We have measured coincident ion pairs produced in the Coulomb explosion of H2 by 8-30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of H2 can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show H2 Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the H2 evolution in the laser pulse.

Access and use of this website and the material on it are subject to the Terms and Conditions set forth at Attosecond strobing of two-surface population dynamics in dissociating H₂+ NRC Publications Record / Notice d'Archives des publications de CNRC:http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=21276171&lang=en http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/ctrl?action=rtdoc&an=21276171&lang=fr READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE.http://nparc.cisti-icist.nrc-cnrc.gc.ca/npsi/jsp/nparc_cp.jsp?lang=en Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n'arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. Questions?Contact the NRC Publications Archive team at PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information. NRC Publications Archive Archives des publications du CNRCThis publication could be one of several versions: author's original, accepted manuscript or the publisher's version. / La version de cette publication peut être l'une des suivantes : la version prépublication de l'auteur, la version acceptée du manuscrit ou la version de l'éditeur. For the publisher's version, please access the DOI link below./ Pour consulter la version de l'éditeur, utilisez le lien DOI ci-dessous.http://doi.org/10.1103/PhysRevLett.98.073003Physical Review Letters, 98, 7, 2007-02-14 The interplay of population between two isolated electronic levels is the major tool in quantum optics and molecular science. In molecular science, controlling electronic excitation controls the bond. This is used to make cold molecules from cold atoms (e.g., [1]). It is also used to break symmetry in molecules [2,3]. Even nonresonant population transfer can control reaction pathways [4].We report the observation of electronic population dynamics -as it occurs. As illustrated in Fig. 1, we use the ground (1s g ) and first excited states (2p u )ofH 2 as our two-level system. The 1s g and 2p u states are isolated from all other electronic levels, dipole coupled, and degenerate in the separated atom limit. Experimentally, we prepare the H 2 by ionizing H 2 in an intense infrared laser pulse. Components of the vibrational wave packet created by tunnel ionization can sweep through both the one-and three-photon resonances between the two surfaces. Population is transferred from 1s g to 2p u via adiabatic rapid passage (the general term given to these processes in quantum optics) or bond softening (the term used in strong field molecular physics) [5,6]. In our experiment, which uses near infrared light, a snapshot of the upper state population is taken within a time window of a few hundred attoseconds via Coulomb explosion imaging. This snapshot is repeated every half las...

We have measured full momentum images of electrons rescattered from Xe, Kr, and Ar following the liberation of the electrons from these atoms by short, intense laser pulses. At high momenta the spectra show angular structure (diffraction) which is very target dependent and in good agreement with calculated differential cross sections for the scattering of free electrons from the corresponding ionic cores.

We have measured angular distributions of ion fragments produced in dissociative double ionization of CO, CO 2 , and C 2 H 2 by intense ultrashort laser pulses. This report extends similar recent studies of O 2 and N 2 to a wider set of molecules. We found that for sub-10-fs pulses of sufficiently low intensity the fragments' angular distributions for all studied molecules are determined by angular dependence of the first ionization step. Those experimental angular distributions were in good agreement with angular dependent ionization probabilities calculated with the molecular tunneling ionization theory. The measured angular distributions directly reflect the symmetry of the corresponding molecular orbitals. For higher laser intensities and longer pulse durations, dynamic alignment and postionization alignment start to affect the angular distributions and ion fragments are preferentially ejected along the laser-polarization direction.Molecules subjected to nonresonant infrared laser pulses of sufficiently high intensity undergo single or multiple ionization often followed by immediate dissociation. Though this process is of great fundamental and practical interest and has been studied extensively for many molecules, its detailed mechanism remains poorly understood. In particular, the nature of strongly anisotropic angular distributions of resulting ion fragments has been a matter of some debate. Specifically, the argument has centered around the relative importance of two possible mechanisms: angular dependence of molecular ionization rates ͑the so-called "geometric" alignment, when molecules of certain orientation are selectively ionized͒; and dynamic alignment by the pulse ͑when molecules actually reorient before dissociating͒. For heavy molecules ͑such as I 2 ͒ geometric alignment is clearly a dominant mechanism ͓1,2͔. For light molecules both mechanisms may play a role and deconvolving their relative contributions is not straightforward.Until very recently most studies of dissociative ionization in diatomic molecules reported strongly asymmetric angular distributions with ion fragments ejected in a narrow angle around the laser polarization direction ͓3-5͔. At the same time, the recently developed theory of molecular tunneling ionization ͑MO-ADK͒ predicts that for some molecules, due to the symmetry of their molecular orbitals, the ionization probability is the highest when the molecular axis is not parallel to the laser electric field ͓6͔. In general, there not need to be a direct correlation between the fragment angular distributions and the angular dependence of the ionization probability, because, in addition to dynamic alignment by the pulse, several ionization steps with different angular dependences may be involved in the process ͓7͔. Nevertheless, a recent report on dissociative ionization of O 2 and N 2 ͓8͔ presents experimental angular distributions that are in almost perfect correlation with the corresponding angle-dependent single ionization probabilities calculated with molecular ADK theory. In...

