IRS Pub 501 would be a good place to start.  Pub 519 would be even better, but it says the same thing for resident aliens and then refers to Pub 501.

Nonresident alien spouse. You are considered

unmarried for head of household purposes
if your spouse was a nonresident alien at any
time during the year and you don't choose to
treat your nonresident spouse as a resident
alien. However, your spouse isn't a qualifying
person for head of household purposes. You
must have another qualifying person and meet
the other tests to be eligible to file as head of
household.