Can't access named property of $obj.psobject.properties, please help.

Hello, I have a script that reads in XMP-data from images, the values are put into a custom object, as far as I understand. 

This is what I do using the Get-ImageMetaData-Module from Joel Bennett:

$datas = (ls C:\MyImage.jpg | Get-ImageMetaData )

Now everything works fine: when I do this afterwards:

$datas.psobject.properties

I get this output (excerpt):

MemberType : NoteProperty
IsSettable : True
IsGettable : True
Value : 3
TypeNameOfValue : System.String
Name : /xmp/xmp:Rating
IsInstance : True 

Now this is what I want to use in my script, I want to read the value of the member named "/xmp/xmp:Rating" into a variable. But I dont know how, because I don't know the way to adress the member named "/xmp/xmp:Rating".

I tried this

$a = $file.psobject.properties."/xmp/xmp:Rating".value

but that doesn't work. Could someone help me here, please? Thanks a lot! Martin.