Quantum manifestations of scattering orbits in the scaled spectrum of a non-hydrogenic atom in crossed fields

Evidence for scattering closed orbits for the Rydberg electron of the singly excited helium atom in crossed electric and magnetic fields at constant scaled energy and constant scaled electric field strength has been found through a quantum calculation of the photo-excitation spectrum. A particular 3D scattering orbit in a mixed regular and chaotic region has been investigated and the hydrogenic 3D closed orbits composing it identified. To the best of our knowledge, this letter reports the first quantum calculation of the scaled spectrum of a non- hydrogenic atom in crossed fields.

Quantum manifestations of closed orbits in the photoexcitation scaled spectrum of the hydrogen atom in crossed fields

The scaled photoexcitation spectrum of the hydrogen atom in crossed electric and magnetic fields has been obtained by means of accurate quantum mechanical calculation using a new algorithm. Closed orbits in the corresponding classical system have also been obtained, using a new, efficient and practical searching procedure. Two new classes of closed orbit have been identified. Fourier transforming each photoexcitation quantum spectrum to yield a plot against scaled action has allowed direct comparison between peaks in such plots and the scaled action values of closed orbits, Excellent agreement has been found with all peaks assigned.

The closed orbits and the photo-excitation scaled spectrum of the hydrogen atom in crossed fields

In a recent Letter to the Editor (J Rao, D Delande and K T Taylor 2001 J. Phys. B: At. Mol. Opt. Phys. 34 L391-9) we made a brief first report of our quantal and classical calculations for the hydrogen atom in crossed electric and magnetic fields at constant scaled energy and constant scaled electric field strength. A principal point of that communication was our statement that each and every peak in the Fourier transform of the scaled quantum photo-excitation spectrum for scaled energy value epsilon = -0.586 538 871028 43 and scaled electric value (f) over tilde = 0.068 537 846 207 618 71 could be identified with a scaled action value of a found and mapped-out closed orbit up to a scaled action of 20. In this follow-up paper, besides presenting full details of our quantum and classical methods, we set out the scaled action values of all 317 closed orbits involved, together with the geometries of many.

High-energy cutoff in the spectrum of strong-field nonsequential double ionization

Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0.8 PW/cm(2) and 1.1 PW/cm(2), where PW equivalent to 10(15) W/cm(2)). Numerical solutions of the full-dimensional time-dependent helium Schrodinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U(p) cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.

Molecular dynamics studies of trinucleotide repeat DNA involved in neurodegenerative disorders.

Expansion of trinucleotide repeat DNA of the classes CAG�·CTG, CGG�·CCG and GAA�·TTC are found to be associated with several neurodegenerative disorders. Different mechanisms have been attributed to the expansion of triplets, mainly involving the formation of alternate secondary structures by such repeats. This paper reports the molecular dynamics simulation of triplet repeat DNA sequences to study the basic structural features of DNA that are responsible for the formation of structures such as hairpins and slip-strand DNA leading to expansion. All the triplet repeat sequences studied were found to be more flexible compared to the control sequence unassociated with disease. Moreover, flexibility was found to be in the order CAG�·CTG > CGG�·CCG = GAA�·TTC, the highly flexible CAG�·CTG repeat being the most common cause of neurodegenerative disorders. In another simulation, a single G�·C to T�·A mutation at the 9th position of the CAG�·CTG repeat exhibited a reduction in bending compared to the pure 15-mer CAGâ�¢CTG repeat. EPM1 dodecamer repeat associated with the pathogenesis of progressive myoclonus epilepsy was also simulated and showed flexible nature suggesting a similar expansion mechanism.