Energy states, oscillator strengths and polarizabilities of many
electron atoms confined by an impenetrable spherical cavity
Yusuf Yakar111*Corresponding authorsTel: +90 382 288 21 67 - Faks: +90 382 288 2299E-mail address:yuyakar@yahoo.com (Y.Yakar)bcakir@selcuk.edu.tr(B.Çakır) Bekir Çakır2* Ayhan
Özmen2
1Physics Department, Faculty of Arts and
Science, Aksaray University, Campus 68100,
2Physics Department, Faculty of Science, Selcuk
University, Campus 42031, Konya-Turkiye
Abstract: The orbital, ground and excited state energies of
many electron atoms confined by an impenetrable spherical cavity with
radius R are calculated using Quantum Genetic Algorithm (QGA) approach
and Hartree-Fock Roothaan (HFR) theory. The important properties such as
static and dynamic polarizability, oscillator strength and static
pressure are investigated as perturbative. The results reveal that
cavity radius and impurity charge have played an important role on the
polarizability, the oscillator strength and pressure of the system. In
addition, it is seen that when cavity radius is extremely large, all
energies and the other physical parameters approach the energies and
physical parameters of unconfined atom. As the dot radius decreases, the
polarizability of system because of the strong spatial confinement
decreases, but the pressure exerting on the system as the cavity radius
R is shrunk increases. In addition, as the impurity charge increases,
the magnitude of the oscillator strength decreases.
Keywords: Orbital energy, static and dynamic
polarizability, oscillator strength, pressure, many electron quantum
dots.
INTRODUCTION
In the past decade, confined quantum systems are of great interest in
many branches of physics, chemistry and engineering due to having unique
properties and potential applications in microelectronic and
optoelectronic devices. When atom is enclosed in a spherical cavity with
impenetrable walls, atom’s electrons are affected by a confining
potential in at least one direction. Such small structures are often
referred to as zero-dimensional structures or quantum dots , in
which the charge carriers are confined in all three dimensions. Quantum
dots (QDs) have discrete energy levels, like real atoms, and they are
also often called artificial atoms. [1] The
energy levels and dot sizes can be controlled by adjusting the potential
barrier. Thus, the quantum confinement effects cause drastic changes in
observable properties of QDs. Technologically, QDs have many potential
uses in microelectronic and optoelectronic devices. In this respect, by
employing different approaches and potential shapes, some researchers
have studied the electronic structure,[2-5]binding energies,[6-8] optical
properties,[9-15] electric and magnetic field
effects, [16-25] and other physical properties[26-30] of single electron QDs. As well known, it
is easy to obtain analytical solutions for QDs with one electron.
However, analytical solutions of many electron QDs are more difficult,
dull and complex than the single electron QDs. Researchers are used
various approach methods to overcome these problems. Recently, several
studies investigating the various properties of two electron QDs have
been published. By employing the Kohn-Sham model, Aquino et
al.[31] calculated energy states of helium atom
inside impenetrable spherical box. Wilson et al.[32] performed the ground state energies for two
electron atoms confined by an impenetrable spherical cavity. Ludeña[33] and Garza et al. [34]reported the Hartree-Fock energy results for the confined many electron
atoms by utilizing Roothaan’s approach. By using QGA and HFR method, the
various excited energy states and ionization energies of two electron
QDs with and without parabolic potential are calculated by our
group.[35] Similarly, for the confined helium
atom, the three lowest S symmetry state energies were computed by
Flores-Riveros et al.[36] by employing
perturbative and variational method. Sarsa and Sech[37] and Sañu-Ginarte et al.[38] studied the ground and excited energy states
of the confined systems such as He, Li and \(C\)atoms by using the variational Monte Carlo and direct variational
method. In 2020, Martinez-Flores and Cabrera-Trujillo[39] calculated the ground and excited state
energies of confined Li -like atoms in an impenetrable
spherical cavity. They implemented Slater’s \(X-\alpha\) approach in
Hartree-Fock theory to obtain the excitation energy spectrum, and they
assumed that the inner electrons do not see the outer electrons. To the
best of our knowledge, there are few theoretical studies related to the
energy spectrum, orbital energy, oscillator strength, static dipole
polarizability and pressure induced by the cavity for\(\text{He}^{-},\ Li,\ \text{Be}^{+},\ B^{++}\) and Be atoms
confined by an impenetrable spherical cavity. We have used QGA procedure
and HFR method to calculate the energies and wave functions, and the
other physical parameters are performed from perturbative calculations.
