Study of Collinear Collision in Impulsive Model

Exact quantum solution of the collinear collision problem is found in the impulsive model. The transition probabilities compare well with those for real systems, except when multiple collisions become important

Analytic pulse design for selective population transfer in many-level quantum systems: maximizing amplitude of population oscillations

State selective preparation and manipulation of discrete-level quantum systems such as atoms, molecules or quantum dots is a the ultimate tool for many diverse fields such as laser control of chemical reactions, atom optics, high-precision metrology and quantum computing. Rabi oscillations are one of the simplest, yet potentially quite useful mechanisms for achieving such manipulation. Rabi theory establishes that in the two-level systems resonant drive leads to the periodic and complete population oscillations between the two system levels. In this paper an analytic optimization algorithm for producing Rabi-like oscillations in the general discrete many-level quantum systems is presented.Comment: Published in Phys.Rev.A. This is the final published versio

Long Lived States in Atom-Molecule Collisions

Understanding of the processes in molecular collisions is of great importance for many areas of research. In chemistry and molecular astrophysics, there is great interest in calculating reaction rates. For elementary particle and nuclear physics, it is of interest to find fundamentally new concepts underlying the mechanism of collisions. And for some, the molecular collision problems are interesting on their own. The most elementary process that is usually studied is a binary collision. If this encounter is brief, we talk about direct collisions. However, if during an encounter, the species are stuck to each other for a long time, we talk about collisions mediated by a long lived state. The theory of collisions mediated by long lived states is very complex and only few basic principles were given in quantum theory; a central place being given to resonances. However, as it will be shown, the theory of such collisions is far from being completed and, in fact, is in its infancy. It is the purpose of this paper to review what is known about the subject and locate the problems

Resolution of the Klein Paradox

We present a resolution of the Klein paradox within the framework of one-particle relativistic quantum mechanics. Not only reflection becomes total but the vacuum remains neutral as well. This is accomplished by replacing the pair production process with virtual negative energy "incidence" within the barrier in a similar manner to what is done with image charges in electrostatic and virtual sources in optics.Comment: 9 pages, 8 figure

Low-energy Antiproton Interaction with Helium

An ab initio potential for the interaction of the neutral helium atom with antiprotons and protons is calculated using the Born-Oppenheimer approximation. Using this potential, the annihilation cross section for antiprotons in the energy range 0.01 microvolt to 1 eV is calculated.Comment: 13 pages, 7 figures, LaTe

Relativistic precession and spin dynamics of an elliptic Rydberg wave packet

Time evolution of wave packets built from the eigenstates of the Dirac equation for a hydrogenic system is considered. We investigate the space and spin motion of wave packets which, in the non-relativistic limit, are stationary states with a probability density distributed uniformly along the classical, elliptical orbit (elliptic WP). We show that the precession of such a WP, due to relativistic corrections to the energy eigenvalues, is strongly correlated with the spin motion. We show also that the motion is universal for all hydrogenic systems with an arbitrary value of the atomic number Z.Comment: Latex2e, uses IOP style files (included), 10 pages, 5 jpg figures, 1 postscript figure. Relation between precession time and radiative liftime added (eq.(12)). Accepted for publication in J. Phys.

Lipid Modifications of Sonic Hedgehog Ligand Dictate Cellular Reception and Signal Response

Sonic hedgehog (Shh) signaling regulates cell growth during embryonic development, tissue homeostasis and tumorigenesis. Concentration-dependent cellular responses to secreted Shh protein are essential for tissue patterning. Shh ligand is covalently modified by two lipid moieties, cholesterol and palmitate, and their hydrophobic properties are known to govern the cellular release and formation of soluble multimeric Shh complexes. However, the influences of the lipid moieties on cellular reception and signal response are not well understood.We analyzed fully lipidated Shh and mutant forms to eliminate one or both adducts in NIH3T3 mouse embryonic fibroblasts. Quantitative measurements of recombinant Shh protein concentration, cellular localization, and signaling potency were integrated to determine the contributions of each lipid adduct on ligand cellular localization and signaling potency. We demonstrate that lipid modification is required for cell reception, that either adduct is sufficient to confer cellular association, that the cholesterol adduct anchors ligand to the plasma membrane and that the palmitate adduct augments ligand internalization. We further show that signaling potency correlates directly with cellular concentration of Shh ligand.The findings of this study demonstrate that lipid modification of Shh determines cell concentration and potency, revealing complementary functions of hydrophobic modification in morphogen signaling by attenuating cellular release and augmenting reception of Shh protein in target tissues