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Re: polarization

Perhaps we could define a few new tokens.  Would somebody be kind enough
to suggest names with rough definitions?  In order to ground the
discussion, attached are the two current definitions from the core
dictionary as they are carried in the mmCIF dictionary.  These tags allow
for definition of polarization referenced to the diffraction plane and the
ratio between two orthogonal polarization components.  If we are going to
reference to gravity or to the normal to the plane of the synchrotron or
some other appropriate reference direction, we should define an explicit
tag. Perhaps we also need to consider measures of polarization other than
a ratio.
  -- Herbert

save__diffrn_radiation.polarisn_norm
    _item_description.description
;              The angle in degrees, as viewed from the specimen, between
               the perpendicular component of the polarisation and the
               diffraction plane. See _diffrn_radiation_polarisn_ratio.
;
    _item.name                  '_diffrn_radiation.polarisn_norm'
    _item.category_id             diffrn_radiation
    _item.mandatory_code          no
    _item_aliases.alias_name    '_diffrn_radiation_polarisn_norm'
    _item_aliases.dictionary      cif_core.dic
    _item_aliases.version         2.0.1
     loop_
    _item_range.maximum           
    _item_range.minimum            .    0.0
                                  0.0   0.0
    _item_type.code               float
    _item_units.code              degrees
     save_

save__diffrn_radiation.polarisn_ratio
    _item_description.description
;              Polarisation ratio of the diffraction beam incident on the
               crystal. It is the ratio of the perpendicularly polarised
               to the parallel polarised component of the radiation. The
               perpendicular component forms an angle of
               _diffrn_radiation.polarisn_norm to the normal to the
               diffraction plane of the sample (i.e. the plane
               containing the incident and reflected beams).
;
    _item.name                  '_diffrn_radiation.polarisn_ratio'
    _item.category_id             diffrn_radiation
    _item.mandatory_code          no
    _item_aliases.alias_name    '_diffrn_radiation_polarisn_ratio'
    _item_aliases.dictionary      cif_core.dic
    _item_aliases.version         2.0.1
     loop_
    _item_range.maximum           
    _item_range.minimum            .    0.0
                                  0.0   0.0
    _item_type.code               float
     save_


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**** *            Herbert J. Bernstein
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On Mon, 11 Sep 2000, Jim Pflugrath wrote:

> I use the polarization that Wolfgang Kabsch defined in his 1988 paper.
> It consists of the degree of polarization and a vector which defines
> a normal to the plane of polarization.  So core CIF almost has it with
> this vector implicit.  The vector is defined is terms of the lab
> coordinate system (remember the MSC system?).
> 
> Jim
> 
> > One thing I haven't seen in the imgCIF/CBF dictionary is a suitable data
> > item for the beam polarization - the core CIF item seems to be based on a
> > four-circle diffractometer equipped with a point detector, as the
> > polarization is referenced to "the normal to the diffraction plane of the
> > sample (i.e. the plane containing the incident and reflected beams)."
> > (from http://www.iucr.org/iucr-top/cif/home.html#dics, section on
> > _diffrn_radiation_polarisn_ratio). 
> > 
> > Would it be more sensible in the case of area detectors to have a
> > definition based on "gravity" (or something more rigorous?).
> 
> Yes, see above.
> 
>

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