THEORY
We have considered lithium/lithium-like and beryllium dots. Dots are
supposed to be spherical with an infinite potential barrier that
confines all particles inside. The time-independent Schrödinger equation
of such a system is given by
\(\hat{H}\Psi=E\Psi\), (1)
where \(\hat{H}\) is the electronic Hamiltonian operator corresponding
to the sum of the kinetic energies plus the potential energies for all
the particles in the system, E and Ψ are
eigenvalue
and eigenfunction of the Hamiltonian operator. The Hamiltonian operator
is given by
\begin{equation}
\hat{H}=\sum_{i=1}^{N}\left[\frac{-\hslash^{2}\nabla_{i}^{2}}{2m}-\frac{Ze^{2}}{4\pi\varepsilon_{0}r_{i}}+\sum_{j=1}^{i-1}{\frac{e^{2}}{4\pi\varepsilon_{0}r_{\text{ij}}}+V_{c}(r_{i})}\right],\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (2)\nonumber \\
\end{equation}where \(m\) and \(\varepsilon_{0}\) are the mass of electron and
dielectric constant of the cavity, Z is the impurity charge, \(r_{i}\)are electron’s position vectors and \(r_{\text{ij}}\) are the
mutual distance between the i th and j th electrons,N is electron number (N =3,4). Here, the first sum on the
right side describes the kinetic energy operators for the electrons, the
second sum represents the Coulomb potential energy for the attraction
between the electrons and impurity charge, the third one denotes the
electron-electron repulsion plus exchange repulsion energy operators.
The last term \(V_{C}\) represents the confining potential term and its
form is defined by
\(V_{C}\left(r_{1},r_{2},r_{3},\ r_{4}\right)=\left\{\par
\begin{matrix}0,\ \ \ \ if\ r_{1},r_{2},r_{3},r_{4}\ <R\\
\infty,\ \ \ \ if\ r_{1},r_{2},r_{3},r_{4}\ \geq R\\
\end{matrix}\right.\ \) , (3)
where R is confinement radius (or dot radius). In three and
four-electron systems, for the ground state configurations \(1s^{2}2s\)and \(1s^{2}2s^{2}\), the total wavefunction of the system is expressed
by the Slater determinant including both space and spin variables
\begin{equation}
\Psi=\left\{\begin{matrix}\left|\phi_{1s}(1){\overset{\overline{}}{\phi}}_{1s}(2)\phi_{2s}(3)\right|\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ for\ N=3\\
\left|\phi_{1s}(1){\overset{\overline{}}{\phi}}_{1s}(2)\phi_{2s}(3){\overset{\overline{}}{\phi}}_{2s}(4)\right|\ \ \ \ \ \ for\ N=4,\\
\end{matrix}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (4)\right.\ \nonumber \\
\end{equation}where \(\phi\) represents the one-electron wave functions consisting
of spin and spatial components. The spatial part of the wavefunction can
be written as the linear combination of Slater type orbitals
(STOs, χ) as follows
\begin{equation}
\phi_{i}=\sum_{k=1}^{\sigma}{c_{\text{ik}}\chi_{k}\left(\zeta_{k},\mathbf{r}\right),}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\left(5\right)\nonumber \\
\end{equation}where \(\sigma\) is the number of basis sets, \(c_{\text{ik}}\ \)is the
expansion coefficients and \(\zeta_{k}\) is the orbital exponents and\(k\rightarrow n_{k}l_{k}m_{k}\) denotes the quantum numbers of basis
