64
R. de Nalda et al.
view of the relevant potential energy curves). Attention will be paid to some crucial
issues that are sometimes overlooked, like the applicability of REMPI schemes in
ultrafast experiments, the role of laser-induced molecular alignment, or the influence
of the optical coupling window on reaction “clocking” times. Additionally, we will
show how these processes can be dramatically altered by the presence of a nearby
molecule. The CH 3 I molecule possesses a significant permanent dipole moment
and readily forms clusters for sufficiently high densities and low temperatures. This
chapter will show the dramatic effects of dimerization on the dissociation dynamics.
4.2 Methodology
4.2.1 The Experiment: Femtosecond Velocity Map Imaging
One of the central ingredients of the experimental strategy employed here is the
use of the velocity map imaging technique, presented for the first time by Eppink
and Parker in 1997 [6]. This technique permits full three-dimensional (3D) spatial
resolution of the velocity distribution of charged particles, which, coupled to femtosecond pump-probe detection, leads to a complete real time elucidation of the
dissociation event. Figure 4.2 shows a typical sketch of the experimental setup.
The irradiation configuration and pulse parameters (central wavelengths and energies) are chosen as a function of the experiment performed as variants of a single
main rig. For most of the experiments described here the laser was a Spectra-Physics
amplified Ti:sapphire system delivering 80 fs, 1 mJ pulses centered at 800 nm with
1 kHz repetition rate, but some of the later experiments (B-band) were performed
with an upgraded system (50 fs, 3.5 mJ). For two-pulse experiments, the fundamental output is split into two arms, one of which is used to pump an optical parametric
amplifier (OPA) tuned to generate signal pulses in the 1.2 µm–1.4 µm region, which
are later frequency quadrupled to constitute a ∼3 µJ beam in the 300–340 nm region
for (2 + 1) REMPI probing of either I atoms or CH 3 fragments. In the non-resonant
experiments, the OPA is not used and the ∼ 800 nm beam constitutes the probe
beam, which is later recombined with the pump beam. The pump beam is generated
by harmonic generation (third harmonic for the A-band, fourth for the B-band) of
the second arm of the fundamental output, yielding 266 nm or 200 nm, respectively.
A computer-controlled, motorized delay stage in the pump arm provides controllable delay between the pump and probe pulses with around 0.3 fs step.
For A-band studies in CH 3 I, performed with a third-harmonic pump pulse, the
time duration of the pump and probe pulses is estimated to be around 100 fs, limited by a ∼ 200 fs cross correlation. Later B-band studies, pumped with the fourth
harmonic at 200 nm, showed a ∼ 400 fs cross correlation. The bandwidth of both
pump and probe lasers is ∼ 3 nm full width at half maximum (FWHM), except the
200 nm beam used for B-band studies, with a bandwidth of only ∼ 0.3 nm FWHM.
Independent polarization control in each arm is provided by the use of half-wave
plates, and telescopes are used to control their focusing geometry on target. The
R. de Nalda et al.
view of the relevant potential energy curves). Attention will be paid to some crucial
issues that are sometimes overlooked, like the applicability of REMPI schemes in
ultrafast experiments, the role of laser-induced molecular alignment, or the influence
of the optical coupling window on reaction “clocking” times. Additionally, we will
show how these processes can be dramatically altered by the presence of a nearby
molecule. The CH 3 I molecule possesses a significant permanent dipole moment
and readily forms clusters for sufficiently high densities and low temperatures. This
chapter will show the dramatic effects of dimerization on the dissociation dynamics.
4.2 Methodology
4.2.1 The Experiment: Femtosecond Velocity Map Imaging
One of the central ingredients of the experimental strategy employed here is the
use of the velocity map imaging technique, presented for the first time by Eppink
and Parker in 1997 [6]. This technique permits full three-dimensional (3D) spatial
resolution of the velocity distribution of charged particles, which, coupled to femtosecond pump-probe detection, leads to a complete real time elucidation of the
dissociation event. Figure 4.2 shows a typical sketch of the experimental setup.
The irradiation configuration and pulse parameters (central wavelengths and energies) are chosen as a function of the experiment performed as variants of a single
main rig. For most of the experiments described here the laser was a Spectra-Physics
amplified Ti:sapphire system delivering 80 fs, 1 mJ pulses centered at 800 nm with
1 kHz repetition rate, but some of the later experiments (B-band) were performed
with an upgraded system (50 fs, 3.5 mJ). For two-pulse experiments, the fundamental output is split into two arms, one of which is used to pump an optical parametric
amplifier (OPA) tuned to generate signal pulses in the 1.2 µm–1.4 µm region, which
are later frequency quadrupled to constitute a ∼3 µJ beam in the 300–340 nm region
for (2 + 1) REMPI probing of either I atoms or CH 3 fragments. In the non-resonant
experiments, the OPA is not used and the ∼ 800 nm beam constitutes the probe
beam, which is later recombined with the pump beam. The pump beam is generated
by harmonic generation (third harmonic for the A-band, fourth for the B-band) of
the second arm of the fundamental output, yielding 266 nm or 200 nm, respectively.
A computer-controlled, motorized delay stage in the pump arm provides controllable delay between the pump and probe pulses with around 0.3 fs step.
For A-band studies in CH 3 I, performed with a third-harmonic pump pulse, the
time duration of the pump and probe pulses is estimated to be around 100 fs, limited by a ∼ 200 fs cross correlation. Later B-band studies, pumped with the fourth
harmonic at 200 nm, showed a ∼ 400 fs cross correlation. The bandwidth of both
pump and probe lasers is ∼ 3 nm full width at half maximum (FWHM), except the
200 nm beam used for B-band studies, with a bandwidth of only ∼ 0.3 nm FWHM.
Independent polarization control in each arm is provided by the use of half-wave
plates, and telescopes are used to control their focusing geometry on target. The