The ejection of triatomic molecular hydrogen ions H-3(+) produced by the interaction of benzene molecules with ultrafast laser pulses

The ejection process of triatomic molecular hydrogen ions produced by the interaction of benzene with ultrafast laser pulses of moderate strong intensity (similar to 10(14) W/cm(2)) is studied by means of TOF mass spectrometry. The H-3(+) formation can only take place through the rupture of two C-H bonds and the migration of hydrogen atoms within the molecular structure. The H-3(+) fragments are released with high kinetic energy (typically 2-8 eV) and at laser intensities >= 10(14) W/cm(2), well above that required for the double ionization of benzene, suggesting that its formation is taking place within multiply charged parent ions. The relative ejection efficiency of H-3(+) molecular hydrogen ions with respect to the atomic ones is found to be strongly decreasing as a function of the laser intensity and pulse duration (67-25 fs). It is concluded that the H-3(+) formation is only feasible within parent molecular precursors of relatively low charged states and before significant elongation of their structure takes place, while the higher multiply charged molecular ions preferentially dissociate into H+ ions. The ejection of H-2(+) ions is also discussed in comparison to the production of H-3(+) and H+ ions. Finally, by recording the mass spectra of two deuterium label isotopes of benzene (1,2-C6H4D2, 1,4-C6H4D2) it is verified that the ejection efficiency of some molecular fragments, such as D2H+, DH+, is dependent on the specific position of hydrogen atoms in the molecular skeleton prior dissociation. (C) 2009 American Institute of Physics. [doi:10.1063/1.3246832]