sets. STOs are preferred in the quantum mechanical analysis of the
electronic structure of confined and unconfined systems, because STOs
represent more correct behavior of the electronic wavefunctions,
especially important in the regions very close to or far from the
impurity. Since the Hamiltonian in Eq.(2) does not include spin terms,
the total energy is unaffected by inclusion of spin factor in the wave
functions. In the HF approximation, the total energy is a sum of
one-electron and two-electron energies. For a three and four electron
system, the ground state energy is written
\(E=\left\{\par
\begin{matrix}{2\epsilon}_{1s}+\epsilon_{2s}+J_{1s1s}+{(2J-K)}_{1s2s}\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ for\ \ \ N=3\\
{2\epsilon}_{1s}+{2\epsilon}_{2s}+J_{1s1s}+J_{2s2s}+{2(2J-K)}_{1s2s}\ \ \ \ \ \ \ for\ \ \ N=4\ ,\\
\end{matrix}\right.\ \) (6)
where \(\epsilon_{i}\) denotes the orbital energies and it is given by
\(\epsilon_{1s}=\left\{\par
\begin{matrix}h_{1s}+J_{1s1s}+({J-\frac{1}{2}K)}_{1s2s}\text{\ \ }\ \ for\ N=3\\
h_{1s}+J_{1s1s}+{(2J-K)}_{1s2s}\text{\ \ \ }\text{\ \ }for\ N=4,\\
\end{matrix}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\right.\ \)\((7a)\)
and
\(\epsilon_{2s}=\left\{\par
\begin{matrix}h_{2s}+{(2J-K)}_{2s1s}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }for\ N=3\\
h_{2s}+J_{2s2s}+{(2J-K)}_{2s1s}\text{\ \ \ }\text{\ \ }for\ N=4\ ,\\
\end{matrix}\text{\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ }\right.\ \)\((7b)\)
in which \(h\) is one-electron energy integrals (kinetic energy plus
impurity attraction energy), \(J_{\text{ij}}\) and \(K_{\text{ij}}\)show two-electron Coulomb and exchange energy integrals. Here, in the
excited states, the valence electron in the 2s sublevel is promoted to
the nlα sublevel, that is \(2s=nl\alpha\). The integrals in
Eqs.(6,7) can be expressed over STOs as follows, in atomic units (au),:
\(h_{p}=\left\langle\phi_{p}(\mathbf{r})\middle|-\frac{\nabla^{2}}{2}-\frac{Z}{r}\middle|\phi_{p}(\mathbf{r})\right\rangle=\sum_{i=1}^{\sigma}{\sum_{j=1}^{\sigma}c_{\text{pi}}}c_{\text{pj}}\left\langle\chi_{p_{i}}\middle|-\frac{\nabla^{2}}{2}-\frac{Z}{r}\middle|\chi_{p_{j}}\right\rangle\),
(8a)
\(J_{p,q}=\left\langle\phi_{p}(\mathbf{r}_{1})\phi_{q}(\mathbf{r}_{2})\middle|\frac{1}{r_{12}}\middle|\phi_{p}(\mathbf{r}_{1})\phi_{q}(\mathbf{r}_{2})\right\rangle\)
\(\ \ \ \ \ \ \ \ =\sum_{i=1}^{\sigma}{\sum_{j=1}^{\sigma}{\sum_{s=1}^{\sigma}{\sum_{k=1}^{\sigma}{c_{\text{pi}}c_{\text{qi}}c_{\text{ps}}c_{\text{qk}}}}}}\left\langle\chi_{p_{i}}\chi_{q_{j}}\middle|\frac{1}{r_{12}}\middle|\chi_{p_{s}}\chi_{q_{k}}\right\rangle\),
(8b)
and
\(K_{p,q}=\left\langle\phi_{p}(\mathbf{r}_{1})\phi_{q}(\mathbf{r}_{2})\middle|\frac{1}{r_{12}}\middle|\phi_{q}(\mathbf{r}_{1})\phi_{p}(\mathbf{r}_{2})\right\rangle\)
\(\ \ \ \ \ \ \ \ \ \ =\sum_{i=1}^{\sigma}{\sum_{j=1}^{\sigma}{\sum_{s=1}^{\sigma}{\sum_{k=1}^{\sigma}{c_{\text{pi}}c_{\text{qi}}c_{\text{ps}}c_{\text{qk}}}}}}\left\langle\chi_{p_{i}}\chi_{q_{j}}\middle|\frac{1}{r_{12}}\middle|\chi_{q_{k}}\chi_{p_{s}}\right\rangle\).
