Laser-based secondary neutral mass spectroscopy: Useful yield and sensitivity Page: 4 of 32
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The bulk sensitivity limit of a technique represents a different type of sensi -
tivity. In the overall context of materials analysis, it might be better character -
ized as near surface analysis. Several factors play a role in this limit. In the
case of duty cycle limited experiments such as LSNMS, there is essentially no
difference between sample limited (surface) analysis and near surface analy -
sis. For the more conventional SNMS techniques [12,14] and for SIMS duty
cycle is not a limiting problem. In these cases the bulk sensitivity limit is a
function of the signal-to-noise ratio of the measurement without regard to
sample consumption. The signal-to-noise ratio will be enhanced for succes -
sively longer averaging times until the bulk sensitivity limit is reached. Often
this limit is due to an isobaric interference. In the best cases this limit arises
from detector dark current.
In this paper, three laser ionization surface analytical instruments will be
compared and contrasted. In this rapidly advancing field, it is impossible to
consider in detail all of the excellent work which is being done and the reader
is asked to take this circumstance into account. Rather than an exhaustive
survey, we endeavor instead to produce an illustrative review which will allow
the reader quick entry into the field. The three instruments that are detailed
are the sputter-initiated resonance ionization spectroscopy (SIRIS) instrument
, the surface analysis by resonance ionization of sputtered atoms (SARISA)
instrument  and the surface analysis by laser ionization (SALI) instrument
Figures 1-3 present the three experimental approaches which will be
discussed in some detail in this paper. The drawings are schematic in nature
and are intended to be illustrative of the underlying principles which each
instrument embodies. Let us examine each of these in turn.
Figure 1 is a generic schematic of the typa of SALI apparatus that has been
used by Becker and Gillen  to nonresonantly ionize sputtered atoms and
then to analyze the photoionized flux by time-of-flight (TOF) mass
spectrometry. The system is contained in a UHV apparatus with a base
pressure  of 10-9 mbar. The timing diagram of fig. 4 should be consulted in
an examination of this apparatus. Those time lines with the label 1 are used in
the SALI apparatus. The experimental sequence of fig. 1 is initiated by an
intense nonresonant laser pulse which ionizes the sputtered flux produced by
a pulsed Ar+ (or other primary ion) beam (2 keV). The laser pulse itself passes
1 mm in front of the target surface and is focused to a 0.2 mm waist. The
ionizing radiation is usually ultraviolet light (in general of wavelength 193 nm
or 248 nm) with an intensity in the range 108 to 1012 W/cm2. The photoions
created are then extracted into a TOF mass analyzer of the reflectron type
Figure 2 is a schematic of an apparatus after Parks et al.  called SIRIS.
Referring to the time lines of fig. 4 which are labeled 2, one finds that the
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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/m1/4/: accessed November 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.