1 Molecular Movies from Molecular Frame Photoelectron Angular
5
tron beams that are used for the ultrafast electron diffraction experiments mentioned above [15]. The key difference here will be that the diffractive information is to be encoded in the ejection of secondary or tertiary electrons from the
molecule, rather than onto the diffraction of the incident high-energy electron
beam.
The organization of the present chapter is as follows. In Sect. 1.2 we present
our efforts on using XUV/X-ray single-photon ionization as a means to generate
fast photoelectrons that encode a (time-evolving) molecular structure. We present
the status of our work at FLASH and LCLS, where we have performed alignmentpump-probe experiments, where a first, alignment laser pulse dynamically aligns
the molecule under investigation, a pump laser pulse photo-excites the molecule
and the FEL pulse ionizes the molecule at a variable time delay, as well as recent
experiments where a high-harmonic generation (HHG) source was used to generate
a comb of XUV laser frequencies reaching up to 50 eV, and where photoionization of a series of small molecules provided insight into the contribution of different molecular orbitals and the onset of the emergence of structural information. In
Sect. 1.3 we present results from our experiments on (atomic) strong field ionization at mid-infrared wavelengths ranging from 4 to 40 µm, where holographic interferences in the measured photoelectron momentum distributions suggest a route
towards a novel technique for measuring (time-resolved) molecular, structural information.
1.2 Molecular Movies Using XUV/X-Ray Photoionization
In the last few years two novel XUV/X-ray short-pulse light sources have come to
the forefront that have significantly changed the opportunities that experimentalists
in atomic and molecular physics research can avail themselves of. On the one hand,
HHG has been developed into a technique that can be implemented in moderatescale laser laboratories on the basis of commercially available, mJoule-level, femtosecond lasers [16–18]. When the pulses from these lasers are focused onto a dense,
gas phase, atomic or molecular target, XUV/X-ray light pulses are formed by means
of an interaction that is commonly described in terms of a three-step mechanism,
where the laser first ionizes the atom/molecule under consideration, then accelerates the ionized electrons and finally drives the electron back towards the ion left
behind, where a recombination can occur that is accompanied by the emission of
XUV/X-ray light [19]. Since this process repeats for every half-cycle of the driving laser field, the output frequencies are restricted to odd harmonics of the driver
laser frequency, explaining the name of the technique. On the other hand, several
XUV/X-ray FEL user facilities have recently become available that provide femtosecond XUV/X-ray pulses with pulse energies that are well beyond the reach of
present-day HHG schemes. The first examples of such facilities have been the Tesla
Test Facility (TTF) and FLASH in Hamburg [20]. More recently, the LCLS at Stanford has come into operation as the world´s first hard X-ray FEL user facility [5].
5
tron beams that are used for the ultrafast electron diffraction experiments mentioned above [15]. The key difference here will be that the diffractive information is to be encoded in the ejection of secondary or tertiary electrons from the
molecule, rather than onto the diffraction of the incident high-energy electron
beam.
The organization of the present chapter is as follows. In Sect. 1.2 we present
our efforts on using XUV/X-ray single-photon ionization as a means to generate
fast photoelectrons that encode a (time-evolving) molecular structure. We present
the status of our work at FLASH and LCLS, where we have performed alignmentpump-probe experiments, where a first, alignment laser pulse dynamically aligns
the molecule under investigation, a pump laser pulse photo-excites the molecule
and the FEL pulse ionizes the molecule at a variable time delay, as well as recent
experiments where a high-harmonic generation (HHG) source was used to generate
a comb of XUV laser frequencies reaching up to 50 eV, and where photoionization of a series of small molecules provided insight into the contribution of different molecular orbitals and the onset of the emergence of structural information. In
Sect. 1.3 we present results from our experiments on (atomic) strong field ionization at mid-infrared wavelengths ranging from 4 to 40 µm, where holographic interferences in the measured photoelectron momentum distributions suggest a route
towards a novel technique for measuring (time-resolved) molecular, structural information.
1.2 Molecular Movies Using XUV/X-Ray Photoionization
In the last few years two novel XUV/X-ray short-pulse light sources have come to
the forefront that have significantly changed the opportunities that experimentalists
in atomic and molecular physics research can avail themselves of. On the one hand,
HHG has been developed into a technique that can be implemented in moderatescale laser laboratories on the basis of commercially available, mJoule-level, femtosecond lasers [16–18]. When the pulses from these lasers are focused onto a dense,
gas phase, atomic or molecular target, XUV/X-ray light pulses are formed by means
of an interaction that is commonly described in terms of a three-step mechanism,
where the laser first ionizes the atom/molecule under consideration, then accelerates the ionized electrons and finally drives the electron back towards the ion left
behind, where a recombination can occur that is accompanied by the emission of
XUV/X-ray light [19]. Since this process repeats for every half-cycle of the driving laser field, the output frequencies are restricted to odd harmonics of the driver
laser frequency, explaining the name of the technique. On the other hand, several
XUV/X-ray FEL user facilities have recently become available that provide femtosecond XUV/X-ray pulses with pulse energies that are well beyond the reach of
present-day HHG schemes. The first examples of such facilities have been the Tesla
Test Facility (TTF) and FLASH in Hamburg [20]. More recently, the LCLS at Stanford has come into operation as the world´s first hard X-ray FEL user facility [5].