Measurement of the rotational alignment in the presence of the second order AC Stark effect

The determination of the rotational quadrupole alignment of diatomic molecu les via REMPI detection is investigated. In this process a high focal inten sity usually increases the detection probability. At high intensities the A C Stark effect may cause a splitting of the normally degenerate mj sublevel s of a rotational state J beyond the spectral width of the exciting radiati on. This leads to a selective detection of only certain mi states with the consequence that deduced alignment factors can be misleading. From the theo retical considerations line profiles are explicitly calculated for dynamic polarizabilities which represent the B(1)Sigma (+)(u) <-- X(1)Sigma (+)(g) transition of H-2, in order to fit an experimental (3+1) REMPI spectrum and to predict (1+1') line shapes as a function of laser intensity. It is furt her shown that the deduced quadrupole alignment factor A(0)((2)) is signifi cantly changed by the second order AC Stark effect when the intensities are chosen high enough to observe asymmetric broadened line profiles. Differen t combinations of relative linear polarizations of the exciting and ionizin g laser beams are discussed.