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Show III. Results and Discussion In this section we discuss our jet-cooled REMPI experiments on 1,2 and 1,4 dichlorobenzene using the apparatus shown in Fig. 2. For purposes of illustration we shall usc 1,4 dichlorobenzene (DCB) as our primary example although very similar results have been obtained for the 1.2 isomer. We shall also summarize the results of our earlier jet-cooled LIF and REMPI studies on the chloronaphthalenes (CN's) and dichloronaphthalenes (DCN's). A. Mass Spectra Fig. 3 shows representative mass spectra of jet-cooled 1,4 DCB taken with the TOF MS operated in the normal (top panel) and reflectron (lower panel) modes. Ionization in this case is accomplished with 1+1 REMPI (or R2PI) through the strong SI-S0 vibronic band at 274.10 nm; the complete REMPI spectrum will be discussed below. The mass spectrum is dominated by the cluster of parent ions corresponding to the naturally occurring isotopes of dichlorobenzene. These are 12C61Lt3SCl2 at 146 arnu. 12CS13CH43SCl2 at 147 amu, 12C6H43SCl37CI at 148 arnu. 12CS13CH43SCl37CI at 149 amu, 12C6H437Cl2 at ISO arnu, and 12C513C1Lt37Cl2 at 151 arnu in the natural abundance ratios of 140: 9.4 : 91 : 6.1 : 15 : 1. The observed parent ion intensities deviate from the abundance ratios because the 274.10 nm vibronic band involves stretching of the C-Cl bond and therefore exhibits chlorine isotope shifts. The isotope shifts mean that each isotope absorbs at a slightly different wavelength and thus the exact isotopic intensities in the mass spectrum depend critically on the laser wavelength. We find that significant ion fragmentation begins at laser pulse energies of -1.0 mJ (with the 20-cm cylindrical lens) and the spectra in Fig. 3 are taken at -3.0 mJ/pulse, a sufficiently high intensity to cause substantial fragmentation. The fragmentation pattern will be discussed in detail below. The greatly improved mass resolution provide by the reflectron can be clearly seen in Fig. 3, especially in the inserts that show the parent ion region. The mass resolution (M/6M) at 146 amu of the normal TOF MS is -200, while for the reflectron it is -600. As the inserts in Fig. 3 show this allows clear resolution of all the parent isotopes that are only partially resolved in the normal TOF mass spectrum. This improvement in resolution comes with almost no increase in size of the instrument; to obtain a similar increase in resolution simply by increasing the drift region of the normal TOF MS would necessitate an unreasonably large apparatus. The reflectron improves the resolution by compensating for the initial spread in ion energies. As an example consider a case in which the ions are created with two energies, high and low. In a normal TOF the two sets of ions diverge in time as they travel along the flight path and the high energy ions reach the detector before the low energy ions. Generalizing this to the case of a range of initial ion energies we can say that the ion packet spreads in time and ultimately limits the resolution. In the reflectron TOF high energy ions penetrate the reflecting field more deeply than do low energy ions and thus spend more time there. By carefully adjusting the reflecting field strength one can create a situation where the high energy ions just catch the low energy ions at the detector, hence 8 |