how to call function when window is close

Question

I am opening a window in asp.net  as(   window.open('ChatWindow.aspx', '_blank', 'top=610, left=1150,width=220,height=450,titlebar:no,toolbar:no,statusbar:no'))

as we know that it has minimize,maximize and close(X mark)....when close(x mark) is clicked how can i call code behind method...pls suggest me clearly