Geometry:TSAXS 3D

In transmission-SAXS (TSAXS), the x-ray beam hits the sample at normal incidence, and passes directly through without refraction. TSAXS is normally considered in terms of the one-dimensional momentum transfer (q); however the full 3D form of the q-vector is necessary when considering scattering from anisotropic materials. The q-vector in fact has three components:

${\begin{alignedat}{2}\mathbf {q} &={\begin{bmatrix}q_{x}\\q_{y}\\q_{z}\end{bmatrix}}\\&={\frac {2\pi }{\lambda }}{\begin{bmatrix}\sin \theta _{f}\cos \alpha _{f}\\\cos \theta _{f}\cos \alpha _{f}-1\\\sin \alpha _{f}\end{bmatrix}}\end{alignedat}}$

This vector is always on the surface of the Ewald sphere. Consider that the x-ray beam points along +y, so that on the detector, the horizontal is x, and the vertical is z. We assume that the x-ray beam hits the flat 2D area detector at 90° at detector (pixel) position $\scriptstyle (x,z)$. The scattering angles are then:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} \theta_f & = \arctan\left[ \frac{x}{d} \right] \\ \alpha_f ^\prime & = \arctan\left[ \frac{z}{d} \right] \\ \alpha_f & = \arctan \left[ \frac{z }{d / \cos \theta_f} \right] \end{alignat} }

where $\scriptstyle d$ is the sample-detector distance, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \alpha_f ^{\prime} } is the out-of-plane component (angle w.r.t. to y-axis, rotation about x-axis), and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \theta_f } is the in-plane component (rotation about z-axis). The alternate angle, Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \alpha_f } , is the elevation angle in the plane defined by Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \theta_f } .

Total scattering

The full scattering angle is defined by a right-triangle with base d and height Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \sqrt{x^2 +d^2}} :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} 2 \theta_s = \Theta & = \arctan\left[ \frac{ \sqrt{x^2 + z^2}}{d} \right] \\ & = \arctan\left[ \frac{ \sqrt{(d \tan \theta_f)^2 + (d \tan \alpha_f^\prime )^2}}{d} \right] \\ & = \arctan\left[ \sqrt{\tan^2 \theta_f + \tan^2 \alpha_f^\prime } \right] \\ & = \arctan\left[ \sqrt{\tan^2 \theta_f + \frac{ \tan^2 \alpha_f }{ \cos^2 \theta_f } } \right] \\ \end{alignat} }

The total momentum transfer is:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = \frac{4 \pi}{\lambda} \sin \left( \theta_s \right) \\ & = \frac{4 \pi}{\lambda} \sin \left( \frac{1}{2} \arctan\left [ \frac{\sqrt{x^2 + z^2}}{d} \right ] \right) \end{alignat} }

Given that:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} \cos( \arctan[u]) & = \frac{1}{\sqrt{1+u^2}} \\ \cos( 2 \theta_s ) & = \frac{1}{\sqrt{1 + (\sqrt{x^2+z^2}/d)^2}} \\ & = \frac{d}{\sqrt{d^2+x^2+z^2}} \end{alignat} }

We can also write:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = \frac{4 \pi}{\lambda} \sin \left( \theta_s \right) \\ & = \pm \frac{4 \pi}{\lambda} \sqrt{ \frac{1-\cos 2\theta_s }{2} } \\ & = \frac{4 \pi}{\lambda} \sqrt{ \frac{1}{2}\left(1 - \frac{d}{\sqrt{d^2+x^2+z^2}} \right) } \\ & = \sqrt{2} \frac{2 \pi}{\lambda} \sqrt{ 1 - \frac{d}{\sqrt{x^2+d^2+z^2}} } \end{alignat} }

Where we take for granted that q must be positive.

In-plane only

If Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \alpha_f = 0 } (and Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \alpha_f ^{\prime} = 0} ), then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle q_z = 0 } , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle 2 \theta_s = \theta_f } , and:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = 2 k \sin \left( \theta_f /2 \right) \\ & = 2 k \sqrt{ \frac{1- \cos(\theta_f)}{2} } \\ & = 2 k \sqrt{ \frac{1}{2} \left( 1 - \frac{1}{\sqrt{1+(x/d)^2}} \right) } \\ & = 2 k \sqrt{ \frac{1}{2} \left( 1 - \frac{d}{\sqrt{d^2+x^2}} \right) } \end{alignat} }

The other component can be thought of in terms of the sides of a right-triangle with angle Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \theta_f = 0 } :

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q_x & = q \cos ( \theta_f /2 ) \\ & = 2k \sin(\theta_f /2 ) \cos ( \theta_f /2 ) \\ & = k \sin(\theta_f)\\ q_y & = - q \sin ( \theta_f /2 ) \\ & = - 2k \sin(\theta_f /2 ) \sin ( \theta_f /2 ) \\ & = - k \left( 1 - \cos \theta_f \right) \\ & = k \left( \cos \theta_f - 1 \right) \\ \end{alignat} }

Summarizing:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbf{q} = \frac{2 \pi}{\lambda} \begin{bmatrix} \sin \theta_f \\ \cos \theta_f - 1 \\ 0 \end{bmatrix} }

