Beam-laser techniques

Fig.9.28. Lifetime measurement with the beam-laser technique using Doppler tuning [9.117]...

Beam-Laser Techniques. By exchanging the exciting foil in the beam-foil teclmique for a focused laser beam, selective excitation can be obtained and the problem of cascades is eliminated [9.142]. By directing the laser beam at a certain angle Q with respect to the ion beam considerable Doppler shifts in the interaction with the fast beam are obtained. The relativistically correct formula for the interaction wavelength A, experienced by the ions that are illuminated by a laser of wavelength g is given by... 

Bernstein R B (ed) 1982 Chemical dynamics via molecular beam and laser techniques The Hinshelwood Lectures (Oxford, 1980) (Oxford Oxford University Press)... 

Because Raman spectroscopy requires one only to guide a laser beam to the sample and extract a scattered beam, the technique is easily adaptable to measurements as a function of temperature and pressure. High temperatures can be achieved by using a small furnace built into the sample compartment. Low temperatures, easily to 78 K (liquid nitrogen) and with some diflSculty to 4.2 K (liquid helium), can be achieved with various commercially available cryostats. Chambers suitable for Raman spectroscopy to pressures of a few hundred MPa can be constructed using sapphire windows for the laser and scattered beams. However, Raman spectroscopy is the characterizadon tool of choice in diamond-anvil high-pressure cells, which produce pressures well in excess of 100 GPa. ... 

Up to the present, a number of conventional film preparation methods like PVD, CVD, electro-chemical deposition, etc., have been reported to be used in synthesis of CNx films. Muhl et al. [57] reviewed the works performed worldwide, before the year 1998, on the methods and results of preparing carbon nitride hlms. They divided the preparation techniques into several sections including atmospheric-pressure chemical processes, ion-beam deposition, laser techniques, chemical vapor deposition, and reactive sputtering [57]. The methods used in succeeding research work basically did not... 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Bernstein, R.B. (ed) (a) Chemical Dynamics via Molecular Beam and Laser Techniques, Clarendon Press, Oxford, New York (1982). (b) Atom-Molecule Collision Theory A guide to experimentalist, Plenum Press, New York (1979). 

Reaction dynamics is the part of chemical kinetics which is concerned with the microscopic-molecular dynamic behavior of reacting systems. Molecular reaction dynamics is coming of age and much more refined state-to-state information is becoming available on the fundamental reactions. The contribution of molecular beam experiments and laser techniques to chemical dynamics has become very useful in the study of isolated molecules and their mutual interactions not only in gas surface systems, but also in solute-solution systems. 

Experiments have also played a critical role in the development of potential energy surfaces and reaction dynamics. In the earliest days of quantum chemistry, experimentally determined thermal rate constants were available to test and improve dynamical theories. Much more detailed information can now be obtained by experimental measurement. Today experimentalists routinely use molecular beam and laser techniques to examine how reaction cross-sections depend upon collision energies, the states of the reactants and products, and scattering angles. 

Bernstein, R. B. Chemical Dynamics via Molecular Beam and Laser Techniques Oxford University London, 1982, p. 45. 

Fig. 8 Al AI2O3 lines ( dark regions ) on a (HAIO) film obtained by two beam interference techniques using a Nd/YAG laser (266 nm, 156mJ/cm, 1 pulse) [32]...

Ullrafine particles (UFPs) of metal and semiconductor nitrides have been synthesized by two major techniques one is the reactive gas condensation method, and the other is the chemical vapor condensation method. The former is modified from the so-called gas condensation method (or gas-evaporation method) (13), and a surrounding gas such as N2 or NII2 is used in the evaporation chamber instead of inert gases. Plasma generation has been widely adopted in order to enhance the nitridation in the particle formation process. The latter is based on the decomposition and the subsequent chemical reaction of metal chloride, carbonate, hydride, and organics used as raw materials in an appropriate reactive gas under an energetic environment formed mainly by thermal healing, radiofrequency (RF) plasma, and laser beam. Synthesis techniques are listed for every heal source for the reactive gas condensation method and for the chemical vapor condensation method in Tables 8.1.1 and 8.1.2, respectively. 

The electronic structure of fluorenes and the development of their linear and nonlinear optical structure-property relationships have been the subject of intense investigation [20-22,25,30,31]. Important parameters that determine optical properties of the molecules are the magnitude and alignment of the electronic transition dipole moments [30,31]. These parameters can be obtained from ESA and absorption anisotropy spectra [32,33] using the same pump-probe laser techniques described above (see Fig. 9). A comprehensive theoretical analysis of a two beam (piunp and probe) laser experiment was performed [34], where a general case of induced saturated absorption anisotropy was considered. From this work, measurement of the absorption anisotropy of molecules in an isotropic ensemble facilitates the determination of the angle between the So Si (pump) and Si S (probe) transitions. The excited state absorption anisotropy, rabs> is expressed as [13] ... 

