10.3 Electron spin and the Zeeman effect

Electron spin and the Zeeman effect are crucial concepts in understanding atomic structure. Spin describes an electron's intrinsic angular momentum, while the Zeeman effect shows how energy levels split in a magnetic field.

These phenomena reveal the quantum nature of atoms and their interactions with external fields. They help explain atomic spectra, magnetic properties, and provide insights into the fundamental behavior of electrons in atoms.

Electron Spin

Quantum Numbers and Spin Properties

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• Spin quantum number (s) describes intrinsic angular momentum of electrons
• Electrons possess a fixed spin value of s = 1/2
• Spin magnetic quantum number (m_s) specifies spin orientation
• m_s can take values of +1/2 (spin-up) or -1/2 (spin-down)
• Spin angular momentum magnitude calculated as $\sqrt{s(s+1)}\hbar$
• Electron spin generates a magnetic moment proportional to its angular momentum
• Magnetic moment magnitude given by $\mu_s = -g_s\mu_B\sqrt{s(s+1)}$
• g_s represents the electron spin g-factor, approximately equal to 2
• μ_B denotes the Bohr magneton, a fundamental unit of magnetic moment

Stern-Gerlach Experiment

• Conducted by Otto Stern and Walther Gerlach in 1922
• Demonstrated the quantization of angular momentum in atoms
• Experimental setup involved a beam of silver atoms passing through an inhomogeneous magnetic field
• Observed beam splitting into two distinct components
• Results confirmed the existence of electron spin
• Beam splitting explained by interaction between electron magnetic moment and applied magnetic field
• Provided experimental evidence for spatial quantization of angular momentum
• Contributed to the development of quantum mechanics and understanding of atomic structure

Spin-Orbit Interaction

Spin-Orbit Coupling Mechanism

• Arises from interaction between electron's spin magnetic moment and orbital magnetic moment
• Electron experiences an effective magnetic field due to its orbital motion around the nucleus
• Spin magnetic moment interacts with this effective magnetic field
• Coupling strength depends on atomic number (Z) and principal quantum number (n)
• Increases for heavier atoms and decreases for higher energy levels
• Leads to energy level splitting, affecting atomic spectra
• Described by spin-orbit coupling constant (ζ)
• ζ proportional to Z^4/n^3 for hydrogen-like atoms

Fine Structure and Energy Level Splitting

• Fine structure results from spin-orbit interaction and relativistic corrections
• Causes splitting of energy levels in atomic spectra
• Introduces small energy shifts compared to gross structure (principal quantum number effects)
• Total angular momentum quantum number (j) combines orbital (l) and spin (s) angular momenta
• j takes values from |l-s| to l+s in integer steps
• Energy level splitting depends on j value
• Fine structure constant (α) characterizes the strength of electromagnetic interactions
• α approximately equal to 1/137, dimensionless quantity
• Fine structure splitting proportional to α^2 times the gross energy level spacing

Zeeman Effect and Magnetic Interactions

Normal and Anomalous Zeeman Effect

• Zeeman effect describes splitting of spectral lines in the presence of an external magnetic field
• Normal Zeeman effect observed in atoms with zero total angular momentum (singlet states)
• Results in three equally spaced spectral lines (triplet)
• Anomalous Zeeman effect occurs in atoms with non-zero total angular momentum
• Produces more complex splitting patterns due to spin-orbit coupling
• Energy level splitting proportional to magnetic field strength
• Selection rules govern allowed transitions between Zeeman-split levels
• Δm_j = 0, ±1 for transitions in the presence of a magnetic field

Larmor Precession and Magnetic Interactions

• Larmor precession describes the motion of magnetic moments in an external magnetic field
• Angular frequency of precession (ω_L) given by $\omega_L = \gamma B$
• γ represents the gyromagnetic ratio, characteristic of the particle
• B denotes the external magnetic field strength
• Precession occurs around the direction of the applied magnetic field
• Energy of magnetic moment in external field given by $E = -\vec{\mu} \cdot \vec{B}$
• Leads to quantization of energy levels in the presence of a magnetic field

g-factor and Magnetic Moment

• g-factor relates magnetic moment to angular momentum
• Different g-factors for orbital (g_l = 1) and spin (g_s ≈ 2) angular momenta
• Landé g-factor (g_J) describes the effective g-factor for total angular momentum
• g_J calculated using the formula $g_J = 1 + \frac{j(j+1) + s(s+1) - l(l+1)}{2j(j+1)}$
• Determines the magnitude of energy level splitting in the Zeeman effect
• Explains the observed intensity patterns in Zeeman-split spectral lines
• Anomalous g-factor of the electron (g_s ≈ 2.002) explained by quantum electrodynamics
• Precise measurements of g-factor provide tests of quantum electrodynamics theory