5. Scattering and
diffraction
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Reflection and refraction of light in the interface between glass with a refractive index 1.5 and air with a refractive index 1.0. TIR = "Total Internal Reflection" 



Xray diffraction is the physical phenomenon that expresses the fundamental interaction between Xrays and crystals (ordered matter). However, to describe the phenomenon, it is advisable to first introduce some physical models that (as all models) do not fully explain reality (as they are an idealization of it), but can be used to help understand the phenomenon.
A wave is an undulatory phenomenon (a
disturbance) that propagates through space and time,
and is regularly repeated.
Waves are usually represented graphically by a sinusoidal function (as shown at right), in which we can determine some general parameters that define it. 


Ondulatory phenomena (waves) propagate at a certain speed (v) and can be modeled to meet the socalled wave equation, scalar or vectorial, depending on the nature of the disturbance. The solutions to this equation are usually combinations of trigonometric terms, each of them characterized by: 1) an amplitude (A), which measures the maximum (or minimum) of the disturbance with respect to an equilibrium value, and 2) a phase Φ:
Φ = 2π(K.r  ν.t + α)
The intensity of an undulatory disturbance, at any point of the wave, is proportional to the square of the disturbance value at that point, and if it is expressed in terms of complex exponentials, this is equivalent to the product of the disturbance by its complex conjugate. The intensity is a measure of the energy flow per unit of time and per unit of area of the wavefront (spherical or flat, depending on the type of wave).
A wave is a regular phenomenon, ie it repeats exactly in time (with a period T) and space (with a period λ, the wavelength), so that λ = ν.T, or λ.ν= v.
In the expression of the phase (Φ), K is the socalled wave vector which gives the sense of progress of the wave (the ray), and is considered with an amplitude 1/λ. Thus, K is the number of repetitions per unit of length.
ν is the frequency (the inverse of the period), that is, the number of repetitions (or cycles) per unit of time. We give the name pulse to the magnitude given by: 2π.ν, which measures the number of repetitions per radian (180/π degrees) of the cycle.
In the full electromagnetic spectrum (ie in the distribution of electromagnetic wavelengths) the hard Xrays (the high energy ones) are located around a wavelength of 1 Angstrom in vacuum (for Cu the average wavelength is 1.5418 Angstrom and for Mo it's 0.7107 Angstrom), while visible light has a wavelength in the range of 4000 to 7000 Angstrom.
t and r are, respectively, the time and the position vector with which we measure the disturbance, and α is the original phase difference relative to the other components of the wave.
We speak of waves being in phase if the difference between the phases of the components is an integer multiple of 2π, and we say that the waves are in opposition of phase if that difference is an odd multiple of π. For an easy mathematical treatment to keep track of the relations between phases of the wave components, these terms are usually expressed in an exponential notation, where the exponential imaginary unit i means a phase difference of +π/2.
Possible states of interference of two
waves shown at the top, having identical amplitude and frequency. The wave drawn at the bottom (bold line) shows the result of the interference, which has maximum amplitude when interfering waves overlap, i.e. they are in phase. Complete destructive interference is obtained (resulting wave vanishes) when the maxima of one of the component waves coincide with the minima of the other, i.e., when the two waves are in phase opposition. Animation taken from The Pennsylvania State University 
Undulatory disturbance corresponding to the combination of two elementary waves (blue and green) of similar wavelengths (λ, λ), with the same amplitude (A, A) and relative difference of phase α. The disturbance is moving from left to right with a velocity v. The sum of these two elementary waves produces a wave (sum of the individual ones) depicted in red (λ). 
The total disturbance of two noncoherent sources is the sum of the individual intensities 
To model the composition of simple trigonometric waves (of type sine or cosine, or in their imaginary exponential form) the Fresnel representation is normally used. In this representation it is assumed that each wave oscilates around the X axis, as the projection of the circular motion of a vector of length equal to its amplitude and with an angular speed equal to the wave pulse ω. In this way, the resultant wave can be obtained by adding the individual vectors and projecting the resultant vector over the same X axis.
Fresnel (or Argand)
representation in which is shown the composition of several
individual waves (f_{j}).
F is the amplitude of the resultant wave and Φ its phase. 
Interaction of Xrays with matter
Xray waves interact with matter through the electrons contained in atoms, which are moving at speeds much slower than light. When the electromagnetic radiation (the Xrays) reaches an electron (a charged particle) it becomes a secondary source of electromagnetic radiation that scatters the incident radiation.
According to the wavelength and phase relationships of the scattered radiation, we can refer to elastic processes (or inelastic processes: Compton scattering), depending if the wavelength does not change (or changes), and to coherence (or incoherence) if the phase relations are maintained (or not maintaned) over time and space.
The exchanges of energy and momentum that are produced during these processes can even lead to the expulsion of an electron out of the atom, followed by the occupation of its energy level by electrons located in higher energy levels.
All these types of interactions lead to different processes in the materials such as: refraction, absorption, fluorescence, Rayleigh scattering, Compton scattering, polarization, diffraction, reflection, ...
The refractive index of all materials in relation to Xrays is close to 1, so that the phenomenon of refraction of Xrays is negligible. This explains why we are not able to produce lenses for Xrays and why the process of image formation, as in the case of visible light, cannot be carried out with Xrays.
Absorption means an attenuation of the transmitted beam, losing its energy through all types of interactions, mainly thermal, fluorescence, inelastic scattering, formation of free radicals and other chemical modifications that could lead to degradation of the material. This intensity decrease follows an exponential model dependent on the distance crossed and on a coefficient of the material (the linear absorption coefficient) which depends on the density and composition of the material.
The process of fluorescence, in which an electron is pulled out of an atom's energy level, provides information on the chemical composition of the material. Due to the expulsion of electrons from the different energy levels, sharp discontinuities in the absorption of radiation are produced. These discontinuities allow local analysis around an atom (EXAFS).
In the Compton effect, the interaction is inelastic and the radiation loses energy. This phenomenon is always present in the interaction of Xrays with matter, but due to its low intensity, its incoherence and its propagation in all directions, its contribution is only found in the background radiation produced through the interaction.
By scattering we will refer here to the changes of direction suffered by the incident radiation, and NOT to dispersion (the phenomenon that causes the separation of a wave into components of varying frequency).
Variation in the absorption of a material according to the wavelength of the incident radiation 
Dispersion of visible light into its nearly monochromatic wavelenghts 
Elastic scattering by an electron
Interaction of a Xray front with an isolated electron, which becomes a new X.ray source, producing the Xrays waves in a spherical mode.  The spherical waves produced by two electrons interact with each other, producing positive and negative interferences. 

