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Show ) ions would be stopped in the ionization region .) Again, this is done in a pulsed fashion; the voltage is dropped after the initial extraction but before the return of the reflected ions. Finally the reflecting field itself consists of a series of 19 guard rings that are intended to minimize the effect of stray electric fields perpendicular to the ion flight axis. Even small stray fields can have a disastrous effect upon the ions in the reflecting field because of the large amount of time spent there. The primary motivation for the use of the reflectron is to provide greatly improved mass resolution over a normal TOF MS of similar size. The basis for this enhanced resolution are discussed in detail in the following section. Signals from the ion detector are amplified by a fast amplifier and then processed in two ways. For the spectral scans, the ion signals corresponding to two different masses (or two mass ranges) are averaged by two boxcar averagers (SRS model 250). Also the doubled-dye-Iaser energy is monitored via a pyroelectric power meter (Laser Precision Rj-7200) coupled to a third boxcar averager. The boxcar averagers are interfaced to a lab computer (IBM PC AT) that is used for data acquisition, processing, and graphics. For mass spectra, the signal from the ion detector is processed by a 125 MHz digital oscilloscope (LeCroy 9400). This device acts as a transient recorder and signal averager that can record the entire mass spectrum on a single laser shot, or, as was done more commonly, can average many laser shots to acquire a composite mass spectrum with higher signal-to-noise. The digital oscilloscope is also interfaced to the lab computer for data transfer and graphics. The jet-cooled LIF and REMPI experiments on the chloronaphthalenes were perfonned in another apparatus. In this apparatus the jet expansion is crossed approximately 50 nozzle diameters downstream at right angles by the UV laser beam. LIF is collected at right angles to both the laser beam and jet axis by a 15-cm focal length lens and imaged onto a photomultiplier tube. Filters are used to suppress scattered laser light while passing the red-shifted fluorescence. A very small TOF MS (-8-cm drift region) can also be positioned in the jet expansion. This device has a very wide open ionization region so as not to perturb the unskimmed free-jet expansion and allows for a crude mass analysis of the photoions produced in the REMPI process. Either the photomultiplier or the ion detector signal is amplified and processed by boxcar averagers and a laboratory computer. Finally, the samples used in these experiments were used as obtained from the suppliers without further purification. 1,2 dichlorobenzene (98.5%) and 1,4 dichlorobenzene (99+%) were obtained from Aldrich Chemical, and the mono- and dichloronaphthalenes were obtained from Ultra Scientific. For the dichlorobenzenes the vapor at room temperature (-1.5 torr) was mixed with helium in the sample container and then admitted to the pulsed valve. For the chloronaphthalenes some heating of the reservoj . and valve were necessary (-50 OC) to maintain adequate vapor pressure. We estimate that the typi~ ." partial pressure of chloronaphthalene in these cases was ~O . l torr. 7 |
