Circular-dichroism effects on photoelectron angular distributions for the 7 P and 8 P states of cesium C. S. Feigerle and R. N. Compton * Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996 L. E. Cue´llar Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996 N. A. Cherepkov ² Falkuta¨t fu¨r Physik, Universita¨t Bielefeld, 33501 Beilefeld, Germany L. V. Chernysheva A.F. Ioffe Physical-Technical Institute, 194021 St. Petersburg, Russia ~Received 5 July 1995; revised manuscript received 26 December 1995! Circular dichroism in the photoelectron angular distribution ~CDAD! of the 7 P J and 8 P J states of cesium have been measured using plane-polarized light to excite and align the atoms and alternately right- and left-circularly polarized light to ionize them. The experiments are compared with the results of theory, calcu- lated using the Hartree-Fock wave functions with many electron correlations included in the random-phase approximation with exchange. A comparison of theory and experiment allows information on the alignment of the resonant state to be extracted from the experimental data. No CDAD effect is observed for the nP 1/2 states that cannot be aligned, whereas CDAD is observed for the 7 P 3/2 and 8 P 3/2 states. The CDAD signal is significantly larger for the 8 P 3/2 state than for the 7 P 3/2 state, indicating a greater retention of the optical alignment of the 8 P 3/2 state within the time sampled by the ionizing laser pulse. The differences in the retention of alignment for the two states are attributed to effects of hyperfine depolarization, caused by coupling of the electronic angular momentum to the nuclear spin. @S1050-2947~96!05306-1# PACS number~s!: 32.80.Fb I. INTRODUCTION The interaction of polarized light with chiral substances, i.e., those that lack a plane of symmetry and an inversion center, forms the basis for a variety of phenomenon impor- tant to the fields of optics, physics, chemistry, biology, and medicine. The year 1995 marks the 150th anniversary of the landmark discovery of Pasteur that solutions made from separated asymmetric crystals of tartaric acid rotate plane- polarized light to the left or right depending on the handed- ness of the crystal dissolved in solution. One of the most common means of probing chirality is through measurements of circular dichroism ~CD!. In general, circular dichroism is a phenomenon where the response of the system to right- and left-circularly polarized light is different. Historically, circu- lar dichroism is observed either as a rotation of plane- polarized light or as a conversion of linearly to elliptically polarized light @1,2#. In the bound-bound region of the spec- trum of a chiral molecule, circular dichroism arises from a difference in the absorption coefficients for right- and left- circularly polarized light. These differences are apparent only when terms beyond the dipole approximation are in- cluded in the theory, yielding an interference term between the magnetic- and electric-dipole moments that has a differ- ent sign for the two polarizations @3#. The extension of circular dichroism to bound-free transi- tions for a free molecule was considered by Ritchie @4#, who derived a theory of circular dichroism in angular distribu- tions ~CDAD! of photoelectrons from oriented molecules. In that analysis, differences in the angular differential cross sec- tion for right- and left-circularly polarized light stemmed from an interference between the electric- and magnetic- dipole terms, similar to traditional CD. Subsequent analyses @5–8# have shown that interferences between the final-state continuum partial waves allow CDAD to exist within the electric-dipole approximation. Theory has predicted that in addition to chiral molecules, CDAD is possible for fixed-in- space linear molecules @5–7# aligned or oriented atoms @8–10# and aligned molecules @11#. Initially, it would appear that each of these systems falls outside the classical defini- tion of a chiral target; however, if one considers the three- vector system of the photon propagation direction, the target alignment ~or orientation! axis, and the electron collection direction, a ‘‘handedness’’ for the photoionization process can be established. The photoelectron intensity will depend on whether this handedness matches or is opposite that of the ionizing photon. Because of this effective chirality require- ment, CDAD cannot exist if the target is isotropic. Thus the very existence of a nonzero CDAD implies alignment or ori- entation ~including fixed-in-space molecules! of the target and the specific shape of the CDAD spectrum provides de- tails about it @12#. The relationship between CDAD and * Also at Chemical Physics Section, Oak Ridge National Labora- tory, Oak Ridge, TN 37831. ² Permanent address: State Academy of Aerospace Instrumenta- tion, 190000 St. Petersburg, Russia. PHYSICAL REVIEW A JUNE 1996 VOLUME 53, NUMBER 6 53 1050-2947/96/53~6!/4183~7!/$10.00 4183 © 1996 The American Physical Society