The most common types of transitions are \( E1 \), \( E2 \) and \( M1 \). The \( E1 \) transition operator is given by \( \lambda=1 \) $$ O(E1) = rY^{(1)}_{\mu }(\hat{r})e_{q} e= \sqrt{\frac{3}{4\pi }}\mathbf{r}e_{q} e. $$ The \( E2 \) transition operator with \( \lambda=2 \) $$ O(E2) = r^{2}Y^{(2)}_{\mu }(\hat{r})e_{q} e, $$