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Show ) achieving a temporal focus of the ion packet at the detector and greatly enhancing the resolution over that of a normal TOF. The parent ions formed in the REMPI process are generated at different points in the extraction region and thus have a range of initial energies. They also have an initial kinetic energy component along the ion axis due to thermal motion. The latter contribution is large for room temperature samples but very small in the molecular beam where the transverse translational temperature is typically less than 10 K. Fragment ions have an additional contribution to their initial energy spread from the recoil velocity of the photofragmentation process. This is clearly evident in Fig. 3. Notice that while the parent ions are coarsely resolved in the normal TOF the fragment ions are completely unresolved (even though they are smaller) due to the kinetic energy release in the fragmentation. However the reflectron can compensate for this energy spread and thus the fragment ions in the lower panel of Fig. 3 are clearly resolved with greater than unit mass resolution for all fragments. Finally we note that with the linear reflectron there is a pronounced loss of ions during their flight due both to transverse ion drift and decreased transmission due to the large number of grids that the ions must pass through, each of which is only -92% transmissive. The former problem can be overcome by the use of more sophisticated ion optics and the latter difficulty can be eliminated by using a reflectron in an angular geometry. 1 0 In Fig. 4 we display a series of reflectron TOF mass spectra, again for 1,4 DCB using one-color REMPI through the 274.10 run band. These spectra illustrate the different fragmentation patterns that can be obtained by varying the ionizing laser conditions in the REMPI process. The top and central panels of Fig. 4 show the effect of increasing UV laser intensity on the mass spectrum. As mentioned above, at pulse energies less than -1 mJ almost no fragmentation of the parent ion is observed, however at -3 mJ/pulse the fragment ions are of comparable intensity to the parent ions. The bottom panel of Fig. 4 demonstrates what happens when the dye laser fundamental at 548.2 nm is also focused into the ionization region so that the intensities in the visible and UV are roughly comparable. The addition of the visible light causes greater fragmentation; the fragment peaks are larger and even atomic carbon ions are now observed. More importantly the addition of the dye fundamental changes the cracking pattern in a pronounced manner. With just the UV laser almost no loss of a single chlorine atom is observed, rather the highest observed fragments correspond to loss of both chlorine atoms from the benzene ring or fission into chlorinated and hydrocarbon fragments. The dye laser strongly promotes the loss of a single chlorine to give the set of fragments clustered around mass III (12C6H4 35CI). This process may be resonantly enhanced by an electronic transition in the 1,4 DCB cation that appears near 521 nm in an argon matrix. 1 1 The fragment ions in the lower two panels of Fig. 4 can be identified readily. The groupings of mass peaks about 12, 27, 37, 50 (except for the mass 47 peak), and 75 amu correspond to hydrocarbon ions of the form CnHm + where n=I-4, and 6 (but probably not 5) and m=I-4. Besides the hydrocarbon fragments we also observe chlorinated fragments that can be easily picked out by the 35CI : 37 CI isotope ratio (which for the particular laser wavelength in the bottom panel of Fig. 4 is 2.1 : 1.0). They are 9 |