Ignore the article Monyer just pasted. Is about visible spectrum. Sadly, RG removed the downvoting option.

You can have a look at the following instead (IR spectrum):

http://pubs.acs.org/doi/pdf/10.1021/j100678a021

http://www.sciencedirect.com/science/article/pii/0038109877906639

Article Some optical properties of graphite from IR to millimetric wavelengths

Probably you can find adsorption coefficients there.

I also agree with the comments of Dr. Fernando . Perhaps the below links may give some answers.

http://adsabs.harvard.edu/full/1983PASJ...35..397T

http://www.sciencedirect.com/science/article/pii/S1877705815002714