PHYSICAL
CHEMISTRY DIVISION
COMMISSION ON MOLECULAR STRUCTURE AND SPECTROSCOPY*
Specification of Components, Methods
and Parameters in Fourier Transform Spectroscopy by Michelson and Related
Interferometers
(Technical Report)
LIST A: For Fourier Transform Spectroscopy at Modest Resolution
> Default assumptions
> Group AI Components, methods and parameters
that are rarely changed
> Group AII Components, methods and parameters
that should be given in all papers
> Group AIII Components, methods and parameters
that should be given when quantitative significance is claimed for absolute
or relative intensities or lineshapes
> Notes about List A
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- List B - List C
Default assumptions
The following components, methods and parameters are defined as the
defaults. They are the most commonly used and need not be specified.
Deviations from them should be specified.
- A Michelson or related interferometer with rapid continuous mirror
scan was used.
- No optical filter was used. If an optical filter was used, specify
its temperature and wavenumber range.
- The apodized interferogram was zero-filled before Fourier transformation,
so that the Fourier transform gave at least two spectral points in
each resolution interval.
- The interferogram was sampled at 0.6328 µm intervals (HeNe
laser wavelength), i.e., the spectral bandwidth was in the range 0
to over 3 950 cm-1 but less than 7 899 cm-1.[see
notes]
- The source was a continuous incandescent source, e.g., a hot globar
or wire, etc..
Group AI Components, methods and
parameters that are rarely changed
These components, methods and parameters should be specified
in an accessible journal in the first publication from an instrument.
This report should be cited in subsequent papers together with any differences
in these specifications.
- The detector. For a cooled detector, its temperature and stated
wavenumber range.
- The beamsplitter used and the angle of incidence upon it.
- Whether the interferogram was one-sided or two-sided.
- The optical retardation velocity (for continuous scan).[see
notes]
- Any other signal-filters used, e.g., electronic or digital filters.
Group AII Components, methods
and parameters that should be given in all papers
- The sample, its temperature, its purity and how this was determined,
the method of sampling.
- The instrument manufacturer and model.
- How water and carbon dioxide were removed from the optical path.
- How the wavenumbers were calibrated.[see notes]
- The maximum optical path difference in the interferogram, Xmax,
or the nominal resolution, 1/Xmax AND the apodization
function.[see notes]
- Other instrumental factors that may reduce the instrumental resolution
achieved.[see notes]
- For step scan, the integration time at each point.
- For continuous scan the number of scans signal-averaged in each
interferogram.
Group AIII Components, methods
and parameters that should be given when quantitative significance is
claimed for absolute or relative intensities or lineshapes
- How the phase correction was done, e.g., multiplicative or convolution
method.
- The type of spectrum calculated from the Fourier transform.[see
notes]
- The PNL measure of photometric non-linearity.[see
notes]
- Details of any arithmetic processing of the spectra after Fourier
transformation and phase correction, with the exception of the use
of a stated reference spectrum to calculate the transmittance spectrum,
T, or -lg T.
Notes About List A
Useful references for the concepts and terms used are:
P. R. Griffiths and J. A. de Haseth. Fourier Transform Infrared
Spectrometry, Wiley-Interscience, New York, 1986.
J. E. Bertie Vibrational Spectra and Structure, Ed. J. R. Durig,
Elsevier, Amsterdam, 1985, 14, 221.
Note that it is not helpful to describe instrument parameters as, e.g.,
UPF=3. The physical significance of the parameter is required.
If the sampling interval is Dx,
wavenumbers from 0 cm-1 to 
=
1/(2Dx) are sampled correctly. If
Dx = 0.6328 µm, =
7 899 cm-1 and a spectral bandwidth from 0 to 7 899
cm-1 is sampled correctly. The spectral intensity may not
extend up to 7 899 cm-1; if the signal and associated
noise do not extend beyond 3 950 cm-1, a larger sampling interval
may be used.
Back to List A
The optical retardation velocity, ORV, is
the rate at which the optical path difference changes. It is twice the
velocity of the moving mirror for a Michelson interferometer, and is
4 or 8 times the mirror velocity for other types of interferometer.
Some manufacturers give the ORV indirectly. Thus, Bio-Rad gives the
optical retardation velocity as a frequency, e.g., 5 kHz which means
that the path difference changes by one HeNe laser wavelength 5000 times
per second. From this, ORV = 5 000 x
0.6328 x 10-4
cm s-1 = 0.32 cm s-1.
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Wavenumber calibration is necessary to obtain
accurate wavenumbers from FT spectrometers, although they give extremely
precise (reproducible) wavenumbers. Calibration is conveniently done
with water vapor or indene-camphor-cyclohexane or polystyrene film (A.
R. H. Cole, Tables of Wavenumbers for the Calibration of Infrared
Spectrometers, 2nd edn, IUPAC, Pergamon Oxford, 1977). Most instruments
give wavenumbers correct to ~±0.1 cm-1 if the laser
wavenumber is entered as 15 798.002 cm-1.
Back to List A
Resolution here means instrumental
spectral resolution, which is not uniquely defined. The nominal resolution
is conveniently defined as the reciprocal of the maximum optical path
difference, Xmax. (Xmax equals the
maximum mirror displacement multiplied by 2 for a Michelson or by some
other factor for certain related interferometers) The actual instrumental
resolution is determined by Xmax and the apodization
function, so Xmax and the apodization function should
both be reported. If these can not be determined, the resolution claimed
by the manufacturer for the settings used may be substituted, although
this is not consistently defined. Note that the spectral resolution
actually observed may be determined by the intrinsic width of the spectral
line if this is greater than the instrumental resolution.
Two factors that can reduce the instrumental resolution,
,
below that determined by Xmax and the apodization
function are the focal length, F, of the collimating mirror and
the diameter, d, of the Jacquinot stop (the limiting aperture
of the instrument, frequently the circular aperture at the first focus
of the source, but the detector element in some instruments).
= d2/(8F2). Typically F 
25
cm and d 12
mm, from which
1 cm-1
between 3 000 and 4 000 cm-1. These factors should
be described if resolution of 1
cm-1 or better is claimed above 3 000 cm-1.
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The type of spectrum calculated from the Fourier
transform may be either the magnitude spectrum, I = 
(C2
+ S2), or the intensity spectrum, which is also called the
phase-corrected amplitude spectrum, I = C cos d
+ S sin d ; here all terms change
with wavenumber and I, C, S, and d
are the calculated intensity, the cosine transform, the sine transform and
the phase angle at the wavenumber in question. Random noise about a zero baseline
is always positive in the (uncommon) magnitude spectrum and has random sign
in the intensity spectrum.
Back to List A
The PNL measure of photometric non-linearity
is defined here as: In a single beam spectrum, the ratio of the average
baseline position to low wavenumber of the detector cut-off to the maximum
signal, times 100%.
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- List B - List C
