Stark Effect In Atomic Spectra 3d5p2c

Stark Effect in Atomic Spectra The splitting of atomic spectral lines as a result of an externally applied electric field was discovered by Stark, and is called the Stark effect. As the splitting of a line of the helium spectrum shows, the splitting is not symmetric like that of the Zeeman effect. The splitting of the energy levels by an electric field first requires that the field polarize the atom and then interact with the resulting electric dipole moment. That dipole moment depends upon the magnitude of Mj, but not its sign, so that the energy levels are split into J+1 or J+1/2 levels, for integer and half-integer spins respectively. The Stark effect has been of marginal benefit in the analysis of atomic spectra, but has been a major tool for molecular rotational spectra.

History The effect is named after Johannes Stark, who discovered it in 1913. It was independently discovered in the same year by the Italian physicist Antonino Lo Surdo, and in Italy it is thus sometimes called the Stark-Lo Surdo effect. The discovery of this effect contributed importantly to the development of quantum theory. Ironically, soon after their discoveries, both Stark and Lo Surdo rejected developments in modern physics and allied themselves with the political and racial programs of Hitler and Mussolini.[3] Inspired by the magnetic Zeeman effect, and especially by Lorentz's explanation of it, Woldemar Voigt[4] performed classical mechanical calculations of quasi-elastically bound electrons in an electric field. By using experimental indices of refraction he gave an estimate of the Stark splittings. This estimate was a few orders of magnitude too low. Not deterred by this prediction, Stark [5] undertook measurements on excited states of the hydrogen atom and succeeded in observing splittings. By the use of the Bohr-Sommerfeld ("old") quantum theory Paul Epstein[6] and Karl Schwarzschild[7] were independently able to derive equations for the linear and quadratic Stark effect in hydrogen. Four years later, Hendrik Kramers[8] derived formulas for intensities of spectral transitions. Kramers also included the effect of fine structure, which includes corrections for relativistic kinetic energy and coupling between electron spin and orbit. The first quantum mechanical treatment (in the framework of Heisenberg's matrix mechanics) was by Wolfgang Pauli.[9] Erwin Schrödinger discussed at length the Stark effect in his third paper[10] on quantum theory (in which he introduced his perturbation theory), once in the manner of the 1916 work of Epstein (but generalized from the old to the new quantum theory) and once by his (first-order) perturbation approach. Finally, Epstein[11] reconsidered the linear and quadratic Stark effect from the point of view of the new quantum theory. He derived equations for the line intensities which were a decided improvement over Kramers' results obtained by the old quantum theory. While first-order perturbation effects for the Stark effect in hydrogen are in agreement for the BohrSommerfeld model and the quantum-mechanical theory of the atom, higher order effects are not[citation needed]. Measurements of the Stark effect under high field strengths confirmed the correctness of the quantum theory over the Bohr model.

Overview An electric field pointing from left to right, for example, tends to pull nuclei to the right and electrons to the left. In another way of viewing it, if an electronic state has its electron disproportionately to the left, its energy is lowered, while if it has the electron disproportionately to the right, its energy is raised. Other things equal, the effect of the electric field is greater for outer electron shells, because the electron is more distant from the nucleus, so it travels further left and further right. The Stark effect can lead to splitting of degenerate energy levels. For example, in the Bohr model, an electron has the same energy whether it is in the 2s state or any of the 2p states. However, in an electric field, there will be hybrid orbitals (also called quantum superpositions) of the 2s and 2p states where the electron tends to be to the left, which will acquire a lower energy, and other hybrid orbitals where the electron tends to be to the right, which will acquire a higher energy. Therefore, the formerly degenerate energy levels will split into slightly lower and slightly higher energy levels.

Quantum-confined Stark effect Main article: Quantum-confined Stark effect In a semiconductor heterostructure, where a small bandgap material is sandwiched between two layers of a larger bandgap material, the Stark effect can be dramatically enhanced by boundexcitons. This is because the electron and hole which form the exciton are pulled in opposite directions by the applied electric field, but they remain confined in the smaller bandgap material, so the exciton is not merely pulled apart by the field. The quantum-confined Stark effect is widely used for semiconductor-based optical modulators, particularly for optical fiber communications.

The Stark effect is the shifting and splitting of spectral lines of atoms and molecules due to presence of an external static electric field. The amount of splitting and or shifting is called the Stark splitting or Stark shift. In general one distinguishes first- and second-order Stark effects. The first-order effect is linear in the applied electric field, while the second-order effect is quadratic in the field. The Stark effect is responsible for the pressure broadening (Stark broadening) of spectral lines by charged particles. When the split/shifted lines appear in absorption, the effect is called the inverse Stark effect. The Stark effect is the electric analogue of the Zeeman effect where a spectral line is split into several components due to the presence of a magnetic field. The Stark effect can be explained with fully quantum mechanical approaches, but it has also been a fertile testing ground for semiclassical methods.

Mechanism [edit]Overview An electric field pointing from left to right, for example, tends to pull nuclei to the right and electrons to the left. In another way of viewing it, if an electronic state has its electron disproportionately to the left, its energy is lowered, while if it has the electron disproportionately to the right, its energy is raised.

