The entry for Pasada in the Illustrated Glossary of Pali Terms has "limpid" as an option, which might align with your use of "pure". Though serene seems like a reasonable option too.
Thanissaro article comparing saṁvega and pasāda.
He says pasāda is a complex emotion encompassing many meanings, including 'confidence' that the Buddha's teaching on the Deathless works.
Thanissaro said elsewhere: