Selenium

Se 3d5/2 binding energy value averages [1].

Se 3d5/2-3d3/2 separation = 0.86 eV [2]
Se 3p3/2: 163 eV
Se 3p1/2: 169 eV
Se 3s: 232 eV

Note that the LMM Auger structure taken with an Al K(alpha) X-ray source for Se is quite extensive (see below) and can cause spectral interferences with a wide variety of other elements. 

XPS spectrum of ground Se showing the extensive LMM Auger structure (Al X-ray source).

References:
[1] C.D. Wagner, A.V. Naumkin, A. Kraut-Vass, J.W. Allison, C.J. Powell, J.R.Jr. Rumble, NIST Standard Reference Database 20, Version 3.4 (web version) (http:/srdata.nist.gov/xps/) 2003.
[2] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben "Handbook of X-Ray Photoelectron Spectroscopy", Perkin-Elmer Corporation, Physical Electronics Division, Eden Prairie, Minn. (1992)

Selenium and Sulphur

Quantification of sulphur in the presence of selenium by XPS is difficult from survey scan data due to the overlap of the S 2p peak with the Se 3p3/2 and Se 3p1/2 peaks and the S 2s peak with the Se 3s peak. It is possible, in some cases, to quantify the amounts of Se and S present using a high-resolution scan of the S 2p and Se 3p region (use a B.E. window of 174 - 154 eV). Using a Se 3p3/2 - 3p1/2 doublet separation of 5.8 eV (+/- 0.2 eV) (Se metal was found here to have a separation of 5.75 eV) and constraining the 3p3/2 - 3p1/2 peaks to a 2:1 ratio (constraint factor of A*0.5), the remaining structure may then be attributed to the S 2p peak(s). Of course this analysis will also give you information on the sulphur species present. Then, by using the correct R.S.F. values, a Se:S ratio can be obtained using a components quantification.

If the survey spectrum is collected at the same time, the survey scan data and high-resolution data can be combined using a "Regions and Comps" calculation (under the Report Spec. / Quantification Parameters window in CasaXPS) to give full quantification of all the elements present. Remember to set the R.S.F. to 0 for the Regions window of the high-resolution spectrum.

A closer look at the Se 3p spectrum shows that the Se 3p3/2-3p1/2 ratio may be closer to 7:3, or an area constraint factor of A*0.43. This may be due to some overlap of the Auger structure with the 3p peaks. A example of the final fit of the S 2p and Se 3p can be found here.

R.S.F. Values
S 2p3/2 = 0.4453
Se 3p3/2 = 0.84933