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 C: For Fourier Transform Raman Spectroscopy
> Default assumptions
> Group CI Components, methods and parameters that
are rarely changed
> Group CII Components, methods and parameters
that should be given in all papers
> Group CIII Components, methods and parameters
that should be given when significance is claimed for absolute or relative
intensities or lineshapes
> Notes about List C
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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.
- The apodized interferogram was zero-filled before Fourier transformation,
so that the Fourier transform gave at least two spectral points in
each resolution interval.
Group CI 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 all subsequent papers together with any
differences in these specifications.
- The detector. For a cooled detector, the temperature and stated
wavenumber range.
- The beamsplitter used.
- Whether the interferogram was one-sided or two-sided.
- The optical retardation velocity (for continuous scan).[see
notes]
- Optical filter used and its useful wavenumber range.
- Any other signal-filters used, e.g., electronic or digital filters.
- The intervals at which the interferogram was sampled.
Group CII Components, methods and
parameters that should be given in all papers
- The sample, its temperature and purity and how these were determined,
the method of sampling.
- The instrument manufacturer and model.
- How the Raman wavenumber shifts were calibrated.[see
notes]
- The excitation wavenumber, and the degree of focussing of the laser
beam.[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 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.
- The scattering geometry, i.e., the propagation and polarization
directions of the incident and scattered light.
- The correction for the throughput of the spectrometer applied to
the spectrum.
- Any other corrections applied to the relative intensities.
Group CIII Components, methods and
parameters that should be given when 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.
Notes About List C
Useful references for the concepts and terms used are:
Recommendations of IUPAC Commission on Molecular Structure
and Spectroscopy: E. D. Becker, J. R. Durig, W. C. Harris and G. J.
Rosasco. Presentation of Raman Spectra in Data Collections.,
Pure Appl. Chem. 53, 1879 (1981).
P. R. Griffiths and J. A. de Haseth. Fourier Transform
Infrared Spectrometry, Wiley-Interscience, New York (1986).
Note that it is not helpful to describe instrument parameters
as, e.g., UPF=3. The physical significance of the parameter is required.
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. Most instruments
give wavenumbers correct to 
±0.1
cm-1 if the laser wavenumber is entered as 15 798.002
cm-1. Recall that in spectroscopic usage "wavenumber"
is short for "wavenumber of the radiation in vacuum". The
refractive index of air changes little between the visible and the infrared,
so use of the vacuum wavenumber of the laser gives the noted accuracy
throughout the infrared.
The accurate wavenumber of the exciting laser is
required to obtain accurate Raman shifts from accurate absolute wavenumbers,
e.g., use of 9 394.2 cm-1 as the vacuum wavenumber of
the Nd YAG laser seems to give Raman shifts accurate to ±0.1
cm-1. The best indicator of accurate calibration is that
corresponding Stokes and anti-Stokes lines have the same Raman shift.
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Resolution here means instrumental
spectral resolution, which is not uniquely defined. It is best measured
as the FWHH of a non-lasing emission line from the laser. Otherwise,
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.
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Two instrumental factors that
can reduce the resolution, 
,
below that determined by the Xmax and 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).
= d2/(8F2). If F
25 cm and d
5 mm,
0.5 cm-1 at 9000 cm-1. These factors should be
specified when significant.
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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.
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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, times100%.
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