How can I add @mention when creating a FeedItem via APEX?

String salesforceHost = System.Url.getSalesforceBaseURL().toExternalForm();

String url = salesforceHost + '/services/data/v23.0/chatter/feeds/record/' + RecordId + '/feed-items';

HttpRequest req = new HttpRequest();

req.setMethod('POST');
req.setEndpoint(url);
req.setHeader('Content-type', 'application/json');
req.setHeader('Authorization', 'OAuth ' + UserInfo.getSessionId());

req.setBody('{ "body" : { "messageSegments" : [ { "type": "mention", "id" : "' + UserId + '" }, { "type": "text", "text" : "' + ' ' + chatterAlert + '" } ] } }');
Http http = new Http();

HTTPResponse res = http.send(req);