Laser-probing technique

Fig. 6. Density of non-condensed fraction of the gas as the trap depth is reduced along the cooling path. The density is measured by the optical resonance shift, and the trap depth is set by the rf frequency. The lines (dash, solid, dot-dash) indicate the BEC phase transition line, assuming a sample temperature of (l/5th, l/6th, l/7th) the trap depth. The scatter of the data reflects the reproducibility of the laser probe technique and is dominated by alignment of the laser beam to the sample...

Measuring Doppler widths of rotational lines by laser-probe techniques gives velocity distributions in just the same way as measuring Doppler widths of atomic lines by conventional means. In this method a laser beam with a very narrow band width is tuned over the spectral line to determine the profile of the Doppler broadened line. The line shape can be interpreted to give the average velocity of the product. As yet, this method has been applied only to rotational energy transfer studies however, with the availability of mode-locked lasers providing narrow band widths, this procedure may become more widely used. 

The formation of CO from O 4- CS has been studied by infrared chemiluminescence (analysis of steady-state populations), by the chemical laser method and by laser probe techniques. Good agreement exists for the relative populations in v = 6-15. The disagreement over the populations of the lower levels recently was resolvedby the discovery that the O 4- CSg reactions, which in most systems accompanies the O 4- CS reaction, yields CO in the lower levels. Following the suggestion of Kelley, the lower levels in Table 2.13 were obtained from extrapolation of the linear... 

Table 3.2. Some Reactions Studied with Chemical Laser-Probe Techniques...

Laser Raman diagnostic teclmiques offer remote, nonintnisive, nonperturbing measurements with high spatial and temporal resolution [158], This is particularly advantageous in the area of combustion chemistry. Physical probes for temperature and concentration measurements can be debatable in many combustion systems, such as furnaces, internal combustors etc., since they may disturb the medium or, even worse, not withstand the hostile enviromnents [159]. Laser Raman techniques are employed since two of the dominant molecules associated with air-fed combustion are O2 and N2. Flomonuclear diatomic molecules unable to have a nuclear coordinate-dependent dipole moment caimot be diagnosed by infrared spectroscopy. Other combustion species include CFl, CO2, FI2O and FI2 [160]. These molecules are probed by Raman spectroscopy to detenuine the temperature profile and species concentration m various combustion processes. 

B2.5.4.2 LASER FLASH PHOTOLYSIS AND PUMP-PROBE TECHNIQUES... 

Figure B2.5.8. Schematic representation of laser-flash photolysis using the pump-probe technique. The beam splitter BS splits the pulse coming from the laser into a pump and a probe pulse. The pump pulse initiates a reaction in the sample, while the probe beam is diverted by several mirrors M tluough a variable delay line.

The small (<1 cm) sizes and brief (<1 //s) lifetimes of the fusion research plasmas preclude the use of most probe techniques. Laser pulse imaging... 

With the use of appropriate transmission optics, high focusing of the laser light is carried out and the extension of the optical probe is considerably reduced. Accordingly, laser-based techniques offer the possibility of measurements of high spatial resolution. 

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample. In addition, the pump and probe pulses are synchronized so that the lime interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

It is particularly important to study process phenomena under dynamic (rather than static) conditions. Most current analytical techniques are designed to determine the initial and final states of a material or process. Instmments must be designed for the analysis of materials processing in real time, so that the cmcial chemical reactions in materials synthesis and processing can be monitored as they occur. Recent advances in nuclear magnetic resonance and laser probes indicate valuable lines of development for new techniques and comparable instmmentation for the study of interfaces, complex hquids, microstmctures, and hierarchical assemblies of materials. Instmmentation needs for the study of microstmctured materials are discussed in Chapter 9. 

In order to relate material properties with plasma properties, several plasma diagnostic techniques are used. The main techniques for the characterization of silane-hydrogen deposition plasmas are optical spectroscopy, electrostatic probes, mass spectrometry, and ellipsometry [117, 286]. Optical emission spectroscopy (OES) is a noninvasive technique and has been developed for identification of Si, SiH, Si+, and species in the plasma. Active spectroscopy, such as laser induced fluorescence (LIF), also allows for the detection of radicals in the plasma. Mass spectrometry enables the study of ion and radical chemistry in the discharge, either ex situ or in situ. The Langmuir probe technique is simple and very suitable for measuring plasma characteristics in nonreactive plasmas. In case of silane plasma it can be used, but it is difficult. Ellipsometry is used to follow the deposition process in situ. 

Absorption and Ensuing Ion-Molecule Reactions via Ultrafast Laser Pump-Probe Techniques.196... 

Another very informative nonlinear experiment involves a typical pump-probe technique, but with varying laser polarization. These experiments, again for isolated chromophores, measure the rotational anisotropy TCF [122]... 

Schoenung, S. M., and R. K. Hanson. 1981. CO and temperature measurements in a flat flame by laser absorption spectroscopy and probe techniques. Combustion Science Technology 24 227-37. 

CO2 is the gas generally used for mass-spectrometric analysis. More recently CO and O2 have also been used in high temperature conversion of organic material and in laser probe preparation techniques. A wide variety of methods have been described to liberate oxygen from the various oxygen-containing compounds. 

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