(8c)
One- and two-electron integrals can be easily evaluated by modifying for
appropriate consideration of the boundaries the expressions of atomic
systems. [40]
In optical transitions between energy levels, the transition probability
of absorption or emission of electromagnetic radiation is defined by
oscillator strength. The oscillator strength (OS) is a dimensionless
quantity and it plays an important role in spectroscopic studies. This
term determines the intensity of a specific spectral line in atomic
spectrum and also offers additional information on the fine
structure.41 OS is given by, in au ,
\(f_{\gamma\rightarrow n}=2\left(E_{n}-E_{\gamma}\right)\left|\left\langle M_{\text{nγ}}\right\rangle\right|^{2}\),
(9)
where \(E_{n}\) (\(E_{\gamma})\) denotes higher (lower) energy states
and \(M_{\text{nγ}}\) is dipole transition matrix
element.25
Dipole polarizability is important for investigating of the pressure
effects on the atom, the theoretical description of interatomic
interactions, the electron atom scattering and the optical properties of
materials. When a confined system in the state \(i\) is subjected to a
time–independent electric field –static case- in thez -direction, the static dipole polarizability (SDP) can be
expressed in terms of OS as follows [42]
\(\alpha_{\gamma}=\sum_{n\neq\gamma}{\frac{f_{\gamma\rightarrow n}}{\left(E_{n}-E_{\gamma}\right)^{2}},\ \ }\)\((10)\)
where \(\gamma\) indicates the level which the polarizability is
computed. Here, the summation on the right side is over all continuum
states. The calculation of the integrals over all continuum states is
very complex for the present numerical
capability.[43] For the 2s polarizability, the
transition from one-shell to three shell dos not change the
polarizability. This is due to the fact that the major contribution to
the 2s polarizability comes from the \(2s\)→2p transition and it is not
significantly modified by appearance of new np states in additional
shells. [44] SDP may be also calculated from the
formula suggested by Kirkwood [38,45], in au,
\(\alpha_{i}^{K}=\frac{4}{9}\sum_{i=1}\left\langle r_{i}^{2}\right\rangle^{2}\). (11)
The other important quantity is dynamic dipole polarizability, which
describes the distortion of the electron and charge distribution of a
system. Dynamic dipole polarizability (DDP) is directly related to the
parameters such as the Van der Waals constants, the frequency dependence
of the refractive index, the dynamic dipole shielding factor, the
Rayleigh scattering cross sections and the mean excitation energies.[46] Dynamic polarizability can be expressed in
terms of OS by
\(\alpha(\nu)=\sum_{n>\gamma}{\frac{f_{\gamma\rightarrow n}}{\left(E_{n}-E_{\gamma}\right)^{2}-\text{hν}^{2}}\ }\).
(12)
were, \(\nu\) is the photon frequency of the applied field in thez -direction. As can be clearly seen in Eq. (12), when thehν energy is equal to the energy difference\(E_{n}-E_{\gamma}\), the dynamic polarizability shows singularities
for any electronic state n . This is particularly important in
cavities where the symmetry breaking due to confinement leads to very
close-nearly degenerate- atomic levels.[44]
The compression of an atomic system leads to an increase in kinetic
energy K as a function of pressure P . As the confinement
radius R decreases, the average pressure that the cavity implements on
the atomic system is given by the virial theorem as[33,47]
\(K(R)=4\pi R_{0}^{3}P(R)-E(R)\). (13)
RESULTS AND DISCUSSION
We have calculated the ground and excited state energies and
wavefunctions of lithium/lithium-like and beryllium atoms confined at
the centre of an impenetrable spherical cavity of dot radius R. To
minimize the total energy over STOs we have used the new variational
method, which is a combination of QGA procedure and HFR method. In order
to maintain the orthogonality of orbital, we have used the same set of
screening parameters for all the one-electron spatial orbital with the
same angular momentum and employed seven basis sets to calculate the
energy expectation value. The calculations are made considering a single
Slater determinant configuration. Therefore, the electron correlation
effects being the second-order and higher order effects are not taken
into account. Our results are given in terms of au (Hartree), and these
results include terms with a fixed magnetic quantum numbersm (i.e., \(m=m^{{}^{\prime}}=0)\).
