ABSTRACT
The dissociative ionization of ethane in intense femtosecond laser fields has been investigated as a function of the laser pulse shape by systematically varying the quadratic spectral phase, i.e. the linear chirp. A very pronounced effect of the sign of the chirp is observed for the parent ion and all fragment ion yields, all ion yields being strongly favored by negative chirp of the laser field. The ratio of the H3+ ion yield to H+ ion yield can also be manipulated by changing the linear chirp, the maximum being observed for a significantly smaller chirp value than that for the individual ion yields. Since the H+ ions and the H3+ ions predominantly originate from the dication of ethane, this indicates control of fragmentation within one charge state of the ethane. Additional experiments performed with d3-ethane demonstrate that the control is operative prior to the statistical scrambling of hydrogen atoms, further supporting the concept of intra-charge-state control. In the case of formation of CH3+ ions two different ensembles occur, one from the monocation, another from the dication. The ratio of these ensembles can again be controlled by means of the linear chirp parameter implying control between the two different charge states (inter-charge-state control).
ABSTRACT
Recent progress in the field of chirality analysis employing laser ionization mass spectrometry is reviewed. Emphasis is given to femtosecond (fs) laser ionization work from the author's group. We begin by reviewing fundamental aspects of determining circular dichroism (CD) in fs-laser ionization mass spectrometry (fs-LIMS) discussing an example from the literature (resonant fs-LIMS of 3-methylcyclopentanone). Second, we present new data indicating CD in non-resonant fs-LIMS of propylene oxide.
ABSTRACT
The circular dichroism (CD) induced by femtosecond laser pulse excitation of 3-methyl-cyclopentanone has been investigated by means of experiment and theory as a function of the laser pulse duration. In the experiment the CD in ion yields is measured by femtosecond laser ionization via a one-photon resonant excited state. In the theoretical part the CD is calculated by solving laser driven quantum electron dynamics for the same resonant excitation based on ab initio electronic structure calculations employing a complete description of the electric field-electric dipole and magnetic field-magnetic dipole interactions. Both the experimentally measured CD in ion yields and the calculated CD in excited state populations exhibit a marked increase of the CD for pulse duration increasing from 50 fs to about 200 fs. Beyond 200 fs pulse duration the CD levels off. The combination of experimental and theoretical evidences indicates that the CD decreases with increasing laser intensity connected to the increased coupling between the excited states.
ABSTRACT
Telling the difference quickly: Femtosecond laser pulses are not only suitable to distinguish structural isomers. They also provide access to the distinction of enantiomers by combination of circular dichroism and mass spectrometry [picture: see text].
