Optical Pumping of Molecules

A technique is described for the optical pumping of gas‐phase molecules based on the orientation dependence of their absorption cross section. It is shown that when the rate of excitation exceeds the relaxation rate of a ground‐state $\mathrm{(\upsilon ″,\; J″)}$ level, the magnetic sublevels are preferentially “burned away” and the steady‐state M‐level population is altered appreciably. An experiment to demonstrate molecular alignment has proved successful using the 4880‐Å line of a cw argon‐ion laser to cause the transition $\text{(υ″\hspace{0.17em}=\hspace{0.17em}3, J″\hspace{0.17em}=\hspace{0.17em}43)→(υ′\hspace{0.17em}=\hspace{0.17em}6, J′\hspace{0.17em}=\hspace{0.17em}43)}$ in the $B^1{\Sigma }_u{\mathrm{\hspace{0.17em}-\hspace{0.17em}X}}^1{\Sigma }_g^{+}$ blue‐green band system of the Na₂ molecule. The establishment of an unequal magnetic sublevel population is detected by monitoring the degree of polarization $P$ of the fluorescence as a function of laser intensity. It is found that $P$ first remains constant then decreases by about 50% as the laser‐beam power increases from 0.1 to 150 mW. In addition, the absorption becomes nonlinear with a power threshold corresponding to the onset of the decrease in $P$ . This optical pumping technique is applicable to a wide class of molecules, and both simple classical and quantum mechanical theories of the pumping process have been formulated. These theories relate the degree of polarization to both the pumping rate and the relaxation time of the pumped level. The time‐dependent as well as steady‐state behavior of $P$ is examined. Under conditions of strong optical pumping it is predicted that the degree of polarization as a function of time will overshoot its steady‐state value, and as the pumping rate increases large negative transients in $P$ will appear.