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Results for “nonperturbative effects”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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17 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

N-point correlation functions in the CfA and SSRS redshift distribution of galaxies

Using counts in cells, we estimate the volume-average N-point galaxy correlation functions for N = 2, 3, and 4, in redshift samples of the CfA and SSRS catalogs. Volume-limited samples of different sizes are used to study the uncertainties at different scales, the shot noise, and the problem with the boundaries. The hierarchical constants S3 and S4 agree well in all samples in CfA and SSRS, with average S3 = 194 +/- 0.07 and S4 = 4.56 +/- 0.53. We compare these results with estimates obtained from angular catalogs and recent analysis over IRAS samples. The amplitudes SJ seem larger in real space than in redshift space, although the values from the angular analysis correspond to smaller scales, where we might expect larger nonperturbative effects. It is also found that S3 and S4 are smaller for IRAS than for optical galaxies. This, together with the fact that IRAS galaxies have smaller amplitude for the above correlation functions, indicates that the density fluctuations of IRAS galaxies cannot be simply proportional to the density fluctuations of optical galaxies, i.e., biasing has to be nonlinear between them.

Gaztanaga, Enrique↗
NASA NTRS1993

Quark and Gluon Relaxation in Quark-Gluon Plasmas

The quasiparticle decay rates for quarks and gluons in quark-gluon plasmas are calculated by solving the kinetic equation. Introducing an infrared cutoff to allow for nonperturbative effects, we evaluate the quasiparticle lifetime at momenta greater than the inverse Debye screening length to leading order in the coupling constant.

Heiselberg, H.↗
NASA NTRS1990

Cosmological baryon and lepton number in the presence of electroweak fermion-number violation

In the presence of rapid fermion-number violation due to nonperturbative electroweak effects certain relations between the baryon number of the Universe and the lepton numbers of the Universe are predicted. In some cases the electron-neutrino asymmetry is exactly specified in terms of the baryon asymmetry. Without introducing new particles, beyond the usual quarks and leptons, it is necessary that the Universe possess a nonzero value of B - L prior to the epoch of fermion-number violation if baryon and lepton asymmetries are to survive. Contrary to intuition, even though electroweak processes violate B + L, a nonzero value of B + L persists after the epoch of rapid fermion-number violation. If the standard model is extended to include lepton-number violation, for example through Majorana neutrino masses, then electroweak processes will reduce the baryon number to zero even in the presence of an initial B - L unless 20 M(sub L) approximately greater than the square root of (T(sub B - L) m(sub P1)) where M(sub L) sets the scale of lepton number violation and T(sub B - L) is the temperature at which a B - L asymmetry is produced. In many models this implies that neutrinos must be so light that they cannot contribute appreciably to the mass density of the Universe.

Harvey, Jeffrey A.↗
NASA NTRS1995

A Green's function method for heavy ion beam transport

The use of Green's function has played a fundamental role in transport calculations for high-charge high-energy (HZE) ions. Two recent developments have greatly advanced the practical aspects of implementation of these methods. The first was the formulation of a closed-form solution as a multiple fragmentation perturbation series. The second was the effective summation of the closed-form solution through nonperturbative techniques. The nonperturbative methods have been recently extended to an inhomogeneous, two-layer transport media to simulate the lead scattering foil present in the Lawrence Berkeley Laboratories (LBL) biomedical beam line used for cancer therapy. Such inhomogeneous codes are necessary for astronaut shielding in space. The transport codes utilize the Langley Research Center atomic and nuclear database. Transport code and database evaluation are performed by comparison with experiments performed at the LBL Bevalac facility using 670 A MeV 20Ne and 600 A MeV 56Fe ion beams. The comparison with a time-of-flight and delta E detector measurement for the 20Ne beam and the plastic nuclear track detectors for 56Fe show agreement up to 35%-40% in water and aluminium targets, respectively.

