20
A. Rouzée et al.
that, under experimental conditions, inevitably takes place in a laser focus. These
arguments are corroborated by the calculations shown in Fig. 1.8(a), which show
the result of generalized SFA calculations for the ionization of metastable Xe atoms
by a 7 µm laser with an peak intensity ranging from 1.9 × 10 11 –7 × 10 11 W/cm
2
[62]. For a constant value of p z , the interference pattern hardly changes as a function of the intensity. This is very different when the wavelength of the ionizing laser
is changed. To illustrate this, Fig. 1.8(b) shows a series of calculations where the
holographic interference is calculated as a function of the laser wavelength under
conditions where the ponderomotive energy (and thus the value of p z where the
high-energy cut-off is observed in the photoelectron spectrum) stays constant. Figure 1.8(b) clearly shows that with increasing laser wavelength the spacing of the
holographic interference fringes narrows, due to the fact that the difference between
t C and t ref
0 scales linearly with laser wavelength, leading to a doubling of the phase
difference between the reference and signal electron wave at a fixed position in the
momentum map [62].
1.4 Outlook
It is in the nature of scientific development that advances are often stimulated
by the emergence of novel technological capabilities. In this respect, the molecular sciences are no exception. At present, the emergence of intense, short pulse
light sources outside the traditional near-infrared to near-UV wavelength range
promises the development of novel techniques that address time-dependent dynamics and that do not rely so much on molecular photo-absorption as on
diffraction of laser light or the photoelectrons that can be generated using these
sources.
On the one hand, at an increasing number of places around the world, XUV/Xray free electron laser sources are being constructed and coming available, that
deliver ultrashort XUV/X-ray laser pulses with unprecedented fluences and peak
intensities, that can be used to develop new ways to study time-resolved molecular dynamics based on use of the diffractive properties of energetic photoelectrons that are ejected from time-evolving molecules upon photo-ionization. In the
last few years pump-probe protocols have been developed that allow to first dynamically align a molecule of interest, thereby fixing its orientational degrees of
freedom in the laboratory frame, before addressing the molecule with a pump
pulse that initiates the photo-dynamics of interest and the XUV/X-ray laser pulse
that ionizes the molecule and/or fragments resulting from the photo-excitation. In
combination with sophisticated 2D or even 3D energy—and angle-resolved photoelectron and—ion detection strategies this promises to lead to the emergence
of a novel way of studying photo-chemical events that complements the present
absorption-based techniques. There are remaining problems that need to be solved,
such as the challenge of adequately synchronizing the FEL light with the output
A. Rouzée et al.
that, under experimental conditions, inevitably takes place in a laser focus. These
arguments are corroborated by the calculations shown in Fig. 1.8(a), which show
the result of generalized SFA calculations for the ionization of metastable Xe atoms
by a 7 µm laser with an peak intensity ranging from 1.9 × 10 11 –7 × 10 11 W/cm
2
[62]. For a constant value of p z , the interference pattern hardly changes as a function of the intensity. This is very different when the wavelength of the ionizing laser
is changed. To illustrate this, Fig. 1.8(b) shows a series of calculations where the
holographic interference is calculated as a function of the laser wavelength under
conditions where the ponderomotive energy (and thus the value of p z where the
high-energy cut-off is observed in the photoelectron spectrum) stays constant. Figure 1.8(b) clearly shows that with increasing laser wavelength the spacing of the
holographic interference fringes narrows, due to the fact that the difference between
t C and t ref
0 scales linearly with laser wavelength, leading to a doubling of the phase
difference between the reference and signal electron wave at a fixed position in the
momentum map [62].
1.4 Outlook
It is in the nature of scientific development that advances are often stimulated
by the emergence of novel technological capabilities. In this respect, the molecular sciences are no exception. At present, the emergence of intense, short pulse
light sources outside the traditional near-infrared to near-UV wavelength range
promises the development of novel techniques that address time-dependent dynamics and that do not rely so much on molecular photo-absorption as on
diffraction of laser light or the photoelectrons that can be generated using these
sources.
On the one hand, at an increasing number of places around the world, XUV/Xray free electron laser sources are being constructed and coming available, that
deliver ultrashort XUV/X-ray laser pulses with unprecedented fluences and peak
intensities, that can be used to develop new ways to study time-resolved molecular dynamics based on use of the diffractive properties of energetic photoelectrons that are ejected from time-evolving molecules upon photo-ionization. In the
last few years pump-probe protocols have been developed that allow to first dynamically align a molecule of interest, thereby fixing its orientational degrees of
freedom in the laboratory frame, before addressing the molecule with a pump
pulse that initiates the photo-dynamics of interest and the XUV/X-ray laser pulse
that ionizes the molecule and/or fragments resulting from the photo-excitation. In
combination with sophisticated 2D or even 3D energy—and angle-resolved photoelectron and—ion detection strategies this promises to lead to the emergence
of a novel way of studying photo-chemical events that complements the present
absorption-based techniques. There are remaining problems that need to be solved,
such as the challenge of adequately synchronizing the FEL light with the output