288 research outputs found
Ionization in a laser assisted ion-ion collision
The ionization of a hydrogen-like heavy ion by impact of a charged projectile
under simultaneous irradiation by a short laser pulse is investigated within
the non-perturbative approach, based on numerical solutions of the
time-dependent Dirac equation. Special emphasis is placed on the question of
whether the laser- and impact-ionization channels interfere with each other,
and how this intereference affects the ionization probability. To answer this
question we performed detailed calculations for the laser-assisted collisions
between hydrogen-like and alpha particles. The results of the
calculations clearly indicate that for the experimentally relevant set of
(collision and laser) parameters, the interference contribution can reach 10%
and can be easily controlled by varying the laser frequency.Comment: 9 pages, 10 figure
K-shell ionization of heavy hydrogen-like ions
A theoretical study of the K-shell ionization of hydrogen-like ions,
colliding with bare nuclei, is performed within the framework of the
time-dependent Dirac equation. Special emphasis is placed on the ionization
probability that is investigated as a function of impact parameter, collision
energy and nuclear charge. To evaluate this probability in a wide range of
collisional parameters we propose a simple analytical expression for the
transition amplitude. This expression contains three fitting parameters that
are determined from the numerical calculations, based on the adiabatic
approximation. In contrast to previous studies, our analytical expression for
the transition amplitude and ionization probability accounts for the full
multipole expansion of the two-center potential and allows accurate description
of nonsymmetric collisions of nuclei with different atomic numbers, . The calculations performed for both symmetric and asymmetric collisions
indicate that the ionization probability is reduced when the difference between
the atomic numbers of ions increases.Comment: 8 pages, 6 figure
Fluctuating Nonlinear Spring Model of Mechanical Deformation of Biological Particles
We present a new theory for modeling forced indentation spectral lineshapes
of biological particles, which considers non-linear Hertzian deformation due to
an indenter-particle physical contact and bending deformations of curved beams
modeling the particle structure. The bending of beams beyond the critical point
triggers the particle dynamic transition to the collapsed state, an extreme
event leading to the catastrophic force drop as observed in the force
(F)-deformation (X) spectra. The theory interprets fine features of the
spectra: the slope of the FX curves and the position of force-peak signal, in
terms of mechanical characteristics --- the Young's moduli for Hertzian and
bending deformations E_H and E_b, and the probability distribution of the
maximum strength with the strength of the strongest beam F_b^* and the beams'
failure rate m. The theory is applied to successfully characterize the
curves for spherical virus particles --- CCMV, TrV, and AdV
Tubulin bond energies and microtubule biomechanics determined from nanoindentation in silico
Microtubules, the primary components of the chromosome segregation machinery,
are stabilized by longitudinal and lateral non-covalent bonds between the
tubulin subunits. However, the thermodynamics of these bonds and the
microtubule physico-chemical properties are poorly understood. Here, we explore
the biomechanics of microtubule polymers using multiscale computational
modeling and nanoindentations in silico of a contiguous microtubule fragment. A
close match between the simulated and experimental force-deformation spectra
enabled us to correlate the microtubule biomechanics with dynamic structural
transitions at the nanoscale. Our mechanical testing revealed that the
compressed MT behaves as a system of rigid elements interconnected through a
network of lateral and longitudinal elastic bonds. The initial regime of
continuous elastic deformation of the microtubule is followed by the transition
regime, during which the microtubule lattice undergoes discrete structural
changes, which include first the reversible dissociation of lateral bonds
followed by irreversible dissociation of the longitudinal bonds. We have
determined the free energies of dissociation of the lateral (6.9+/-0.4
kcal/mol) and longitudinal (14.9+/-1.5 kcal/mol) tubulin-tubulin bonds. These
values in conjunction with the large flexural rigidity of tubulin
protofilaments obtained (18,000-26,000 pN*nm^2), support the idea that the
disassembling microtubule is capable of generating a large mechanical force to
move chromosomes during cell division. Our computational modeling offers a
comprehensive quantitative platform to link molecular tubulin characteristics
with the physiological behavior of microtubules. The developed in silico
nanoindentation method provides a powerful tool for the exploration of
biomechanical properties of other cytoskeletal and multiprotein assemblie
- …