Resonant Two-Photon Ionization RTPI Combined with Mass Spectrometry

4 Resonant Two-Photon Ionization (RTPI) Combined with Mass Spectrometry 

When used in combination with mass spectrometry, RTPI allows mass- and wavelength-selective spectroscopy, even if the spectral lines of the different species overlap. This is particularly important for molecular isotopes with dense spectra, which overlap for the different isotopes. This is illustrated by Fig. 1.40, where the differences in the line positions in the spectra of Li3 and Li3 are caused partly by the different masses but mainly by the different nuclear spins. Such isotope-selective spectra give detailed information on isotope shifts of vibrational and rotational levels and facilitate the correct assignment of the spectral lines considerably. Furthermore, they yield the relative isotopic abundances. 

Time-of-flight mass spectrometers with pulsed lasers are convenient since they allow the simultaneous but separate recording of the spectra of different isotopes 

1 Doppler-Limited Absorption and Ruorescence Spectroscopy with Lasers 

For measurements of cluster-size distributions in cold molecular beams (Sect. 4.3), or for monitoring the mass distribution of laser-desorbed molecules from surfaces, these combined techniques of laser ionization and mass spectrometry are very useful [105, 106]. For the detection of rare isotopes in the presence of other much more abundant isotopes, the double discrimination of isotope-selective excitation by the first laser LI and the subsequent mass separation by the mass spectrometer is essential to completely separate the isotopes, even if the far wings of their absorption lines overlap [107]. The combination of resonant multiphoton ionization (REMPI) with mass spectrometry for the investigation of molecular dynamics and fragmentation is discussed in Chap. 5. 

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