POLARIZATION DEPENDENCE OF THE AC STARK-EFFECT IN MULTIPHOTON TRANSITIONS OF DIATOMIC-MOLECULES

The (2 + 2) resonance-enhanced multiphoton ionization (REMPI) of N2 via the a 1-PI(g) (v = 1,J) levels shows a strong dependence on the polarization of the laser beam causing this process. This behavior is attributed to the ac Stark effect produced by the near resonance of the N2 o3 1-PI(u) (V = 0,J) levels with the sum of the first three photons. The multiphoton transitions are broadened and asymmetric in appearance; one level is even split. The line profiles change markedly as the polarization of the laser beam is varied from linear to circular. A general theory is presented for the ac Stark effect in a diatomic molecule undergoing a multiphoton transition. When the sum of the photon energies is resonant with an allowed transition, a splitting of the line is observed (Autler-Townes effect). Off resonance, the magnetic sublevels are shifted by different amounts, causing the line profile to be broadened and distorted. This theoretical treatment is able to explain in a satisfactory manner the observed behavior of (2 + 2) REMPI of N2 via the a 1-PI(g)-X 1-SIGMA(g)+ transition and the two-photon laser-induced fluorescence of CO via the A 1-PI-X 1-SIGMA+ transition.