Design of a high-resolution mass spectrometer for studying the photodissociation of organic ions in the gas phase

Abstract
The design and operating conditions of an apparatus to study photodissociation of ions is described. Ions in a large, double-focusing, mass spectrometer are irradiated with photons from an argon-ion laser. A path length of 706 mm for interaction between the ion and laser beams is obtained by passing the radiation through the ion source and then along the same path as that followed by the ions in the first field-free region of the mass spectrometer. By scanning the voltage of the electric sector, different fragment ions resulting from photodissociation are transmitted in turn to an electron multiplier. They are distinguished from ions resulting from unimolecular dissociation by mechanically chopping the laser beam and using the technique of phase-sensitive detection. The photodissociations of the molecular, positive ions of the three isomers of nitrotoluene have been investigated. It is found that, after excitation of the ions with 2.541 eV photons, the resulting fragmentation pathways are different from those for the corresponding unimolecular dissociations.