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Show There is one more very important point that must be mentioned before moving on to the descriptions of our apparatus and results. As shown schematically by the levels in Fig. 1 each electronic state has associated with it a large number of vibronic levels corresponding to the many vibrational modes present in a large molecule. The molecular and isomeric selectivity of either LIF or REMPI depends on the fact that at a given photon energy only one isomeric variant of a particular molecule fulfills the resonant condition in which the spacing between vibronic levels in So and S 1 exactly matches the photon energy. By using tunable lasers the entire LIP or REMPI spectrum of a sample is mapped out and it is possible to differentiate between species based on the spectral location of their vibronic bands. Large chlorinated aromatics have vibrational modes, such as C-CI stretches, bends, and ring bending modes, that are very low in energy. At room temperature a large fraction of So molecules reside in these low-lying vibronic levels and therefore there are many possibilities for resonant matches to vibronic levels in S 1. This results in a spectroscopic condition known as sequence congestion in which the LIF or REMPI spectra exhibit very broad band structure that limits selectivity because it reduces the ability to distinguish between two molecules that have similar, closely spaced spectral features. The situation is further complicated by the fact that each vibronic band in the spectrum is broadened as a result of molecular rotation in the ground and excited states (i.e. each vibrational state has associated with it many rotational levels that are thermally populated). Thermal spectral broadening grows much worse with increasing temperature so that at high temperature (typical of incinerator exhaust streams) the spectrum is hopelessly washed out On the other hand when the molecule is cold the spectrum becomes dramatically simpler and the bands much narrower due to the lack of sequence congestion and rotational broadening. There are several techniques available for cooling molecules including cryrogenic matrix isolation and free-jet expansion cooling. We have used the latter approach in which the sample molecule is freely co-expanded with an inert, high pressure carrier gas (helium) into a vacuum. This expansion is isentropic and, in effec~ converts the random thennal motion of the carrier gas into directed translational motion along the jet axis. As a result the transverse translational temperature becomes very low; helium temperatures of less than 1 K have been obtained using free-jet expansions.7 The expansion becomes supersonic because the speed of sound is so low at these temperatures that the ratio of flow velocity to sound speed (Mach number) becomes very high. Because collisions occur between the carrier gas and the sample molecules in the high-density region of the jet, the molecules are also cooled by this process although the molecular degrees of freedom do not all cool to an equal extent. Rotational temperatures typically track the translational temperature and can routinely be made less than 10 K so that rotational broadening is negligible. Vibrational temperatures are somewhat higher, typically -100 K; this is normally more than cold enough to relieve sequence congestion. The remainder of the paper is organized as follows. In the next section we describe the coupling of free-jet expansion cooling to LIF and REMPI spectroscopy and describe a compact molecular beamrrOF MS apparatus that has improved mass resolution over conventional TOF MS instruments of similar size. 4 |