Serialising a Linq Query to Json

IEnumerable<Employee> empJson = from emp in employees
                                where emp.Department == "IT Department"
                                select emp;
var ser = new DataContractJsonSerializer(typeof(IEnumerable<Employee>));
using (stream = new MemoryStream()) {
    ser.WriteObject(stream, empJson);
    string json = Encoding.Default.GetString(stream.ToArray());
    return json;

}