Laser spectroscopy of jet-cooled chlorinated aromatic hydrocarbons

The ultrasensitive and isomerically selective detection of chlorinated aromatic hydrocarbons is currently a problem of particular concern due to the toxic and/or carcinogenic nature of these species and their widespread presence in the environment. In this work laser-induced fluorescence and resonantly enhanced multi photon ionization (REMPI) are applied to a series of mono- and dichloronaphthalenes that are rotationally cooled in a free jet expansion. Both techniques provide isomeric selectivity in the S1-S0 spectral region; however 1 + 2 REMPI is more sensitive. In the REMPI spectra of the dichloronapthalenes (DCNs) the S1-S0 origins of different positional isomers are separated by as much as 424 cm-1. Low resolution time-of-flight (TOF) mass spectra of the DCNs show the REMPI ion fragmentation pattern to be isomer dependent. The additional selectivity that isomer-dependent frag mentation provides is demonstrated by the different relative intensities observed in the parent and fragment ion REMPI spectra of a three-component DCN mixture. Possible extensions of the REMPI technique, including two-color, 1 + 1 REMPI for enhanced sensitivity and high-resolution TOF mass spectrometry for enhanced isomeric selectivy, are discussed.