Fundamentals and Features of Analytical Laser Mass Spectrometry with Ultrashort Laser Pulses

Among the various ionization techniques used in mass spectrometry, resonant multiphoton ionization (REMPI) is the method of choice if the analysis demands high temporal resolution together with the ability to detect several preselected compounds simultaneously in a complex mixture or matrix. However, REMPI can only fulfill these demands if the excited molecular state(s) involved in the multiphoton ionization process are not significantly depleted by fast energy relaxation or by fragmentation processes during the duration of the ionizing laser pulse. Up to now, only lasers with pulse lengths in the nanosecond range were available for analytical applications of laser mass spectrometry. It turns out that, under these conditions, no useful mass spectra can be obtained for a broad variety of compounds such as organometallic compounds, biomolecules, chemical warfare agents and explosives. A possibility of extending the advantageous features of REMPI to these important groups of substances in particular, and to quickly relaxing molecules in general, is the application of ultrashort but intense laser pulses, which complete the ionization process so quickly that relaxation processes cannot become important. The mass spectra obtained in this way are characterized by an intense molecular ion signal accompanied by structure-specific fragments. Using selected examples of environmentally relevant substances, the influence of laser pulse duration on the features of the laser mass spectra is discussed. Furthermore, it is demonstrated that, even under the high laser intensities used in this experiment, non-resonant processes do not play a significant role and selective ionization is still possible.