A comparison of femtosecond and nanosecond multiphoton ionization and dissociation for some nitro‐molecules

Abstract

Resonance-enhanced multiphoton ionization spectrometry (REMPI), using time-of-flight mass spectrometers and tuned lasers, has proved an important ultra-sensitive analytical technique. Nevertheless, conventional nanosecond REMPI suffers from a number of shortcomings: most importantly, REMPI often fails through rapidly (pre)dissociating states. In the case of thermally labile molecules, which include the nitro-molecules, either no or very small parent or high/mass fragment ion peaks exist, making the interpretation of the mass spectra ambiguous at best and often impossible. Femtosecond laser mass spectrometry (FLMS) can often 'defeat' these dissociative states, resulting in large parent or high-mass fragment ion peaks which make the interpretation less ambiguous. In the present paper, nanosecond and femtosecond multiphoton ionization and fragmentation are compared using time-of-flight mass spectrometry for NOz gas and a number of different nitro-molecules: nitromethane, nitrobenzene, rn-nitrotoluene, dinitrotoluene and trinitrotoluene.

Femtosecond laser photoionization time-of-flight mass spectrometry of nitro-aromatic explosives and explosives related compounds

“…These results agree well with femtosecond measurements of Kosmidis et al [25] at 375 and 750 nm, and those of Ledingham et al [24] at 375 nm. The results of this study, and that of Tasker et al [28], differ considerably in the relative amounts of…”

“…Differences in the mass spectra again occur at 325 nm excitation, as this set of experimental conditions produces a small amount of the molecular ion, in contrast to the 206 nm experiments of Weickhardt and Tonnies [29] and Tonnies et al [43] The same holds true in comparison to the 375 nm, 50 fs duration work of Ledingham et al [24] where the M ϩ ion of TNT is absent. Examination of the TNT mass spectra in Figure 2 again reveals that the UV and visible wavelengths are better at molecular ion formation than is 795 nm irradiation, even though the degree of fragmentation and the fragment ion species are similar.…”

“…Ledingham et al [24] have also studied the fs ionization/dissociation of NB at 375 nm using 50 fs pulses and they indicate that the ion peaks observed in the mass spectrum of NB are evidence of a ladder switching mechanism where the dissociative channels below the ionization energy are somewhat "defeated" by the rapid up-pumping provided by the ultra-fast laser pulse. Again, the mass spectrum obtained at 325 nm in the present experiments agrees qualitatively with their results.…”

“…Two plausible explanations for the appearance of the M ϩ ion at 332 nm are the better signal to noise ratio of the 332 nm experiments, or alternatively, a wavelength-dependent ionization process. Ledingham et al [24] used 50 fs irradiation at 375 nm to study DNT and obtained mass spectra, and found that the highest molecular weight species in the mass spectrum was (M Ϫ OH) ϩ [165 m/z], with high yields of H ϩ , NO ϩ , C 5 H n ϩ , C 6 H n ϩ , and C 7 H n ϩ present. Since the two experiments are conducted at unique wavelength and power densities, it is rather difficult to draw conclusions regarding the presence of the parent molecular ion, although it is quite likely that the outcome of the ionization and dissociation of these photo-labile molecules is sensitive to the product of the wavelength and power density used.…”

“…It was therefore desirable to devise a strategy for the ionization of these species that resulted in molecular ion formation, and/or molecular ion fragments that are strongly correlated to the precursor species. Two laserbased ionization techniques have shown promise in this regard; ultrafast laser ionization in conjunction with time-of-flight mass spectrometry as pioneered by Ledingham et al [24] and more recently, laser singlephoton ionization time-of-flight mass spectrometry [38].…”