Out-of-plane only

If Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \theta_f = 0 } , then Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle q_x = 0 } , Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \scriptstyle \alpha_f^{\prime} = \alpha_f = 2 \theta_s } , and:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = 2 k \sin \left( \alpha_f /2 \right) \\ & = 2 k \sqrt{ \frac{1- \cos(\alpha_f)}{2} } \\ & = 2 k \sqrt{ \frac{1}{2} \left( 1 - \frac{d}{\sqrt{d^2+z^2}} \right) } \end{alignat} }

The components are:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q_z & = q \cos ( \alpha_f /2 ) \\ & = 2k \sin(\theta_f /2 ) \cos ( \theta_f /2 ) \\ & = k \sin(\alpha_f)\\ q_y & = - q \sin ( \alpha_f /2 ) \\ & = k \left( \cos \alpha_f - 1 \right) \\ \end{alignat} }

Summarizing:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbf{q} = \frac{2 \pi}{\lambda} \begin{bmatrix} 0 \\ \cos \alpha_f - 1 \\ \sin \alpha_f \end{bmatrix} }

Components

For arbitrary 3D scattering vectors, the momentum transfer components are:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q_x & = \frac{2 \pi}{\lambda} \sin \theta_f \cos \alpha_f \\ q_y & = \frac{2 \pi}{\lambda} \left ( \cos \theta_f \cos \alpha_f - 1 \right ) \\ q_z & = \frac{2 \pi}{\lambda} \sin \alpha_f \end{alignat} }

In vector form:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \mathbf{q} = \frac{2 \pi}{\lambda} \begin{bmatrix} \sin \theta_f \cos \alpha_f \\ \cos \theta_f \cos \alpha_f - 1 \\ \sin \alpha_f \end{bmatrix} }

Total magnitude

Note that this provides a simple expression for q total:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = \sqrt{ q_x^2 + q_y^2 + q_z^2 } \\ & = \frac{2 \pi}{\lambda} \sqrt{ \sin^2 \theta_f \cos^2 \alpha_f + \left ( \cos \theta_f \cos \alpha_f - 1 \right )^2 + \sin^2 \alpha_f } \\ \left( \frac{q}{k} \right)^2 & = \sin^2 \theta_f \cos^2 \alpha_f + \cos^2 \theta_f \cos^2 \alpha_f -2 \cos \theta_f \cos \alpha_f + 1 + \sin^2 \alpha_f \\ & = \cos^2 \alpha_f (\sin^2 \theta_f + \cos^2 \theta_f) +\sin^2 \alpha_f -2 \cos \theta_f \cos \alpha_f + 1 \\ & = \cos^2 \alpha_f (1) +\sin^2 \alpha_f -2 \cos \theta_f \cos \alpha_f + 1 \\ & = 2 -2 \cos \theta_f \cos \alpha_f \\ q & = \sqrt{2}k \sqrt{ 1 - \cos \theta_f \cos \alpha_f } \\ \end{alignat} }

Check

As a check of these results, consider:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} q & = \frac{4 \pi}{\lambda} \sin \left( \theta_s \right) \\ & = \frac{4 \pi}{\lambda} \sqrt{ \frac{1-\cos 2\theta_s }{2} } \\ \left( \frac{q}{k} \right)^2 & = \frac{4}{2} \left( 1-\cos 2\theta_s \right) \\ & = 2 \left( 1-\frac{1}{\sqrt{1+\left( \sqrt{\tan^2 \theta_f + \frac{ \tan^2 \alpha_f }{ \cos^2 \theta_f } } \right) ^2}} \right) \\ & = 2 \left( 1-\frac{1}{\sqrt{1+\tan^2 \theta_f + \frac{ \tan^2 \alpha_f }{ \cos^2 \theta_f } }} \right) \\ & = 2-\frac{2}{\sqrt{1+\frac{\sin^2 \theta_f}{\cos^2 \theta_f} + \frac{ \sin^2 \alpha_f }{ \cos^2 \alpha_f \cos^2 \theta_f } }} \\ \end{alignat} }

And:

Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/":): {\displaystyle \begin{alignat}{2} & \left( 1+\frac{\sin^2 \theta_f}{\cos^2 \theta_f} + \frac{ \sin^2 \alpha_f }{ \cos^2 \alpha_f \cos^2 \theta_f } \right) ^{-1/2} \\ = & \left( \frac{\cos^2 \alpha_f \cos^2 \theta_f}{\cos^2 \alpha_f \cos^2 \theta_f}+\frac{\cos^2 \alpha_f \sin^2 \theta_f}{\cos^2 \alpha_f \cos^2 \theta_f} + \frac{ \sin^2 \alpha_f }{ \cos^2 \alpha_f \cos^2 \theta_f } \right) ^{-1/2} \\ = & \left( \frac{\cos^2 \alpha_f \cos^2 \theta_f + \cos^2 \alpha_f \sin^2 \theta_f + \sin^2 \alpha_f}{\cos^2 \alpha_f \cos^2 \theta_f} \right) ^{-1/2} \\ = & \left( \frac{\cos^2 \theta_f \cos^2 \alpha_f }{\cos^2 \alpha_f \cos^2 \theta_f + \cos^2 \alpha_f \sin^2 \theta_f + \sin^2 \alpha_f} \right) ^{+1/2} \\ = & \frac{\cos \theta_f \cos \alpha_f }{ \sqrt{ \cos^2 \alpha_f (\cos^2 \theta_f + \sin^2 \theta_f) + \sin^2 \alpha_f }} \\ = & \cos \theta_f \cos \alpha_f \end{alignat} }