Molecular alignment from circular dichroic photoelectron angular distributions in (n+1) resonance enhanced multiphoton ionization

The theory for determination of molecular alignment from circular dichroism in photoelectron angular distributions is generalized to treat the case in which the excitation polarization direction and the laboratory z axis do not coincide. A new method of data analysis is presented here. Alignment created by surface scattering or photofragmentation should be obtainable by these procedures. For studies of orientation with elliptically polarized excitation, differential cross sections at a given collection angle are found to be, to a good approximation, independent of excited-state alignment. Orientation can thus be obtained from differential cross sections by the methods developed by Kummel, Sitz, and Zare [J. Chem. Phys. 88, 6707 (1988)]

Extraction of alignment parameters from circular dichroic photoelectron angular distribution (CDAD) measurements

In a previous paper, we showed that circular dichroism in photoelectron angular distributions (CDAD) can be used to probe alignment in gas phase atoms and linear molecules. Often this alignment is parametrized through the moments of alignment A(2), A(4), etc., which are commonly extracted from fluorescence polarization measurements. In this paper we show how these can be simply extracted from CDAD spectra. This technique can be used in principle to extract the moments to any order

(1+ 1) CDAD: A new technique for studying photofragment alignment

We report a new technique for measuring photofragment alignment in the gas phase by observing circular dichroism in photoelectron angular distributions (CDAD). This technique is well suited for determining the gas phase alignment of linear molecules. The experiment involves excitation of the photofragment with linearly polarized light followed by photoionization with left or right circularly polarized light. The difference between the photoelectron angular distributions for these two cases is the CDAD spectrum. By measuring CDAD through two different excitation branches, one can obtain the ground state photofragment alignment A(2)0 using a simple analytical formula independent of the photoionization dynamics

Circular dichroism in photoelectron angular distributions from two‐color (1+1) REMPI of NO

A detailed experimental and theoretical study of dichroic effects in photoelectron angular distributions is reported for (1+1), two‐color REMPI of NO via the A^ 2Σ^+, v=0 state. Optically aligned A state rotational levels are probed through ionization by circularly polarized light. 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 M_J 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

Studies of angle-resolved photoelectron spectra from oriented NiCO: A model for adsorbed CO

We present a study of angle-resolved photoelectron spectra from oriented, linear NiCO. We address the question: How well do simple cluster models such as oriented NiCO simulate adsorbate molecules with respect to photoemission? The photoemission cross sections are obtained using Hartree-Fock electronic continuum orbitals. For the bonding 5σ̃ orbital, we find oriented NiCO to be a better model than oriented CO. The large magnitude of the 5σ̃ photoionization cross sections relative to the 4σ̃ cross section cannot, however, be explained by our calculations without consideration of backscattering of the photoelectrons ejected "downward" into the detector

Atomic and molecular alignment from photoelectron angular distributions in (n+1) resonantly enhanced multiphoton ionization

Two distinct (n+1) REMPI techniques for obtaining the alignment of gas phase atoms and molecules from photoelectron angular distributions are presented. In both methods, the alignment is extracted from the angular distributions independently of the photoionization dynamics. The first method, which takes advantage of circular dichroism in the angular distributions (CDAD) has already been established experimentally as a useful probe of state alignment. The theory outlined in previous work is expanded here. The second method involves photoionization with light linearly polarized along the photoelectron collection direction and is presented here for the first time

Circular dichroism in photoelectron angular distributions as a probe of atomic and molecular alignment

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 2Sigma+ 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

Dynamics of Single- and Multi-photon Ionisation Processes in Molecules

Single-photon ionisation and resonant multiphoton ionisation studies, which can now be carried out using synchrotron radiation and pulsed dye lasers respectively, are providing important dynamical information on molecular photoionisation. In this paper we discuss some results of our recent studies of several, single- and multi-photon ionisation processes in molecules. The results will be taken from our studies of (i) single-photon ionisation of linear molecules with emphasis on the role of shape and autoionising resonances on these cross· sections, (ii) photoionisation from oriented NiCO as a simple but realistic model of photoemission in adsorbate-substrate systems, and (iii) resonant multiphoton ionisation of H_2

Condition-specific outcome measures for low back pain: Part II: Scale construction

A literature review of the most widely used, condition-specific, self-administered assessment questionnaires for low back pain has been undertaken. In part I, technical issues such as validity, reliability, availability and comparability were analyzed for the nine most widely used outcome tools. This second part focuses on the content and wording of questions and answers in each of the nine questionnaires, and an analysis of the different score results is performed. The issue of score bias is discussed and suggestions are given in order to increase the construct validity in the practical use of the individual questionnaire