|
In order to produce a supersonic expansion, we must be able to expand into a vacuum. To this end we have 5 vacuum chambers in our lab. Two of these chambers are setup for fluorescence detection and three for ion detection (the specifics of these two methods are discussed in the next section, Ultraviolet Spectroscopy). Although each chamber is different, an example of each is given in the descriptions below.
|
The Ion Chamber
|
|
The supersonic expansion is created by sending the gas through the pulsed valve at the top. The molecules travel down to the ion source region, which is marked by the 2-stage acceleration plates and the ground plate. In between the 2 acceleration plates, the molecules interact with the UV laser and ionize. The molecules are now ions, and are positively charged. The positively charged acceleration plates repel the ions. The ions now have about 1.5 meters to travel. Because heavier molecules will travel this distance slower than light molecules, we can determine the masses of the ions by how long they take to reach the detector.
|
|
Passing out of the first chamber, the ions encounter an Einzel lens, which acts to "focus" the ions toward the detector all the way at the other end. We also have the option of using a set of steering plates, which can use adjustable positive voltage to steer the ions. Near the detector at the end of the chamber is the mass gate pulser, a grid of very fine wire that carries both positive and negative voltage and can be turned on and off in a matter of nanoseconds. This allows us to reject certain masses and allow others through.
|
|
Our other two ion chambers are not equipped with mass gate pulsers, but the rest of the chamber basics are the same. One of the remaining chambers is pumped using diffusion pumps, while the newest is equipped with Pfeiffer Turbo pumps.
|
The LIF chamber
|
|
Laser induced fluorescence (LIF) is a simpler and more sensitive technique than R2PI, but lacks mass resolution. An LIF chamber is considerably smaller and less complex than the chamber shown above, because creating and collecting photons is much simpler than creating and collecting ions! To collect the fluorescence, one needs only a lens and a sensitive detector. It is also necessary to filter out scattered light, the light from the laser that bounces off surfaces in the chamber and reaches the detector by accident. Tuning the UV laser through the region of interest and collecting fluorescence signal as a function of wavelength gives an ultraviolet spectrum!
|
|
We have one other LIF chamber in our lab. It has the same design, but the collection optics allow for approximately a 30% gain in fluorescnece signal.
|
