PubMed Health⌕ Search

Biomedical subjects

Chris H Greene

Publications and source records attributed to Chris H Greene.

18 recordsLinked to original sources
PubMed2006

Theoretical progress and challenges in H3+ dissociative recombination.

We discuss the Jahn-Teller mechanism for dissociative recombination in low energy collisions between electrons and H3+ ions, in energy ranges relevant to the processes underway in interstellar clouds. While theory has become capable of predicting recombination rates in reasonable agreement with storage ring experiments, some discrepancies remain with them, and a long-standing discrepancy with stationary afterglow measurements remains troubling. Speculations about the desirable improvements in both theory and experiment are presented.

Journal Article↗
PubMed2006

Radiation damage to DNA: electron scattering from the backbone subunits.

In the context of damage to DNA by low energy electrons, we carry out calculations of electron scattering from tetrahydrofuran and phosphoric acid, models of the subunits in the DNA backbone, as a first step in simulating the electron capture process that occurs in the cell. In the case of tetrahydrofuran, we also compare with previous theoretical and experimental data. A comparison of the shape of the resonant structures to virtual orbitals is also performed to gain insight into the systematic connections with electron scattering from similar molecules and dissociative electron attachment experiments.

DNA Damage↗
PubMed2006

Low-energy electron scattering from DNA and RNA bases: shape resonances and radiation damage.

Calculations are carried out to determine elastic-scattering cross sections and resonance energies for low-energy electron impact on uracil and on each of the DNA bases (thymine, cytosine, adenine, and guanine), for isolated molecules in their equilibrium geometry. Our calculations are compared with the available theory and experiment. We also attempt to correlate this information with experimental dissociation patterns through an analysis of the temporary anion structures that are formed by electron capture in shape resonances.

Biophysics↗
PubMed2005

High-accuracy optical clock via three-level coherence in neutral bosonic 88Sr.

An optical atomic clock scheme is proposed that utilizes two lasers to establish coherent coupling between the 5s2 1S0 ground state of 88Sr and the first excited state, 5s5p 3P0. The coupling is mediated by the broad 5s5p 1P1 state, exploiting the phenomenon of electromagnetically induced transparency. The effective linewidth of the clock transition can be chosen at will by adjusting the laser intensity. By trapping the 88Sr atoms in an optical lattice, long interaction times with the two lasers are ensured; Doppler and recoil effects are eliminated. Based on a careful analysis of systematic errors, a clock accuracy of better than 2 x 10(-17) is expected.

Journal Article↗
PubMed2005

Electron-molecule scattering calculations in a 3D finite element R-matrix approach.

We have implemented a three-dimensional finite element approach, based on tricubic polynomials in spherical coordinates, which solves the Schrodinger equation for scattering of a low energy electron from a molecule, approximating the electron exchange as a local potential. The potential is treated as a sum of three terms: electrostatic, exchange, and polarization. The electrostatic term can be extracted directly from ab initio codes (GAUSSIAN 98 in the work described here), while the exchange term is approximated using different local density functionals. A local polarization potential approximately describes the long range attraction to the molecular target induced by the scattering electron.

Journal Article↗
PubMed2004

Triatomic dissociative recombination theory: Jahn-Teller coupling among infinitely many Born-Oppenheimer surfaces.

The coupling of many Rydberg surfaces poses a stringent challenge for theoretical description. Yet simplifications arise because quantum defect ideas can be applied, which circumvents some of the usual difficulties that arise. We describe a theoretical technique capable of handling these complex interactions, which has already been applied with success to the dynamics of H3 Rydberg states. The extensions necessary to treat the other isotopomers H2D+ and HD2+ are then discussed, along with a preliminary application.

Journal Article↗
PubMed2003

Xenon clusters in intense VUV laser fields.

A simple model is developed that quantitatively describes intense interactions of a vacuum ultraviolet (VUV) laser pulse with a xenon cluster. We find good agreement with a recent experiment [Nature (London) 420, 482 (2002)]]. In particular, the large number of VUV photons absorbed per atom, at intensities significantly below 10(16) W/cm(2), is now understood.

Journal Article↗
PubMed2003

Atom-molecule laser fed by stimulated three-body recombination.

Using three-body recombination as the underlying process, we propose a method of coherently driving an atomic Bose-Einstein condensate (BEC) into a molecular BEC. Superradiantlike stimulation favors atom-to-molecule transitions when two atomic BECs collide at a resonant kinetic energy, the result being two molecular BEC clouds moving with well-defined velocities. Potential applications include the construction of a molecule laser.

Journal Article↗
PubMed2003

Multichannel cold collisions between metastable Sr atoms.

