These general considerations find practical application in the fact that many atoms have low-lying fine-structure levels that are prone to collisional excitation. The existence of such levels stems from the spin-orbit interaction. Following our discussion of the hydrogen atom in § 2.1, the relative motion of any orbiting electron and a charged nucleus creates a torque on the magnetic moment associated with the electron's intrinsic spin. The electrons of hydrogen and helium are described by single-particle wavefunctions that have no associated orbital angular momentum (l = 0). Hence, the electronic ground states of these atoms lack the internal torque and exhibit no fine-structure splitting. We must turn to oxygen, the next most abundant element.
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