The Application of Infrared Synchrotron Radiation to the Study of Interfacial Vibrational Modes

Abstract

Synchrotron radiation provides an extremely bright broad-band source in the infrared which is ideally suited to the study of surface and interface vibrational modes in the range 50-3000 cm-1. Thus it covers the important range of molecule-substrate interactions, as well as overlapping with the more easily accessible near-ir region where molecular intemal modes are found. Compared to standard broadband infrared sources such as globars, not only is it 10f_ times brighter, but its emittance matches the phase-space of the electrochemical cell leading to full utilization of this brightness advantage. In addition, the source is more stable even than water-cooled globars in vacuum for both short-term and long-term fluctuations. Thus one can work at high resolution and use isotopic shifts to identify modes as well as study very weak modes. We will summarize the properties of synchrotron radiation in the infrared, in particular pointing out the distinct differences between this and the x-ray region. We will use experimental data in discussing important issues of signal to noise and will address the unique problems and advantages of the synchrotron source. Thus we will emphasize the important considerations ne_ssary for developing new facilities. This analysis will then lead to a discussion of phasespace matching to electrochemical cells, and to other surfaces in vacuum. Finally we will show several examples of the application of infrared synchrotron radiation to surface vibrational spectroscopy. The examples will ali be for metal crystal surfaces in ultrahigh vacuum and will include CO/Cu(100) and (111) and LO/K/Cu(100). The experiments will show how the stability of the synchrotron source allows subtle changes in the background to be observed in addition to the discrete vibrational modes. These changes are due to electronic states induced by the adsorbate. In some cases we have seen interferences between these and the discrete vibrational modes, leading to a breakdown of the dipole selection rules, and the observation of additional modes. These important experiments serve as sensitivity limit indicators and thus as a guide to future applications in the electrochemistry field.