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Molecular-Frame Angular Distributions of Resonant Auger Electrons Print

 

Molecular-frame electron angular distribution (MFAD) measurements provide access to an unprecedented level of detailed information about phenomena involving quantum coherence, such as phases of photoelectron waves, symmetry breaking in molecular dissociation, core-hole localization in molecules, and molecular double-slit interference, all of which are hidden in conventional gas-phase electron spectroscopy, owing to the random orientation of the molecules. While most MFAD studies to date have focused on photoelectrons, an international team of scientists from Western Michigan University, the ALS, and Tohoku University in Japan has successfully used a novel approach to determine for the first time the molecular-frame angular distributions of resonantly excited Auger electrons in carbon monoxide.

Simplifying Molecular Measurements

To know what it’s like to whirl, twirl, and dive for a seeming eternity while braving a thrill ride at an adventure park, it's much better to be the rider than the spectator. For those studying the intimacies of the quantum world at the molecular level, some of the same spirit applies: it's much better to be in the molecular frame of reference—on the molecule as it were—than in the laboratory looking on. Phenomena that are otherwise obscured by the random orientation of the many molecules in a gaseous sample under investigation become clear and obvious when measurements are made in this way. Easier said than done, however; after all, scientists and their experimental equipment are in the laboratory, and how do you isolate one molecule at a time to " ride" it.

Testing a new approach to making measurements in the molecular frame of reference that was recently developed by a U.S.–Japan collaboration, Rolles et al. have now verified that there is a comparatively simple way out in experiments involving x-ray "excitation" of molecules. By correctly choosing the placement of a small number of detectors used to track the electrons and ions (charged atoms, molecules, and molecular fragments) that result when molecules absorb x rays, the number of measurements required to "see" in the molecular frame of reference are considerably reduced, making such experiments feasible to conduct and providing the basis for new insights in the dynamic world of molecular phenomena.

The molecular frame is the natural reference frame for the study of molecules and their interaction with electromagnetic radiation or charged particles, but in order to experimentally determine MFADs, the molecules have to be ‘‘fixed’’ in space, usually by angle-resolved coincidence measurements of the electron(s) and ion(s) resulting from photoexcitation of the molecule and subsequent decay processes. Because of their element and site specificity, Auger electrons are often used as a probe for the atomic environment in large molecules and solids. In the case of resonant excitation (in which there are no photoelectrons), Auger electrons represent the only way (apart from fluorescence) to obtain information.

Figure 1. Comparison of three-dimensional MFADs of the h and i groups of the Auger spectrum for carbon monoxide molecules oriented perpendicular to the electric field vector E of the exciting radiation (white sticks) for experiment and theory. The intensity is zero at the intersection of the electric field vector E and the molecular C–O axis.

There have been no reports of MFAD experiments for resonantly excited Auger electrons, in part because measurements require rotating either the electron spectrometer or the polarization vector of the exciting radiation and hence are substantially more challenging and time consuming than for photoelectron MFADs. Recently, a new analytical framework was developed at Tohoku University that makes it possible to obtain full three-dimensional MFADs from measurements of electrons at only two angles in combination with momentum-resolved detection of the ions over the full 4π solid angle.

To test the new approach the research team applied it to identifying the molecular states involved in two groups (labeled h and i) from the Auger spectrum following the resonant CO:C(1s)→ π* excitation. Measurements of Auger electrons in coincidence with C+ (if the molecule fragments following Auger emission) or CO+ (if it does not) ions were made at ALS Beamline 4.0.2, with the ALS operating in the two-bunch mode. The apparatus consisted of an ion-momentum-imaging spectrometer and two electron time-of-flight (TOF) analyzers mounted in the plane perpendicular to the light propagation direction at 0° and 54.7°. With the newly developed transformation method, this was sufficient to determine the three-dimensional MFADs from the measurements.

Fig. 2 Calculated Auger electron MFADs for the three overlapping Auger lines within group i (note that the 4 2∏ line here is the “tail” of the 4 2∏ line in group h).

The measured MFADs of the h and i groups show distinct differences. In particular, the MFAD of group h displays a strong asymmetry with a preferential emission of the electrons along the molecular axis in the direction of the carbon atom, while the MFAD of group i is more isotropic and has a large fraction of its total intensity perpendicular to the molecular axis in the plane through the origin. The different shapes of the MFADs are well reproduced by ab initio calculations. They provide an unambiguous assignment of group h to two states, 3 2∏ and 4 2∏, which have almost identical MFADs. Group i appears in the energy region where the calculations predict transitions to the 5 2∏ and 1 2Φ states.

In sum, the measurements yield very different angular distributions for the close-lying h and i groups, both of which could be identified through comparison with theoretical predictions. Moreover, contributions from experimentally unresolved Auger transitions into the i group could be identified. The team’s calculations were able to pass the stringent test of reproducing well the experimental angular distributions, thus providing the basis for further theoretical advances, new insights into the dynamics of molecular Auger decay, and identification of different molecular states.

 

 

Research conducted by D. Rolles, Z.D. Pesic, and I. Dumitriu (Western Michigan University and ALS); G. Prümper, H. Fukuzawa, X.-J. Liu, and K. Ueda (Tohoku University, Japan); R.F. Fink (University of Würzburg, Germany); A.N. Grum-Grzhimailo (Tohoku University, Japan, and Moscow State University, Russia); and N. Berrah (Western Michigan University).

Research Funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES);
the Japanese Society for the Promotion of Science; the Japanese Ministry of Education; and the Alexander von Humboldt Foundation.

Publication about this research: D. Rolles,G. Prumper, H. Fukuzawa, X.-J. Liu, Z.D. Pesic, R.F. Fink, A.N. Grum-Grzhimailo, I. Dumitriu, N. Berrah, and K. Ueda,"Molecular-frame angular distributions of resonant CO:C(1s) Auger electrons," Phys. Rev. Lett. 101, 263002 (2008).

 

 

ALSNews Vol. 298, May 27, 2009

 

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