Hi Li,

To add to Daniel's response.  Depending on what UV wavelength you are
interested in you may be able to get values of k from the Henke X-ray
atomic scattering factor tables.  You can find information on how to
calculate k from the the atomic scattering factors along with a large
list of the wavelength dependent atomic scattering factors online at
http://www-cxro.lbl.gov/optical_constants/ .
Cheers,
Marie