NASA Discipline Radiation Health↗
NASA NTRS2021

Spillway Preheating

In traditional models only an order one fraction of energy is transferred from the inflaton to radiation through nonperturbative resonance production in preheating immediately after inflation, due to backreaction effects. We propose a particle production mechanism that could improve the depletion of the inflaton energy density by up to four orders of magnitude. The improvement comes from the fast perturbative decays of resonantly produced daughter particles. They act as a “spillway” to drain these daughter particles, reducing their backreaction on the inflaton and keeping the resonant production effective for a longer period. Thus we dub the scenario “spillway preheating”. We also show that the fraction of energy density remaining in the inflaton has a simple inverse power-law scaling in the scenario. In general, spillway preheating is a much more efficient energy dissipation mechanism, which may have other applications in model building for particle physics.

Cosmology of Theories beyond the SM↗
NASA NTRS1999

Coherent Pump-Probe Interactions and Terahertz Intersubband Gain in Semiconductor Quantum Wells

In recent years there has been considerable interest in intersubband-transition-based infrared semiconductor quantum well (QW) lasers because of their potential applications. In the mid-infrared range, both electrically-injected quantum cascade lasers [1] and optically-pumped multiple QW lasers [2] have been experimentally realized. In these studies, optical gain is due to population inversion between the lasing subbands. It was also proposed that stimulated Raman scattering in QW systems can produce net infrared optical gain [3j. In such a nonlinear optical scheme, the appearance of optical gain that may lead to intersubband Raman lasers does not rely on the population inversion. Since, in tile resonant Raman process (Raman gain is the largest in this case), the pump field induces population redistribution among subbands in the QW s ystem, it seems that a realistic estimate of the optical gain has to include this effect. Perturbative calculations used in the previous work [3] may overestimate the Raman gain. In this paper we present a nonperturbative calculation of terahertz gain of optically-pumped semiconductor step quantum wells. Limiting optical transitions within the conduction band of QW, we solve the pump-field-induced nonequilibrium distribution function for each subband of the QW system from a set of coupled rate equations. Both intrasubband and intersubband relaxation processes in the quantum well system are included. Taking into account the coherent interactions between pump and THz (signal) waves, we we derive the susceptibility of the QW system for the THz field. For a GaAs/AlGaAs step QW, we calculate the Thz gain spectrum for different pump frequencies and intensities. Under moderately strong pumping (approximately 0.3 MW/sq cm), a significant THz gain (approximately 300/m) is predicted. It is also shown that the coherent wave interactions (resonant stimulated Raman processes) contribute significantly to the THz gain.

Liu, Ansheng↗
NASA NTRS2010

Free-Free Transitions in the Presence of Laser Fields at Very Low Incident Electron Energy

We study the free-free transition in electron-hydrogenic systems in ground state in presence of an external laser field at very loud incident energies. The laser field is treated classically while the collision dynamics is treated quantum mechanically. The laser field is chosen to be monochromatic, linearly polarized and homogeneous. The incident electron is considered to be dressed by the laser in a nonperturbative manner by choosing a Volkov wave function for it. The scattering weave function for the electron is solved numerically by taking into account the effect of the electron exchange, short-range as well as of the long-range interactions to get the S and P wave phase shifts while for the higher angular momentum phase shifts the exchange approximation has only been considered. We calculate the laser assisted differential cross sections (LADCS) for the aforesaid free-free transition process for single photon absorption/emission. The laser intensity is chosen to be much less than the atomic field intensity. A strong suppression is noted in the LADCS as compared to the field free (FF) cross sections. Unlike the FF ones, the LADCS exhibit some oscillations having a distinct maximum at a low value of the scattering angle depending on the laser parameters as well as on the incident energies.