When a nonpolarized Xray beam (that is, when its electromagnetic field is vibrating at random in all directions perpendicular to the propagation), interacts with an electron, the interaction takes place primarily through its electric field. Thus, in a first approximation, we can neglect both the magnetic and nuclear interactions. According to the electromagnetic theory of Maxwell, the electron scatters electric waves which propagate perpendicular to the electric field, in such a way that the scatterd energy (which crosses the unit of area perpendicular to the direction of propagation and per unit of time) is:
I_{e}(K_{s}) = I_{0} [e^{4} / R_{0}^{2} m^{2} c^{4}] [( 1 + cos^{2} 2θ) / 2] 
Thomson scattering model

Ks is the scattering vector, R_{0} is the distance to the observation point, 2θ is the angle between the incident direction and the direction where the scattering is observed; e and m are the charge and mass of the electron, respectively, and c is the speed of propagation of radiation in the vacuum.
The equation above describes the Thomson's model (1906) for the spherical wave elastically scattered by a free electron, which is similar to the Rayleigh scattering with visible light. The scattered wave is elastic, coherent and spherical. The mass factor (m) in the denominator justifies neglecting the nuclear scattering.
The binding forces between atom and electron are not considered in the model. It is assumed that the natural frequencies of vibration of the electron are much smaller than those of the incident radiation. In this "normal" scattering model (in contrast to the anomalous case in which those frequencies are comparable) the scattered wave is in opposition of phase with the incident radiation.
The second factor (in brackets equation above) which depends on the θ angle, is known as the polarization factor, because the scattered radiation becomes partially polarized, which creates a certain anisotropy in the vibrational directions of the electron, as well as a reduction in the scattered intensity (depending of the direction). The scatterd intensity shows symmetry around the incident direction. As the scattered wave is spherical, the inverse proportionality to the squared distance makes the energy per unit of solid angle a constant.

