Vinod Prasad

Intense field induced excitation and ionization of an atom confined in a dense quantum plasma

Exponential cosine screened Coulomb potential (ECSCP) has been widely used in various branches of physics e.g., solid-state physics, nuclear physics and plasma physics. The atomic photoionization processes under plasma shielding can serve as an efficient tool for study of plasma properties in various environments ranging from nano-scale devices to astrophysical objects. In the present study, ECSCP has been used to characterize a dense quantum plasma and its effect on the spectrum of an atom encaged in a spherical box has been investigated. The work has further been extended to study the response of such a system to a periodic laser field. Photoexcitation and ionization probabilities of the system have been studied as a function of applied laser field parameters using the non-perturbative Floquet technique. As the Floquet method requires exact energy values and oscillator strengths, the spectrum of confined system has been calculated using Bernstein-polynomial method. The variation of energy spectrum and oscillator strengths with screening as well as confinement parameters has also been explored.

Ionization of the H atom in ultrashort chirped laser pulses

The response of an atomic hydrogen system in the presence of an intense ultrashort laser pulse is investigated by direct integration of the time-dependent Schrodinger equation. The target wavefunctions are expanded in terms of an accurate and discrete L2 basis, which is optimized for convergence, making the approach more realistic. We have studied the variation of probability of ionization with laser pulse parameters such as the intensity, frequency, chirp rate and pulsewidth. The need for ultrashort pulses is highlighted by the results and we also found that chirped pulses provide a more efficient way of population transfer when compared with a non-chirped pulse.

Pulse shape effect of delayed pulse on non-adiabatic rotational excitation.

We examine the time evolution of Non-adiabatic excitation of polar molecule in static field exposed to a combination of delayed pulses. The delayed pulse pair consists of half cycle pulse (HCP) and an another delayed pulse (either ultrashort half cycle pulse or zero area pulse). We describe how Non-adiabatic rotational excitation (NAREX) due to Gaussian HCP pulse alone can be greatly modified by applying ultrashort HCP/zero area pulse. It is also shown that non-adiabatic rotational excitation can be controlled by various laser parameters, out of which pulse shape plays the most significant role for controlling the dynamics. Time dependent Schrödinger equation (TDSE) of NAREX dynamics, are studied using efficient fourth order Runge Kutta method.

Pulse train induced rotational excitation and orientation of a polar molecule

We investigate theoretically the rotational excitation and field free molecular orientation of polar HBr molecule, interacting with train of ultrashort laser pulses. By adjusting the number of pulses, pulse period and the intensity of the pulse, one can suppress a population while simultaneously enhancing the desired population in particular rotational state. We have used train of laser pulses of different shaped pulse envelopes. The dynamics and orientation of molecules in the presence of pulse train of different shapes is studied and explained.

Adsorbed molecules in external fields: Effect of confining potential

We study the rotational excitation of a molecule adsorbed on a surface. As is well known the interaction potential between the surface and the molecule can be modeled in number of ways, depending on the molecular structure and the geometry under which the molecule is being adsorbed by the surface. We explore the effect of change of confining potential on the excitation, which is largely controlled by the static electric fields and continuous wave laser fields. We focus on dipolar molecules and hence we restrict ourselves to the first order interaction in field-molecule interaction potential either through permanent dipole moment or/and the molecular polarizability parameter. It is shown that confining potential shapes, strength of the confinement, strongly affect the excitation. We compare our results for different confining potentials.

Dynamics of orientation of adsorbed polar molecules in static electric and laser field

The rotational states of adsorbed polar molecule in the presence of static electric and laser field are investigated. In this study, we have taken two types of polar molecules, namely, one with large dipole moment and moderate rotational constant (LiCl), and the other with moderate dipole moment and large rotational constant (HBr). The adsorbed molecule is considered as rigid rotor in finite conical potential well. The eigenvalues are calculated by numerically solving the Schrödinger equation. We have shown that various properties of rotational states of the molecules are significantly influenced by the static electric field, laser field, conical potential well height and the hindrance angle. Also the use of two different polar molecules for the investigation is justified by the variation in various properties due to change in molecule.

Controlling rotational dynamics and alignment of molecule by infrared laser pulse

We investigate the effects of delayed infrared laser (IRL) pulse shape on the non-adiabatic rotational excitation and alignment of a polar molecule. We suggest a control scheme for choosing populations of molecular rotational states by wave packet interference. The rotational wave packets of polar molecule (here HBr) excited non-adiabatically by orienting pulse is controlled actually using the second delayed IRL pulse. By adjusting the time delay between the two laser pulses and the shape of delayed IRL pulse, constructive or destructive interference among these wave packets enables the population to be enhanced or repressed for the specific rotational state. We have used fourth order Runge-Kutta method to study the non-adiabatic rotational excitation (NAREX) dynamics.

Laser induced vibration–rotation excitation of hydroxyl ions: OH− and OH+

The quantum dynamics of vibration–rotation excitation of hydroxyl ions (e.g., OH− and OH+ in the gaseous phase) in the presence of a laser beam is investigated. The nonperturbative Floquet method is used to solve the equation of motion with an explicitly time‐dependent Hamiltonian. Convergent results are obtained for the long‐time averaged transition probabilities for various transitions by including a sufficiently large number of rovibrational and photon states. In order to understand the roles played by power broadening, dynamic stark shifts, and non‐Lorentzian line shapes, higher laser intensities (in the GW/cm2 range) are considered in the molecular excitation process.

Asymmetric effects on the optical properties of double-quantum well systems

Abstract. Linear, nonlinear, and total absorption coefficient and refractive index changes of double-quantum well (DQW) systems are studied theoretically in the presence of external static electric field applied along the growth direction. The analytical expression for the linear and nonlinear optical properties is obtained using density matrix method. Emphasis is laid on the effect of asymmetry in the shapes of DQW system on optical properties. Some interesting results are obtained and explained.

The two-photon process in an atom using the pseudostate summation technique

An accurate pseudostate summation technique is used to evaluate the multiphoton absorption coefficient for an atom as a function of the frequency of incident photons. The effect of polarization of the incident radiation on the absorption coefficient is shown by comparing the results obtained for the linearly polarized light with those obtained for the circularly polarized light.