8
A. Rouzée et al.
distinguishable variants exist, namely adiabatic alignment, where the molecule is
exposed to a laser pulse that is significantly longer than the rotational period of the
molecule [27], and impulsive alignment, where the molecule is exposed to a laser
pulse that is significantly shorter than the rotational period [28]. The advantage of
the latter method is that it leads to the formation of aligned molecular samples under laser field-free conditions (i.e. after the alignment laser pulse is over), although
with a degree of alignment that is lower than in the adiabatic case. Hybrid schemes
combining adiabatic and impulsive alignment have also been proposed [29], and—
in combination with state-selection techniques—allow the preparation of molecular
samples with a very high-degree of alignment and orientation [30] that can be used
in experiments aimed at observing the emission of photoelectrons in the molecular
frame.
Recently the experimental setup shown in Fig. 1.2 has been used to perform such
an experiment [31]. A series of small molecules (CO 2 , N 2 , O 2 and CO) were exposed to the sequence of an IR laser pulse that dynamically aligned the molecules
and an XUV pulse generated by HHG that ionized the molecules at a variable time
delay. Photoelectrons and fragment ions resulting from the latter photoionization
process were recorded on a velocity map imaging detector, i.e. accelerated towards
a two-dimensional detector consisting of a set of micro-channel plates, a phosphor
screen and a CCD camera, thereby allowing the measurement of a 2D projection
of the 3D velocity distribution. The 3D velocity distribution was determined from
the 2D projection by means of an iterative Abel inversion routine [32]. An important feature of the experiment was the fact that a very high count rate could be
achieved (up to ca. 10 6 counts/second), due to the use of a very efficient gas injection system, which was integrated in the repeller electrode of the velocity map
imaging spectrometer [33]. This allowed achieving very high signal-to-noise ratios
in the data acquisition, which were crucial for observing the small differences in
the photoelectron angular distribution of aligned and non-aligned (or anti-aligned)
molecules.
Figure 1.3 provides an overview of the dynamic alignment that was achieved in
the experiment. The experimental angular distributions of high energy O + , resp. N +
fragments resulting from XUV-induced dissociative ionization and/or Coulomb explosion are plotted as a function of the time delay between the impulsive alignment
by the IR laser and the XUV ionization by the HHG laser. The angular distributions
are expressed by means of cos 2 θ 2D , where θ 2D is the angle between the measured
velocity of the fragment ion in the plane of the 2D detector and the common polarization axis of the XUV and IR beams. Perfect alignment of the molecular axes
corresponds to θ 2D = 0, whereas θ 2D = π/2 corresponds to molecules that are antialigned, i.e. having their internuclear axis perpendicular to the polarization axis of
the alignment laser. θ 2D is not to be confused with θ , the angle between the 3D fragment ion velocity and the laser polarization axis. The degree of molecular alignment
is given by cos 2 θ .
As Fig. 1.3 shows, an IR-laser induced alignment occurs shortly after the excitation by the IR laser pulse, and is then followed by a series of alignment revivals
A. Rouzée et al.
distinguishable variants exist, namely adiabatic alignment, where the molecule is
exposed to a laser pulse that is significantly longer than the rotational period of the
molecule [27], and impulsive alignment, where the molecule is exposed to a laser
pulse that is significantly shorter than the rotational period [28]. The advantage of
the latter method is that it leads to the formation of aligned molecular samples under laser field-free conditions (i.e. after the alignment laser pulse is over), although
with a degree of alignment that is lower than in the adiabatic case. Hybrid schemes
combining adiabatic and impulsive alignment have also been proposed [29], and—
in combination with state-selection techniques—allow the preparation of molecular
samples with a very high-degree of alignment and orientation [30] that can be used
in experiments aimed at observing the emission of photoelectrons in the molecular
frame.
Recently the experimental setup shown in Fig. 1.2 has been used to perform such
an experiment [31]. A series of small molecules (CO 2 , N 2 , O 2 and CO) were exposed to the sequence of an IR laser pulse that dynamically aligned the molecules
and an XUV pulse generated by HHG that ionized the molecules at a variable time
delay. Photoelectrons and fragment ions resulting from the latter photoionization
process were recorded on a velocity map imaging detector, i.e. accelerated towards
a two-dimensional detector consisting of a set of micro-channel plates, a phosphor
screen and a CCD camera, thereby allowing the measurement of a 2D projection
of the 3D velocity distribution. The 3D velocity distribution was determined from
the 2D projection by means of an iterative Abel inversion routine [32]. An important feature of the experiment was the fact that a very high count rate could be
achieved (up to ca. 10 6 counts/second), due to the use of a very efficient gas injection system, which was integrated in the repeller electrode of the velocity map
imaging spectrometer [33]. This allowed achieving very high signal-to-noise ratios
in the data acquisition, which were crucial for observing the small differences in
the photoelectron angular distribution of aligned and non-aligned (or anti-aligned)
molecules.
Figure 1.3 provides an overview of the dynamic alignment that was achieved in
the experiment. The experimental angular distributions of high energy O + , resp. N +
fragments resulting from XUV-induced dissociative ionization and/or Coulomb explosion are plotted as a function of the time delay between the impulsive alignment
by the IR laser and the XUV ionization by the HHG laser. The angular distributions
are expressed by means of cos 2 θ 2D , where θ 2D is the angle between the measured
velocity of the fragment ion in the plane of the 2D detector and the common polarization axis of the XUV and IR beams. Perfect alignment of the molecular axes
corresponds to θ 2D = 0, whereas θ 2D = π/2 corresponds to molecules that are antialigned, i.e. having their internuclear axis perpendicular to the polarization axis of
the alignment laser. θ 2D is not to be confused with θ , the angle between the 3D fragment ion velocity and the laser polarization axis. The degree of molecular alignment
is given by cos 2 θ .
As Fig. 1.3 shows, an IR-laser induced alignment occurs shortly after the excitation by the IR laser pulse, and is then followed by a series of alignment revivals