Selective Laser-Induced Resonant Two-Photon Ionization and Fragmentation of Substituted Nitrobenzenes at Atmospheric Pressure.

摘要

Resonant two-photon ionization (R2PI) at 1 atm in helium is studied as a selective and specific method for softly ionizing or fragmenting substituted nitrobenzenes. R2PI 266 nm in 1 atm He is shown to produce relatively soft ionization where either the molecular ion, M+, is observed or a specific fragment ion such as (M-NO)+ or (M-OH)+. As the laser power is increased, additional fragments may be produced, however, ions due to fragmentation of the aromatic ring are rarely observed even at the highest laser powers used (>10 to the ninth power W/cm2). The ions produced by R2PI are shown to depend on the laser photon energy, and it is demonstrated that by selecting specific frequencies of light, specific ions can be enhanced in the mass spectrum of a given molecule. It is also shown that under the same conditions distinctly unique ions are produced for the para, meta, and ortho isomers of p-nitroaniline, thus allowing easy discrimination among the isomers. In addition, the effect of the atmospheric pressure buffer gas in producing relatively soft ionization is demonstrated by performing a comparative study under vacuum conditions where extensive fragmentation is generally obtained. Also the ion products obtained under atmospheric pressure conditions are shown to be uniquely different than those obtained by conventional methods such as electron impact or chemical ionization under vacuum conditions.