Well, first I would factor out exp(-a), as that is constant.
Next I put it into http://www.yqcomputer.com/
which says that "Mathematica could not find a formula for your
But that only does indefinite integrals, and this seems like a
case where the definite integral might be possible to evaluate
even though the indefinte integral doesn't have an analytical
solution. (Not that you can find a numeric solution for an