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Show light and passes UV (Coming UG-5). The UV beam is attenuated to pulse energies in the 1-5 mJ range with a variable allenuator (NRC model 935-10) before being focused with a 20-cm-focal-Iength cylindrical lens into the ionization region of the reflectron TOF MS where it crosses the I-mm-diameter molecular beam. We have also used the same laser system in our two-color REMPI experiments by spliuing off -20% of the 532-nm pump beam before it enters the dye laser and frequency doubling this beam in a temperature-tuned KDP crystal to obtain the 266-nm fourth harmonic of the Nd:Y AG laser. This wavelength is ideal for use as a second color 0,,2 in Fig. 1) for the dichlorobenzenes. The 266-nm beam is reduced to a diameter of 4 mm with a telescope and is temporally delayed with an optical delay line so that it arrives at the ionization region a few nanoseconds after the tunable UV beam. The 266-nm beam counterpropagates relative to the tunable UV beam and the two beams are overlapped with each other and the molecular beam by means of several x,y ,z translation stages and beam steering optics. Finally in some cases it is useful to add the dye laser fundamental to the UV beam; this is accomplished by simply removing the visible blocking filter and allowing both the dye and UV beams to propagate collinearly through the focusing lens. Ions formed by one of the schemes shown in Fig. 1 are mass analyzed in the reflectron TOF MS. This instrument is a modified version of a commercially available linear reflectron8 (R. M. Jordan Co.) and can be operated in two modes. In the first mode the TOF MS is operated "normally", that is the ions are extracted vertically upward (Le. toward the top of the page in Fig. 2) in the classic two-stage acceleration9 to a total energy of -900 eV and then allowed to drift in a 40-cm field-free region before striking a dual microchannel plate ion detector (R. M. Jordan Co.). The molecules in the beam have a substantial velocity component (1.76 x lOS cm/s) along the beam axis that is at right angles to the ion path. For heavy ions with long flight times this velocity component causes the ions to miss the ion detector. Accordingly we use an electric field provided by a pair of plates in the drift region to deflect the ions back on course. In the second, or "reflectron" mode of operation, the ions are initially extracted downward with an energy of -675 eV, drift in a IO-cm field-free region, and then enter a reflecting field. This reflector provides a potential barrier that decelerates the ions, brings them to a complete stop, and then accelerates them upward. The ions pass back through the lO-cm field-free region, through the ionization region, and then through the 4O-cm drift region before finally striking the ion detector. It is necessary to play three tricks to ensure that the ions make it to the detector. The frrst is deflecting the ions to compensate for the molecular beam velocity as described above and is accomplished with a pair of plates in the first drift region. This deflecting field is pulsed so that it is on when the ions pass downward but off when they come back up. This deflection is critical to the overall ion transmission because the greatly increased flight times in the reflectron demand complete compensation for any off-axis velocity components. The second trick is to drop the voltage on the repeller plate in the ionization region (second from top in Fig. 2) low enough that the ions pass through on their return trip. (If this voltage remained at its initial value the 6 |