I have the following integral I wish to compute, it transforms a quantum position wave function into momentum space:
ϕ(p)=∫d3r(2πℏ)3/2e−ip⋅rψ(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(2aℏ)3/2π2∫π0∫2π0∫∞0e−ip⋅x/h−rx/adrxdφxdθx
Where the dot product in spherical coordinates is:
p⋅x=rxrp[sinθpsinθxcos(φp−φx)+cosθpcosθx]
Substitute and integrate, we get:
ϕ(rp,φp,θp)=1(2aℏ)3/2π2irp∫2π0∫π0dθxdφxsinθpsinθxcos(φp−φx)+cosθpcosθxNow let: a=sinθpcos(φp−φx)b=cosθpϕ=1(2aℏ)3/2π2irp∫π0ln(√a2+b2−a)−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(2aℏ)3/2π2∫2π0dφ∫∞0r2dr∫π0sin(θ)dθeiprcos(θ)e−r/a=1√2(aℏ)3/2π∫∞0r2dr2sin(pr)pre−r/a=4a3/2√2ℏ3/2π(a2p2+1)2
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