In Table 1 and Table 2 show the orbital energies and total energies for
the confined Lithium atom’s electronic configurations \({1s}^{2}2s\)(2S) and \({1s}^{2}2p\) (2P) for
various dot radii. As seen in tables, both the orbital energy and the
total energy increase as the confinement radius R decreases. In Table 1,
in all confinement regions, our results are in good agreement with the
results calculated by Ludeña.[33] However, there
are small discrepancies between the literature results obtained by
Flores and Trujillo.[39] This difference is
slightly increase in 1s-orbital energy \(\epsilon_{1s}\). This is
originated from their calculation method. They assume that the inner
electrons do not see the outer electron in the electronic configuration\({1s}^{2}2s\). It can be remarked that the confinement radius becomes
very large, all energy states approach the corresponding states of
unconfined lithium atom. For example, for the ground state, at R=10, we
have obtained the results as \(\epsilon_{1s}=-2.47672au\),\(\epsilon_{2s}=-0.19366au\) and \(E_{{1s}^{2}2s}=-7.42992au\).
Unconfined Lithium atom’s orbital and ground state energies are\(\epsilon_{1s}=-2.477739au\), \(\epsilon_{2s}=-0.196321au\) and\(E_{{1s}^{2}2s}=-7.432723au.\) 40 Similarly, for
the first excited state, in Table 2, at R=10, we have obtained the
results as \(\epsilon_{1s}=-2.50153au\),\(\epsilon_{2p}=-0.12788au\) and \(E_{{1s}^{2}2p}=-7.37611au\).
The literature value of \(E_{{1s}^{2}2p}\) is\(-7.3865au\).[48] It is noted that all energies
increase when the spatial confinement is stronger. This is the result of
the Heisenberg uncertainty principle. Similarly, for the excited state\({1s}^{2}2p\) , total energy results are better than the results
obtained by Sañu-Ginarte. However, there are slight differences between
our results and the results of Martinez et al. due to the reason
explained above.
In Tables 3-5, we have illustrated the orbital energies, the ground and
excited states energies for the confined\(\text{He}^{-},\text{Be}^{+}\), \(B^{++}\ \)and Be atoms.
As seen from tables, when the dot radius becomes very large, the orbital
and total energy levels approach the values of a free space atoms. For
example, in Table 3, at R=10, for \({1s}^{2}2s\) and \({1s}^{2}2p\)configurations of \(\text{Be}^{+}\) dot, we have obtained the numerical
values as \(\epsilon_{1s}=-5.13779\text{au}\),\(\epsilon_{2s}=-0.66581\text{au}\),\(E_{{1s}^{2}2s}=-14.27373au\) and\(\epsilon_{1s}=-5.15158\text{au}\), \(\epsilon_{2p}=-0.51796au\),\(E_{{1s}^{2}2p}\)= -14.12896, respectively. The literature values for
the excited state of unconfined \(\text{Be}^{+}\) atom are\(\epsilon_{1s}=\)-5.1383425\(,\ \ \epsilon_{2s}=\)-0.6661462 and\(E_{{1s}^{2}2p}=-14.277394au.\)[49] We have
also showed the orbital and ground state energies of Be dot
in Table 3. The same situations are seen here. That is, as the dot
radius increases, all energies approach the values that are equal to the
corresponding energies of Be atom. For the orbital and ground state
energy for Be dot, our results are consistent with the
results of Ludeña and are lower than Sanu-Ginarte. In Table 4, we have
illustrated the orbital, ground and excited state energies of\(B^{++}\) dot. It is worth to note that, while \(R\gtrsim\)1.5, the
speed of energy change is slowing down due to the strong attractive
force. There are slight differences between our results and the
literature results. On the other hand, in table 5, for \(\text{He}^{-}\)dot, the similar behaviours are obtained here. We could not compare our
results with the literature data.
