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Show as igncd as follows: CCl+ (47 and 49 amu), C2HC1+ (60 and 62 amu), C2H2CI+ (61 and 6J amll). probably C3H2CI+ (73 and 75 amu) although these are overlapped by hydrocarbon fragments, C4H2CI+ ( S and 87 amu), CSH2CI+ (97 and 99 amu), and as mentioned above C6H4Cl+ (111 and 113 amu). No atomic chlorine ions are observed. The observation of chlorinated hydrocarbon fragments indicates that the multiphoton ionization/fragmentation mechanism includes both loss of the chlorine atoms from the ring followed by ring destruction and the direct breakup of the ring into chlorinated fragments. Just as in conventional electron impact ionization the REMPI ion "cracking pattern" can in principle be used as an additional aid in determining the species present in an unknown sample. This approach has not been widely used with REMPI mass spectra largely because the wide range of possible ionizing laser conditions (pulse energy, pulse duration, fluence, intensity, wavelength, etc.) and the fact that most REMPI mass spectra have been taken at relatively low resolution where it is not possible to observe all hydrocarbon fragments. The flexibility in the REMPI technique makes it possible to generate a very large number of ion fragmentation patterns. The task of inventorying the REMPI cracking patterns for a wide variety of species is formidable but not impossible. Such information could be cataloged readily for a specific laser{fOF MS system such as the one described here and subsequently applied as an aid to the identification of species in an unknown sample run on the same instrument. B. REMPI and LIF (optical) Spectra In Fig. 5 we exhibit three examples of 1+1 REMPI (or R2P1) spectra taken for 1,4 DCB. Each of these spectra was taken with the TOF MS operated in the normal (low resolution) mode, and the ion channel that is monitored is predominantly the parent ion 12C6H435C12+ (although some of the 13C parent ion channel is also detected). The tunable UV laser energy in each of the spectra is -1 mJ/pulse, just below the threshold for ion fragmentation. The top and central panels compare one-color (A 1 + AI) REMPI spectra taken at room temperature (298 K) and under jet-cooled, molecular beam conditions. The room temperature spectrum is taken by allowing 1,4 DCB vapor to leak directly into the mass spectrometer chamber to give a pressure of -4xI0-6 torr. The room temperature spectrum displays the broadened vibronic features discussed in the Sec. I. As discussed there each vibronic line is broadened due to molecular rotation and many vibronic lines are present in the spectrum due to sequence congestion. The sequence congestion is most notable to the red of the strong 274.10 nm band where almost all of the observed intensity is due to "hot bands", i.e. transitions that arise from vibronic levels other than the vibration less level of the ground state (v"=O). As can be seen in the central panel the effect of jet-cooling on the REMPI spectrum is dramatic. The sequence congestion due to hot bands is virtually eliminated and each vibronic band sharpens due to the extremely cold rotational temperature in the beam. Under jet-cooled conditions even complicated regions of the spectrum (i.e. 271-272 nm) containing many bands can be resolved and assigned. We know from previous room temperature absorption spectra of 1,4 DCB that there are strong S I-SO absorption bands to 10 |
