序列化形式在引导模式下不起作用

我正在尝试使用jquery / ajax在对话框内序列化表单，但是表单提交的是普通表单，而不是通过ajax提交的，这是我的javascript：

$('.modal-content').on('submit', "send_pm", function() {
    $.ajax({
        type: $(this).attr('method'),
        url: $(this).attr('action'),
        headers: {
            'X-Requested-With': 'XMLHttpRequest'
        },
        dataType: 'json',
        success: function(data) {
            if (!data.success) {
                $.notify(data.error, "error");
            } else {
                $('#globalModal').dialog("close");
                $.notify(data.success, "success");
            }
        }
    });
    return false;
});

我的对话框

<div class="modal" id="globalModal" tabindex="-1" role="dialog" aria-labelledby="myModalLabel">
    <div class="modal-dialog">
        <div class="modal-content">
            <div class="modal-body">
             </div>
        </div>
    </div>
 </div>

我在jquery上返回false，因此它应该正常提交，这是我的表格：

<form action="<?php echo config::get('URL'); ?>messages/send_message" method="POST" id="send_pm">
    <div class="panel-default m-t-20">
        <div class="panel-body p-t-30">
            <div class="form-group" data-intro="<?php echo Tutorials::tutorial('message_compose_sender_to'); ?>" data-step="2">
                <input type="text" class="form-control" value="<?php echo system::escape($this->to); ?>" id="recipient" name="to">
            </div>
            <div class="form-group" data-intro="<?php echo Tutorials::tutorial('message_compose_subject'); ?> " data-step="3">
                <input type="text" class="form-control" name="subject" value="<?php echo system::escape($this->subject); ?>" placeholder="Subject">
            </div>
            <div class="form-group" data-intro="<?php echo Tutorials::tutorial('message_compose_message_body'); ?> " data-step="4" data-position="top">
                <textarea id="compose-textarea" name="message" class="form-control" style="height: 300px">
                    <?php echo System::wysiwyg($this->
                            message); ?>
                </textarea>
            </div>
            <div class="pull-right" data-intro="<?php echo Tutorials::tutorial('message_compose_discard_message'); ?>" data-step="1">
                <button type="submit" class="btn btn-purple waves-effect waves-light"><span><?php echo System::translate("Send"); ?>
                            </span><i class="icon-mail-send"></i></button>
            </div>
        </div>
    </div>
</form>