Paschen-Back effect

Paschen-Back effect spect An effect on spectral lines obtained when the light source is placed in a very strong magnetic field the anomalous Zeeman effect obtained with weaker fields changes over to what is, in a first approximation, the normal Zeeman effect. pash-on bak i,fekt ... 

The Extreme Paschen-Back Effect.—If the magnetic field is very strong, the interactions that cause the spins of the electrons to combine to a resultant spin and the orbital moments to combine to a resultant orbital moment are broken. Then each electron orients its spin independently in the magnetic field, having two possible values, and... 

Fig. IV-3.—Diagram representing orientations of the spin vectors of the two electrons and the orbital angular momentum vectors of the two electrons in the extreme Paschen-Back effect for an atom with two 2 electrons. The two spins orient themselves separately in the vertical magnetic field, as do also the two orbital angular momentum vectors. Each electron spin can assume orientations such that the component of angular momentum along the field direction is represented by the quantum number m, + or — and each orbital angular momentum may orient itself in such a way that the component of the orbital angular momentum along the field direction is represented by the quantum number m +1, 0, or —1.

BACK-GOUDSMIT EFFECT. An effect closely related to the Zeeman effect. It occurs in the spectrum of elements having a nuclear magnetic and mechanical moment. See also Hyperfine Structure and Paschen-Back Effect. 

Closely related to the Zeeman effect are two others, the Paschen-Back effect, produced by very strong magnetic fields, and the Back-Goudsmit effect, observed with the spectra of elements having a nuclear magnetic moment, such as bismuth. See also Chemical Elements Electron Theory Paschen-Back Effect and Stark Effect. 

Figure 4 Anomalous Zeeman effect on a 2P-2S transition (weak field). The strong-field limit is called the Paschen-Back effect. The splitting of the levels is not drawn to scale.

Carrying on the investigation of the anomalous Zeeman effect and the Paschen-Back effect on the spectra of the alkali atoms, Pauli postulated that an electron in an external magnetic field has to be described by four independent quantum numbers. Moreover, in order to justify the Bohr-Sommerfeld Aufbau (building-up) principle of the periodic system of the elements, he came up with his famous exclusion principle (Ausschliefiungsprinzip). In its original formulation it reads 10... 

In order to identify the location of the carbon sources, Zeeman spectroscopy has turned out to be a valuable tool. The principles of the influence of the magnetic field on carbon and oxygen ions in the fusion edge plasma (B = 1 to 10 T) have been outlined in [19,20]. The method works well when the Zee-man (Paschen-Back) effect plays an important, or dominant, role in relation to other broadening mechanisms. In general the line splitting is given by ... 

Fig. 7.—Vector model for the Paschen-Back effect (transition to the normal Zeeman effect). Since the energy of the orbital moment and spin-moment in the magnetic field is greater than the magnetic interaction between orbit and spin, the orbital and spin-moments process separately round the field direction.

We therefore in this case obtain a term-splitting with the term-separation corresponding to the normal Zeeman effect. Thus when the magnetic field is steadily increased, a gradual transition takes place from the anomalous to the normal Zeeman effect the transitional zone is referred to as that of the Paschen-Back effect. The nomen-... 

We note in passing that this definition of a strong field is not the one found in textbooks, which relates to the relative strengths of the Zeeman splitting and multiplet structure (the Paschen-Back effect). A field capable of inducing angular-momentum uncoupling can still be a weak field under the present definition.2... 

Paschen-Back effect - In atomic spectroscopy, the decoupling of electron spin from orbital angular momentum as the strength of an external magnetic field is increased. 

In strong magnetic fields the magnetic energy of an atom will be higher than the splitting of the lines. Then the L-S interaction will become negligible and the anomalous Zeeman effect resembles the Zeeman triplet (the Paschen-Back effect). 

At very high magnetic fields a splitting pattern known as the Paschen-Back effect occurs. In this pattern, named for the German spectroscopists Friedrich Paschen and Ernst Back in 1912, the basic pattern returns to that of the normal Zeeman effect, but with each line split up into a set of closely spaced lines. This occurs because the total orbital and spin angular momentum vectors of the atom, denoted L and S respectively, precess independently about the direction of the magnetic field. 

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