Photon absorbed by H2

[justdancin89]

I need help with my quantum mechanics homework. I can't seem to figure out the appropriate equation t use to answer the following question:

An ultraviolet photon (wavelength = 58.4nm) from a He discharge tube is absorbed by H2 at rest. If momentum is conserved, what is the velocity of the hydrogen molecule after absorbing the photon?

[dextercioby]

I would say that you need to pick up your book and read about (relativistic) conservation laws. Then you'd get an idea how to start the solution to the problem.