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Dynamics of a particle in confined-harmonic potential, subjected to external static electric and time-dependent laser fields is studied. The energy levels and wave functions of unperturbed harmonic oscillator are evaluated using B-polynomial Galerkin method. Matrix formulation is used throughout the procedure. This procedure is very simple and efficient in comparison with other methods. Modifications of wave functions and energy levels due to static electric field are also calculated. Finally, absorption spectra of such a driven oscillator are studied and explained.

The systems for which exact quantum mechanical solutions for Schrödinger equation can be found are few in number, for example, the harmonic oscillator potential and nonrelativistic hydrogen atom. The harmonic oscillator potential is a model of great practical importance, as it approximates any arbitrary potential close to equilibrium. In nanotechnology, potentials of simple shape such as quantum dots are often well approximated by such parabolic potentials. In fact, almost all exactly solvable problems in Quantum Mechanics are harmonic oscillator problems in disguise.

The confined-harmonic oscillator potential plays an important role in many applications of Quantum Mechanics. Such a potential is extensively used to describe the bound states of nonrelativistic systems. It also plays a basic role in chemical and molecular physics. In quantum chemistry, simple harmonic potential is used as a simplified model to describe vibrational motion of two atoms, where, more precise model is the Morse potential. In nonrelativistic quantum mechanics, the Schrödinger equation for this potential has been studied for systems ranging from 1-Dimensional to D-Dimensional Space [1-4]. Such a system has been widely studied as it can be exactly solved and is a very relevant system [

The perturbation of quantum harmonic oscillators with external fields has recently attracted a renewed interest due to different aspects of the problem, catalysed by recent developments as follows: 1) quantum dynamics of ion in a Paul trap [

The effects of external fields on systems under the effect of other types of potentials like pseudo-harmonic oscillator potential have also been explored in literature. For example, the effect on energy levels of a 2D Klein Gordon particle under pseudo-harmonic oscillator interaction has been studied [

Time-dependent perturbations of such systems have also been studied extensively [15,16]. Explicit wave functions and geometric phases of time-dependent harmonic oscillator in external time-dependent magnetic and electric field have been derived [

An electron in confined-harmonic oscillator potential exposed to an external electric field is equivalent to a charged harmonic oscillator in a uniform electric field or a harmonic oscillator in an external dipole field. Such a system has an important role in quantum chemical applications [

The traditional techniques of studying such quantum mechanical systems have lately been supplemented by finite basis set methods like B-spline [3,24-26] and Bernstein-polynomial (B-polynomial) methods [27-29]. Recently, Heidari et al. [

Polynomials are incredibly useful mathematical tools as they can be calculated very easily and accurately on computer systems. Their evaluation is also fast. They are capable of representing a tremendous variety of functions, can be differentiated and integrated quite easily, and can be pieced together to form spline curves that can approximate any function to any desired accuracy. The B-polynomial method is, therefore, much simpler and efficient. Recently, J. Liu et al. have proposed a new numerical method based on B-polynomials expansion for solving one dimensional elliptic interface problems [

In this paper, the dynamics of an electron in a confined-harmonic potential in static electric and strong laser fields is studied. We have used B-polynomial Galerkin method to solve static field modified harmonic oscillator system. The populations of states modified by static electric field are calculated. The eigenenergies, eigenfunctions and dipole matrix elements of the system are also calculated. The interaction of static field modified confined-harmonic oscillator system with the laser field is taken into account by non-perturbative quasi-energy technique [37-40]. The sequence of the paper is as follows. In Section 2, necessary description of B-polynomials is given. In Section 3, the model under consideration is defined and methods adopted for solving the time-independent as well as time-dependent Schrödinger equation are given. Section 4 deals with interpretation of results and finally, in Section 5, concluding remarks are made.

The B-polynomials [

for, where

There are, n-th degree B-polynomials. For mathematical convenience, we usually set if or. These B-polynomials of degree n form a complete basis over the interval. The B-polynomials can be generated by a recursive relation [

More details of these polynomials are available in literature [24,28,29,32,35,42].

The B-polynomial Galerkin method is employed to solve Schrödinger equation for the present case. In the area of numerical analysis, Galerkin methods are a class of methods for converting a problem such as a differential equation to a linear system of equations. A few of the related formulas used are mentioned here for reference.

(6) (7)

where the are expressed as

Consider an electron under the effect of a confined-harmonic oscillator potential subjected to an external static electric field, where is the strength of the electric field. The units used throughout are the atomic units, i.e.,. The confining potential is given by

where is a positive constant representing the strength of the potential called the force constant. If the electric dipole moment of the electron is denoted by, the potential energy of the electron due to electric field is given by. The electric dipole moment of the electron is given by, representing the position vector of the electron with respect to the origin and, the charge.

