Procedures

The following is a series of procedures used in order to align, adjust or replenish the apparatus.

3.4.1 Atomic Beam Path Alignment

Before an atomic beam was generated, it was necessary to align all of the components of the system. A HeNe laser was used to define the atomic beam path in the following manner. The laser was directed backwards along the atomic beam path from the detection system to the oven chamber, as shown in figure 3.8, with the chopper, quartz block and detection slit removed from the beam path. The laser intensity entering the oven chamber was maximised to determine a central path through the skimmers. The process was more convenient with the removal of the oven.

Components could then by replaced individually, and aligned using the HeNe beam.

3.4.2 Oven Alignment

The oven was located in its mount using three screws. After removal, the oven could be easily relocated if only one screw was loosened during removal. However, this method of relocation was occasionally known to fail due to the thread cut down the sides of the oven. To ensure precise relocation, the HeNe laser beam was used to define the atomic beam path as in section 3.4.1. A small mirror (similar to a dentist's mirror) was inserted into the oven chamber from above to allow a view of the aperture. The oven was then aligned so that the oven aperture was illuminated by the HeNe beam. Note that this method correctly aligns the oven when cold. When heated, the oven needed to be lowered by approximately 0.5mm to account for thermal expansion. The final oven alignment procedure was to maximise the detection signal for the unobstructed atomic beam.

Chapter 3 Experimental Development

3.3.3 Quartz Block Alignment

The alignment of the quartz block required the alignment of three parameters: • The pitch. To ensure the two ends of the quartz block were at the same height. • The yaw. To ensure that the quartz surface was vertical.

• The elevation.

The alignment of the pitch and yaw were very important, as it was critical that the laser beams illuminating the evanescent fields were perpendicular to the bevelled entry surfaces on the quartz block.

First, the quartz block was roughly located so that the HeNe beam defining the atomic beam path skimmed the face. The pitch was aligned by ensuring that the HeNe beam ran parallel to the edge of the quartz block.

The yaw was then aligned. It depended upon the atomic beam path. The atomic beam path (HeNe beam) elevation was measured at both ends of the interaction chamber. The elevation of the atomic beam path was then extrapolated to find the elevation of the atomic beam at a distance of 600mm from the quartz block. A mirror was placed 600mm from the quartz block at an angle of 45° to the atomic beam path, directly in front of the bevelled edge of the quartz block. A laser spot (from the 699) was located on the mirror at an elevation equal to the atomic beam path elevation at 600mm from the quartz block. The yaw of the quartz block and the mirror were adjusted so that the 699 laser beam was retroreflected from the bevelled edge on the quartz block.

Finally, the elevation of the quartz block is adjusted so that the HeNe beam skims past a clean and damage free area.

Chapter 3 Experimental Development

3.4.4 Detection System Alignment

The detection system alignment required the alignment of: • the simulated atomic beam

• the detection laser • the microscope

The atomic beam was simulated by using a short length of wire which was attached to the detection slit and extended horizontally under the microscope. The atomic beam path was defined, as described in section 3.4.1. The short wire was then placed in the HeNe beam, so that it was illuminated along it's length. The reflection of the HeNe beam off the wire simulated the LIF from the atomic beam.

The detection laser was then aligned by intersecting the detection laser with the simulated atomic beam while minimising the scattered detection laser light in the chamber. Next, it was ensured that the initial detection laser direction was parallel to the detection apparatus motion. This was accomplished by quickly moving the

detection system along its range of motion, and checking the detection laser spot on the detection mirror was stationary.

Before the microscope was installed, it was adjusted so that the output was collimated for an object at a working distance of 7mm. The alignment of the microscope was then reduced to locating the simulated atomic beam which was illuminated by the detection laser only. This was achieved by removing the photomultiplier and moving the microscope until the illumination was seen by eye. The mirror at the top of the microscope could also be adjusted at this stage.

Finally, after closing the system and pumping down, the microscope could be fine adjusted using the three motorised micrometers to maximise signal for the unobstructed atomic beam.