Far be it from me to disagree with bob, but in this case, you specified that the secondary application activate after Access exits and not before...that will not be possible if you activate it from the On_Close event of a form. As Access closes, it first closes all open objects, then the db, then exits the application itself, so we could be looking at an issue of semantics, but I assumed that you meant after the msaccess.exe process ended.
I have several modules that use the File System Object to write the script files I need, then I execute the scripts using the Windows Scripting Host command line (wscript.exe [flags] [path]). It sounds overdone, but there's no noticeable lag time in creation and execution and it allows me access to parts of the system that Access is less than helpful with.