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Partial wave analysis : ウィキペディア英語版
Partial wave analysis
Partial wave analysis, in the context of quantum mechanics, refers to a technique for solving scattering problems by decomposing each wave into its constituent angular momentum components and solving using boundary conditions.
== Preliminary scattering theory ==
The following description follows the canonical way of introducing elementary scattering theory. A steady beam of particles scatters off a spherically symmetric potential V(r), which is short ranged so that for large distances r\to\infty, the particles behave like free particles. In principle, any particle should be described by a wave packet but we describe the scattering of a plane wave traveling along the z-axis \exp(ikz) instead, because wave packets are expanded in terms of plane waves and this is mathematically simpler. Because the beam is switched on for times long compared to the time of interaction of the particles with the scattering potential, a steady state is assumed. This means that the stationary Schrödinger equation for the wave function \Psi(\vec x) representing the particle beam should be solved:
:\left(+ V(r)\right )\Psi(\vec x) = E\Psi(\vec x)
We make the following ''ansatz'':
:\Psi(\vec x) = \Psi_0(\vec x) + \Psi_s(\vec x)
where \Psi_0(\vec x) \propto \exp(ikz) is the incoming plane wave and \Psi_s(\vec x) is a scattered part perturbing the original wave function.
It is the asymptotic form of \Psi_s(\vec x) that is of interest, because observations near the scattering center (e.g. an atomic nucleus) are mostly not feasible and detection of particles takes place far away from the origin. At large distances, the particles should behave like free particles and \Psi_s(\vec x) should therefore be a solution to the free Schrödinger equation. This suggests that it should have a similar form to a plane wave, omitting any physically meaningless parts. We therefore investigate the plane wave expansion:
:e^ = \sum_^\infty (2 \ell + 1) i^\ell j_\ell(k r) P_\ell(\cos \theta).
The spherical Bessel function j_\ell(kr) asymptotically behaves like
: j_\ell(kr) \to \frac 1 r \left(\exp(i(kr-l\pi/2)) - \exp(-i(kr-l\pi/2))\right).
This corresponds to an outgoing and an incoming spherical wave. For the scattered wave function, only outgoing parts are expected. We therefore expect \Psi_s(\vec x)\propto \frac 1 r \exp(ikr) at large distances and set the asymptotic form of the scattered wave to
:\Psi_s(\vec x)\to f(\theta, k)\frac
where f(\theta, k) is the so-called ''scattering amplitude'', which is in this case only dependent on the elevation angle \theta and the energy.
In conclusion, this gives the following asymptotic expression for the entire wave function:
:\Psi(\vec x)\to \Psi^(\vec x) = \exp(ikz) + f(\theta, k)\frac.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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