These states describe the spin configurations of electrons in atoms or molecules and are fundamental in spectroscopy and photochemistry.
Key Concepts of Electronic States:
Electron Spin:
Electrons have an intrinsic angular momentum, spin, with values of +½ (spin-up) or -½ (spin-down).
Pauli Exclusion Principle:
No two electrons in the same orbital can have identical quantum numbers; thus, electrons in the same orbital must have opposite spins.
Multiplicity:
Defined as 2S+12S + 12S+1, where SSS is the total spin quantum number (sum of unpaired electron spins).
Indicates the number of possible spin states for a given configuration.
Singlet, Doublet, and Triplet States:
Singlet State:
Electron Configuration: No unpaired electrons; all are spin-paired.
Total Spin (SSS): 0
Multiplicity: 2(0)+1=12(0) + 1 = 12(0)+1=1
Example: Ground state of most closed-shell molecules (1X^1X1X, e.g., 1S^1S1S).
Doublet State:
Electron Configuration: One unpaired electron (common in radicals).
Total Spin (SSS): ½½½
Multiplicity: 2(½)+1=22(½) + 1 = 22(½)+1=2
Example: Radical species (2X^2X2X).
Triplet State:
Electron Configuration: Two unpaired electrons with parallel spins.
Total Spin (SSS): 1
Multiplicity: 2(1)+1=32(1) + 1 = 32(1)+1=3
Example: Excited states of many molecules (3X^3X3X, e.g., 3P^3P3P).
Jablonski Diagram
A Jablonski diagram illustrates the electronic states of a molecule and the transitions between them:
Vertical Transitions: Represent absorption of photons leading to excitation.
Horizontal Transitions: Indicate non-radiative processes like internal conversion and intersystem crossing.
Downward Transitions: Depict emission processes such as fluorescence and phosphorescence.