A solid
angle is the angle in threedimensional space that an object subtends
at a point. It is a measure of how big that object appears to an
observer looking from that point. Metrically it is the constant ratio between the intersecting areas of concentric spheres with a cone, and the corresponding squared radii of the spheres: A1/R1^{2} = A2/R2^{2} = A3/R3^{2} = ... = solid angle in steradians 

The factor of the geometric "difference of phase"
With regard to the phenomenon of diffraction and interference, it is important to consider the phase relationship between two waves due to their different geometric paths. This affects the difference of phase α of the resultant wave:
Φ = 2π(K_{0}.r  ν.t
+ α)
where K_{0} is the wave vector of the incident wave, K_{s} is the wave vector in the direction of propagation and r_{ij} is the vector between the two propagation centres which produces the phase difference.
If we have several disturbance centers whose phase differences are measured from a common origin, and we consider the position vectors r_{j} of their phase differences, the phase difference of one of the centers can be written (using unit vectors in the directions of propagation with λK = s) as:
This means that all r_{j} points in which the product (s  s_{0}) r_{j} has a constant value (cte) , will have the same phase, given by:
Scattering by an atom
An atom that can be considered as a set of Z electrons (its atomic number) can be expected to scatter Z times that which an electron does. But the distances between the electrons of an atom are of the order of the Xrays wavelength, and therefore we can also expect some type of partial destructive interferences among the scattered waves. In fact, an atom scatters Z times (what an electron does) only in the direction of the incident beam, decreasing with the increasing of the θ angle (the angle between the incident radiation and the direction where we measure the scattering). And the more diffuse the electronic distribution of electrons around the nucleus, the greater the reduction.

Diagram showing the variation of the amplitudes scattered by an electron, without considering the polarization (left figure), and an atom (right figure). The amplitude scattered by an atom decreases with increasing scattering angle. 
We give the name atomic
scattering factor to the ratio between the
amplitude scattered by an atom and a single electron. As the speed of
electrons in the atom is much greater than the variation of the
electric vector of the wave, the incident radiation only "sees" an
average electronic cloud, which is characterized by an electron density
of charge ρ(r). If
this distribution is considered spherically symmetric, it will just
depend on the distance to the nucleus, so that, with:
H = 2 sin θ / λ (which is the length of the scattering vector H = K_{s} K_{0} = (s  s_{0}) / λ):
f(H) = 4π∫_{(0 → ∞)} r^{2} ρ(r) (sin H r / H r) dr
Thus, the atomic scattering factor will represent a number of electrons (the effective number of electrons of a particular atom type) that scatter in phase in that direction, so that θ = 0 and f(0) = Z. The hypothesis of isotropy, ie that this atomic factor does not depend on the direction of H, appears to be unsuitable for transition momentum in which d or f orbitals are involved, nor for the valence electrons.
By quantummechanics calculations we can obtain the values for the atomic scattering factors, and we can derive analytical estimates of the type:


When the frequency of the incident radiation is close to the natural vibration of the electron linked to the atom, we have to make some corrections (Δ) due to the phase differences that occur between the individual waves scattered by electrons, whose vibration (due to the incident wave) is affected by that linking. Thus:

Decrease of the atomic scattering factor due to the thermal vibration 
Scattering by a set of atoms
Xrays scattered by a set atoms produce Xray radiation in all directions, leading to interferences due to the coherent phase differences between the interatomic vectors that describe the relative position of atoms. In a molecule or in an aggregate of atoms, this effect is known as the effect of internal interference, while we refer to an external interference as the effect that occurs between molecules or aggregates. The scattering diagrams below show the relative intensity of each of these effects:

