Journal Article

Electron localization following attosecond molecular photoionization

Kelkensberg, F., Benedetti, E., Swoboda, M., Vrakking, M. J. J., L’Huillier, A., Lépine, F., Ghafur, O., Ivanov, M. Yu, Zherebtsov, S., Znakovskaya, I., Kling, M. F., Morales, F., Ferrari, F., Pérez-Torres, J. F., Sanz-Vicario, J. L., Sansone, G., Siu, W., Johnsson, P., Nisoli, M., Martín, F.

2010

Overview

Attosecond-scale electron localization The primary event in photoexcitation — involved in processes such as photosynthesis and photoisomerization — is an electronic response that occurs on attosecond (1 as = 10 −18 s) timescales, a realm recently made accessible to spectroscopic investigation by the development of attosecond-scale light pulses. Sansone et al . report an experimental study in which electron localization in molecules is measured on attosecond timescales using pump–probe spectroscopy. H 2 and D 2 are dissociatively ionized by the sequence of an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends on the delay between the pump and probe pulses. This work demonstrates that combined experimental and computational efforts enable the use of attosecond pulses for the exploration of electron localization. Attosecond (10 −18 s) laser pulses make it possible to peer into the inner workings of atoms and molecules on the electronic timescale — phenomena in solids have already been investigated in this way. Here, an attosecond pump–probe experiment is reported that investigates the ionization and dissociation of hydrogen molecules, illustrating that attosecond techniques can also help explore the prompt charge redistribution and charge localization that accompany photoexcitation processes in molecular systems. For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10 −15 -s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale 1 , 2 , 3 , 4 has become possible only with the recent development of isolated attosecond (10 −18 -s) laser pulses 5 . Such pulses have been used to investigate atomic photoexcitation and photoionization 6 , 7 and electron dynamics in solids 8 , and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H 2 and D 2 was monitored on femtosecond timescales 9 and controlled using few-cycle near-infrared laser pulses 10 . Here we report a molecular attosecond pump–probe experiment based on that work: H 2 and D 2 are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends—with attosecond time resolution—on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump–probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born–Oppenheimer approximation.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/638/563/981 / 639/766/36 / Asymmetry / Atom and Molecular Physics and Optics / Atom- och molekylfysik och optik (Här ingår: Kemisk fysik, kvantoptik) / Atomic and molecular physics / Atoms / Autoionization, photoionization, and photodetachment / Exact sciences and technology / Fundamental areas of phenomenology (including applications) / Fysik / Humanities and Social Sciences / Ionization / letter / Methods / Molecular dynamics / Molecular properties and interactions with photons / multidisciplinary / Natural Sciences / Naturvetenskap / Nonlinear optics / Optics / Photoionization / Photon interactions with molecules / Physical Sciences / Physics / Properties / Science / Science (multidisciplinary) / Ultrafast processes; optical pulse generation and pulse compression