2 XUV Lasers for Ultrafast Electronic Control in H 2
27
relative importance of different ionization channels. We also present theoretical predictions on phenomena induced by intense XUV fs laser pulses that are inexistent
in atoms.
This document is organized as follows. Section 2.2 briefly reviews the experiments performed in the last few years on single and double ionization of hydrogen
molecules subject to different radiation sources. In Sect. 2.3, we report the most relevant theoretical contributions aimed at exploring excitation and ionization of H 2 ,
followed by a brief description of the time-dependent theoretical methods employed
for the calculations presented here. Sections 2.4 and 2.5 discuss in detail some of
these theoretical results in the framework of the current and future laser capabilities.
We conclude with a short discussion on the current limitations of these methods to
describe ultrafast molecular phenomena and on upcoming studies devoted to understand fundamental concepts in small molecules.
2.2 Experimental Set-Ups
A complete understanding of molecular photoionization, even in the simplest systems, is not yet accomplished. Ionization and dissociation may take place simultaneously in time-scales of few femtoseconds (fs) and several competing channels
may be favored/suppressed by tuning the radiation parameters. Synchrotron radiation sources have been used to explore photoionization of H 2 , with particular interest in the region where doubly excited states (DES) are significantly populated
(60 > ω > 25 eV). Interference phenomena due to the coupled motion of electrons
and nuclei have been observed in single photoionization of H 2 . For instance, synchrotron radiation experiments, with diverse light and detection arrangements, have
explored the anisotropies in the angular distributions of emitted electrons, which are
the consequence of autoionization from different DES [20–23]. Also, a noticeable
number of works have focused on photodissociation of DES into two neutral fragments [24–31]. These last works measure the fluorescence from the decay of excited
H atoms, which enables to distinguish dissociation arising from vibrationally excited states of the ion H
+
2 from that arising from DES that dissociate faster than they
ionize. Synchrotron radiation experiments have thus provided a significant amount
of structural information that allows one to disentangle the complexity of molecular
photoionization.
Nevertheless, emerging pulsed radiation sources are required for a time-resolved
imaging of the molecular dynamics that eventually may lead to manipulation and
control of these processes. High harmonic generation (HHG) techniques or freeelectron laser (FEL) and X-ray FEL sources are able to generate intense ultrashort
laser pulses in the UV and XUV regions. In the last few years, diverse pump-probe
experiments have provided images with unprecedented time and spatial resolution
on photo-induced atomic and molecular dynamics. In this context, pioneering experiments using XUV pulses were performed in two-photon single ionization of
hydrogen molecules [32], where dissociative above threshold ionization (ATI) was
explored.
27
relative importance of different ionization channels. We also present theoretical predictions on phenomena induced by intense XUV fs laser pulses that are inexistent
in atoms.
This document is organized as follows. Section 2.2 briefly reviews the experiments performed in the last few years on single and double ionization of hydrogen
molecules subject to different radiation sources. In Sect. 2.3, we report the most relevant theoretical contributions aimed at exploring excitation and ionization of H 2 ,
followed by a brief description of the time-dependent theoretical methods employed
for the calculations presented here. Sections 2.4 and 2.5 discuss in detail some of
these theoretical results in the framework of the current and future laser capabilities.
We conclude with a short discussion on the current limitations of these methods to
describe ultrafast molecular phenomena and on upcoming studies devoted to understand fundamental concepts in small molecules.
2.2 Experimental Set-Ups
A complete understanding of molecular photoionization, even in the simplest systems, is not yet accomplished. Ionization and dissociation may take place simultaneously in time-scales of few femtoseconds (fs) and several competing channels
may be favored/suppressed by tuning the radiation parameters. Synchrotron radiation sources have been used to explore photoionization of H 2 , with particular interest in the region where doubly excited states (DES) are significantly populated
(60 > ω > 25 eV). Interference phenomena due to the coupled motion of electrons
and nuclei have been observed in single photoionization of H 2 . For instance, synchrotron radiation experiments, with diverse light and detection arrangements, have
explored the anisotropies in the angular distributions of emitted electrons, which are
the consequence of autoionization from different DES [20–23]. Also, a noticeable
number of works have focused on photodissociation of DES into two neutral fragments [24–31]. These last works measure the fluorescence from the decay of excited
H atoms, which enables to distinguish dissociation arising from vibrationally excited states of the ion H
+
2 from that arising from DES that dissociate faster than they
ionize. Synchrotron radiation experiments have thus provided a significant amount
of structural information that allows one to disentangle the complexity of molecular
photoionization.
Nevertheless, emerging pulsed radiation sources are required for a time-resolved
imaging of the molecular dynamics that eventually may lead to manipulation and
control of these processes. High harmonic generation (HHG) techniques or freeelectron laser (FEL) and X-ray FEL sources are able to generate intense ultrashort
laser pulses in the UV and XUV regions. In the last few years, diverse pump-probe
experiments have provided images with unprecedented time and spatial resolution
on photo-induced atomic and molecular dynamics. In this context, pioneering experiments using XUV pulses were performed in two-photon single ionization of
hydrogen molecules [32], where dissociative above threshold ionization (ATI) was
explored.