Bhatia, A. K.↗
NASA NTRS2009

Free-Free Transitions in the Presence of Laser Fields at Very Low Incident Electron Energy

We study the free-free transition in electron-hydrogenic systems in ground state in presence of an external laser field at very low incident energies. The laser field is treated classically while the collision dynamics is treated quantum mechanically. The laser field is chosen to be monochromatic, linearly polarized and homogeneous. The incident electron is considered to be dressed by the laser in a nonperturbative manner by choosing a Volkov wave function for it The scattering wave function for the electron is solved numerically by taking into account the effect of the electron exchange, short-range as well as of the long-range interactions to get the S and P wave phase shifts while for the higher angular momentum phase shifts, the exchange approximation has only been considered. We calculate the laser-assisted differential cross sections (LADCS) for the aforesaid free-free transition process for single photon absorption/emission. The laser intensity is chosen to be much less than the atomic field intensity. A strong suppression is noted in the LADCS as compared to the field free (FF) cross sections. Unlike the FF ones, the LADCS exhibit some oscillations having a distinct maximum at a low value of the scattering angle depending on the laser parameters as well as on the incident energies.

Bhatia, Anand K.↗
NASA NTRS1983

Inclusion of discrete-to-continuum coupling in multiphoton excitation and dissociation calculations

A nonperturbative approach to multiphoton excitation and dissociation of molecules is presented in which coupling to the continuum is treated explicitly. Transitions among continuum levels are not modeled directly, but something of their effect is represented by assuming that the continuum population density is so low as to be effectively zero at all times. Two trial applications are briefly discussed.

Zimmerman, I. H.↗
NASA NTRS2012

Free-Free Transitions in the Presence of Laser Fields and Debye Potential at Very Low Incident Electron Energies

We study the free-free transition in electron-helium ion in the ground state and embedded in a Debye potential in the presence of an external laser field at very low incident electron energies. The laser field is treated classically while the collision dynamics is treated quantum mechanically. The laser field is chosen as monochromatic, linearly polarized and homogeneous. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing Volkov wave function for it. The scattering wave function for the incident electron on the target embedded in a Debye potential is solved numerically by taking into account the effect of electron exchange. We calculate the laser-assisted differential and total cross sections for free-free transition for absorption/emission of a single photon or no photon exchange. The results will be presented at the conference.

Bhatia, Anand↗
NASA NTRS2012

Free-Free Transitions of the e-H System Inside a Dense Plasma Irradiated by a Laser Field at Very Low Incident-Electron Energies

The free-free transition is studied for an electron-hydrogen atom in ground state when a low-energy electron (external) is injected into hydrogenic plasma in the presence of an external homogenous, monochromatic, and linearly polarized laser field. The effect of plasma screening is considered in the Debye-Huckel approximation. The calculations are performed in the soft photon limit. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the electron exchange. The laser-assisted differential and total cross sections are calculated for single-photon absorption or emission and no-photon exchange in the soft photon limit, the laser intensity being much less than the atomic field intensity. The calculations have been carried out for various values of Debye parameter, ranging from 0.005 to 0.12. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situation. A significant difference is noted for the singlet and triplet cross sections. The suppression is much more in the triplet states.

Bhatia, A. K.↗
NASA NTRS2012

Free-Free Transitions of e-H System Inside a Dense Plasma Irradiated by a Laser Field at Very Low Incident Electron Energies

The free-free transition is studied for an electron-hydrogen in the ground state at low incident energies in the presence of an external homogenous, monochromatic, and linearly polarized laser-field inside a hot dense plasma.The effect of plasma screening is considered in the Debye-Huckel approximation. The calculations are performed in the soft photon limit, assuming that the plasma frequency is much higher than the laser frequency. The incident electron is considered to be dressed by the laser field in a nonperturbative manner by choosing the Volkov solutions in both the initial and final channels. The space part of the scattering wave function for the electron is solved numerically by taking into account the electron exchange. The laser-assisted differential and total cross sections are calculated for single-photon absorption /emission and no photon exchange in the soft photon limit, the laser intensity being much less than the atomic field intensity. The calculations have been carried out for various values of Debye parameter, ranging from 0.005 to 0.12. A strong suppression is noted in the laser-assisted cross sections as compared to the field-free situation. A significant difference is noted for the singlet and triplet cross sections. The suppression is much more in the triplet states.

