amounts. Or a measurement of the fluorescence intensity as the laser frequency is scaimed can give an absorption spectrum. This may allow much higher resolution than is easily obtained with a traditional absorption spectrometer. In other experiments the fluorescence may be dispersed and its spectrum detennined with a traditional spectrometer. In suitable cases this could be the emission from a single electronic-vibrational-rotational level of a molecule and the experimenter can study how the spectrum varies with level. 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

Better detection limits are obtained using fluorescence, particularly when using a laser as an excitation source. When using fluorescence detection, a small portion of the capillary s protective coating is removed and the laser beam is focused on the inner portion of the capillary tubing. Emission is measured at an angle of 90° to the laser. Because the laser provides an intense source of radiation that can be focused to a narrow spot, detection limits are as low as 10 M. 

Interaction of an excited-state atom (A ) with a photon stimulates the emission of another photon so that two coherent photons leave the interaction site. Each of these two photons interacts with two other excited-state molecules and stimulates emission of two more photons, giving four photons in ail. A cascade builds, amplifying the first event. Within a few nanoseconds, a laser beam develops. Note that the cascade is unusual in that all of the photons travel coherently in the same direction consequently, very small divergence from parallelism is found in laser beams. 

In a skimmed supersonic jet, the parallel nature of the resulting beam opens up the possibility of observing spectra with sub-Doppler resolution in which the line width due to Doppler broadening (see Section 2.3.4) is reduced. This is achieved by observing the specttum in a direction perpendicular to that of the beam. The molecules in the beam have zero velocity in the direction of observation and the Doppler broadening is reduced substantially. Fluorescence excitation spectra can be obtained with sub-Doppler rotational line widths by directing the laser perpendicular to the beam. The Doppler broadening is not removed completely because both the laser beam and the supersonic beam are not quite parallel. 

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