He laughed, and paused awhile; he saw her turn, and knew that even for a while, he held her attention.
A While [THINK: noun phrase (two words)]
Awhile [THINK: adverb (one word)]
“The two-word expression a while is a noun phrase, consisting of the article a and the noun while, defined as “a period or interval of time.”
“…The noun phrase a while can and often does follow a preposition, such as for or in: “He said he would be home in a while.”
“The one-word awhile is an adverb that means “for a short time or period.””
“The adverb awhile cannot follow a preposition, a rule that makes sense if you revisit the definition of the term and drop it into a sentence…“He said he would be home in for a short time or period.””
[read more about it on blog.dictionary.com]