a. A.V. + 자 => "as," "soon after" b. A.V. + 자마자. . . => "as soon as"

a.	A.V. + 자
	=> "as," "soon after"
	b.	A.V. + 자마자. . .
		=> "as soon as"

-자 and -자마자 are very close in meaning, but -자마자 seems to have more immediacy than -자. -자 cannot be used in a command or a "let's . . ." construction. 

(a)

• 서울에 도착하자 은행으로 갔다.
  => Shortly after I arrived in Seoul, I went to a bank.
  
• 영화가 시작하자 아기가 울기 시작했다.
  => As the movie started, a baby started to cry.
  

(b)

• 그 소식을 듣자마자 전화를 걸었다.
  => As soon as I heard the news, I made a phone call.
  
• 엄마를 보자마자 아기가 웃었다.
  => The baby smiled as soon as she saw her mother.