Saturday, 22 July 2017

integration - Fourier transform in three dimensions getting out of hand



I have the following integral I wish to compute, it transforms a quantum position wave function into momentum space:



ϕ(p)=d3r(2π)3/2eiprψ(r)
Where ψ(r)=e|r|/a/πa3, the wave equation for the ground state of hydrogen.



So I thought about converting it to polar coordinates first then evaluate the integrals, but it turned out to be unexpectedly complicated.




Below is my attempt (note, since this is physics related, θ is swapped with φ in polar coordinates):






We first rewrite the integral in polar coordinates:
ϕ(rp,φp,θp)=1(2a)3/2π2π02π00eipx/hrx/adrxdφxdθx



Where the dot product in spherical coordinates is:
px=rxrp[sinθpsinθxcos(φpφx)+cosθpcosθx]




Substitute and integrate, we get:
ϕ(rp,φp,θp)=1(2a)3/2π2irp2π0π0dθxdφxsinθpsinθxcos(φpφx)+cosθpcosθxNow let: a=sinθpcos(φpφx)b=cosθpϕ=1(2a)3/2π2irpπ0ln(a2+b2a)ln(a2+b2+a)a2+b2dφx







The second integral is evaluated from wolfram alpha. From here I'm quite stuck. The final integral looks rather hopeless. Is there any tricks to Fourier transforms in space or did I make a mistake somewhere?



Edit: I think I accidentally substituted 2π instead of π, but the correct version is even more complicated.


Answer



Symmetry is your friend. Since ψ is radially symmetric, so is ϕ.
So wlog we can assume p=pk, and p.r=prcos(θ). Then



ϕ(pk)=1(2a)3/2π22π0dφ0r2drπ0sin(θ)dθeiprcos(θ)er/a=12(a)3/2π0r2dr2sin(pr)prer/a=4a3/223/2π(a2p2+1)2


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