Velocities, distribution of atom

With narrow band LQ -radiation it was possible to measure the spatial velocity distribution of atomic hydrogen generated by the photodissociation of HI . Other applications of radiation included the... 

Lastly, there remains a significant lack of information concerning the velocity dependence for molecular secondary ion emission, a fact that can be related back to the greater variability in secondary neutral distributions (see Section 3.2.1 and subsections contained within). This, and the remaining questions concerning the velocity distribution of atomic secondary ions, thus signifies the need for further studies in this area. 

The laser control of the velocity distribution of atoms or molecules at particular quantum levels that emerged in the course of development of saturation spectroscopy free of Doppler broadening (Lamb 1964) is fairly close to the ideas considered in this book. I myself started to work on the problem of laser elimination of Doppler broadening as far back as 1965 and gradually progressed to ideas of laser confinement of atomic motion within a volume of about A . Therefore, I have decided to include a brief description of the ideas of laser velocity-selective control of atoms and molecules. 

The goal of this book is to present in a coherent way the problems of the laser control of matter at the atomic-molecular level, namely, control of the velocity distribution of atoms and molecules (saturation Doppler-free spectroscopy) control of the absolute velocity of atoms (laser cooling) control of the orientation, position, and direction of motion of atoms (laser trapping of atoms, and atom optics) control of the coherent behavior of ultracold (quantum) gases laser-induced photoassociation of cold atoms, photoselective ionization of atoms photoselective multiphoton dissociation of simple and polyatomic molecules (vibrationally or electronically excited) multiphoton photoionization and mass spectrometry of molecules and femtosecond coherent control of the photoionization of atoms and photodissociation of molecules. 

Figure B2.5.12 shows the energy-level scheme of the fine structure and hyperfme structure levels of iodine. The corresponding absorption spectrum shows six sharp hyperfme structure transitions. The experimental resolution is sufficient to detennine the Doppler line shape associated with the velocity distribution of the I atoms produced in the reaction. In this way, one can detennine either the temperature in an oven—as shown in Figure B2.5.12 —or the primary translational energy distribution of I atoms produced in photolysis, equation B2.5.35.

Initializing the initial kinetic energy and temperature of the system it is necessary to start the motion at some level, eg, assume a Boltzmann (random) distribution of atomic velocities, at 300 K. 

Fig. 8-3. Distribution of atomic (or molecular) velocities from ihe rotating disc.

Fig. 10. Velocity distributions of the H-atom fragment recorded at perpendicular (X) and parallel ( ) configurations in the reaction of H2S photolysis. For clarity, the perpendicular one is shifted up but not normalized and the base lines for both cases are indicated by dotted lines.

The quantum-mechanical ionization cross section is derived using one of several approximations—for example, the Born, Ochkur, two-state, or semi-classical approximations—and numerical computations (Mott and Massey, 1965). In some cases, a binary encounter approximation proves useful, which means that scattering between the incident particle and individual electrons is considered classically, followed by averaging over the quantum-mechanical velocity distribution of the electrons in the atom (Gryzinski, 1965a-c). However, Born s approximation is the most widely used one. This is discussed in the following paragraphs. 

For agiven system of metal/alloy and atomization gas, the 2-D velocity distributions of the gas and droplets in the spray can be then calculated using the above-described models, once the initial droplet sizes and velocities are known from the modeling of the atomization stage, as described in the previous subsection. With the uncoupled solution of the gas velocity field in the spray, the simplified Thomas 2-D nonlinear differential equations for droplet trajectories may be solved simultaneously using a 4th-orderRunge-Kutta algorithm, as detailed in Refs. 154 and 156. 

These classical formulas still do not account for the motion of the bound electrons in the atom or molecule. To be more appropriate to the interaction of an incident electron with the bound target electron, one must recognize that the velocity vector of the bound electron can be randomly oriented with respect to the incident electron providing a broadening of the energy of the secondary electron as calculated by the modified Mott cross section. If one integrates over the velocity distribution of the bound electron, the more familiar binary encounter approximation is derived that, in its simplest form, is given by Kim and Rudd [39] as... 

Schindler, R. N., M. Liesner, S. Schmidt, U. Kirchner, and Til. Benter, Identification of Nascent Products Formed in the Laser Photolysis of CH,OCI and HOC1 at 308 nm and around 235 nm. Total Cl-Atom Quantum Yields and the State and Velocity Distributions of CI(2P(), J. Photochem. Photobiol. A Chem., 107, 9-19 (1997). 

The photochemical dynamics of H2S has been studied in its first absorption band between 180 and 260 nm (2) using LIF measurements to determine the quantum state distribution of the SH fragment (169-171), as well as TOF measurements of the velocity distribution of H atom fragment (172). In the former case, the vibrational and rotational distribution of the SH fragment was only measured in the v" = 0 level because fewer radicals with v" > 0 are produced and the LIF technique does not efficiently detect these excited radicals. 

The values of the parameter y corresponding to experimental dependences of zeff on the charge Z, have been found in Ref. 178. Using the statistical Fermi-Thompson model of an atom, the authors have calculated the velocity distribution of external electrons for atoms of heavy elements. Using the Bohr criterion, they have obtained a formula for zeff of the form (5.7) with the exponent of Z, equal to For this value of y the calculated values of zeff for heavy fission fragments in a hydrogen medium practically coincide with the experimental ones. 

By numerical integration of the Bloch equations describing the classical trajectory of an atom from the nozzle through the standing wave field at 243 nm to the detector and integration over all possible trajectories and over the velocity distribution of the atoms, a theoretical line shape is deduced which is then fitted to the experimental data. The solid lines in Fig. 2 are obtained from this fitting procedure. 

Various methods have been proposed for measuring the product velocity distributions from the Doppler absorption profile of the laser-induced fluorescence from the products. In one case, it has been suggested [115] that the velocity distribution of the H atom from... 

Fig. 2. The three dimensional velocity distribution of all the atoms after impact of a cold cluster of 125 Ar atoms at a surface at a velocity of 25 km/s (= 0.25 A/fs). Shown for comparison is a Meixwell-Boltzmann functional form for the same mean energy. Even at this high velocity of impact, the velocity distribution after the impact is essentially isotropic. See Sec. 4.

Figure 23 is a typical output from a MD simulation. It is for an Ari2s cluster at an impact velocity of 20 km s . It is seen that by 80 fs the velocity distribution of the cluster atoms looks like a three dimensional Maxwell-Boltzmann one. 

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