2 XUV Lasers for Ultrafast Electronic Control in H 2
37
channels more easily. Indeed, the short wavelength of UV/XUV pulses ensures that
the dynamics of the nuclear wave packets that is being probed is entirely due to the
unperturbed molecular potential [78]. These pump-probe schemes would also bring
the possibility to explore the Σ +
g DES, as well as diverse phenomena related to transitions through singly excited states of the neutral. Such processes are discussed in
the following section.
2.5 Control and Non-linear Effects in Multiphoton Single
Ionization
Investigations on few-photon single ionization of hydrogen molecules are very
scarce. Experimental studies are restricted to above threshold ionization processes
[32, 79], and most theoretical studies, in the multiphoton regime, have made use
of the fixed nuclei approximation (FNA) [40–42]. This approach is valid to quantitatively predict total few-photon ionization probabilities for photon energies below
∼ 11 eV. For higher photon energies, the (1 + 1)-REMPI (resonant enhanced multiphoton ionization) channel in H 2 opens and FNA approaches underestimate ionization [51, 80]. In the region where two-photon ionization is the dominant process,
the proper inclusion of the nuclear degrees of freedom in the theoretical description leads to a dramatic increase of the (1 + 1)-REMPI probability and its variation with photon energy is smoother with respect to that predicted within the FNA
[see Fig. 2.5(a)]. More importantly, fully dimensional theoretical calculations have
shown that, for a given range of photon energies, dissociative ionization may become the dominant process. This observation is unexpected because, in all reported
studies on H 2 photoionization, dissociative ionization (DI) is always orders of magnitude smaller than non-dissociative ionization (NDI). Moreover, it indicates that
by manipulating the radiation parameters one can actually control the ratio between
different ionization channels. This will be discussed in the first part of this section.
In the second subsection, we explore the different mechanisms that are triggered at
high intensities and that allow for such control in the two-photon single ionization
processes. Finally, in order to shed some more light on (1 + 1)-REMPI processes in
molecules, results obtained for a pump-probe scheme with two identical UV pulses
are shown in the third subsection. There, we discuss the observables that allows
one to retrieve information and characterize the vibronic wave packets associated to
singly excited states of the neutral.
2.5.1 Control of Single Ionization Channels by Means of VUV
Pulses
The ratio between DI and NDI channels can be tailored by appropriately tuning the
laser parameters. This is illustrated in Fig. 2.5, where DI, NDI and total ionization
37
channels more easily. Indeed, the short wavelength of UV/XUV pulses ensures that
the dynamics of the nuclear wave packets that is being probed is entirely due to the
unperturbed molecular potential [78]. These pump-probe schemes would also bring
the possibility to explore the Σ +
g DES, as well as diverse phenomena related to transitions through singly excited states of the neutral. Such processes are discussed in
the following section.
2.5 Control and Non-linear Effects in Multiphoton Single
Ionization
Investigations on few-photon single ionization of hydrogen molecules are very
scarce. Experimental studies are restricted to above threshold ionization processes
[32, 79], and most theoretical studies, in the multiphoton regime, have made use
of the fixed nuclei approximation (FNA) [40–42]. This approach is valid to quantitatively predict total few-photon ionization probabilities for photon energies below
∼ 11 eV. For higher photon energies, the (1 + 1)-REMPI (resonant enhanced multiphoton ionization) channel in H 2 opens and FNA approaches underestimate ionization [51, 80]. In the region where two-photon ionization is the dominant process,
the proper inclusion of the nuclear degrees of freedom in the theoretical description leads to a dramatic increase of the (1 + 1)-REMPI probability and its variation with photon energy is smoother with respect to that predicted within the FNA
[see Fig. 2.5(a)]. More importantly, fully dimensional theoretical calculations have
shown that, for a given range of photon energies, dissociative ionization may become the dominant process. This observation is unexpected because, in all reported
studies on H 2 photoionization, dissociative ionization (DI) is always orders of magnitude smaller than non-dissociative ionization (NDI). Moreover, it indicates that
by manipulating the radiation parameters one can actually control the ratio between
different ionization channels. This will be discussed in the first part of this section.
In the second subsection, we explore the different mechanisms that are triggered at
high intensities and that allow for such control in the two-photon single ionization
processes. Finally, in order to shed some more light on (1 + 1)-REMPI processes in
molecules, results obtained for a pump-probe scheme with two identical UV pulses
are shown in the third subsection. There, we discuss the observables that allows
one to retrieve information and characterize the vibronic wave packets associated to
singly excited states of the neutral.
2.5.1 Control of Single Ionization Channels by Means of VUV
Pulses
The ratio between DI and NDI channels can be tailored by appropriately tuning the
laser parameters. This is illustrated in Fig. 2.5, where DI, NDI and total ionization