Assume that the electric field is along direction, therefore the potential energy term becomes. The Hamiltonian for the system can be written as

Therefore, the Schrödinger equation for the system becomes

A fixed interval is chosen to study the system. The desired solution may be expanded in terms of a set of continuous polynomials over the closed interval and is given by

where s are the coefficients of expansion and are B-polynomials of degree as defined in Section 2. Substituting Equation (12) into Equation (11), taking scalar product with on both sides and using Equation (7), Equation (11) becomes

where the matrix elements, , and assume closed forms by applying the formulas in Section 2 [

where the column matrix can be determined by solving this symmetric generalized eigenvalue problem.

The interval is assumed to be and the number of B-polynomials is taken to be 26. The accuracy and efficiency of the method depend on the number of B-polynomials chosen to construct the approximate solutions. In the present case, the number of B-polynomials is taken to be 26 as there is not much gain in accuracy beyond this value., , and in Equation (14) are 26 × 26 matrices. The standard Fortran EISPACK library is used to solve the generalized eigenvalue problem and find the eigenvalues and eigenvectors. The eigenvalues give the energy levels of the system. The initial eigenvalues for have been found to be correct to five places of decimal. The eigenvectors are used to calculate the corresponding wave functions using Equation (12). These wave functions are the dressed states of the system and are denoted by.

The system is now exposed to a time-dependent laser field polarized along x-axis, where is the strength and is the frequency of the laser field. The corresponding Hamiltonian becomes

where is given by Equation (10). The time-dependent Schrödinger equation for the system is now written as

The solution of Equation (16) in quasi-energy formalism can be written as [

where are defined as quasi-energies and are time-independent eigenvectors to be determined. is the lowest energy level of the system under the effect of static electric field and is the number of levels considered. The are the dressed states of the system in presence of laser field. The first six energy levels are taken into account and the range of and is chosen such that three of them are bound. Substituting the above form of the solution into Equation (16), multiplying by and integrating over for to results in a set of six homogeneous coupled equations in. Using the orthogonality property of wave func tions and applying the exact rotating wave approximation [

The s are the dipole matrix elements and s are the energies of first six levels. The s are defined as

and can be easily evaluated using the calculated wave functions. The set of Equations (18) can be solved to determine the quasi-energies and the corresponding eigenvectors. These eigenvectors can be used to determine the new dressed state wave functions given by Equation (17). In order to solve the set of Equations (18), it is written in matrix form and the corresponding matrix, called the quasi-energy matrix, is diagonalized using standard Fortran subroutines. The calculated eigenvectors are used to determine the transition probabilities to study the absorption spectra. The transition probability from ground state 0 to final state j can be computed from the eigenvectors of the quasi-energy matrix as [45,46]

The photoionization probability, , i.e., the probability of electron to come out of bound states, is given by

where is the sum of the probabilities of the system being in various bound states. Using Equation (21) the phenomenon of photoionization is also studied.

A single electron in a confined-harmonic oscillator potential is considered to be under the effect of a static electric field. The B-polynomial Galerkin method is used to calculate the dressed states of the confined electron as discussed in Section 3. The variation of eigenvalues for the first six energy states with the static electric field and force constant has been studied for this perturbed system. The values have been plotted in

According to the standard result from perturbation theory for a charged harmonic oscillator in electric field, the energy levels are always lowered by an amount (in atomic units) due to the field. The “dressed” potential [

which is just a shift of the harmonic potential. From

The effect of and on the dipole matrix elements, , , and can be seen from

Since with the introduction of electric field the system is perturbed, this relation would not be valid. With increase in the value of, the dipole elements diverge from the corresponding values for. For example, increases marginally for some values but

decreases for all. The effect of electric field is much less for higher values. The pattern followed by these values is again related to the change in wave functions.

The dipole matrix elements have been plotted with respect to in

The system is now exposed to laser field. The response of the perturbed system is now investigated by varying different control parameters like force constant, static electric field, laser field strength and laser frequency. The variation of transition probabilities for first four energy states with respect to has been depicted in

The phenomenon of photoionization also shows up for some values. The peaks for the fourth state, i.e., the first level in the continuum, represent photoionization probability. It is observed that the blue shift for this case is much more than that for the second excited state. It is evident that the first excited state peaks show exact resonance as the probability reaches 0.5. For the peaks corresponding to the second excited state and the fourth level, there is variation in peak strength. This is because the particular frequencies do not represent the condition of exact resonance, i.e., they are slightly off-resonant.

By keeping as a.u. and as a.u., the variation of transition probabilities with respect to has been shown for different values of force constant in

The probability for photoionization can be seen more clearly if total probability of bound states and continuum are represented separately.

bility of bound states and free states as a function of laser frequency for a.u. and a.u. at different values of. With increase in electric field, blue shift in frequency for photoionization is observed. Similarly, blue shift is observed in

The dynamics of an electron in confined-harmonic oscillator potential under the effect of static electric field and strong laser field is studied. The method based on Bpolynomial basis set is employed to solve the Schrö- dinger equation for the charged confined-harmonic oscillator. The static electric field modifies the wave functions and energies of such confined oscillator and hence the response of the oscillator to external applied laser field gets affected. Photoionization probabilities show strong dependence on the applied static as well as laser field parameters.