I don't think it is that simple. There is heat from absorption (ie dark surfaces absorb more heat that light surfaces) and radioactivity to contend with as well. Then there is photoionisation processes affecting molecular makeup and bonds.
I have a book 'Comet Science' by Jacques Crovisier and Therese Encrenaz (both professionals in the field) (ISBN 0521645913 or 0521641799) with a section titled 'sublimation of cometary ices' with some formulae and charts detailing sublimation rates of various compound for distances from the sun as well as the molecular evolution and production rates observed at various distances.
If you are interested then drop me a line and I can scan them for you (just 2 sub A4 sized pages) and post them. It is for education purposes afterall - isn't it
Cheers