Scattering diagrams of a monoatomic
material in different states.
In the intensity axis we have neglected the background contribution. The figures mainly represent the effect of the external interference, while the internal interference (in this case due to a single atom only) is simply reflected by the relative intensity of the maxima. Note how the thermal movement in the liquid softens and reduces the scattering profile, and how the maxima produced by the glass also decrease. In the crystal, where the phase relations are fixed and repetitive, the scattering profile becomes sharp with well defined peaks, whereas in the other diagrams the peaks are broad and somewhat continuous. In the crystal case the scattering effect is known as diffraction. Note how the scattering phenomenon reflects the internal order of the sample  the positional correlations between atoms. 
In the case of monoatomic gases, the effects of interference between atoms m and n lead (in terms of the intensity scattered by an electron) to:
which, when averaged over the duration of the experiment and in all k directions of space, gives rise to the Debye formula:
Geometry of the scattering produced by a set of identical atoms 
In the case of monoatomic liquids some effects appear at short distances, due to correlations between atomic positions. If the density of atoms per unit of volume (at a distance r from any atom with spherical symmetry) is, on average, ρ(r), then the expression 4π r^{2}ρ(r) is known as the radial distribution, and the Debye formula becomes:
<I(H)> = I_{e}(H) N f^{2}(H) [ 1 + ∫_{(0 → ∞)} 4πr^{2}ρ(r) sin (2πH r) / 2πH r dr ]
All these relationships allow the analysis of the Xray scattering in amorphous, glassy, liquid and gaseous samples.
No matter the possible complexity with which the phenomenon of Xray scattering is presented. The nonspecialist reader should only remember some simple ideas that are outlined below (drawings taken from the lecture by Stephen Curry)...













Scattering by a monoatomic lattice: Diffraction
When the set of atoms is structured as a regular threedimensional lattice (so that the atoms are nodes of the lattice), the precise geometric relationships between the atoms give rise to particular phase differences. In these cases, cooperative effects occur and the sample acts as a threedimensional diffraction grid. Under these conditions, the effects of external interference produce a scattering structured in terms of peaks with maximum intensity which can be described in terms of another lattice (reciprocal of the atomic lattice) which shows typical patterns, such as those you can see when you look at a streetlight through an umbrella or a curtain.

Relationship between two 2dimensional lattices, direct lattice (on the left) and reciprocal lattice (on the right). The repetition parameters in reciprocal space carry the * superscript and k is a scale factor that depends on the experiment. d_{10} and d_{01} are the corresponding direct lattice spacings. Note that the figures show a direct unit cell and a reciprocal unit cell only, corresponding to the diffraction patterns shown on the left side of the page. See also direct and reciprocal lattices. 
Structured in a lattice, any atom can be defined by a vector, referred to a common origin:
R _{j,m1,m2,m3} = m1 a + m2 b + m3 c
where R_{j} represents the position of the j node in the lattice; m1, m2, m3, are integers and a, b and c are the vectors defining the lattice. According to this, the intensity scattered by a material would be:
I(H) = I_{e}(H) Σ_{m1}Σ_{m'1}Σ_{m2}Σ_{m'2}Σ_{m3}Σ_{m'3} f_{j}(H) f_{j'}(H) exp [2πi (s  s_{0}) r_{m,m'} / λ]
r_{m,m'} = R_{m1,m2,m3}  R_{m'1,m'2,m'3} = (m1m'1) a + (m2  m'2) b + (m3  m'3) c
And calculating this sum we have:
In this expression, M1, M2, M3 represent the number of unit cells contained in the crystal along the a, b and c directions, respectively, so that in the total sample the number of unit cells would be M = M1.M2.M3 (around 10^{15 }in crystals of an average thickness of 0.5 mm).
I_{L}(H) is the factor of external interference due to the monoatomic lattice. It consists of several products of type (sin^{2} Cx) / sin^{2} x, where C is a very large number. This function is almost zero for all x values, except in those points where x is an integer multiple of π, where it takes its maximum value of C^{2}. The total value would be a maximum value only when all three products are other than zero, where it will take the value of M^{2}. That is, the diffraction diagram of the direct lattice is another lattice that takes nonzero values in its nodes and that, due to the I_{e}(H) factor, varies from one place to another...
Due to the finite size of the samples, the small chromatic differences of the incident radiation, the mosaic of the sample, etc., the maxima show some type of spreading around them. Therefore, in order to set the experimental conditions for measurement, one needs a small sample oscillation around the maximum position (rocking) to integrate all these effects and to collect the total scattered energy.