Bhatia, A. K.↗
NASA NTRS1989

Enhanced resolution particle size distributions by multiple angle photon correlation spectroscopy

Photon correlation spectroscopy (PCS) has become a method of choice for measuring submicrometer particles. It is capable of rapid, accurate measurements of mean particle size. Since the measurements are nonperturbing, it is ideal for monitoring systems undergoing dynamic changes. Despite its widespread acceptance, the information content of PCS measurements for particle size distributions is low and provides limited resolution. A method is presented whereby PCS measurements made at several scattering angles plus the angular distribution of light scattered from the particles are combined in a single simultaneous analysis to effect an enhanced resolution particle size distribution. The efficacy of the method is assessed by recovering size distributions from computer simulated data and by comparisons of conventional PCS measurements of polystyrene spheres with those made by the new method.

Bott, Steven E.↗
NASA NTRS1988

On the choice of the electron-electron potential in relativistic atomic physics

In the calculation of relativistic effects in the structure of many-electron atoms there are two popular choice for the operator V used to represent the electron-electron interaction. One of these, V(I), is associated with the use of the Coulomb gauge propagator for photons; the other, V(II), is associated with the use of the Feynman gauge propagator. In contrast to V(I), the nonperturbative use of V(II) leads to energy levels which are already incorrect in order (alpha exp 4)m. This fact provides a quantitative argument for preferring V(I) to V(II).

Sucher, J.↗
NASA NTRS1991

Line-frequency measurements and analysis of N2O between 900 and 4700/cm

The N2O infrared spectrum from 900 to 4700/cm has been studied with a high-resolution Fourier-transform spectrometer. Measurements were made of line frequencies of several N2O isotopes for several ground-state and hot bands. These bands include the 1530-0330, 2330-0330, 0441-0440, 0441-0330, 0531-0330, and 1331-0330 bands of (N-14)2(O-16). Also, perturbation-enhanced transitions in the 0710-0110 and 0730-0110 bands were assigned and measured. The frequencies have been analyzed to obtain a unique set of effective vibration-rotation parameters for each vibrational state. The results obtained from this research are of high accuracy for the ground-state, and the first-excited-state bands of (N-14)2(O-16), in which the computed frequencies derived from the least-squares fits, are known to an absolute uncertainty of only +/-4 x 10 exp -5 for nonperturbed transitions.

Toth, Robert A.↗
NASA NTRS1999

Terahertz Optical Gain Based on Intersubband Transitions in Optically-Pumped Semiconductor Quantum Wells: Coherent Pumped-Probe Interactions

Terahertz optical gain due to intersubband transitions in optically-pumped semiconductor quantum wells (QW's) is calculated nonperturbatively. We solve the pump- field-induced nonequilibrium distribution function for each subband of the QW system from a set of rate equations that include both intrasubband and intersubband relaxation processes. The gain arising from population inversion and stimulated Raman processes is calculated in a unified manner. We show that the coherent pump and signal wave interactions contribute significantly to the THz gain. Because of the optical Stark effect and pump-induced population redistribution, optical gain saturation at larger pump intensities is predicted.

Liu, Ansheng↗
NASA NTRS1978

Semiclassical theory of unimolecular dissociation induced by a laser field

A semiclassical nonperturbative theory of direct photodissociation in a laser field is developed in which photon absorption and dissociation are treated in a unified fashion. This is achieved by visualizing nuclear dynamics as a representative particle moving on electronic-field surfaces. Methods are described for calculating dissociation rates and probabilities by Monte Carlo selection of initial conditions and integration of classical trajectories on these surfaces. This unified theory reduces to the golden rule expression in the weak-field and short-time limits, and predicts nonlinear behavior, i.e., breakdown of the golden rule expression in intense fields. Field strengths above which lowest-order perturbation theory fails to work have been estimated for some systems. Useful physical insights provided by the electronic-field representation have been illustrated. Intense field effects are discussed which are amenable to experimental observation. The semiclassical methods used here are also applicable to multiple-surface dynamics in fieldfree unimolecular and bimolecular reactions.

Yuan, J.-M.↗