Yes, users can include diacritics - à, é, ç, å, ï, and ø, for example - in their search queries. However, the search engine will strip out the diacritics and use the simple, non-diacritics version of the letter instead. So à becomes a, é becomes e, and so on, as far as the search engine is concerned.
There are two primary reasons ProQuest chose this approach:
Much of the older content in our system has had diacritics stripped out already, so by searching specifically for “niño,” results that include “nino,” with the diacritic stripped out, won’t be included in the search results.
Users don’t always have access to a keyboard that has the ability to enter diacritics so they need to be able to search for “nino” and retrieve entries with “niño.”