Your topic is so rare that I couldn’t pass by and not challenge myself by trying to find some sources.
I haven’t succeeded much as I recon. However, there are two papers I think will be interesting for you concerning your issue:
This one about the espionage against the US but by our citizens. The authors write about motivation as well, so I think you can make a reverse analysis and get what you need.
This one is about two basic motivation factors of the espionage.
And here is a draft of what is titled as a guide on espionage psychology. Doesn’t look much reliable but all is good in a famine.