Calculation of scattering amplitude ratio for two different angles

[GAGS]

Hi all, since scattering amplitude is given as:
f(theta) = summation over l(from o to infinity)(2l+1)/k exp(i(phi))sin(phi) P_l(cos theta).But what happen if i want to calculate the ratio of f(theta=0)/f(theta = pi/2).
Can anyone tell me the value of P_l(0).
Thanks

 

[malawi_glenn]

It is a Legendre polynomial, http://mathworld.wolfram.com/LegendrePolynomial.html

For all odd l; the scattering amplitude is 0 for cos(theta) = 0