In Fig.1, we have plotted the ground (\({1s}^{2}2s,\)2S) and excited (\({1s}^{2}2p,\) 2P)
state energies of the confined\(\text{He}^{-},\ \ Li,\ \ \text{Be}^{+}\), \(B^{++}\) atoms as a
function of dot radius. For each nl state, as the confinement
radius R decreases, the energies increase more quickly. It is seen that
when the confinement radius is extremely small (in the strong
confinement region\(\ R\lesssim 1.5\)), the spatial confinement has a
very strong influence on the impurity energy states. In this region,
when the confinement radius continuously decreases, the wavefunctions of
the localized states penetrates into the outside region of the quantum
dot more and more. Therefore, the energies become leger and larger. On
the other hand, in this region,\(\ \)the degeneracy of the energy
spectrum is completely disappeared, and the energy states separate from
each other. For the same principle quantum number, as the dot radius
decreases, the energy of the level with smaller l increases more
quickly than the bigger l level. The reason is that, most of time
the confined electron in small l level distributes itself at the
more outer part of the impurity than the electron in big l level.
In Fig.2, we display the orbital energies \(\epsilon_{1s}\),\(\epsilon_{2s}\) and \(\epsilon_{2p}\) of the confined lithium-like
atoms as a function of the confinement radius R. As it will be seen in
(a), when the impurity charge Z increases, the orbital energy\(\epsilon_{1s}\) increases as negative. The energy \(\epsilon_{1s}\)becomes negative while R\(\gtrsim 2.3\), R\(\gtrsim 1.1\),
R\(\gtrsim 0.65\) and R\(\gtrsim 0.5\) for\(\text{He}^{-},\ \ Li,\ \ \text{Be}^{+}\) and \(B^{++}\),
respectively. Here, the negative sign indicates that we need to give the
electron energy to move it from the confined atom. In the weak
confinement region \(R\geq 2.5\ ,\) while the spatial confinement
effect on the energy \(\epsilon_{1s}\) is very weak, this effect becomes
significant in the region \(R\leq 1\), in which is known as the strong
confinement region. As for in (b), similarly, the orbital energies\(\epsilon_{2s}\) and \(\epsilon_{2p}\) increase with the decrease of
confinement radius until reaching the continuum. It is seen that while
going to large dot radii, \(\epsilon_{2s}\) (dashed line) is deeper than
the \(\epsilon_{2p}\) (solid line). As seen in (b), \(\epsilon_{2s}\)and \(\epsilon_{2p}\) energy states have the crossing points, which are
highlighted by circles. These crossing points (or critical cavity
radius) are at \(R\sim 4.9,\ \ R\sim 3.2\), \(R\sim 2.4\) and\(R\sim 2\) for \(\text{He}^{-},\ \ Li,\ \ \text{Be}^{+}\) and\(B^{++}\) dots, respectively. For the critical dot (cavity) radii, our
results are in good agreement with the results reported by Flores and
Trujillo [39] , which are 3.4, 2.5 and 2.1 for\(Li,\ \ \text{Be}^{+}\) and \(B^{++}\), respectively. As the
confinement radius is reduced, after these crossing points, this
situation is exactly reversed, that is, \(\epsilon_{2p}\) energy state
becomes higher than \(\epsilon_{2s}\) state. It is worth nothing that
since the 2s-orbital is occupied with an electron in the initial ground
state electronic configuration, for cavity with R lower than the
critical cavity radius, one would have photon emission instead of
absorption for initial electronic configuration. Thus, the excited
electron from the initial s- level to the final p- level
returns to ground state by the photon emission. That is, as the dot
radius decreases, if the p -level lies energetically below the
corresponding s -level, this transition occurs. This situation is
mostly seen in dipole oscillator transitions, and the oscillator
strength become positive for excitations. As seen in \(\epsilon_{2s}\)and \(\epsilon_{2p}\) orbital energy curves, the crossing points shift
toward smaller dot radii as the impurity charge Z, and in large dot
radii the difference between \(\epsilon_{2s}\) and \(\epsilon_{2p}\)orbital energies increases with the increase of Z.