Graphical representation of one of the
products of the I_{L}(H) function between two consecutive maxima.
Note the transformation from scattering to diffraction, that is, from broad to very sharp peaks, as the number of cells M1 increases. The maxima are proportional to M1^{2} and the first minimum appears closer to the maximum with increasing M1. 
When the material is not structured in terms of a monoatomic lattice, but is formed by a group of atoms of the same or of different types, the position of every atom with respect to a common origin is given by:
= T_{m1,m2,m3} + r_{j} 
Reduction inside a unit cell of the absolute position of an atom through lattice translations. 
that is, that to go from the origin to the atom, at position R, we first go, through the T translation, to the unit cell origin, and from there with the vector r we reach the atom.
As the atom is always included within a unit cell, its coordinates referred to the cell are smaller than the axes, and often are expressed as fractions of them:
r
= X a
+ Y b + Z c = X/a a
+ Y/b b + Z/c c
= x a + y b + z c
where x, y, z, as fractions of axes, are now between 1 and +1.
Then, under the conditions initially raised, ie with a monochromatic and depolarised Xray beam (as a plane wave, formed by parallel rays of a common front wave), perpendicular to the propagation unit vector s_{0} that completely covers the sample, the kinematic model of interaction indicates that the sample produces diffracted beams in the direction s with an intensity given by:
I(H) = I_{e}(H) I_{F}(H) I_{L}(H)
where I_{e} is the intensity scattered by an electron, I_{L} is the external interference effect due to the threedimensional lattice structure, and I_{F} is the square of the socalled structure factor, a magnitude which takes into account the effect of all internal interferences due to the geometric phase relationships between all atoms contained in the unit cell. This internal structural effect is:
I_{F}(H) =  F^{2}(H)  = F(H) F*(H)
As a consequence of the complex representation of waves, mentioned at the beginning, the square of a complex magnitude is obtained by multiplying the complex by its conjugate. Thus, specifically, we give the name structure factor, F(H), to the resultant wave from all scattered waves produced by all atoms in a given direction :
F(H) = Σ_{(1 → n)} f_{j}(H) exp [2π(s  s_{0}) r_{j} / λ]
As already stated, the phase differences due to geometric distances R are proportional to (s  s_{0}) R / λ. This means that if we change the origin, the phase differences will be produced according to the geometric changes, in such a way that as the exponential parts of the intensity functions are conjugate complexes, they will affect the intensities in terms of a proportionality constant only. Thus, a change of origin is not relevant to the phenomenon.
In the equation of the total intensity, I(H), the conditions to get a maximum lead to the following consequences:

