Enhancement of the Molecular Ion Peak from Halogenated Benzenes and Phenols Using Femtosecond Laser Pulses in Conjunction with Supersonic Beam/Multiphoton Ionization Mass Spectrometry

Halogenated benzenes and phenols are measured by multiphoton ionization mass spectrometry using femtosecond (150, 500 fs) and nanosecond (15 ns) laser pulses. The molecular ion is strongly enhanced for monohalobenzenes when the pulse width of the ionization laser is shorter than the lifetimes of their excited states. This is attributed to the reduction of intersystem crossing by a spin-orbit interaction, the so-called internal heavy-atom effect, and to rapid dissociation from the triplet state. A femtosecond laser pulse was deemed to be useful for the efficient ionization of dichlorobenzene and trichlorobenzene although their lifetimes are unknown, since polychlorinated benzenes are thought to have shorter lifetimes as the result of a stronger spin-orbit interaction. The ionization efficiencies of o-chlorophenol and p-chlorophenol are also obtained using femtosecond and nanosecond pulses. In the case of o-chlorophenol, intersystem crossing occurs more efficiently by stabilization of the triplet state by intramolecular hydrogen bonding, and as a result, the femtosecond pulse is more effective in ionizing o-chlorophenol, which has a shorter lifetime. These results indicate that an ultrashort laser pulse is very useful in improving the ionization efficiency for a molecule with a short lifetime, such as polychlorinated dioxins and their precursors.