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October 27, 2004

 

 

Spin-Resolved Photoelectron Spectroscopy

Inner-shell electrons, localized on a single atom, are sensitive site-specific probes of the molecular environment, unlike valence electrons, which can be delocalized over the whole molecule. Binding energies of inner-shell electrons for an atom vary depending on the electron density around that atom modified by the electron-donating and -withdrawing properties of the surrounding atoms. Furthermore, the molecule's geometry can remove the degeneracy of inner-shell electrons, separating them depending on the orientation of their orbitals within the molecule. Such phenomena do not occur in atoms, and hence the investigation of inner-shell photoionization in molecules is considerably more complex. At ALS Beamline 4.0.2, a group of scientists has, for the first time, used spin-resolved photoelectron spectroscopy to study molecular effects on the inner-shell electrons of two simple sulfur-containing molecules, carbonyl sulfide and hydrogen sulfide. The authors show that this technique is very sensitive to molecular environment in the selected cases by comparing with previous atomic results.

An "Environmentally
Sensitive" Probe

How does the presence of neighboring atoms affect the local environment of an atom in a molecule? Within the close quarters of a molecule, even electrons in the tightly bound inner shell of an atom feel the effects of the complicated molecular field generated by the atoms next door. Carbonyl sulfide and hydrogen sulfide are both simple triatomic molecules that contain a single sulfur atom in different configurations. Both are well-studied molecules found in planetary atmospheres and are of great interest in biological, astronomical, and fundamental research. Scientists study the molecules' interactions with various forms of radiation, including soft x rays, to better understand phenomena such as the chemistry of the sulfur cycle in the Earth's atmosphere or spectroscopic data from interstellar space. Experiments that measure the energies and ejection angles of electrons emitted when the molecules are exposed to soft x rays have long been used to obtain detailed information about the molecules' electronic structures. With this work, Turri et al. have now demonstrated the ability to further probe the molecular environment with exquisite sensitivity by also detecting the spin of ejected inner-shell photoelectrons.

Carbonyl sulfide (OCS) and hydrogen sulfide (H2S) are simple sulfur-containing molecules of fundamental, biological, and astrophysical interest. These molecules have different geometries: OCS is linear with a terminal sulfur atom, whereas H2S is bent with a central sulfur atom. The 2p inner-shell level of the isolated sulfur atom has six electrons arranged into three orbitals separated by spin-orbit splitting: one sulfur 2p1/2 level with two electrons and two degenerate sulfur 2p3/2 levels with four electrons. In the anisotropic field of the OCS and H2S molecules, however, the degeneracy of the sulfur 2p3/2 level is removed, splitting it into two levels with slightly different binding energies and resulting in two closely spaced peaks in the photoelectron spectrum. These molecular effects obviously cannot be explained using a purely atomic model, but rather require the use of more complex molecular models.

Measurements were performed using circularly polarized light from the elliptically polarizing undulator (EPU) at Beamline 4.0.2. Spin-resolved photoelectron spectra were measured using a time-of-flight (TOF) electron spectrometer, combined with a Mott polarimeter of the Rice type. In the Mott detector, electrons emerging from the TOF spectrometer are accelerated up to 25 keV and scattered off of a thorium foil. Depending on their spin, the electrons will preferentially scatter into one of two detectors, thus determining their spins.

 

Photoionizatin spectrum

Photoionization spectrum of sulfur 2p shell of H2S as measured at 54.7° (magic angle) with 200-eV photon energy. The continuous and dashed curves are the result of a least-squares fitting procedure. Lines labeled A, B, and C are associated with the H2S ion in its ground vibrational state.

The spin polarizations of the two field-split components of the sulfur 2p3/2 levels were graphed as a function of photoelectron kinetic energy for OCS and H2S. A negative spin polarization here indicates that the electrons are preferentially emitted with spin parallel, rather than antiparallel, to the direction of photon propagation. To explore the effects of the molecular environment, the researchers compared their experimental results with values calculated for a fully atomic model. In both molecules, the sulfur atom has a filled outer shell and thus has the same electronic configuration as argon. Calculations of the spin polarization for argon 2p photoionization have been published previously and are depicted in the graph as solid lines. The atomic model is in good agreement with the spin polarization of both field-split sulfur 2p3/2 lines in OCS, while those for H2S deviate both from each other and from the calculations over a wide energy range. However, when compared with data for the unresolved sulfur 2p3/2 lines (shown as red asterisks on the graph), the argon calculations do reproduce the observed spin polarization in H2S.

Spin polarization

Spin polarization as a function of the photoelectron kinetic energy. Experimental points are for the ground vibrational state of the molecular field-split sulfur 2p3/2 (B = green, C = blue). For the H2S molecule, the unresolved doublet 2p3/2 (red) is also shown. Solid lines are calculations for argon 2p photoionization.

It is clear that the different molecular environments of the sulfur atoms in OCS and H2S affect the spin polarization of the photoelectrons from the 2p shell. The molecular effects disappear when the results are integrated over the field-split sulfur 2p3/2 levels, restoring somehow the spherical symmetry found in the atomic case. Similar, albeit less intense, behavior has been observed for the angle-resolved photoionization cross sections for these molecules.

Research conducted by G. Turri, G. Snell, and S.E. Canton (Western Michigan University and Berkeley Lab); B. Langer (Max-Born-Institute, Germany); M. Martins (Universität Hamburg, Germany); E. Kukk (University of Oulu, Finland); N. Cherepkov (State University of Aerospace Instrumentation, Russia); J.D. Bozek and A.L. Kilcoyne (Berkeley Lab); and R.C. Bilodeau and N. Berrah (Western Michigan University).

Research funding: U.S. Department of Energy, Office of Basic Energy Sciences (BES) and Research Council for the Natural Sciences of the Academy of Finland. Operation of the ALS is supported by BES.

Publication about this research: G. Turri, G. Snell, B. Langer, M. Martins, E. Kukk, S.E. Canton, R.C. Bilodeau, N. Cherepkov, J.D. Bozek, A.L. Kilcoyne, and N. Berrah, "Probing the molecular environment using spin-resolved photoelectron spectroscopy," Phys. Rev. Lett. 92, 013001 (2004).

ALSNews Vol. 246, October 27, 2004

 

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