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Cooper, J.

Publications and source records attributed to Cooper, J..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Redistribution of radiation in the absence of collisions

Redistribution and depolarization of near-resonant radiation is studied for the He 2(1P)-3(1D) line (668 nm) and some data for H-alpha (656 nm) are also presented. A Nd:YAG pumped dye laser, with a bandwidth of 0.1/cm and a power density of 100 kW/sq cm, is used in the experiment. The transitions have lower levels with natural line widths that are large compared with their upper-level natural widths, and, under experimental conditions, their collisional widths are also small. The ratio of fluorescence to Rayleigh intensity is measured and the results indicate that while the Rayleigh emission has the same temporal development as the laser pulse (5 ns FWHM), the fluorescent intensity has a decay time determined by the radiative decay lifetime and by collisional quenching. The prediction that redistribution of radiation occurs in the absence of collisions for transitions having significant lower-level radiative widths is confirmed. Depolarization rates by collisions with helium are also inferred from the measurements.

Lombardi, G. G.↗
NASA NTRS1984

Modifications to population rate equations resulting from correlations between collisional and radiative processes

There are many systems of physical interest for which a set of rate equations for level populations can provide insight. If the system has two (or more) different mechanisms for effecting transition between levels, total rates of transfer are usually taken as the sum of rates that the individual mechanisms would cause acting alone. Using the example of a hydrogen atom subjected to (ionic and electronic) collisions and external radiation, it is shown that when these individual mechanisms overlap, the total transfer rates must be modified to account for correlations between collisional and radiative processes. For a broad-band radiation field the modified rates have a simple physical interpretation. In the case of a narrow-band field the overlapping events may cause new coherences to appear and interpretation of the modified 'rates' is more complicated.

Ballagh, R. J.↗
NASA NTRS1984

Radiative transfer equations in broad-band, time-varying fields

A derivation of the equation of transfer is obtained by starting with Maxwell's equations in the 'slowly varying envelope' form. Particular attention is paid to characterizing the intensity that is 'seen' by the atom (which is found to be related to a Wigner distribution of the electric field). The equation of transfer is found to be valid for 'broad-band' slowly varying radiation fields.

Cooper, J.↗
NASA NTRS1983

On the line profile coefficient for stimulated emission

It is pointed out that the line profile coefficients for spontaneous and stimulated emission are identical in low-intensity radiation fields. In more intense radiation fields, however, the stimulated emission profiles in the radiative transfer equation and in the rate equations for the atomic level populations may differ from each other, owing to their different physical nature. A seeming discrepancy between the stimulated emission profiles of the usual semi-classical aproach and a recent quantum mechanical approach by Cooper et al. (1982), which should also be valid for intense 'broadband' fields, is discussed and shown to have negligible consequences for low-intensity radiation fields.

Cooper, J.↗
NASA NTRS1983

Theory of time-dependent intense-field collisional resonance fluorescence

The time-dependent theory of Courtens and Szoke (1977) is generalized using the approach of Burnett et al. (1982) to derive time-dependent spectral intensities of resonance fluorescence from atoms driven by a pulsed laser in the presence of collisions. These results are valid both for laser detunings inside and outside the usual impact region of the spectrum, including Zeeman degeneracy effects. This theory is applied to a simple but important example (J = 0 to J = 1) to obtain quantitative predictions for the observable scattered-light spectrum which can be directly compared with recent experiments.

Kleiber, P. D.↗
NASA NTRS1983

Redistribution - Why half a collision is better than a whole one

The study of spectral line shapes has traditionally been mainly concerned with the measurement and interpretation of absorption or emission profiles. Often only the line widths are studied. The present investigation has the objective to evaluate the additional information which can be obtained by scattering light (usually from a laser) from an atomic system which is being perturbed by collisions. A scattering experiment is discussed. The scattered light consists of two components, a (coherent) Rayleigh component and a redistributed (fluorescent) component. In order to obtain the absorption spectrum, questions regarding the probability of photon absorption are considered. By observing the fluorescence subsequent to absorption during a collision it is found possible to obtain information on the evolution of the system from the point of absorption to the completion of the collision. The information on the intracollisional evolution is the justification for the title of the study, namely 'Why half a collision is better than a whole one'.

Cooper, J.↗
NASA NTRS1982

On redistribution and the equations for radiative transfer

The derivation of the equations of statistical equilibrium are outlined, starting from the quantum density-matrix equations, drawing particular attention to the approximations and assumptions used in the development of tractable expressions. Then, using the quantum-fluctuation-regression theorem, emission and absorption coefficients are obtained for multilevel atomic systems which are nondegenerate except for m-substates. These coefficients are valid to first order in the incident intensity. Possible extensions to higher intensity broadband incoherent fields are suggested.

Cooper, J.↗
NASA NTRS1982

Non-urban mobile radio market demand forecast

A national nonmetropolitan land mobile traffic model for 1990-2000 addresses user classes, density classes, traffic mix statistics, distance distribution, geographic distribution, price elasticity, and service quality elasticity. Traffic demands for business, special industrial, and police were determined on the basis of surveys in 73 randomly selected nonurban counties. The selected services represent 69% of total demand. The results were extrapolated to all services in the non-SMSA areas of the contiguous United States. Radiotelephone services were considered separately. Total non-SMSA mobile radio demand (one way) estimates are given. General functional requirements include: hand portability, privacy, reduction of blind spots, two way data transmission, position location, slow scan imagery.

Castruccio, P. A.↗
NASA NTRS1982

Collisional redistribution of radiation in strong fields - Modification of the collision dynamics

The theory of light scattering from an atom undergoing collisions is extended to the case of strong laser fields. In particular, the conditions necessary to relate the observable quantities to intense-field collisional rate constants in a dressed-state basis are considered. It is shown that by studying the spectrum emitted by the atoms in the presence of a strong field one may measure the collisional rates for transfer between the dressed states of atom plus radiation field.