We present a multichannel-scattering calculation of elastic and inelastic cold collisions between two low-field seeking, metastable 88Sr [(5s5p)3P2] atoms in the presence of an external magnetic field. The scattering physics is governed by strong anisotropic long-range interactions, which lead to pronounced coupling among the partial waves of relative motion. As a result, nonadiabatic transitions are shown to trigger a high rate of inelastic losses. At relatively high energies, T>100 microK, the total inelastic collision rate is comparable with the elastic rate. However, at lower collisional energy, the elastic rate decreases, and at T approximately 1 microK, it becomes substantially smaller than the inelastic rate. Our study suggests that magnetic trapping and evaporative cooling of 88Sr [(5s5p)3P2] atoms, as well as 40Ca [(4s4p)3P2], in low-field seeking states will prove difficult to achieve experimentally.

Journal Article↗
PubMed2003

Single-stage sub-Doppler cooling of alkaline earth atoms.

We report the first experimental study of sub-Doppler cooling in alkaline earth atoms (87Sr) enabled by the presence of nuclear spin-originated magnetic degeneracy in the atomic ground state. Sub-Doppler cooling in a sigma(+)-sigma(-) configuration is achieved despite the presence of multiple, closely spaced excited states. This surprising result is confirmed by an expanded multilevel theory of the radiative cooling force. Detailed investigations of system performance have shed new insights into (sigma(+)-sigma(-)) cooling dynamics and will likely play an important role in the future development of neutral atom-based optical frequency standards.

Journal Article↗
PubMed2003

Theory of dissociative recombination of d(3h) triatomic ions applied to h+3.

We propose a fully quantal description of H+3 dissociative recombination. The new method combines multichannel quantum defect theory, the adiabatic hyperspherical approach, and the techniques of outgoing-wave Siegert pseudostates. The vibrational, rotational, nuclear spin degrees of freedom of the ion, in their full dimensionality, are all taken into account for the first time in any calculation of polyatomic dissociative recombination. Our calculation of the recombination rate confirms that the Jahn-Teller effect is responsible for the large rate in H+3. The resulting theoretical rates for dissociative recombination of H+3 are in good general agreement with experiment.

Journal Article↗
PubMed2003

Recombination of three ultracold fermionic atoms.

Three-body recombination of identical, spin-polarized fermionic atoms in the ultracold limit is investigated using model interactions. The mechanisms for recombination are parametrized by the "scattering volume" V(p) and described in the framework of the adiabatic hyperspherical representation. We have calculated the recombination rate K3 as a function of V(p) and have found K3 proportional, variant |V(p)|(8/3) for small |V(p)|. Recombination near a two-body Feshbach resonance can thus be significant.

Journal Article↗
PubMed2003

Nonclassical paths in the recurrence spectrum of diamagnetic atoms.

Using time-independent scattering matrices, we study how the effects of nonclassical paths on the recurrence spectra of diamagnetic atoms can be extracted from purely quantal calculations. This study reveals an intimate relationship between two types of nonclassical paths: exotic ghost orbits and diffractive orbits. This relationship proves to be a previously unrecognized reason for the success of semiclassical theories, such as closed-orbit theory, and permits a comprehensive reformulation of the semiclassical theory that elucidates its convergence properties.

Journal Article↗
PubMed2002

Competition among molecular fragmentation channels described with Siegert channel pseudostates.

To describe multiple interacting fragmentation continua, we develop a method in which the vibrational channel functions obey outgoing wave Siegert boundary conditions. This paper demonstrates the utility of the Siegert approach, which uses channel energy eigenvalues that possess a negative imaginary part. The electron scattering energy in each such channel is rotated upward, giving it an equal and opposite imaginary part. This permits a natural inclusion of vibrational continua without requiring them to appear as explicit channels in the scattering matrix. Calculations illustrate the application of this theory to photoionization, photodissociation, and dissociative recombination.

Journal Article↗
PubMed2002

Formation of atomic tritium clusters and bose-einstein condensates.

We present an extensive study of the static and dynamic properties of systems of spin-polarized tritium atoms. In particular, we calculate the two-body |F,m(F)>=|0,0> s-wave scattering length and show that it can be manipulated via a Feshbach resonance at a field strength of about 870 G. Such a resonance might be exploited to make and control a Bose-Einstein condensate of tritium in the |0,0> state. It is further shown that the quartet tritium trimer is the only bound hydrogen isotope and that its single vibrational bound state is a Borromean state. The ground state properties of larger spin-polarized tritium clusters are also presented and compared with those of helium clusters.

Journal Article↗
PubMed2001

Phase modulation of ultrashort light pulses using molecular rotational wave packets.

We demonstrate experimentally how the time-dependent phase modulation induced by molecular rotational wave packets can manipulate the phase and spectral content of ultrashort light pulses. Using impulsively excited rotational wave packets in CO2, we increase the bandwidth of a probe pulse by a factor of 9, while inducing a negative chirp. This chirp is removed by propagation through a fused silica window, without the use of a pulse compressor. This is a very general technique for optical phase modulation that can be applied over a broad spectral region from the IR to the UV.

Journal Article↗