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ALSNews           Vol. 57           July 24, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. RESEARCHERS CHALLENGE COMMON VIEW OF SURFACE BOND CHEMISTRY
    3. USERS' TOWN MEETING AUGUST 14

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam availability for the last two weeks was 91.1% overall and 90% for user shifts. All problems were repaired and were of short duration. A power dip caused a 3-1/2-hour interruption to operations on July 19.

Operations Summary for July 24 - August 12

Jul 24, 00:00-24:00           Accelerator Physics
Jul 25, 00:00- Jul 29, 07:15  1.5-GeV/400-mA/320-bunch user operations
Jul 29, 07:30-24:00           Maintenance & Startup
Jul 30, 00:00-24:00           Accelerator Physics
Jul 31, 00:00-08:00           User Scrubbing & Special Operations
Jul 31, 08:00- Aug 05, 07:15  1.5-GeV/400-mA/320-bunch user operations
Aug 05, 07:30-24:00           Maintenance & Startup
Aug 06, 00:00-24:00           Accelerator Physics
Aug 07, 00:00-08:00           User Scrubbing & Special Operations
Aug 07, 08:00- Aug 12, 07:15  1.1-GeV/400-mA/320-bunch user operations

2. RESEARCHERS CHALLENGE COMMON VIEW OF SURFACE BOND CHEMISTRY
(contact: anders.nilsson@fysik.uu.se)

The bonding of molecules to surfaces is of interest to a wide variety of researchers in surface and interface science. Recently the common textbook view of the surface bond received a challenge from a research group using x-ray emission spectroscopy to study the surface chemical bond between molecular nitrogen (N2) and a nickel surface.

Chemistry textbooks and the published literature in surface/interface science describe the bonding (adsorption) of a molecule to a transition metal surface in terms of the interaction of the metal's d orbitals with the adsorbed molecule's outer molecular orbitals (lowest unoccupied molecular orbital or LUMO, and highest occupied molecular orbital or HOMO). Testing this model involves taking a look (via spectroscopy) at the adsorbed molecule's molecular orbitals. Such testing has been extremely difficult, however, since photoemission spectroscopy (the conventional method for such experiments) shows many overlapping states, with the metal's d states dominating the spectrum, so that little can be discerned about the adsorbed molecule's states.

A research group at the ALS led by Anders Nilsson and Nial Wassdahl of Uppsala University, Sweden, overcame this problem using x-ray emission spectroscopy (XES). The advantage of XES for this experiment is that by tuning the incoming photon energy, experimenters can selectively excite electrons from one nitrogen atom or the other of the adsorbed N2 molecule, and not from the nickel surface. (Since N2 bonds to nickel in a "standing-up" position, the energies of the two atoms' core states are shifted slightly by their differing proximity to the nickel surface.) The group used this approach to produce separate spectra for each nitrogen atom. The results show that all valence states are affected by the formation of the adsorption bond, even though the bond is relatively weak (0.4 eV). All spectral peaks (representing the 2p projected molecular orbitals) exhibit different intensities or shapes for the inner and outer nitrogen atoms. Also, the spectra show new molecular states that arise from interaction of molecular pi states with nickel d states. These new states are the most important ones for the surface chemical bond. The nitrogen molecules' sigma states change spatially; for example, the 4-sigma molecular orbital becomes polarized toward the inner nitrogen atom, and the 3-sigma and 5-sigma orbitals become predominantly located on the outer nitrogen atom.

Two characteristics of the ALS undulator beam the experimenters used--high intensity and a small wavelength spread--make it particularly useful for this experiment. XES experiments require a high-intensity source because x-ray emission yields only about one photon emitted per thousand photons absorbed. Selective-excitation experiments require a highly monochromatic source in order to produce selective excitation of only one nitrogen atom in the N2 molecule. Indeed, according to Nilsson, it was the possibility of performing this kind of experiment that brought his research group to the ALS in the first place.

3. USERS' TOWN MEETING AUGUST 14

The next town meeting of ALS management and users will occur on August 14 from 3 to 5 p.m. in the Building 4 conference room. The primary topic will be the long-term operations schedule for the period from November 6, 1996 to April 27, 1997. Users are invited to come with comments or requests regarding energy and other aspects of operations. There will also be a short presentation by the accelerator physics group on operating conditions, brightness, beam lifetime, etc. Please send any issues you may wish to raise at the meeting to ALS Users' Executive Committee Chair Jeff Bokor (jbokor@eecs.berkeley.edu) or to Fred Schlachter (fred_schlachter@lbl.gov) in advance of the meeting.