Other things equal, the effect of the electric field is greater for outer electron shells, because the electron is more distant from the nucleus, so it travels further left and further right. The Stark effect can lead to splitting of degenerate energy levels. For example, in the Bohr model, an electron has the same energy whether it is in the 2s state or any of the 2p states. However, in an electric field, there will be hybrid orbitals (also called quantum superpositions) of the 2s and 2p states where the electron tends to be to the left, which will acquire a lower energy, and other hybrid orbitals where the electron tends to be to the right, which will acquire a higher energy. Therefore, the formerly degenerate energy levels will split into slightly lower and slightly higher energy levels. [edit]Classical

electrostatics

The Stark effect originates from the interaction between a charge distribution (atom or molecule) and an external electric field. Before turning to quantum mechanics we describe the interaction classically and consider a continuous charge distribution ρ(r). If this charge distribution is non-polarizable its interaction energy with an external electrostatic potential V(r) is

. If the electric field is of macroscopic origin and the charge distribution is microscopic, it is reasonable to assume that the electric field is uniform over the charge distribution. That is, V is given by a twoterm Taylor expansion,

, with the electric field:

,

where we took the origin 0 somewhere within ρ. Setting V(0) as the zero energy, the interaction becomes

. Here we have introduced the dipole moment μ of ρ as an integral over the charge distribution. In case ρ consists of N point charges qj this definition becomes a sum

. [edit]Perturbation

theory

Turning now to quantum mechanics we see an atom or a molecule as a collection of point charges (electrons and nuclei), so that the second definition of the dipole applies. The interaction of atom or molecule with a uniform external field is described by the operator

This operator is used as a perturbation in first- and second-order perturbation theory to for the first- and second-order Stark effect.

[edit]First order Let the unperturbed atom or molecule be in a g-fold degenerate state with orthonormal zeroth-order state functions . (Non-degeneracy is the special case g = 1). According to perturbation theory the first-order energies are the eigenvalues of the g x g matrix with general element

If g = 1 (as is often the case for electronic states of molecules) the first-order energy becomes proportional to the expectation (average) value of the dipole operator ,

Because a dipole moment is a polar vector, the diagonal elements of the perturbation matrix Vint vanish for systems with an inversion center (such as atoms). Molecules with an inversion center in a non-degenerate electronic state do not have a (permanent) dipole and hence do not show a linear Stark effect. In order to obtain a non-zero matrix Vint for systems with an inversion center it is necessary that some of the unperturbed functions have opposite parity (obtain plus and minus under inversion), because only functions of opposite parity give non-vanishing matrix elements. Degenerate zeroth-order states of opposite parity occur for excited hydrogen-like (one-electron) atoms. Such atoms have the principal quantum number n among their quantum numbers. The excited state of hydrogen-like atoms with principal quantum number n is n2-fold degenerate and

where is the azimuthal (angular momentum) quantum number. For instance, the excited n = 4 state contains the following states,

The one-electron states with even are even under parity, while those with odd are odd under parity. Hence hydrogen-like atoms with n>1 show first-order Stark effect. The first-order Stark effect occurs in rotational transitions of symmetric top molecules (but not for linear and asymmetric molecules). In first approximation a molecule may be seen as a rigid rotor. A symmetric top rigid rotor has the unperturbed eigenstates

with 2(2J+1)-fold degenerate energy for |K| > 0 and (2J+1)fold degenerate energy for K=0. Here DJMK is an element of

the Wigner D-matrix. The first-order perturbation matrix on basis of the unperturbed rigid rotor function is non-zero and can be diagonalized. This gives shifts and splittings in the rotational spectrum. Quantitative analysis of these Stark shift yields the permanentelectric dipole moment of the symmetric top molecule. [edit]Second order As stated, the quadratic Stark effect is described by secondorder perturbation theory. The zeroth-order problems

are assumed to be solved. It is usual to assume that the zeroth-order state to be perturbed is nondegenerate. If we take the ground state as the nondegenerate state under consideration (for hydrogenlike atoms: n = 1), perturbation theory gives

with the components of the polarizability tensor α defined by

The energy E(2) gives the quadratic Stark effect. Because of their spherical symmetry the polarizability tensor of atoms is isotropic,

which is the quadratic Stark shift for atoms. For many molecules this expression is not too bad an approximation, because molecular tensors are often reasonably isotropic. [edit]Problems The perturbative treatment of the Stark effect has some problems. In the presence of an electric field, states of atoms and molecules that were previously bound (square-integrable), become formally (non-squareintegrable) resonances of finite width. These resonances may decay in finite

time via field ionization. For low lying states and not too strong fields the decay times are so long, however, that for all practical purposes the system can be regarded as bound. For highly excited states and/or very strong fields ionization may have to be ed for. (See also the article on the Rydberg atom).

Stark spectroscopy (sometimes known as electroabsorption/emission spectroscopy) is a form of spectroscopy based on the Stark effect. In brief, this technique makes use of the Stark effect (or electrochromism) either to reveal information about the physiochemical or physical properties of a sample using a well-characterized electric field or to reveal information about an electric field using a reference sample with a well-characterized Stark effect. The use of the term "Stark effect" differs between the disciplines of chemistry and physics. Physicists tend to use the more classical definition of the term (see Stark effect), while chemists usually use the term to refer to what is technically electrochromism, i.e., Electrochromism is the phenomenon displayed by some materials of reversibly changing color when a burst of charge is applied. Various types of materials and structures can be used to construct electrochromic devices, depending on the specific applications. In the former case, the applied electric field splits the atomic energy levels and is the electric field analog of the Zeeman effect. However, in the latter case, the applied electric field changes the molar absorption coefficient of the sample, which can be measured using traditional absorption or emission spectroscopic methods. This effect is known as electrochromism.