The efficiencies of resonant and nonresonant multiphoton ionization in the femtosecond region.

Ionization efficiency in mass spectrometry was examined for three types of molecules under different ionization schemes, i.e., resonant/nonresonant and two-photon/three-photon ionization, using an ultraviolet (UV) femtosecond laser at different wavelengths, pulse energies, and pulse widths. The efficiency of nonresonant ionization could be improved substantially by decreasing the laser pulse width. The effect of resonance was minimal when an ultrashort optical pulse of less than 100 fs was employed for ionization in the UV region. Three-photon ionization was less efficient than two-photon ionization, but the difference was not significant at shorter pulse widths in the femtosecond region. Although the excess energy can be decreased by optimizing the laser wavelength in the case of nonresonant ionization, fragmentation was not suppressed in the cases studied here. However, fragmentation was drastically suppressed when the laser pulse width was decreased. Thus, this approach of using an ultrashort optical pulse would provide a new tool for soft ionization and then for more reliable identification of an analyte.