Abstract: A molecular jet separator includes a body which defines an evacuation chamber. A bore in the body traverses the chamber so that the bore has two segments on opposite sides of the chamber. Respective tubes are seated in these bore segments, with their opposed ends in direct or indirect engagement within the chamber, in a firm fit so that the bores of the tubes are substantially aligned. The bores of the tubes are open at the aforesaid opposed ends, and at least one of these opposed ends is shaped to define a predetermined gap between the opposed open ends of the tube bores.