Precession induced modulation of fluorescence

A technique is presented with which the alignment of molecular angular momenta in molecular beam scattering can be measured. In principle the method can be applied to any molecule as long as the polarization of an absorbed or emitted photon is related directly to the orientation of the molecular angular momentum and as long as the orientation of the molecular angular momentum can be rotated by precession in a magnetic field. The main advantage of the method is that the relative change in fluorescence intensity, as modulated by the magnetic field, is directly proportional to the alignment of the angular momenta. Both magnitude of the alignment and direction of the principal axis of the orientational distribution of the angular momenta can be determined from the field dependence of the fluorescence intensity. The applicability of the method is demonstrated with some experimental results in which the alignment of Na2 is determined after scattering by He.