The interest in the study of the photoionization and photoabsorption process of polyhalomethane molecules of atmospheric relevance has increased considerably in the last years.  Halogenated molecules such as, CH₂Cl₂,CH₂BrCl, CH₂ICl, and many others, have been observed in the troposphere, and have consequently been considered important sources of reactive halogens in the atmosphere. Several experimental techniques have been employed to the investigation of these important compounds.
 
The threshold photoelectron photoion coincidence (TPEPICO) experiment involves photoionization of a room temperature gas sample in the source of a time of flight (TOF) mass spectrometer. Threshold electrons with energies less than about 5 meV are extracted by a low electric field and detected, providing the start signal for the ion TOF determination.  The ions are energy selected according to the conservation equation, h v = E_ion + KE_el, where hv is the photon energy and KE_el is the electron kinetic energy.  Ion TOF distributions are collected at a number of photon energies from which we obtain the fractional abundance of parent and fragment ions. Accurate ionization energies and dissociation onsets are obtained with this technique. The experimental results are analyzed by using ab initio calculations and statistical RRKM theory in order to extract accurate thermochemical information.
 
Additional electron-ion coincidence techniques (PEPICO, PEPIPICO, PE3PICO) and ion yield spectroscopy, combined with synchrotron radiation have been employed in the elucidation of the ionic fragmentation pathways following valence and core excitation of several polyatomic molecules. Photoionization branching ratios and kinetic energy release for the ionic fragments have also been determined.