Burnett, K.↗
NASA NTRS1981

Redistribution of radiation for the wings of Lyman-alpha

Earlier work on redistribution of radiation by collisions for isolated lines is extended to overlapping lines, and an explicit expression for the frequency redistribution is given for Lyman-alpha. This expression is valid, even when the emitted photon is in the (non-impact) line wings. A simple physical explanation of the result is possible.

Yelnik, J.-B.↗
NASA NTRS1981

Far wing depolarization of light - Generalized absorption profiles

An absorption (and/or emission) event which takes place during a strong collision is called a 'correlated event'. It is discussed how correlated events affect the far red wing depolarization of fluorescence. Attention is given to an atomic vapor which is irradiated by linearly polarized light of a frequency on the red side of the resonance line. Two limiting cases are considered, corresponding to excitation in the impact region and in the quasi-static wing. In the quasi-static wing, absorption of a photon followed by fluorescence (rather than Rayleigh scattering), occurs mostly during a collision. Correlated events dominate the scattering process. Expressions derived for the polarization of the fluorescent light are applied to far red wing depolarization. It is found that the polarization of the fluorescent light does not go to zero in the far wing, but depends crucially on the detailed nature of the anisotropy in the long-range part of the interatomic potential.

Thomann, P.↗
NASA NTRS1980

Collisional redistribution of radiation. I - The density matrix

As a first step in deriving accurate, calculable expressions for collisional redistribution of radiation, valid outside the impact regime a method for calculating an accurate binary-collision density operator for an atom in the presence of a driving field and perturbers is presented. Projection-operator techniques to establish first the precise validity of the Markoff approximation for radiative relaxation are used. Using the same techniques, it is shown how the collisional-relaxation problem may be analyzed in a practical manner outside the Markoff approximation. The effect of correlations between radiative and collisional events is included in a consistent way and the physical implications of these correlation effects are demonstrated for a simple example, the two-level atom with nondegenerate levels.

Burnett, K.↗
NASA NTRS1980

Collisional redistribution of radiation. II - The effects of degeneracy on the equations of motion for the density matrix. III - The equation of motion for the correlation function and the scattered spectrum

The effect of correlations between an absorber atom and perturbers in the binary-collision approximation are applied to degenerate atomic systems. A generalized absorption profile which specifies the final state of the atom after an absorption event is related to the total intensities of Rayleigh scattering and fluorescence from the atom. It is suggested that additional dynamical information to that obtainable from ordinary absorption experiments is required in order to describe redistributed atomic radiation. The scattering of monochromatic radiation by a degenerate atom is computed in a binary-collision approximation; an equation of motion is derived for the correlation function which is valid outside the quantum-regression regime. Solutions are given for the weak-field conditions in terms of generalized absorption and emission profiles that depend on the indices of the atomic multipoles.

Burnett, K.↗
NASA NTRS1980

Collisional redistribution of radiation. III - The equation of motion for the correlation function and the scattered spectrum

Computations were made of the scattering of monochromatic radiation by a degenerate atom in the binary-collision approximation for field strengths whose products of the Rabi frequency for atomic transition and the duration of a strong collision are much less than 1. An expression of motion for the correlation function is derived which does not exclude the region where thermal correlations may be neglected; the equation is valid outside the quantum-regression regime, and has a straightforward solution for practical cases. Solutions for the weak-field linear response regime are presented in terms of generalized absorption and emission profiles which depend on the indices of the atomic multipoles.

Burnett, K.↗
NASA NTRS1980

Observation of dynamic correlations in collisional redistribution and depolarization of light

Theory shows that to explain the polarization of light collisionally redistributed from the far line wings of an atomic transition, one must consider correlated events in which absorption during a collision, and propagation to the final Zeeman-state superposition at the end of the collision is important. Polarizations of up to about 40% have been measured in the far line wings, substantially confirming this prediction, and showing that scattering experiments cannot just be characterized by simple absorption or emission profiles.

Thomann, P.↗
NASA NTRS1980

Zeeman degeneracy effects in collisional intense-field resonance fluorescence

Resonance fluorescence due to intense laser fields from a Zeeman degenerate atom being perturbed by collisional interactions is calculated in the impact regime by using the quantum-fluctuation-regression theorem. Various interesting effects are found. For example, the scattered intensity spectrum for a J = 0 to J = 1 transition for polarization parallel to the laser polarization is essentially an asymmetric triplet, whereas for a perpendicular polarization due to collisions the spectrum is essentially a doublet (whose frequencies do not correspond with any of those of the triplet). Further, the width of the fluorescent component (whose frequency is close to the unperturbed frequency) actually decreases with increasing laser power.

Cooper, J.↗
NASA NTRS1980

A multiphoton treatment of near resonant light scattering in intense fields

The scattering by a two-state atom of monochromatic light of arbitrary intensity and detuning, is treated by solving Schroedinger's equation directly for the state of the scattered field. The calculation is performed in a basis of dressed atom states, which include to all orders the coupling of the incident field to the atom. Amplitudes describing sequences of scattering events are found by a nonperturbative method. A scattered intensity spectrum, directly related to the rate of change of energy in any given scattered field mode, is constructed in terms of bilinear products of the amplitudes representing all possible sequences of photon emission, and in the steady state limit the spectrum is found to be identical to the widely quoted results of Mollow. A discussion is devoted to understanding in physical terms the various quantities appearing in the expression for the intensity spectrum.

Ballagh, R. J.↗