function search(searchString){
    location.replace('index.html?searchString=' + searchString);
}

function respondToEnterKey(event) {
        if (Event.KEY_RETURN == event.keyCode) {
                search($('searchString').getValue());
                Event.stop(event)
        }
}

Event.observe(window, 'load', function() {
        $('searchString').observe('keypress', respondToEnterKey);

});