Solution

ID: past-exam-of-the-physics-course-of-the-year-4-of-the-natural-sciences-course-of-the-university-of-cambridge/2016/paper-1/section-a/a9/solution

For hyperfine coupling for a spin-one nucleus, let the electron spin be and the nuclear spin . First couple , giving , then , giving .
Allowed
For each , the projection is ; electron projections similarly satisfy . Summing gives 18 states for and 30 for , agreeing with .
Notation qualification: if the printed leading “2” denotes the principal quantum number, a hydrogenic bound state does not exist because ; a physical state needs . If a doublet term symbol or was intended, its leading 2 instead means , and the angular-momentum table applies without fixing . The answers above use the explicitly specified values, while retaining this ambiguity.[ref]

New to topics? Read the docs here!