Manohar Awasthi

Non-perturbative solution of the time-dependent Schrödinger equation describing H2 in intense short laser pulses

A method for solving the time-dependent Schrodinger equation describing the electronic motion of molecular hydrogen exposed to very short intense laser pulses has been developed. The fully correlated three-dimensional time-dependent electronic wavefunction is expressed in terms of field-free wavefunctions. These are obtained from a configuration-interaction calculation where the one-electron basis functions are built from B splines. The reliability of the method is tested by comparing results in the low-intensity regime to the prediction of lowest order perturbation theory. The onset of non-perturbative effects is shown for higher intensities and the validity of the single-active electron approximation is briefly discussed. Finally, the ability of the method to calculate photoelectron spectra including above-threshold-ionization peaks is demonstrated.

Internuclear-distance dependence of ionization of H2 in strong laser fields

The ionization yield of molecular hydrogen exposed to intense laser pulses is studied as a function of the internuclear distance R. The results obtained by means of the full solution of the three-dimensional time-dependent Schrodinger equation (TDSE) describing electronic motion are compared with those predicted by the quasi-static approximation (QSA). It is found that for laser pulses with a wavelength of 800 nm and peak intensities between 3.5 × 1013 W cm−2 and 1 × 1014 W cm−2 the QSA predicts an R dependence that is in qualitative agreement with the TDSE results, provided a vertical ionization potential is used in the QSA model. The quantitative agreement depends, however, strongly on the laser intensity. While the yields for 800 nm vary smoothly with R, this is not the case for 266 nm (and 3.5 × 1013 W cm−2). As expected, for these parameters the ionization dynamics is better described as a multi-photon process; it is strongly influenced by channel closing and the appearance of resonantly enhanced multi-photon ionization.

Photoionization of the alkali dimer cations Li+2, Na+2 and LiNa+

Photoionization cross sections for the three alkali dimer cations (Li+2, Na+2 and LiNa+) were calculated at the equilibrium internuclear distance for parallel, perpendicular and isotropic orientations of the molecular axis with respect to the field. A model-potential method was used for the description of the cores. The influence of the model-potential parameters on the photoionization spectra was investigated. Two different methods, a time-independent and a time-dependent one, were implemented and used for computing the cross sections.

Light and thermally induced metastabilities in electrochemically etched nanocrystalline porous silicon

We observe that light soaking for short durations and thermal quenching in nanocrystalline porous silicon (PS) produce metastable states. These metastable states show higher dark and photo currents, large photoluminescence and a weaker electron spin resonance (ESR) signal. However, long exposures to light produce the opposite effect. The metastable states are stable against sub-band gap light exposures. These metastable states can be removed by annealing at 150°C for 1 h. ESR shows the presence of a-Si phase (g ∼ 2.0058, 6.4 G) in PS sample, but it is not sufficient to explain all the experimental results. Rather, our experiments suggest that light soaking causes more than one type of defects in porous silicon. The structural changes involving the movement of hydrogen present at the surface of PS or at the PS/a-Si interface may be responsible for these effects.

Single-active-electron approximation for molecules in strong laser fields : Test application to H2

A new method for solving the electronic three-dimensional time-dependent Schrodinger equation (TDSE) for molecules in ultrashort intense laser fields was developed. In this method the molecules are described within the single-active-electron (SAE) approximation using density-functional theory (DFT). The method and its implementation is tested for H2 for which a full six-dimensional two-electron solution is obtained via a time-dependent configuration-interaction approach. For larger molecular systems (for which no full solution is possible) the novel SAE method can, e.g., be used to test the validity of simplified SAE-based models like the molecular strong-field approximation (MO-SFA) or the molecular-orbital Ammosov-Delone-Krainov (MO-ADK) model.