If you need object ID matte: select the Emitter (or any other objects) assign a cyObject tag on it -> in the "Object Compositing Setting" tab you'll find ObjectID counter.
then in the Render Settings, Go to the "Render Elements" tab and check "Object IDs" under the "IDs" sub-tab. you good to go.
If you just need an alpha on your render: go to render settings under Settings -> Film enable the "Transparent Background" checkbox. then save your renders with alpha.