Scattering rate
# Scattering rate

A formula may be derived mathematically for the rate of scattering when a beam of electrons passes through a material.

## The interaction picture

Define the unperturbed Hamiltonian by $H_{0}$, the time dependent perturbing Hamiltonian by $H_{1}$ and total Hamiltonian by $H$.

The eigenstates of the unperturbed Hamiltonian are assumed to be

    $H=H_{0}+H_{1}\$
    $H_{0}|k\rangle =E(k)|k\rangle$

In the interaction picture, the state ket is defined by

    $|k(t)\rangle _{I}=e^{iH_{0}t/\hbar }|k(t)\rangle _{S}=\sum _{k'}c_{k'}(t)|k'\rangle$

By a Schrödinger equation, we see

    $i\hbar {\frac {\partial }{\partial t}}|k(t)\rangle _{I}=H_{1I}|k(t)\rangle _{I}$

which is a Schrödinger-like equation with the total $H$ replaced by $H_{1I}$.

Solving the differential equation, we can find the coefficient of n-state.

    $c_{k'}(t)=\delta _{k,k'}-{\frac {i}{\hbar }}\int _{0}^{t}dt'\;\langle k'|H_{1}(t')|k\rangle \,e^{-i(E_{k}-E_{k'})t'/\hbar }$

where, the zeroth-order term and first-order term are

    $c_{k'}^{(0)}=\delta _{k,k'}$
    $c_{k'}^{(1)}=-{\frac {i}{\hbar }}\int _{0}^{t}dt'\;\langle k'|H_{1}(t')|k\rangle \,e^{-i(E_{k}-E_{k'})t'/\hbar }$

## The transition rate

The probability of finding $|k'\rangle$ is found by evaluating $|c_{k'}(t)|^{2}$.

In case of constant perturbation,$c_{k'}^{(1)}$ is calculated by

    $c_{k'}^{(1)}={\frac {\langle \ k'|H_{1}|k\rangle }{E_{k'}-E_{k}}}(1-e^{i(E_{k'}-E_{k})t/\hbar })$
    $|c_{k'}(t)|^{2}=|\langle \ k'|H_{1}|k\rangle |^{2}{\frac {sin^{2}({\frac {E_{k'}-E_{k}}{2\hbar }}t)}{({\frac {E_{k'}-E_{k}}{2\hbar }})^{2}}}{\frac {1}{\hbar ^{2}}}$

Using the equation which is

    $\lim _{\alpha \rightarrow \infty }{\frac {1}{\pi }}{\frac {sin^{2}(\alpha x)}{\alpha x^{2}}}=\delta (x)$

The transition rate of an electron from the initial state $k$ to final state $k'$ is given by

    $P(k,k')={\frac {2\pi }{\hbar }}|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{k'}-E_{k})$

where $E_{k}$ and $E_{k'}$ are the energies of the initial and final states including the perturbation state and ensures the $\delta$-function indicate energy conservation.

## The scattering rate

The scattering rate w(k) is determined by summing all the possible finite states k' of electron scattering from an initial state k to a final state k', and is defined by

    $w(k)=\sum _{k'}P(k,k')={\frac {2\pi }{\hbar }}\sum _{k'}|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{k'}-E_{k})$

The integral form is

    $w(k)={\frac {2\pi }{\hbar }}{\frac {L^{3}}{(2\pi )^{3}}}\int d^{3}k'|\langle \ k'|H_{1}|k\rangle |^{2}\delta (E_{k'}-E_{k})$
