Laser-based secondary neutral mass spectroscopy: Useful yield and sensitivity Page: 3 of 32
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The past several years has seen a rapid development in laser-related surface
measurement techniques. In particular, laser multiphoton ionization (MPI) of
sputtered atoms has proven to be a sensitive probe, both of the sputtering
process [1,2] and of surface composition [3-7]. This laser ionization version of
secondary neutral mass spectrometry (LSNMS) has a threefold advantage in
surface analysis when compared to Secondary Ion Mass Spectrometry (SIMS).
First, laser ionization techniques measure the dominant fraction of the
sputtered flux (for metals and semiconductors ) - neutral atoms. Second,
because the dominant neutral portion of the sputtered flux is measured, large
changes in signal due to minor surfz.ae chemical effects may be minimized .
However, in cases where neutral atoms do not represent the dominant portion
of the sputtered flux, such as when oxygen is absorbed on the surface in the
presence of extended sputtering  or for oxide materials , this may not be
the case. Finally in the case of Resonance Ionization Spectroscopy (RIS), the
laser ionization process is so species specific  that the stringent
requirements for high mass resolution spectrometers are strongly alleviated
making it possible to achieve much better transmission.
A comparison of LSNMS to other sputtered neutral mass spectrometric
(SNMS) techniques such as the plasma ionization method of Oechsner [121
has been drawn previously  based on their relative ionization efficiency,
the range of species ionized, and the experimental duty cycle. In this paper, a
more detailed comparison of the various LSNMS techniques will be made and
a few of the most exciting new results will be reviewed.
Basically, LSNMS represents an extremely efficient, though low-duty factor,
ionization method. For sufficiently large laser powers, most elements may be
ionized with unit efficiency. This can be compared to electron ionization effi -
ciencies of -0.01%  and hot electron gas ionization efficiencies of 1% .
For most SNMS techniques the ionization efficiency is reasonably mass
independent [131. For LSNMS the ionization can be species unspecific (non-
resonant) or very species specific (resonant). Finally, one must consider the
duty cycle of the experiment. Most SNMS techniques operate with unit duty
factor, while this is not the case with LSNMS. The LSNMS duty factor is
limited by the laser repetition rate of commercially available pulsed lasers to
10-4 - a significant disadvantage for routine analysis.
The discussion of LSNMS techniques in the present paper will focus on two
important quantities for surface analysis - the useful yield, V, and the bulk
sensitivity limit. The useful yield is defined as
W= atoms detected/atoms sputtered
This important quantity represents the key limit for micro characterization of
samples with atomic dimensions, e. g., surface monolayers.
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Young, C.E.; Pellin, M.J.; Calaway, W.F.; Joergensen, B.; Schweitzer, E.L. & Gruen, D.M. Laser-based secondary neutral mass spectroscopy: Useful yield and sensitivity, article, January 1, 1986; United States. (digital.library.unt.edu/ark:/67531/metadc1113129/m1/3/: accessed September 26, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.