Difference Between Zeeman Effect and Stark Effect

Main Difference – Zeeman Effect vs Stark Effect

Zeeman Effect and Stark Effect are two concepts in chemistry that were discovered by scientists in late 1900’s. The Zeeman effect and stark effect can be observed regarding the atomic spectra of an atom. Atomic spectra can be either absorption spectra or emission spectra. When energy is given to atoms, the atoms become excited and electrons move to higher energy levels by absorbing this energy. This absorption gives the absorption spectra. However, since a higher energy level is not stable, these electrons fall back to the ground energy level, releasing the absorbed energy as radiation. This results in emission spectra. The main difference between Zeeman effect and Stark effect is that Zeeman effect is observed in the presence of an external magnetic field whereas Stark effect is observed in the presence of an external electrical field.

Key Areas Covered

1. What is Zeeman Effect
      – Definition, Different Types
2. What is Stark Effect
      – Definition, Different Types
3. What is the Difference Between Zeeman Effect and Stark Effect
      – Comparison of Key Differences

Key Terms: Absorption, Anomalous Zeeman Effect, Atomic Spectrum, Diamagnetic Zeeman Effect, Electromagnetic Radiation, Emission, Linear Stark Effect, Magnetic Field, Magnetic Moment, Normal Zeeman Effect, Quadratic Stark Effect, Stark Effect, Zeeman Effect

What is Zeeman Effect

Zeeman effect describes the splitting of the spectral lines of an atom in the presence of a strong magnetic field. It is named after the Dutch scientist Pieter Zeeman. This effect describes the effect of a magnetic field on atoms or ions. Now, let’s find out what is a spectral line.

An atomic spectrum is the spectrum of frequencies of electromagnetic radiation emitted or absorbed during transitions of electrons between energy levels within an atom. Emissions lead to emission spectra, and absorption leads to absorption spectra. This spectrum is a characteristic property of elements. The spectrum is composed of a collection of spectral lines for each and every emission/ absorption. Each and every spectral line stands for the energy difference between two energy levels of the atom. Pieter Zeeman observed that these spectral lines undergo splitting when the atom is kept in the presence of an external magnetic field. Zeeman effect is the result of the interaction between the magnetic moment of the atom and the external magnetic field.

The following image shows the atomic emission spectra for hydrogen. When energy is given to an atom, the electrons can absorb energy and move to a higher energy level. But, a higher energy level is an unstable state for an atom. Therefore, the electron comes back to a lower energy level releasing the absorbed energy. This gives an emission spectral line. But when this is studied under applied magnetic field, there we can see three spectral lines instead of one. This is the Zeeman effect.

Figure 1: Emission Spectra for Hydrogen in the Absence and the Presence of a Magnetic Field

Types of Zeeman Effect

There are three types of Zeeman effect. They are the normal effect, anomalous effect, and diamagnetic effect. The normal Zeeman effect is caused by the interaction with the orbital magnetic moment. The anomalous Zeeman effect is caused by the interaction with combined orbital and intrinsic magnetic moments. The diamagnetic Zeeman effect is caused by the interaction with the field-induced magnetic moment.

What is Stark Effect

Stark effect is the splitting of spectral lines observed when the radiating atoms, ions, or molecules are subjected to a strong electric field. This effect was first discovered by the German scientist Johannes Stark. The effect was named after him. The Stark effect may include both shifting and splitting of spectral lines. The electric field first polarizes the atom and then interacts with the resulting dipole moment.

Figure 2: Stark Splitting in Hydrogen

Types of Stark Effect

Stark effect arises due to the interaction between the electric moment of the atom and the external electric field. This effect can be observed in two types as linear Stark effect and quadratic Stark effect. The linear Stark effect arises due to a dipole moment that arises from a naturally occurring non-symmetric distribution of electrical charge. The quadratic Stark effect arises due to a dipole moment that is induced by the external field.

Difference Between Zeeman Effect and Stark Effect

Definition

Zeeman Effect: Zeeman effect describes the splitting of the spectral lines of an atom in the presence of a strong magnetic field.

Stark Effect: Stark effect is the splitting of spectral lines observed when the radiating atoms, ions, or molecules are subjected to a strong electric field.

Applied Field

Zeeman Effect: Zeeman effect can be observed in an applied magnetic field.

Stark Effect: Stark effect can be observed in an applied electrical field.

Cause

Zeeman Effect: Zeeman effect is the result of the interaction between the magnetic moment of the atom and the external magnetic field.

Stark Effect: Stark effect arises due to the interaction between the electric moment of the atom and the external electric field.

Conclusion

Zeeman effect was discovered by a Dutch scientist Pieter Zeeman. Stark effect was discovered by the German scientists Johannes Stark. The main difference between Zeeman effect and Stark effect is that Zeeman effect is observed in the presence of an external magnetic field whereas Stark effect is observed in the presence of an external electrical field.

References:

1. “Zeeman effect.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 20 June 2011, Available here.
2. “Zeeman Effect in Hydrogen.” Zeeman Effect, Available here.

Image Courtesy:

“Stark splitting” (Public Domain) via Commons Wikimedia