We have used momentum imaging techniques to measure in high resolution the kinetic energy release spectra and angular distributions of coincident O + and N + ion pairs produced by short laser pulses (8-35 fs) on targets of N 2 and O 2 at peak intensities between 1 and 12 × 10 14 W cm −2. We record the full momentum vectors of both members of each pair and achieve a kinetic energy release resolution of less than 0.3 eV. We find that the process proceeds through well-defined electronic states of the excited molecular dications. Using linear and circularly polarized light, we identify two mechanisms for the production of these states, rescattering and sequential ionization. By using 8 fs pulses, we observe that the internuclear distance can be frozen during the pulse. For low intensities and 8 fs pulses, emission from N 2 is strongly directed along the polarization vector, while that for O 2 is not, a result we interpret as being due to the different symmetries of the outer orbitals of these molecules. For high intensities and longer pulses, the distributions increasingly fold towards the polarization vector, ultimately peaking at zero degrees for both molecules. For oxygen, a local peaking for molecules aligned at right angles to the polarization vector is seen. A discussion and interpretation of the results are presented.

We demonstrate a method for determining the full three-dimensional molecular-frame photoelectron angular distribution in polyatomic molecules using methane as a prototype. Simultaneous double Auger decay and subsequent dissociation allow measurement of the initial momentum vectors of the ionic fragments and the photoelectron in coincidence, allowing full orientation by observing a three-ion decay pathway, (H þ , H þ , CH þ 2 ). We find the striking result that at low photoelectron energies the molecule is effectively imaged by the focusing of photoelectrons along bond directions. DOI: 10.1103/PhysRevLett.108.233002 PACS numbers: 33.80.Eh, 33.60.+q Imaging molecular structure is a critical challenge in chemical physics recently highlighted by the emergence of techniques that, similar to ultrafast electron diffraction [1] or x-ray diffraction [2], have the potential to be taken to the time domain and thereby ultimately be used to make ''movies'' of chemical reactions on their natural time scale. Of particular interest is the development of such techniques that can be applied to the dynamics of isolated molecules. Here, the full three-dimensional orientation of a polyatomic molecule is measured simultaneously with the three components of the momentum of a photoelectron ejected from it with no underlying assumptions of symmetry or geometry. We present three-dimensional images of a polyatomic molecule measured with this technique, demonstrating an effect predicted [3] for polyatomic molecules with a heavy central atom bonded to hydrogens, namely that low-energy photoelectrons can directly image the molecular potential and bond structure.When a photoelectron is launched by photoabsorption of an inner shell, the outgoing photoelectron wave is then scattered by the aggregate potential of the molecule. The final angular distribution in the body-fixed frame of the molecule is an exquisitely sensitive probe of molecular structure and initial electronic state, which has been recently argued and demonstrated [4,5]. However, observing molecular-frame photoelectron angular distributions (MFPADs) at high resolution requires accurate orientation of the molecule in the gas phase. Three-dimensional laser alignment [6,7] can accomplish such orientation prior to photoionization but is limited to molecules with an asymmetric polarizability. In the case of simple diatomic molecules, orientation can also be accomplished by detecting the photoelectron in coincidence with positively charged fragments that emerge following prompt Auger decay and dissociation [8]. Progress has also been made toward threedimensional MFPAD measurement using coincidence detection and velocity map imaging [9]. Here we present photoelectron imaging of methane molecules, where both the photoelectron momentum and corresponding body frame of the polyatomic molecule are fully determined in all three dimensions.For many molecules, including CH 4 , core ionization opens a strong simultaneous double Auger decay channel that produces a trication that then can disso...

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We have measured with high resolution the full vector momenta of the doubly charged recoil ions produced when Ar and Ne atomic targets are ionized by intense circularly polarized few-cycle ͑ϳ8 fs͒ laser pulses in the sequential ionization region. The momentum spectra of the ions were found to exhibit structure which is characteristic of the relative binding energies of the two electrons and the sequential nature of the emission process. We demonstrate that the measured spectra can be used to extract subcycle time information regarding the sequential release of the two electrons.