Multiphoton ionization of molecules

Abstract The multiphoton ionization techniques for molecules were developed in parallel with the studies into the resonance multistep ionization of atoms, but their aim was to achieve photo selective laser detection of molecular traces in combination with mass spectrometry of the photoions produced, that is, to create what is called a “laser two- dimensional optical mass spectrometer” (Letokhov 1976). However, the more complex structure of molecular spectra, the high photoionization potentials of molecules (8–12 eV), and the absence of tunable laser sources operable in the UV region of the spectrum made it necessary to use the available UV excimer lasers capable of photoionizing molecules by “brute force.” In that case, the low selectivity of the optical channel was compensated by the extra selectivity of the mass spectrometer. Figure 10.1 schematically illustrates the main methods of multiphoton ionization of molecules: (a) resonance stepwise ionization, (b) resonance-enhanced two-photon (or multiphoton) ionization (RETPI or REMPI), and (c) nonresonance multiphoton ionization. The stepwise photoionization technique provides good spectral selectivity; it requires a tunable laser radiation source of moderate power output (kW–MW/cm2). The REMPI technique is of limited selectivity and requires higher radiation powers (MW–GW/cm2); it is especially convenient where use is made of ultrashort laser pulses. And, finally, the nonresonance multiphoton ionization (MPI) technique features no spectral selectivity; it is a “brute-force” method, where the necessary chemical selectivity is attained solely in the mass-spectrometer channel. Therefore, the last two techniques are unsuitable for the detection of trace molecules in a mixture of organic molecules, because many of the molecules simultaneously suffer photoionization and photo fragmentation. However, when combined with the cooling of molecules in a pulsed supersonic jet, which prevents their distribution among many sublevels and thus makes their spectra sharper, the REMPI technique proves very effective (Reisler and Wittig 1985; Lubman 1990).