qpms/notes/cylinderT.lyx

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#LyX 2.4 created this file. For more info see https://www.lyx.org/
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\begin_body
\begin_layout Standard
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LatexCommand input
filename "makrot.lyx"
literal "false"
\end_inset
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\begin_layout Title
T-matrix of an axially symmetric particle
\end_layout
\begin_layout Standard
So we have
\begin_inset CommandInset citation
LatexCommand cite
after "(9.12)"
key "kristensson_scattering_2016"
literal "false"
\end_inset
:
\begin_inset Formula
\begin{align*}
R_{nn'} & =ik^{2}\iint_{S_{s}}\left(\frac{\eta}{\eta_{1}}\wfkcreg_{n}\left(k\vect r\right)\times\wfkcreg_{\overline{n'}}\left(k_{1}\vect r\right)+\wfkcreg_{\overline{n}}\left(k\vect r\right)\times\wfkcreg_{n'}\left(k_{1}\vect r\right)\right)\cdot\uvec{\nu}\,\ud S,\\
Q_{nn'} & =ik^{2}\iint_{S_{s}}\left(\frac{\eta}{\eta_{1}}\wfkcout_{n}\left(k\vect r\right)\times\wfkcreg_{\overline{n'}}\left(k_{1}\vect r\right)+\wfkcout_{\overline{n}}\left(k\vect r\right)\times\wfkcreg_{n'}\left(k_{1}\vect r\right)\right)\cdot\uvec{\nu}\,\ud S,
\end{align*}
\end_inset
where
\begin_inset Formula $S_{s}$
\end_inset
is the scatterer surface,
\begin_inset Formula $\uvec{\nu}$
\end_inset
is the outwards pointing unit normal to it, and the subscript
\begin_inset Formula $_{1}$
\end_inset
refers to the particle inside; then
\begin_inset Formula
\begin{equation}
T_{nn'}=-\sum_{n''}R_{nn''}Q_{n''n}^{-1}.\label{eq:T matrix from R and Q}
\end{equation}
\end_inset
\end_layout
\begin_layout Standard
Let us consider the case with full rotational symmetry around the
\begin_inset Formula $z$
\end_inset
axis and parametrise the integral in terms of polar angle
\begin_inset Formula $\theta$
\end_inset
.
Let
\begin_inset Formula $\beta$
\end_inset
be the angle between the surface normal
\begin_inset Formula $\uvec{\nu}$
\end_inset
and the coordinate radial direction
\begin_inset Formula $\uvec r$
\end_inset
.
The infinitesimal surface area element is then
\begin_inset Formula
\[
\ud S\left(\theta\right)=\frac{\left(r\left(\theta\right)\right)^{2}\sin\theta}{\cos\beta\left(\theta\right)}\ud\theta\,\ud\phi
\]
\end_inset
and the surface normal in local coordinates
\begin_inset Formula
\[
\uvec{\nu}\left(\theta\right)=\uvec r\cos\beta\left(\theta\right)+\uvec{\theta}\sin\beta\left(\theta\right),
\]
\end_inset
which also sets a convention for the sign of
\begin_inset Formula $\beta$
\end_inset
.
\end_layout
\begin_layout Standard
For fully axially symmetric particles the integrals vanish for
\begin_inset Formula $m\ne-m'$
\end_inset
due to the
\begin_inset Formula $e^{i\left(m+m'\right)}$
\end_inset
asimuthal factor in the integrand.
One then has
\begin_inset Formula
\begin{equation}
T_{nn'}=-\sum_{n''}R'_{nn''}Q'_{n''n}^{-1}\label{eq:T-matrix from reduced R and Q}
\end{equation}
\end_inset
where
\begin_inset Formula
\begin{align*}
R'_{nn'} & =\int_{0}^{\pi}\left(\frac{\eta}{\eta_{1}}\wfkcreg_{n}\left(k\vect r\right)\times\wfkcreg_{\overline{n'}}\left(k_{1}\vect r\right)+\wfkcreg_{\overline{n}}\left(k\vect r\right)\times\wfkcreg_{n'}\left(k_{1}\vect r\right)\right)\cdot\left(\uvec r\cos\beta\left(\theta\right)+\uvec{\theta}\sin\beta\left(\theta\right)\right)\frac{\left(r\left(\theta\right)\right)^{2}\sin\theta}{\cos\beta\left(\theta\right)}\ud\theta,\\
Q'_{nn'} & =\int_{0}^{\pi}\left(\frac{\eta}{\eta_{1}}\wfkcreg_{n}\left(k\vect r\right)\times\wfkcreg_{\overline{n'}}\left(k_{1}\vect r\right)+\wfkcreg_{\overline{n}}\left(k\vect r\right)\times\wfkcreg_{n'}\left(k_{1}\vect r\right)\right)\cdot\left(\uvec r\cos\beta\left(\theta\right)+\uvec{\theta}\sin\beta\left(\theta\right)\right)\frac{\left(r\left(\theta\right)\right)^{2}\sin\theta}{\cos\beta\left(\theta\right)}\ud\theta
\end{align*}
\end_inset
where
\begin_inset Formula $\vect r=\vect r\left(\theta\right)=\left(r\left(\theta\right),\theta,0\right)$
\end_inset
.
Matrices
\begin_inset Formula $Q',R'$
\end_inset
differ from the original
\begin_inset Formula $R,Q$
\end_inset
matrices in
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LatexCommand eqref
reference "eq:T matrix from R and Q"
plural "false"
caps "false"
noprefix "false"
\end_inset
by a factor of
\begin_inset Formula $2\pi ik^{2}$
\end_inset
, but this cancels out in the matrix product.
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Parametrisation of cylindrical particle surface.
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For cylindrical particle of radius
\begin_inset Formula $R$
\end_inset
and height
\begin_inset Formula $h$
\end_inset
, we can divide the parametrisation into three intervals
\begin_inset Formula $\left(0,\theta_{1}\right),\left(\theta_{1},\theta_{2}\right),\left(\theta_{2},\pi\right)$
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where
\begin_inset Formula $\theta_{1}=\tan^{-1}\left(2R/h\right),\theta_{2}=\pi-\tan^{-1}\left(2R/h\right)$
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:
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\begin_layout Enumerate
In the first section,
\begin_inset Formula $0<\theta<\theta_{1}$
\end_inset
,
\begin_inset Formula
\begin{align*}
r & =\frac{h}{2\cos\theta},\\
\beta & =-\theta.
\end{align*}
\end_inset
\end_layout
\begin_layout Enumerate
In the second section,
\begin_inset Formula $\theta_{1}<\theta<\theta_{2}$
\end_inset
,
\begin_inset Formula
\begin{align*}
r & =\frac{R}{\cos\left(\theta-\pi/2\right)}=\frac{R}{\sin\theta},\\
\beta & =-\theta+\pi/2.
\end{align*}
\end_inset
\end_layout
\begin_layout Enumerate
In the third section,
\begin_inset Formula $\theta_{2}<\theta<\pi$
\end_inset
,
\begin_inset Formula
\begin{align*}
r & =\frac{h}{2\cos\left(\theta-\pi\right)}=-\frac{h}{2\cos\theta},\\
\beta & =-\theta+\pi.
\end{align*}
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