Closing the opener window opened by a <form>

Hi all,

I have a form goes to a bank's payment page and the form opens this page in a blank page.
<form target="_blank" method="post" action="https://bankspage.com" onSubmit="return checkCheckBox(this)">

the bank gets the money from the credit card and returns the results to my page. what I want is when the client closes this result page, the main page redirected to a URL.

So basicly on unload of a blank page opened by a form, redirect the opener page to a new URL.

Any idea?

Thanks
telmessos