A_ de~ail~d ex~erimental and theoretical study of dichroic effects in photoelectron angular d1stnbut10ns IS reported for ( 1 + 1), two-color REMPI of NO via the A 2 L +, v = 0 state.?ptically aligned A state rotational levels are probed through ionization by circularly polarized hght. Resultant photoelectron angular distributions exhibit significant left-right asymmetry, the phase and magnitude of which are shown to be related to the curvature of the excited state MJ distribution. Theoretical calculations involving a full ab initio treatment of the ionization dynamics result in circularly dichroic angular distribution ( CDAD) parameters in good agreement with those derived experimentally. Additional effects including hyperfine depolarization and coherence are also discussed in relation to the observed CDAD data.
Two-color ( 1 + 1) REMPI (resonantly enhanced multiphoton ionization) photoelectron spectroscopy is used to probe the NO photofragments produced by the UV photodissociation of methyl nitrite, i.e., CH 3 0NO + hv ...... CH 3 0NO*(S 2 ) ...... CH 3 0· (X)+ NO (X, v, J). The photofragments are produced in their ground electronic states but with high rotational and translational energy. NO fragment angular distributions, rotational state distributions, and spatial alignment are determined by photoion and photoelectron detection. The initial state alignment is obtained by the CDAD (circularly dichroic angular distribution) technique for the first time. CDAD measurements for rotational levels with 35.5 = -0.4. This alignment is consistent with an "impulsive" dissociation mechanism in which photofragment recoil along the CH 3 0-NO bond imparts substantial rotational angular momentum to the NO molecule resulting in a high-J state distribution and preferential rotation in the plane of dissociation. These measurements clearly establish the utility of the CDAD method for probing chemical processes in which spatial alignment plays an important role. Photoion angular distributions are used to probe correlations between the CH 3 0NO transition dipole moment, NO fragment velocity, and angular momentum. These correlations reveal additional details of the photolysis mechanism.
In this paper we show that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe atomic and molecular alignment in the gas phase. Careful choice of photon (left or right circularly polarized) propagation and photoelectron collection directions breaks the cylindrical symmetry of the target, giving rise to dichroic effects. CDAD exists in the electric dipole approximation. We illustrate the sensitivity of CDAD to alignment by considering photoionization of the A 2Σ+ state of NO. Most of the cases of alignment we consider are created by multiphoton absorption while the others, more general, might be created in fragmentation, desorption, etc. The alignment created by n-photon absorption quickly reaches a classical limit which is reflected in the CDAD spectrum. Finally, we show that CDAD is also a sensitive probe of gas phase atomic state alignment by considering photoionization of the 7P3/2 state of cesium created by single photon absorption from the ground state.
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