To clarify what has been said above, the reader can analyse further objects and their corresponding diffraction patterns through this link. Additionally we suggest you to watch the video prepared by the Royal Institution to demonstrate optically the basis of diffraction using a wire coil (representing a molecule) and a laser (representing an Xray beam).
Laue equations, Bragg's interpretation and Ewald's geometric diffraction model
We have seen that the diffraction diagram of a direct lattice defined by three translations, a, b and c, can be expressed in terms of another lattice (the reciprocal lattice) with its reciprocal translations: a*, b* and c*, and these translation vectors (direct and reciprocal) meet the conditions of reciprocity:
a a* = b b* = c c* = 1 and a b* = a c* = b c* = 0
and they also meet that (for instance):
a*
= (b x
c)
/ V
(x means vectorial or cross product)
where V is the volume of the direct unit cell defined by the 3 vectors of the direct cell, and therefore:
a* = N_{100} / d_{100}
where N_{100}
is a unit vector perpendicular to the planes of indices
h=1,
k=0,
l=0,
and where
d_{100}
is the corresponding interplanar spacing. And similarly with b*
and c*.
H*_{hkl}
= h a*
+ k b*
+ l c*
= N_{hkl}
/ d_{hkl}
On the other hand, we have seen that the maxima in the diffraction diagram of a crystal correspond to the maximum function I_{L}(H), meaning that each of the products that define this function must be individually different from zero, as a sufficient condition to obtain a maximum for the diffracted intensity. If we remember that H = (s  s_{0}) / λ, this also means that the three socalled Laue equations must be fulfilled:
where h, k, l are integers Laue equations 
Max von Laue (18791960) 
(There is also a less formal way to derive and/or to understand the Laue equations, and therefore we invite interested readers to visit this link ...) 
These three conditions are met if the vector H represents a vector of the reciprocal lattice, so that:
H = h a* + k b* + l c*
since due to the properties of the reciprocal lattice, it can be stated that:
H_{hkl}
a =
h,
H_{hkl}
b
= k,
H_{hkl}
c
=
l
But taking into account that geometrically we can consider spacings of type d_{hkl}/2, d_{hkl}/3, and in general d_{hkl}/n (ie, d_{nh,nk,nl}, where n is an integer), the W.L. Bragg’s equation (Nobel Prize in Physics in 1915 ) would be in the form:
λ = 2 (d_{hkl} /n) sin θ_{nh,nk,nl}
where n is an integer number Bragg's Law 
William L. Bragg (18901971) 
(There is also a less formal way to derive and/or to understand Bragg's Law, and therefore we invite interested readers to visit this link ...) 
Moreover, if the conditions of Laue are fulfilled (as explained in the following figure) all atoms located on the sequence of planes parallel to the one with indices hkl at a given distance (D_{P}) from the origin (D_{P} being an integer multiple of d_{hkl}) will diffract in phase, and their geometric differenceofphase factor will be:
(s  s_{0}) r = n λ
and consequently a diffraction maximum will be produced in the direction:
s = s_{0} + λ H*_{hkl}
Geometrical model to interpret
the diffraction (in phase) of all parallel planes with indices hkl
and a constant interplamar spacing d_{hkl}, when the Laue conditions (or their
equivalent, Bragg's Law) are fulfilled.
N_{hkl} is the unit vector perpendicular to the hkl planes and in the diffraction conditions is given by: N_{hkl} = H*_{hkl} d_{hkl} The plane equation can, therefore, be written as: H*_{hkl }r = H*_{hkl} r_{i}= H*_{hkl} r_{i} cos (H*_{hkl} , r_{i}) = (1/d_{hkl}) D_{P} = n 
Bragg's equation has a very simple interpretation... When in the crystalradiation interaction a maximum of diffraction occurs, it is equivalent to say that the incident beam reflects on the crystal planes of indices hkl and interplanar spacing d_{hkl}. That's why in talking about diffraction maxima, sometimes we use the phrase Bragg's reflection.
Moreover, this equation holds all the traditional relations of reciprocity of diffraction, between spacingdirection or positionmomentum: the shorter spacing, the larger angle and vice versa; direct lattices with large unit cells produce very close diffracted beams, and vice versa.


Readers with installed Java Runtime tools can play with Bragg's model using this applet.
On the other hand, we have seen that in general:

Paul Peter Ewald (18881985) 
This figure describes Ewald's geometric model. When a reciprocal point , P*(hkl), touches the surface of Ewald's sphere, a diffracted beam is produced starting in the centre of the sphere and passing through the point P*(hkl). Actually the origin of the reciprocal lattice, O*, coincides with the position of the crystal and the diffracted beam will start from this common origin, but being parallel to the one drawn in this figure, exactly as it is depicted in the figure below. 

To obtain all possible diffracted beams that a sample can provide, using a radiation of wavelength λ, it is sufficient to conveniently orient the crystal and make it turn, so that its reciprocal points will have the opportunity to lay on the surface of Ewald's sphere. In these circumstances, diffracted beams will originate as described above. With larger wavelengths, the volume of the reciprocal space that can be explored will be smaller, but the diffracted beams will appear more separated.
Ewald's model showing how diffraction occurs. The incident Xray beam, with wavelength λ, shown as a white line, "creates" an
imaginary Ewald's sphere of diameter 2/λ
(shown in green).
The reciprocal lattice (red points) rotate as the crystal rotates, and
every time that a reciprocal point cuts the sphere surface a diffracted
beam is produced from the center of the sphere (yellow arrows).

This Java application, that can be downloaded from this link,is also based on the concept of the reciprocal lattice and allows playing with the Ewald's model to understand the diffraction. Original by Nicolas Schoeni and Gervais Chapuis of the Ecole Polytechnique Fédéral de Lausanne (Switzerland). 
Considering that the interplanar spacings d_{hkl} are a characteristic of the sample, by reducing the wavelength, Bragg's Law indicates that the diffraction angles (θ) will decrease; the spectrum shrinks, but on the other hand, more diffraction data will be obtained, and therefore a better structural resolution will be achieved.
According to Ewald's model, the amount of reciprocal space to be measured can be increased by reducing the wavelength, that is, by increasing the radius of the Ewald's sphere 


