RESONANCE-ENHANCED MULTI-PHOTON IONIZATION AND FRAGMENTATION OF MOLECULAR-BEAMS - NO, I2, BENZENE, AND BUTADIENE

Using the technique of laser ionization mass spectrometry, studies have been carried out of multiphoton ionization (MPI) and fragmentation of molecules under collision-free conditions. MPI spectra and mass fragmentation patterns are obtained via irradiation of isolated molecules by a pulsed, tunable dye laser focused down on a molecular beam traversing the ion-source region of a quadrupole mass filter. At each resonance, corresponding to m-photon ionization of an n-photon-excited intermediate state, the fragmentation pattern of the ions is measured. Resonance-enhanced MPI/mass spectra provide detailed information on the identity and relative abundances of the ionic fragments formed in the overall n+m photoionization process. From a knowledge of the appearance potentials of the various fragment ions, the minimum number of photons absorbed per molecule can be deduced. Increasing the laser pulse peak energy (or fluence) increases the average number of photons absorbed and thus the extent of fragmentation. However, even at the highest laser peak power densities used, the ionization (and fragmentation) remains extremely wavelength selective; the nonresonant contribution to the ionization is only a small fraction of the resonance-enhanced ion yields. Detailed experimental results on the 2+m photon ionization of benzene (the Johnson band, near 391 nm) are reported. Under strong focusing conditions, the most abundant ion is found to be C+, which implies the absorption of as many as 9 photons per molecule during the 6 ns pulse length of the laser. For I2, two different (n=2) bands were studied: for the Dalby system (near 372 nm), m=1 and 2, while for the Goodman band (near 585 nm), m=4. For trans-1,3-butadiene (near 387 nm), products requiring m=4 are observed. It is estimated that under the conditions of the present experiments ≃10-4 of the beam molecules within the focal region are ionized by the laser pulse. © 1979 American Institute of Physics.