Recovery System for Xenon Chloride Laser

In a xenon chloride excimer laser, an electric glow discharge causes hydrogen chloride and xenon gases to react, and excites the resulting xenon chloride, XeCl, so that it lases. As the laser operates, its power gradually diminishes. When the loss of power renders the laser unsuitable for its intended use, the gases in the lasing chamber must be replaced with fresh hydrogen chloride and possibly xenon. This is frequently inconvenient and requires a costly shutdown of the laser.

We have discovered that xenon chloride lasers can be operated continuously without any loss of power, and without the addition of fresh hydrogen chloride, if the gases in the lasing chamber are continuously circulated through a reforming chamber. The reforming chamber contains a metal chloride of less chemical stability than hydrogen chloride, at a temperature between the condensation temperature of hydrogen chloride and the dissociation temperature of the metal chloride used.

The accompanying drawing is a diagrammatic view of a certain presently preferred embodiment of apparatus according to this invention for maintaining the power output of a xenon chloride laser.

In the drawing, a laser 1 consists of a lasing chamber 2 which is filled with a mixture of gases 3, which includes xenon, hydrogen chloride, and an inert buffer gas (preferably helium, because it is inexpensive and has high thermal conductivity). Two electrodes 4 and 5 create an electric glow discharge that results in the formation of xenon chloride in the lasing chamber. The electric glow discharge excites the xenon chloride and produces a high repetition rate laser pulse between fully reflecting mirror 6 and partially transmitting mirror 7, resulting in laser output 8. A blower 9 continuously circulates the gases in lasing chamber 2 through conduits 10, 11, and 12 and reforming chamber 13. Reforming chamber 13 contains a metal chloride 14 of less chemical stability than hydrogen chloride. Electric resistance heater 15 maintains the temperature in reforming chamber 13 between the condensation temperature of hydrogen chloride and the dissociation temperature of the metal chloride used.

While we do not wish to bound by any theories, we believe that the method and apparatus of this invention overcome the problem of the gradual diminution of power in xenon chloride lasers by reforming hydrogen chloride to replace hydrogen chloride that is lost. We believe that the loss of hydrogen chloride occurs when hydrogen chloride in the lasing chamber dissociates into hydrogen and chlorine, and the free chlorine reacts with metals in the lasing chamber, such as nickel, and is thereby depleted. In the process of this invention, the excess hydrogen that results is reacted with the metal chloride in the reforming chamber to reform hydrogen chloride. This reaction in the reforming chamber can be described by the equation:

where MCl.sub.x represents the metal halide and x is the valence of M. Any metal chloride can be used in the reforming chamber that is less stable than hydrogen chloride. Examples of such metal chlorides include ReCl.sub.3, MoCl.sub.2, MoCl.sub.4, WCl.sub.2, WCl.sub.3, and WCl.sub.5. The preferred metal chloride is MoCl.sub.2 because it is readily available and is much less stable than is hydrogen chloride. The metal chloride must be placed in the reforming chamber so that the gases from the lasing chamber pass in intimate contact with it. This means that the metal chloride should be in a form that has a high surface area, such as a fine powder. It is useful to place the metal chloride on a suspending agent, such as quartz wool, to prevent the powder from being compacted. The gases from the lasing chamber can also be used to fluidize the metal chloride power. The temperature in the reforming chamber must be higher than the temperature at which hydrogen chloride condenses to prevent the condensation of hydrogen chloride in the reforming chamber, but the temperature must be below the temperature at which the metal chloride dissociates. The flow rate of the gases through the reforming chamber is not critical, and as long as the laser power output does not fall, the flow rate should be considered to be adequate. The amount of metal chloride in the reforming chamber is also not critical but, as it will gradually be depleted, less frequent replacements will be needed if a larger amount is used.