"Escape-A", for example, is
send \033A
"Control-c", for example, is
send \003
http://expect.sourceforge.net/FAQ.html#q53 http://wombat.doc.ic.ac.uk/foldoc/foldoc.cgi?ASCII+character+table
See also "How Expect sees function keys".
Helmut Giese mentions that "[t]o go from an arbitrary ASCII character (say X) to its 'control' code (Ctrl X) just do
set ctrl [expr ($char & 01xF)]
"