2 XUV Lasers for Ultrafast Electronic Control in H 2
43
excited states. We will also explore which observables actually contain significant
information to unravel the dynamics of these states.
2.5.3 Probing Nuclear Wave Packets in Molecular Excited States
The present pump-probe set-up with two identical pulses is outlined in Fig. 2.8(a).
A pump pulse excites the molecular target, creating a NWP in the B 1 Σ +
u state that
field-free evolves in time until a second photon is absorbed, thus mapping it into the
ionization channels.
As in the previous sections, we have chosen a photon energy of 12.2 eV, which
is in resonance with the B 1 Σ +
u electronic state in a direct vertical transition from
the ground state, such that the (1 + 1)-REMPI process dominates. We use a laser
intensity of 10 12 W/cm 2 , which is low enough to avoid field-induced distortion of
the molecular potential, and a 2 fs pulse duration, whose broad bandwidth ensures
the creation of an energetically wide vibrational wave packet. Similar experimental
set-ups have been already used with XFEL sources and applied to probe the NWP
dynamics launched in molecular ions by detecting the fragments after the Coulomb
explosion (CE) of H 2 and D 2 [33, 34]. In principle, exploring singly excited states
has the added complication of disentangling several single ionization channels (DI
and NDI associated to several ionization thresholds). Also, an easy mapping into the
final energy-differential observables is not trivial, in contrast with standard CE experiments in [33]. However, as we will show below, it is still possible to distinguish
those channels by looking at angular-differential quantities. Indeed, by measuring
the molecular frame induced asymmetries in the electron ejection in (1 + 1)-REMPI
it is possible to clock the evolution of the pumped NWPs. In a delayed two-photon
absorption one could expected a symmetry breaking of the ejected electrons with
respect to the molecular frame as that observed in experiments on photoionization
mediated through autoionization [21, 36, 37, 91, 92]. The up-down (or left-right)
asymmetry of electron ejection in molecules that dissociatively ionize is a wellknown process [17, 93] that arises from the superposition of two states with different
parity. Such superposition is achieved in those experiments through the time-delay
decay of DES. Analogously, it can be generated by a time-delayed two-photon ionization involving the single excited states of the neutral.
As schematically represented in Fig. 2.8, the pump pulse creates a NWP in the
B 1 Σ +
u single excited state. In this potential energy curve, whose oscillating period is
around 25–30 fs [76], the molecule stretches. Consequently, the NWP in the B 1 Σ +
u
state travels to larger internuclear distances, such that when the probe pulse is absorbed the ionization potential to reach the second threshold (2pσ u state in H
+
2 ) is
lower. Because of the purely dissociative character of the 2pσ u ionization threshold, a direct mapping of the NWP appears in this channel, which is unfortunately
obscured by the larger contribution of the first ionization threshold 1sσ g to the DI.
This is shown in Fig. 2.9(a), where the PKE distribution is plotted at different time
delays. Since the 1sσ g contribution is appreciable larger, only a subtle signature of
43
excited states. We will also explore which observables actually contain significant
information to unravel the dynamics of these states.
2.5.3 Probing Nuclear Wave Packets in Molecular Excited States
The present pump-probe set-up with two identical pulses is outlined in Fig. 2.8(a).
A pump pulse excites the molecular target, creating a NWP in the B 1 Σ +
u state that
field-free evolves in time until a second photon is absorbed, thus mapping it into the
ionization channels.
As in the previous sections, we have chosen a photon energy of 12.2 eV, which
is in resonance with the B 1 Σ +
u electronic state in a direct vertical transition from
the ground state, such that the (1 + 1)-REMPI process dominates. We use a laser
intensity of 10 12 W/cm 2 , which is low enough to avoid field-induced distortion of
the molecular potential, and a 2 fs pulse duration, whose broad bandwidth ensures
the creation of an energetically wide vibrational wave packet. Similar experimental
set-ups have been already used with XFEL sources and applied to probe the NWP
dynamics launched in molecular ions by detecting the fragments after the Coulomb
explosion (CE) of H 2 and D 2 [33, 34]. In principle, exploring singly excited states
has the added complication of disentangling several single ionization channels (DI
and NDI associated to several ionization thresholds). Also, an easy mapping into the
final energy-differential observables is not trivial, in contrast with standard CE experiments in [33]. However, as we will show below, it is still possible to distinguish
those channels by looking at angular-differential quantities. Indeed, by measuring
the molecular frame induced asymmetries in the electron ejection in (1 + 1)-REMPI
it is possible to clock the evolution of the pumped NWPs. In a delayed two-photon
absorption one could expected a symmetry breaking of the ejected electrons with
respect to the molecular frame as that observed in experiments on photoionization
mediated through autoionization [21, 36, 37, 91, 92]. The up-down (or left-right)
asymmetry of electron ejection in molecules that dissociatively ionize is a wellknown process [17, 93] that arises from the superposition of two states with different
parity. Such superposition is achieved in those experiments through the time-delay
decay of DES. Analogously, it can be generated by a time-delayed two-photon ionization involving the single excited states of the neutral.
As schematically represented in Fig. 2.8, the pump pulse creates a NWP in the
B 1 Σ +
u single excited state. In this potential energy curve, whose oscillating period is
around 25–30 fs [76], the molecule stretches. Consequently, the NWP in the B 1 Σ +
u
state travels to larger internuclear distances, such that when the probe pulse is absorbed the ionization potential to reach the second threshold (2pσ u state in H
+
2 ) is
lower. Because of the purely dissociative character of the 2pσ u ionization threshold, a direct mapping of the NWP appears in this channel, which is unfortunately
obscured by the larger contribution of the first ionization threshold 1sσ g to the DI.
This is shown in Fig. 2.9(a), where the PKE distribution is plotted at different time
delays. Since the 1sσ g contribution is appreciable larger, only a subtle signature of