Bonus question: (10 points) Use two-level Einstein coefficients to show that stimulated emission can only occur when there is population inversion. Sketch two common lasing schemes (Hint: use multi- levels, arrows with key labels). Why is the spin-orbit coupling needed for intersystem crossing? Bonus question: (10 points) Use two-level Einstein coefficients to show that stimulated emission can only occur when there is population inversion. Sketch two common lasing schemes (Hint: use multi- levels, arrows with key labels). Why is the spin-orbit coupling needed for intersystem crossing