Applicability of resonant two-photon ionization in supersonic beam mass spectrometry to halogenated aromatic hydrocarbons

In this work, problems are studied that may arise in the use of resonant two-photon ionization as an ionization source in supersonic beam mass spectrometry. The particular classes of molecules studied herein include mono- and disubstituted halogenated benzenes and halogenated toluene, pheno, and aniline derivatives. In a simple one-color R2PI scheme many of the ortho-substituted compounds are found not to ionize efficiently. This appears to be due to a combination of steric and coulombic interactions between adjacent substituent groups that may raise the ionization potential of the molecule relative to the unsubstituted case and also decrease the Franck-Condon transition probability for absorption thus making ionization difficult. These effects have been qualitatively related to the overall electron releasing and withdrawing properties of the substituent groups so that reasonable predictions can be made regarding which substituted benzenes will be amenable to this technique. In para-substituted compounds in which there is little interaction between substituent groups, ionization is generally found to occur efficiently. However, there are cases such as the iodo-substituted benzenes where ionization does not occur apparently due to ultrafast radiationless processes. 57 references, 5 figures, 5 tables.