"Diffraction by helical structures"

Date:
1956-1957
Reference:
PP/CRI/H/2/17
Catalogue details

Licence: In copyright

Credit: "Diffraction by helical structures". Source: Wellcome Collection.

    11/98
    artificial case where we have only one atom per sub-unit at s s j s s °* Then « ! ♦ Our unrolled helical pattern in real space will simply be an array of points C.l*«4*èe@ net points), which Bear has called a helix net, and it will be reciprocal to the set of points in the (n- # <£) plot# It is clear from this point of view that we may join up the net points in an arbitrary way to choose a unit cell«^ for the structure. All descriptions will be equivalent, since they correspond to the same basic net pattern» (ÍW Prp.'jeotj.pfl When the atoas are not all at the same radius } the simple result given above will not hold, since we cannot perfora the factorisation in equation (4). The contribution of one Bessel function term of order n to the scattered amplitudes on a layer line -C is then repre sented by a complex number G a ^ (R ) whose modulus and phase varies con tinuously with B, where oi!> » i M^\)V W 4S] (*> In this notation the scattered amplitude on a layer»plan® is given by K^A) - \ (7) where the sum is, of course, subject to the usual selection rule (2)» We should like to have a convenient way of estimating the contri bution of the various atoms to ®q 9 ¿ (H)* H ow each atom contributes a Bessel function multiplied by a phase factor involving only its <ip and s co-ordinates# The phase factor is the same form for all atoms, and is essentially scale-free, not depending on the radius# Me can