IUVSTA 15th International Vacuum Congress (IVC-15), AVS 48th International Symposium (AVS-48), 11th International Conference on Solid Surfaces (ICSS-11)
    Surface Science Monday Sessions
       Session SS1-MoA

Paper SS1-MoA10
A Fourier Transform-Based TOF HREELS Spectrometer

Monday, October 29, 2001, 5:00 pm, Room 120

Session: Innovations in Surface Science
Presenter: B.G. Frederick, University of Maine
Authors: R.H. Jackson, University of Maine
L.J. LeGore, University of Maine
Y. Yang, University of Maine
P. Kleban, University of Maine
B.G. Frederick, University of Maine
Correspondent: Click to Email
A major limitation of the conventional, dispersive sector, electron energy analyzer is that it is inherently a serial device, leading to long data acquisition times in high resolution electron energy loss spectroscopy (HREELS). A throughput advantage of order 500-1000 can be achieved by combining time-of-flight (TOF) methods with pseudo-random (PRBS) modulation of the back-scattered electron beam. We present the first results of this new approach to HREELS and compare with simulations of the TOF spectrometer. Although PRBS modulation techniques have been applied to a number of other areas,@footnote 1,2@ the Poisson noise distribution arising from pulse-counting detection and the sub-nanosecond time resolution required for meV resolution in HREELS place several demands upon the chopper, detection electronics and digital signal processing software. We will discuss how the chopper, based upon the "interleaved comb" device,@footnote 3@ limits the minimum gating time and the nominal time resolution of the spectrometer. Traditional cross correlation methods, utilized to recover the signal in PRBS modulation, obtain a throughput advantage over single pulse TOF at the expense of resolution; however, the methods are not strictly valid when the noise correlates with signal power. To overcome these limitations, we have developed maximum likelihood methods which account for the Poisson noise distribution and achieve a resolution enhancement of up to a factor of 8, relative to the nominal TOF time resolution, depending upon the signal to noise ratio. @FootnoteText@ @footnote 1@ Y. Uehara, T. Ushiroku, S. Ushioda, Y. Murata, Jap. J. App. Phys., 29 (1990) 2858-2863. @footnote 2@ J. L. Buevoz, G. Roult, Rev. de Phys. Appl., 12 (1977) 59 7. @footnote 3@ P. R. Vlasak, D. J. Beussman, M. R. Davenport, C. G. Enke, Rev. Sci. Instrum., 67 (1996) 68-72.