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K L Sebastian

Publications and source records attributed to K L Sebastian.

8 recordsLinked to original sources
PubMed2006

Rate processes with dynamical disorder: a direct variational approach.

Using path integral approach, we develop variational approximations to the calculation of survival probability for rate processes with dynamical disorder. We derive both upper and lower bounds to the survival probability using Jensen's inequality. The inequalities involve the use of a trial action for which the path integrals can be evaluated exactly. Any parameter in the trial action can be varied to optimize the bounds. We have also derived a lower bound to the rate of the process. As a simple illustration, we apply the method to the problem of a particle undergoing Brownian motion in a harmonic potential well, in the presence of a delta function sink, for which one can calculate the exact survival probability numerically. The calculation confirms the two inequalities. The method should be very useful in similar but more complex problems where even numerical solution is not possible.

Algorithms↗
PubMed2006

Opening of a weak link in a semiflexible ring polymer.

The dynamics of contact formation between different parts of a long chain molecule is of considerable interest in biology. The related processes of opening of a loop or closing to form a loop also are of considerable interest and have attracted the attention of experimentalists/theorists. For closing, results are available in the completely flexible limit. However, this limit is not realized in many cases. Recently, there have been investigations for the semiflexible case too. We develop an approach, which leads to an easy description of the dynamics, incorporating semiflexibility rigorously into account. With this approach, the dynamics of a semiflexible polymer ring formed by a weak bond between the two ends can be modeled as the escape of a particle over a barrier in a multidimensional potential energy surface. We then calculate the rate of opening using a multidimensional transition state theory. Effects of friction on the rate are also taken into account using the standard coupling to a bath of harmonic oscillators. We find that for shorter chains (i.e., semiflexible), the rate of opening is strongly length dependent and is well described by the equation A(L/lp)v exp(Blp/L), with L as the length, and lp as the persistence length, A, B as the constants, and v approximately 1.2.

Biopolymers↗
PubMed2006

Theory of photoinduced heterogeneous electron transfer.

We consider electron injection into the conduction band of a semiconductor, from an electronically excited state of a dye molecule, adsorbed on its surface. For arbitrary width of the conduction band, the survival probability of the excited state can be calculated using a Green's-function approach. We show that the existence of a split-off state can play an important role in the total injection probability. In the wide band limit, the survival probability decays exponentially, but for finite band widths it does not. We further investigate the effect of vibrations on the process. A Green's operator technique may be used to solve this too exactly. We show that the problem may be reduced to a non-Hermitian eigenvalue problem for the vibrational states alone. Exact results can be obtained for arbitrary bandwidth and for a few vibrational degrees of freedom. In the wide band limit, the dynamics is particularly simple and we find that (1) the survival probability of the excited state is unchanged by the inclusion of vibrational motion, but (2) each vibrational state now has a finite lifetime. Numerical results are presented for the effects of reorganization energy, energy of the injecting level, and the variation of the matrix element for the electron injection, on the survival probability of the electron in the excited state. As an illustration of the approach, we also present results of numerical calculation of the absorption spectrum of perylene adsorbed on TiO(2) and compare it with experimental results.

Journal Article↗
PubMed2005

The dynamics of solvation of an electron in the image potential state by a layer of polar adsorbates.

Recently, ultrafast two-photon photoemission has been used to study electron solvation at a two-dimensional metalpolar adsorbate interfaces [A. Miller et al., Science 297, 1163 (2002)]. The electron is bound to the surface by the image interaction. Earlier we have suggested a theoretical description of the states of the electron interacting with a two-dimensional layer of the polar adsorbate [K. L. Sebastian et al., J. Chem. Phys. 119, 10350 (2003)]. In this paper we have analyzed the dynamics of electron solvation, assuming a trial wave function for the electron and the solvent polarization and then using the Dirac-Frenkel variational method to determine it. The electron is initially photoexcited to a delocalized state, which has a finite but large size, and causes the polar molecules to reorient. This reorientation acts back on the electron and causes its wave function to shrink, which will cause further reorientation of the polar molecules, and the process continues until the electron gets self-trapped. For reasonable values for the parameters, we are able to obtain fair agreement with the experimental observations.

Journal Article↗
PubMed2000

Pulling a polymer out of a potential well and the mechanical unzipping of DNA.

Motivated by experiments on DNA under torsion, we consider the problem of pulling a polymer out of a potential well by a force applied to one of its ends. If the force is less than a critical value, then the process is activated, and has an activation energy proportional to the length of the chain. Above this critical value, the process is barrierless and will occur spontaneously. We use the Rouse model for a description of the dynamics of the peeling out, and study the average behavior of the chain by replacing the random noise by its mean. The resultant mean-field equation is a nonlinear diffusion equation, and hence rather difficult to analyze. We use physical arguments to convert this to a moving boundary value problem, which can then be solved exactly. The result is that the time t(po) required to pull out a polymer of N segments scales like N2. For models other than the Rouse model, we argue that t(po) approximately N1+nu.

DNA↗
PubMedDate not supplied

The hypolipidemic effect of onion (Allium cepa Linn) in sucrose fed rabbits.

Long term administration of sucrose significantly increased triglyceride levels in normal rabbits. Effects of an aqueous extract of onion on the sucrose fed rabbits have been investigated. Administration of onion extract significantly reduced serum, liver and aorta triglycerides and serum and liver proteins. On the other hand liver free amino acids have been significantly increased in the onion treated group as compared to the sucrose fed control. The effects of onion have been ascribed to its sulfur containing principles which oxidize thiol compounds either present free or combined in a protein and NADPH which are necessary for lipid synthesis.

Amino Acids↗