They are different. They can propagate or not. I could not tell much. See for instance :
"Free, quasifree, momentarily trapped, and self-trapped exciton states in molecular crystals", Singh, Jai; Matsui, A., Physical Review B (Condensed Matter), Volume 36, Issue 11, October 15, 1987, pp.6094-6098.
I am new to XPS Data analysis, and I have a .sle file that I got out of XPS Machine Software, and I am using CasaXPS to analyse the data. This software takes only .vms files. I want to convert the .sle to a .vms file for analysis. How or where can I do this? Any help on this will be deeply appreciated.