The laser table is located near the center of our lab. A Krypton ion laser is located in the left center of the photo. The red dot is light scattered from the 676-nm beam emitted by this laser. Two mirrors steer the light into the color-center laser located on the front corner of the table. This red light excites a lithium-doped RbCl crystal that is housed inside the shiny metal dewar at the far end of the color-center laser. Liquid nitrogen inside the dewar keeps the color center crystal cold and suppresses the recombination and annihilation of the optically active color center sites in the laser gain medium. Other items visible on the laser table include a wavemeter for absolute wavelength determination, two Fabry-Perot interferometers with free spectral ranges of 300MHz and 1.5GHz, and detectors for laser power measurements. (1GHz corresponds to about 0.03 cm^-1, and 1MHz corresponds to about 0.00003 cm^-1 in the mid-infrared.) The oscilloscope on the right displays the signal from one of the scanning Fabry-Perot interferometers and indicates that the color center laser is lasing on a single longitudinal mode. The molecular beam machine is located behind the laser table. The end of the ultrahigh vacuum main chamber and the light blue stand on which it rests are visible near the large LN₂ dewar.

Taking a couple of steps to the left the lasers are in full view.

The long black Coherent laser (left) is the krypton ion laser that pumps the Burleigh F-center laser (right). The molecular beam machine is visible in the background.

Walking around the right hand side of laser table, we see the beam machine in full glory.

The machine is supported by 3 separate stands (light blue framework). The main chamber is in the foreground with the interconnected second and first differential pumping stages and the molecular beam source chamber extending to the left.

The differential pumping stages serve two purposes. First, they reduce the amount of gas in the molecular beam that reaches the main chamber. This significantly reduces the amount of pumping required to maintain "good" vacuum in the surface science chamber. Second, collimating apertures between the differential stages ensure that nearly all molecules interacting with the nickel surface in the main chamber originate from the supersonic expansion, and not from background gases that may be present in the molecular beam source.

Surface chemistry is highly sensitive to the presence of contaminants on the surface of interest. In order to ensure that our surface is clean and free of unknown contamination, the nickel surface we study is housed in an ultrahigh vacuum chamber (the main chamber), whose pressure is maintained at about 1x10^-10 Torr. This level of vacuum minimizes the number of gas-phase species that may interact with, stick to, and contaminate the surface under study. The (brown) manipulator on top of the main chamber and the cold finger extending down from it support the nickel sample. We use a 1-cm disk cut from a single crystal of metallic nickel and polished to expose the (111), or hexagonal-close-packed crystalline face. The manipulator allows the nickel crystal to be rotated about a vertical axis and translated in three dimensions within the UHV environment. The RF box in front of the main chamber provides RF and DC potentials to the quadrupole mass spectrometer (QMS) in the main chamber. The QMS is located on the machine's beam axis. It is mounted on the end of the main chamber by the small flange visible just above the RF box.

All four compartments of the machine are independently pumped by diffusion pumps (DPs). The DPs are backed by direct drive mechanical pumps that are located in a separate pump room behind the white wall. The main chamber and second stage are protected from backstreaming DP oil by liquid nitrogen-cooled (LN₂) traps. The white trap under the main chamber is clearly visible in the picture. The first stage is protected by a water baffle. The source chamber has no trap to keep the pumping speed high.

The first and second stages and the main chamber can be isolated from their DPs by gate valves. The main chamber and second stage have pneumatic gate valves that are individually interlocked to the DP's cooling water flow and DP temperature, the foreline pressure, and the cryogen level in the corresponding LN₂ trap.

Walking toward the left along the machine, we end up at the source chamber end. The following picture shows the machine with the source in the foreground.

The source chamber is pumped by the large blue diffusion pump. It houses the supersonic molecular beam source, which consists of a metal nozzle with a 25um diameter orfice and a 1mm diameter skimmer. The plexiglass box on top of the source chamber contains a step-down tranformer that provides the high current needed to heat the nozzle resistively.

The first differential pumping stage, located in the middle section of the machine, contains a multi-pass cell where infrared light from the color-conter laser will intersect the molecular beam. The first differential stage also contains a mechanical shutter for precise control of molecular beam exposure times.

The second differential pumping stage lies between the end of the chamber's middle section and a vertical wall welded inside the right-hand part of the chamber. This wall separates the second stage from the main chamber. The second differential stage contains a slotted chopper wheel that can rotate at speeds up to 400Hz. This wheel modulates the molecular beam and, in conjunction with a quadrupole mass spectrometer located on the beam axis in the main chamber, permits time-of-flight measurements that characterize the translational energy of molecules in the supersonic molecular beam. The second differential stage also contains a bolometric detector for direct measurement of infrared absorption by molecules in the molecular beam. A sliding valve in the second differential chamber can completely isolate the ultrahigh vacuum main chamber from the source and differential pumping stages.

From this point, turning to the left, we can view some of the machine's control electronics.

All the controls at this end of the machine are home built. The chopper motor driver (top right and top left panels), the automatic LN₂-trap controller (red rectangles), and the foreline pressure gauges (middle right) are not labeled in the picture.

Looking along the back side of the machine, one sees the remaining controls next to the main chamber. The machine itself is now on your immediate right.

The left rack (gray) contains all the controls for the electron gun and hemispherical electron energy analyzer that comprise the auger electron spectrometer, as well as the ion gauge controllers for the 2nd differential pumping stage and main chamber. The right rack (blue) contains the QMS controller. Variable transformers for the main chamber bake-out heaters and an ion pump controller are also located in these racks. On the right hand side of the picture one can see the closest of the pneumatic gate valves and a part of the hemispherical analyzer that is wrapped up in aluminum foil for bake-out.

Turning to the right and looking into the main chamber through a large window, one sees the nickel crysal at the end of the cold finger.

The nickel crystal is suspened from two tungsten rods extending from the bottom of the cold finger. It is surrounded by a copper cold shield that minimizes radiative heating of the crystal by the chamber walls. The cold shield is in thermal contact with a LN₂ reservoir. A thoriated tungsten filament located behind the crystal permits electron bombardment heating of the crystal. Liquid nitrogen cooling coupled with electron bombardment heating permits the crystal temperature to be varied from 78K to over 1000K. The entire crystal support assembly can be rotated around a vertical axis and translated in 3 dimensions. Combinations of crystal rotation and translation position the crystal for dosing (the molecular beam enters the main chamber from the right in this picture), argon ion sputter cleaning, or for auger electron spectroscopy, electron energy loss spectroscopy, or temperature programmed desorption measurements.