in an instant
within an instant

The rest of his body went in the next instant.

In the next instant the second man would have moved.

The other two men were on him in an instant.

He must have died in an instant, but his body did not fall.

In an instant the man was off the bed and across the room.

A big play on the field can change everything in an instant.

His death did in an instant what he'd spent the last few months trying to do.

But in an instant he's a fish out of water.

Or had come and gone in an instant, while I was thinking of something else.

I could have been free of the place in an instant.