Invade, incline, illumine, illustrate, immerse, impress, irradiate. The meaning of the prefix is in, into.

What is the root word of immersed?

The Latin background of immersion includes the root mergere, "to plunge." The idea of immersion as a physical submergence came first and the idea of something being absorbed in a situation came about later.

Is it immerse or emerse?

As adjectives the difference between immerse and emerse

is that immerse is (obsolete) immersed; buried; sunk while emerse is feminine plural form of emerso.