Fig.3 shows the dipole oscillator transition
1s22s\(\rightarrow\)1s22p for\(Li,\ \ \text{Be}^{+}\) and \(B^{++}\) dots as a function of R. For
all dots, the oscillator curves exhibit similar behaviors. ForLi dot, in large dot radii, which is known as the weak
confinement region, as the confinement radius decreases, the OS begins
to reduce as positive until R\(\approx\)3.2, in which the crossing point
occurs, and then it continues to decrease as negative until reaching to
a negative constant. For Li dot ,at R=10, we find a value of\(f_{2S\rightarrow 2P}\)=0.7489. The literature value of unconfinedLi atom is 0.7488.[50] As mentioned
above, while R\(\gtrsim\)3.2, while the oscillator transition\(f_{2S\rightarrow 2P}\) occurs by the photon absorption, it occurs by
the photon emission after R\(\lesssim\)3.2. For \(\text{Be}^{+}\) and\(B^{++}\) dots, we have observed similar behaviors. That is, the OS
rapidly increases as the dot radius increases and then reaches to a
limit value in larger dot radii. At R=10, we have obtained the values of\(f_{2s\rightarrow 2p}\) =0.5337 and 0.4102 for the \(\text{Be}^{+}\)and \(B^{++}\) dots. The literature result of unconfined\(\text{Be}^{+}\) atom is 0.5505.[51] Similarly,
the oscillator transitions become negative after R\(\approx\)2.4 and
R\(\approx\)1.95 for \(\text{Be}^{+}\) and \(B^{++}\) dots, near the
radii for which the crossing points occur. While R is smaller than the
crossing points, in which point \(\epsilon_{2p}\) is lower than\(\epsilon_{2s}\), the oscillator transitions become by photon emission
induced by the pressure cavity. As seen on the OS curves, the impurity
has a strong effect on the oscillator transitions. The OS decreases with
the increase of Z and shifts toward the smaller dot radius. The reason
is that as the impurity charge increases, electrons are
strongly attracted toward impurity. The similar results are obtained by
Flores and Trujillo.[39]
In Fig.4, we have displayed the SDP of \(Li,\ \ \text{Be}^{+}\) and\(B^{++}\) dots as a function of dot radius. For 2s orbital, the SDP
has been calculated from the Kirkwood formula in (a) and the oscillator
strength in (b). As seen in (a), in large dot radii, it is worth nothing
that the static polarizability of neutral Li dot is higher
than the others since the electron cloud is deformed easily by the
applied field. For Li dot, the static polarizability is very
weak until \(R\sim 3\), and more after it increases monotonically first
up to \(R\sim 12\) and then reaches a saturation value in large dot
radii. In the strong confinement region, as the dot radius is reduced,
the static polarizability decreases due to the fact that the
localization causes a less polarizability of the charge distribution.
When compared the SDP of Li with the SDP of\(\text{Be}^{+}\text{and\ }B^{++}\), the effect of impurity charge Z is
clearly seen on the SDP curves. As the Z increases, the polarizability
rapidly decreases since the charge cloud cannot be easily deformed. In
other words, the polarizability is associated with the binding energy of
the electron. When the electrons get closer to impurity, the electron’s
binding energy increases, and thus the polarizability of the system
starts to reduce due to increasing the binding energy. The opposite is
also true. That is, when the binding energy of the electron is very
weak, in very large dot radii, the polarizability becomes maximum. By
using Kirkwood formula, for R=15, we have obtained the polarizability
values as 171.7730au, 16.9604au and 4.5015au for\(Li,\ \ \text{Be}^{+}\) and \(B^{++}\) dots. The literature values for
unconfined Li atom are 164au.[52] In
(b), in which the SDP has been calculated from the oscillator formula,
for neutral Li dot, as the confinement radius reduces, the
static polarizability decreases until reaching the crossing point\(R\cong 3.2.\) It is seen that static polarization changes its sign at
the critical dot radius, \(R\cong 3.2\). We have calculated the SDP
values as -63.54585au for R=3 and 52.80064au for R=3.5. After the
critical dot radius \(R\cong 3.2\), as the confinement radius
decreases, the polarizability approaches zero from negative value.
Similar behaviors are seen on \(\text{Be}^{+}\) dot in (b). The static
polarizability of \(\text{Be}^{+}\) dot is smaller than that of neutralLi. The sign of SDP in \(\text{Be}^{+}\) dot changes at the
critical dot radius \(R\cong 2.3\). When compared Li with\(\text{Be}^{+}\), as the impurity charge increases, the critical dot
radius at which SDP changes its sign shifts toward smaller dot radii.
For R=15, we have calculated the SDP values as 167.6403au and 27.2986au
for Li and\(\ \text{Be}^{+}\) dots. The literature values are
171.188au and 27. 3836au for unconfined Liand\(\ \text{Be}^{+}\) atoms.[39]
Fig.5 shows that the dynamic dipole polarizability for Li andBe dots as a function of photon energy at R=1, 1.5 and 2. The
frequency step interval is taken as 0.01au. Singularities (or jumps)
appearing on dynamic polarizability curves describe the frequency
corresponding the energy difference in Eq.(12). As can be seen from
Eq.(12), as the photon energy hν increases, the DDP increases
until singularity point, in which point is at\(h\nu=E_{n}-E_{\gamma}\). The presence of this pole leads to the
sign inversion of the polarizability. It should be noted that the SDP
peak’s magnitude and its position which is equal to the singularity
frequencies vary continuously according to the confinement radius. When
compared Li with \(\text{Be}^{+}\), for a fixed R, as the
impurity charge increases, the peak positons of the DDP shift toward
smaller photon energy.
The numerical values of kinetic energy and pressure that the cavity
exerts on the system as the dot radius R (or cavity radius) is shrunk
are presented in Table 6 and also the change of pressure and kinetic
energy is displayed in Fig.6 as a function of the cavity radius R for\(Li,\ \ B^{++}\) and Be dots. As seen in Table 6 and Fig.6,
in weak confinement region or large dot radii, the pressure is very
weak. As the dot radius decreases, both the pressure and the kinetic
energy start to increase monotonically. In the strong confinement
region, the smaller the dot radius, the higher the pressure will be. It
can be said that the pressure is associated with the static
polarizability. That is, in the strong confinement region, while the
polarizability is very weak, the pressure is large. When the cavity
radius is very large, in the weak confinement region, the pressure is
very weak, but the polarizability becomes maximum. On the other hand, As
the cavity radius decreases, both the pressure and the kinetic energy
increases. According to the uncertainty principle, as the electron
approaches the impurity, the uncertainty in the speed of the electron
increases and so its kinetic energy increases. As a result of this, the
pressure increases. It is also worthwhile to note that, for the same R,
the pressure is the highest for Be, decreases forLi and is the lowest for \(B^{++}\). This is because the
beryllium atom is more diffuse in its 2s orbital according to the
others, so the same cavity radius induces a high pressure. However, for
the \(B^{++}\), owing to increasing impurity charge, the boron ion has
already compacted its 2s electron, so the same cavity radius induces a
smaller pressure on the ionic system.[39] On the
other hand, when compared Li with \(B^{++}\), the effect of
impurity charge on pressure is clearly seen here. That is, for the same
confinement radius R, the pressure of \(B^{++}\) is lower than the
pressure of Li. However, for kinetic energy, the situation is
opposite. For Be dot, our results are consistent with the
literature results.[38]
CONCLUSIONS
In this work, we have calculated the ground and excited state energies
and the orbital energies of three and four electron QDs such as\(\text{He}^{-},\ Li,\ \ \text{Be}^{+}\), \(B^{++}\) and Bedots. We also carried out the static and dynamic dipole polarizability,
oscillator strength and pressure induced by the cavity as a function of
dot radius and impurity charge. The results show that both dot radius
and impurity charge have a great influence on total energy, orbital
energy, polarizability, oscillator strength and pressure of the system.
While the polarizability is very weak due to the strong spatial
confinement in small dot radii, it increases monotonically while going
to large dot radii and then reaches a saturation value. For the ground
state polarizability, the approximation of Kirkwood in many electron
dots gives good results. It is found that while the oscillator
transition \(f_{2s\rightarrow 2p}\) occurs by the photon absorption
until a critical dot radius, it occurs by the photon emission after the
critical dot radius. As the dot radius decreases, both the pressure and
the kinetic energy increases, but polarizability of the system
decreases. To our knowledge, there are very little reports including the
calculation of orbital energies, static polarizability, oscillator
strength and pressure of many electron QDs. With respect to the lack of
such studies, we believe that our study makes an important contribution
to the literature. Also, theoretical investigation of electronic and
optical properties of many electron QDs will lead to a better
understanding of the properties of low dimensional structures. Such
theoretical studies may have profound consequences about practical
applications of the spectroscopic studies, and the results of this study
will contribute to the research on related subjects.