Knowledge Bank

Although the most intense transitions in resonance fluorescence from $A~2B2$ to the $X~2A1$ ground state of $NO2$ are the expected `parallel’ $(\Delta K=0)$ transitions, the total intensity contains an important secondary contribution from first-order forbidden transitions with $\Delta \ne 0$. This component of the system divides into two further categories, one where $\Delta K$ has odd parity, the other having $\Delta K$ even but different from zero. The first-order forbidden even-parity transitions range from $\Delta K=2$ up to and including $\Delta K=6$. Both types of perturbation are considered to originate from perturbations in the $A~$ state, and reasons for their notably high incidence will be discussed.