window.onload = prepareLinks;
function prepareLinks() {
var links = document.getElementsByTagName("a");
for (var i=0; i<links.length; i++) {
if (links[i].getAttribute("rel") == "external") {
links[i].onclick = function() {
popUp(this.getAttribute("href"));
return false;
}
}
}
}

function popUp(winURL) {
window.open(winURL,"popup","width=500,height=550");
}