has_post_thumbnail() need to output URL only

Hi, I use has_post_thumbnail() to output custom sized images into slider. Uunfortunatley has_post_thumbnail() outputs array of attirbutes but i need URL only. So I used

<?php
$domsxe = simplexml_load_string(get_the_post_thumbnail());
$thumbnailsrc = $domsxe->attributes()->src;
?>

And it worked great exept when I’m calling it in the loop below it outputs thumbnail image 150×150 instead of my custom-sized image ‘slider’.

<?php if (has_post_thumbnail()){ ?>
<?php
$domsxe = simplexml_load_string(get_the_post_thumbnail());
$thumbnailsrc = $domsxe->attributes()->src;
?>

<li class="sliderImage">
                <a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>">
<img src="<?php echo $thumbnailsrc ?>" alt="<?php the_title() ?>" width="560" height="229" />

</a>

If I use has_post_thumbnail(‘slider’) withing the loop instead of defining it as a $thumbnailsrc above, then I’m getting the deisred image, but them with the image i’m getting all other attribs like with, alt etc and I don’t need it.
Any help is greatly appreciated.