序列化的AJAX $ _POST不为空，但单个$ _REQUEST为空

I'm making an AJAX Call like this:

$( "#submit_jobs_preview" ).click(function(e) {
    var postData = $('#new_job_form').serialize();
    var formURL = "ajax.xxx.php";

    $.ajax({
    type: 'POST',
    url: formURL,
    data: {submit_jobs_preview: postData },
    success: function(data){
        alert(data);
        }   
    });     
    e.preventDefault(); //STOP default actio
});

In php i echo:

echo $_POST['submit_jobs_preview'];

This $_POST shows all values i put in my form correctly like this:

new_job_city=Berlin&new_job_name=Manager etc.

But if i want a single $_REQUEST from this $_POST like this:

 if($_POST['submit_jobs_preview']){
    echo $city = mysql_real_escape_string($_REQUEST['new_job_city']);
 }

the alert is empty.

Why is that? :)

Thanks in advanced!

UPDATE

Full return of data:

XHR Loaded (autocomplete.php - 200 OK - 105.99994659423828ms - 354B) VM2106:3
new_job_city=Halle&new_job_job=Flugbegleiter&new_job_other_job=&job_zielgruppe=Auszubildende&new_job_phone=4921663965439&new_job_email=kleefeld%40dsc-medien.de&new_job_detailhead=F%C3%BCr+unser+zentrales+Marketing+mit+Sitz+in+der+Hauptverwaltung+in+der+City+von+D%C3%BCsseldorf+suchen+wir+zum+n%C3%A4chstm%C3%B6glichen+Termin+eine%2Fn&new_job_time=Vollzeit&teilzeit_std=&new_job_tasks=&new_job_profile=&new_job_advantage=&new_job_creatorId=1&new_job_creationDate=1390569795&new_job_scope=Airport+Service&new_job_niederlassung=7&new_job_active=0 

You don't need to use {submit_jobs_preview: postData } if it is not necessary. I will give you another hint. Use following ajax;

$( "#submit_jobs_preview" ).click(function(e) {
    var postData = $('#new_job_form').serialize();
    var formURL = "ajax.xxx.php";

    $.ajax({
    type: 'POST',
    url: formURL,
    data: postData,
    success: function(data){
        alert(data);
        }   
    });     
    e.preventDefault(); //STOP default actio
});

and on php side;

if($_POST['new_job_city']){
    echo $city = mysql_real_escape_string($_REQUEST['new_job_city']);
}

In this case you can use field names directly.

Another hint:

You can put a hidden value on your form like,

<input type="hidden" name="submit_jobs_preview" value="true"/>

and js side will be same as above in my answer, and in php side you can check like;

if($_POST['submit_jobs_preview']){
    echo $city = mysql_real_escape_string($_REQUEST['new_job_city']);
}

As you can see, you can send your post value as hidden field and make your check

$_POST['submit_jobs_preview'] holds a string and nothing else so you just can't access it that way. Use parse_str() to unserialize $_POST['submit_jobs_preview']. That way you can access its properties.

$_POST['submit_jobs_preview']['new_job_city']