1 Molecular Movies from Molecular Frame Photoelectron Angular
13
whereas the latter implies that in experiments requiring high time-resolution additional measurements (e.g. electro-optical measurements [39] or cross-correlation
schemes based on transient X-ray induced reflectivity modification [40]) are needed
to measure the jitter between the FEL and a 2nd laser on a shot-by-shot basis. Moreover, the development of new protocols is required that allow to find the temporal
and spatial overlap of the FEL laser beam and the other laser beam(s) that are used
in the experiment. The availability of a velocity map imaging spectrometer provides
very useful opportunities for doing this, given that the detector can be used both in
a spatial and a velocity map imaging mode, while at the same time providing high
quality time-of-flight information [41].
The alignment-pump-probe approach with velocity map imaging detection of
high energy photoelectrons described above is in principle very suitable for application at FELs. In contrast with the use of a reaction microscope, the velocity map
imaging technique allows the recording of rather large signals before space-charge
distortions of the measured angular and velocity distribution set in. For example,
when an FEL is focused to a spot diameter of about 100 µm and intersects the
molecular beam containing the target molecules over a length of about 1 mm, then
as many as 10 3 photoelectrons can be generated and measured per laser shot, before one exceeds the empirical threshold of ca. 10 8 photoelectrons/cm 3 where space
charge effects start to cause serious problems.
Our first activity at the FLASH FEL was to introduce the use of velocity map
imaging (VMI) [42, 43]. As far as the use of XUV/X-ray photoionization for the
time-resolved observation of molecular dynamics is concerned, we have so far developed an alignment-pump-probe experiment where small molecules like Br 2 are
dynamically aligned using the fundamental 800 nm wavelength of a Ti:Sa laser,
photo-dissociated using the 2nd harmonic of this laser [44] and then ionized by the
FEL. Figure 1.5(a)–(c) shows 2D momentum maps of Br 2+ fragments in the presence of only the FEL (a), with both the 400 nm and the FEL beam present (b), and
when all three pulses are present (c) [45, 46]. In the presence of the FEL pulse,
the 2D velocity distribution is composed of concentric rings originating from dissociative ionization and Coulomb explosion of the molecule. The prominent new
contribution observed in Fig. 1.5(b) results from the ionization by the FEL pulse
of fragments of the dissociation initiated by the 400 nm pulse. When adding the
800 nm pulse, the angular distribution peaks along the laser polarization axis (see
Fig. 1.5(c)), which indicates that the molecules are aligned prior to dissociation
and ionization. First attempts have been made to record photoelectron angular distributions under these conditions. Figure 1.5(d) shows a differential photoelectron
momentum map similar to the data shown in Fig. 1.4, where in the present case the
difference is shown between a photoelectron momentum map recorded before and
after dissociation by the 400 nm photo-excitation laser pulse. From the difference
image a clear signature of the result of the dissociation process can be identified
through the shift of the resulting photoelectron energies and changes in the angular distribution of photoelectrons from the 3d shell. Recording of photoelectron data
with the time resolution required for a complete investigation of the Br 2 dissociation
dynamics has not been completed yet.
13
whereas the latter implies that in experiments requiring high time-resolution additional measurements (e.g. electro-optical measurements [39] or cross-correlation
schemes based on transient X-ray induced reflectivity modification [40]) are needed
to measure the jitter between the FEL and a 2nd laser on a shot-by-shot basis. Moreover, the development of new protocols is required that allow to find the temporal
and spatial overlap of the FEL laser beam and the other laser beam(s) that are used
in the experiment. The availability of a velocity map imaging spectrometer provides
very useful opportunities for doing this, given that the detector can be used both in
a spatial and a velocity map imaging mode, while at the same time providing high
quality time-of-flight information [41].
The alignment-pump-probe approach with velocity map imaging detection of
high energy photoelectrons described above is in principle very suitable for application at FELs. In contrast with the use of a reaction microscope, the velocity map
imaging technique allows the recording of rather large signals before space-charge
distortions of the measured angular and velocity distribution set in. For example,
when an FEL is focused to a spot diameter of about 100 µm and intersects the
molecular beam containing the target molecules over a length of about 1 mm, then
as many as 10 3 photoelectrons can be generated and measured per laser shot, before one exceeds the empirical threshold of ca. 10 8 photoelectrons/cm 3 where space
charge effects start to cause serious problems.
Our first activity at the FLASH FEL was to introduce the use of velocity map
imaging (VMI) [42, 43]. As far as the use of XUV/X-ray photoionization for the
time-resolved observation of molecular dynamics is concerned, we have so far developed an alignment-pump-probe experiment where small molecules like Br 2 are
dynamically aligned using the fundamental 800 nm wavelength of a Ti:Sa laser,
photo-dissociated using the 2nd harmonic of this laser [44] and then ionized by the
FEL. Figure 1.5(a)–(c) shows 2D momentum maps of Br 2+ fragments in the presence of only the FEL (a), with both the 400 nm and the FEL beam present (b), and
when all three pulses are present (c) [45, 46]. In the presence of the FEL pulse,
the 2D velocity distribution is composed of concentric rings originating from dissociative ionization and Coulomb explosion of the molecule. The prominent new
contribution observed in Fig. 1.5(b) results from the ionization by the FEL pulse
of fragments of the dissociation initiated by the 400 nm pulse. When adding the
800 nm pulse, the angular distribution peaks along the laser polarization axis (see
Fig. 1.5(c)), which indicates that the molecules are aligned prior to dissociation
and ionization. First attempts have been made to record photoelectron angular distributions under these conditions. Figure 1.5(d) shows a differential photoelectron
momentum map similar to the data shown in Fig. 1.4, where in the present case the
difference is shown between a photoelectron momentum map recorded before and
after dissociation by the 400 nm photo-excitation laser pulse. From the difference
image a clear signature of the result of the dissociation process can be identified
through the shift of the resulting photoelectron energies and changes in the angular distribution of photoelectrons from the 3d shell. Recording of photoelectron data
with the time resolution required for a complete investigation of the Br 2 dissociation
dynamics has not been completed yet.