TB) which can be detected with a CCD array and a laser beam reflection technique. This system was able to resolve easily a 100 nm change in thickness and was also able to measure the dynamic change of thickness. The preload applied to the pin on TB of l mm diameter was varied from 0.08 g to 1.0 g with a weight (TW). From this, zero-preload deformation was obtained by extrapolation. The Poisson ratio was about 0.08 in water at the longitudinal load of 0.12 MPa, which was close to or lower than that obtained from the width change. 

LEE, YUAN T. (1936-). Awarded the Nobel prize in chemistry in 19X6 jointly with John C. Polanyi and Dudley R. Herschbach for their contributions concerning the dynamics of chemical elementary processes. A former student of Herschbach. Lee relined molecular-beam and laser techniques, comhining them with theory to perform definitive studies of reactions of individual complex molecules. Lee received his Doctorate from the University of California at Berkeley in 1965. 

Previous workers had used the molecular beam TOF technique (134) and the VUV flash photolysis LIF technique (135). Ling and Wilson (136) had suggested that either the A(2n) state of CN is produced in the original photolysis process or that I atoms were produced in the Pi/2 and 3/2 states. It had been previously shown (135), by collisional quenching studies, that the A state of CN was not produced. This earlier work has been reviewed by Baronvaski (137) but recently both he and others have done further work on this molecule using excimer laser sources in both static gases and pulsed molecular beams. 

Low energy initiation techniques [179, 180, 181] (near infrared, ultrasonic radiation and line tuneable pulse laser) have lately emerged to be better alternatives to the high-energy radiations (y-irradiation and e-beam). Laser-induced polymerisation of monomers have attracted significant attention in recent years generating a considerable literature published on both pulsed... 

On the experimental side, the chemical dynamics on the state-to-state level is being studied via molecular-beam and laser techniques [2]. Alternative, and complementary, techniques have been developed in order to study the real-time evolution of elementary reactions [3]. Thus, the time resolution in the observation of chemical reactions has increased dramatically over the last decades. The race against time has recently reached the ultimate femtosecond resolution with the direct observation of chemical reactions as they proceed along the reaction path via transition states from reactants to products. This spectacular achievement was made possible by the development of femtosecond lasers, that is, laser pulses with a duration as short as a few femtoseconds. In a typical experiment two laser pulses are used, a pump pulse and a probe... 

The current status of prediction and modelling in the area of fuel spray combustion requires, among other parameters, the measurement of droplet or solid particle size distribution and the relative velocity between the fuel and the surrounding gas. Many optical techniques, based on laser light scattering, have been investigated to this purpose (Refs.1,2,2,]+,, 6 and j), but the only system able to simultaneously determine the size and the velocity is the dual-beam laser Doppler velocimeter shown in Figure 1. 

The yield of dissociation products may be small, but sensitive methods of detection can be used. One of these is laser-induced fluorescence, shown schematically in Figure 9.31, in which a second, probe, laser is used to excite fluorescence in one of the products of dissociation. The C02 and probe laser beams are at 90° to each other and the fluorescence is detected by a photomultiplier at 90° to both beams. This technique has been used, for example, to monitor the production of NH2 from the dissociation of hydrazine (N2H4) or methylamine (CH3NH2). The probe laser was a tunable dye laser set at a wavelength of 598 nm corresponding to absorption in the 2g band, where v2 is the bending vibration, of the A2 A, —X2Bl electronic system of NH2, and total fluorescence from the 29 level was monitored. 

Abstract. Laser spectroscopy of hydrogen-like and helium-like ions is reviewed. Emphasis is on the fast-beam laser resonance technique, measurements in moderate-/ ions which provide tests of relativistic and quantum-electrodynamic atomic theory, and future experimental directions. 

Fig. 1. (a) Energy levels of F8+ relevant to the measurement of the n = 2 Lamb shift (b) Schematic illustrating the fast-beam laser resonance technique... 

For various ions the 2S /2 — 2P1/2 and 2,S - /2 — 2P3/2 transitions match the wavelengths of certain efficient lasers enabling spectroscopy with the fast-beam laser-resonance technique [9], The most precise measurement of this type, achieved after considerable experimental development, was of the 2Si/2 — 2P3/2 transition in P14+ [10], where an uncertainty equivalent to 0.15% of the 2Si/2 — 2Pi/2 interval was obtained. A high-power pulsed dye-laser producing intensities of over 10 MW/cm-2, but with a duty cycle of about 10-5, was required to obtain a good signal-to-background ratio. 

---