Table of Contents:
Subject of this work are new findings for femtosecond laser ionization processes that can be assigned to three different but nevertheless connected topics: influence of the spectral phase on the ionization and dissociation behavior, distinguishing structural isomers by means of variation of the spectral phase, and distinguishing enantiomers due to their circular dichroism in ion yields. To complement earlier work on the influence of the spectral phase on the ionization and dissociation of ethane, the time-of-flight signal form of H3+-fragment ions have been discussed. For this fragment ion, a monoenergetic kinetic energy distribution at about 3 eV resulting from the coulomb explosion of the doubly charged parent ion is known in the literature. By varying the electric field strength at the ionization point of the mass spectrometer this work shows an additional ionic species above 300 V/cm with same mean time of flight but lower kinetic energy. Furthermore, at these electric field strengths the signal at m/z = 3 shows a dependence on the angle between laser polarization and the axis of the mass that has not been observed in earlier work. This angular dependence cannot be detected at lower field strengths.
A comparison of the ionization behavior of methane, propane, methanol, d4-methanol, phenylpropylenoxide, difluorobenzene, and limonene as molecular species and argon, krypton and xenon as atomic species on the chirp parameter – in particular the linear chirp parameter –reveals that they possess different chirp effects. Some of the molecular ions show a sign-dependent chirp effect which is similar to the effect that has been observed for ethane: The maximum ion yield occurs in for negatively linear chirped laser pulses with values up to −875 fs2 (propane at 30 µJ). This sign-dependent effect decreases for increasing laser energy and becomes a mostly intensity-driven effect in most cases. However, for methane this intensity-dependent behavior was much less pronounced and only lead to a small displacement of the dependence on the linear chirp parameter to lower values and different inclinations on both sides of the maximum ion yields. The sign-dependent chirp effect is observable in resonant excitation (phenylpropylenoxide – this classification is not unambiguous – and difluorobenzene) as well as in non-resonant ionization. The participation of a HOMO leads to a mostly intensity-driven chirp dependence as the relevant orbital only has negligible molecular characteristics. Therefore, the chirp dependence of methanol resembles the ionization behavior of the rare gas atoms. All chirp dependences can be described with a fit model. This model consists of a sigmoidal term describing the sign dependence and an intensity-driven term. It seems reasonable that Raman processes underlie the observed ionization behavior in a ladder climbing ionization scheme. The comparison of different fragment ions reveal similarities in their chirp dependencies allowing to identify common precursor ions or fragmentation channels. This has been shown for fragment ions of methane and benzenediamine.
The variation of the spectral phase is also suitable to distinguish between o-, m- and p-benzenediamine and difluorobenzene. The distinction of all three isomers was successful in both cases. However, different fragment ion ratios have to be taken into account. The benzenediamine isomers can be distinguished by the doubly-charged parent ion in comparison to the singly-charged parent ion. This can be attributed to different ionization energies. The absolute ion ratios differ, whereas the trends of the ratios as function of the linear chirp parameter are similar referring to similarities in the ionization dynamics. A sequential double ionization leading to the observed sign-dependent chirp effect seems reasonable. The differences in the ionization energies of the difluorobenzene isomers are smaller, therefore, other fragment ion ratios have to be taken into account: The elimination of acetylene and hydrogen fluoride are suitable fragmentation channels to distinguish between all three isomers. The HF-elimination can be rationalized by means of statistical dissociation. However, the elimination of C2H2 is the result of a coherent control scheme.
The enantiomers of 2-butanol, glycidol, carvone, fenchone and limonene have successfully been distinguished by means of the circular dichroism in ion yields. The non-resonant excitation is challenging as several vibronic and electronic states with different values and signs can contribute to the overall circular dichroism. Therefore, the enantiomers of glycidol cannot be distinguished at 807 nm, especially without a suitable achiral reference molecule. Such a reference allows to correct systematic errors as has been shown for different misalignments of the λ/4-plate in the case of fenchone. For carvone it has been demonstrated that the temperature dependence of the circular dichroism resulting from different populations of conformers with, e. g. different signs, can also be observed for the circular dichroism in ion yields. Furthermore, the distinction of enantiomers of limonene has revealed an enlargement of the circular dichroism in ion yields for the fragmentation of limonene leading to two isoprene species in a photo retro diels alder reaction.