Showing papers by "Till Jahnke published in 2013"
TL;DR: A mass spectrometry approach is demonstrated that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionization–induced Coulomb explosion.
Abstract: Bijvoet's method, which makes use of anomalous x-ray diffraction or dispersion, is the standard means of directly determining the absolute (stereochemical) configuration of molecules, but it requires crystalline samples and often proves challenging in structures exclusively comprising light atoms. Herein, we demonstrate a mass spectrometry approach that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionization-induced Coulomb explosion. This technique is applied to the prototypical chiral molecule bromochlorofluoromethane and the isotopically chiral methane derivative bromodichloromethane.
234 citations
TL;DR: The first direct measurement of the temporal evolution of ICD is presented using a novel experimental approach and is predicted to be highly complex, as its efficiency strongly depends on the distance of the atoms involved and this distance typically changes during the decay.
Abstract: During the past 15 years a novel decay mechanism of excited atoms has been discovered and investigated. This so-called interatomic Coulombic decay (ICD) involves the chemical environment of the electronically excited atom: the excitation energy is transferred (in many cases over long distances) to a neighbor of the initially excited particle usually ionizing that neighbor. It turned out that ICD is a very common decay route in nature as it occurs across van der Waals and hydrogen bonds. The time evolution of ICD is predicted to be highly complex, as its efficiency strongly depends on the distance of the atoms involved and this distance typically changes during the decay. Here we present the first direct measurement of the temporal evolution of ICD using a novel experimental approach.
60 citations
TL;DR: The results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses, and the technique described is robust and provides a powerful tool for ultrafast science.
Abstract: Electron motion in chemical bonds occurs on an attosecond timescale. This ultrafast motion can be driven by strong laser fields. Ultrashort asymmetric laser pulses are known to direct electrons to a certain direction. But do symmetric laser pulses destroy symmetry in breaking chemical bonds? Here we answer this question in the affirmative by employing a two-particle coincidence technique to investigate the ionization and fragmentation of H₂ by a long circularly polarized multicycle femtosecond laser pulse. Angular streaking and the coincidence detection of electrons and ions are employed to recover the phase of the electric field, at the instant of ionization and in the molecular frame, revealing a phase-dependent anisotropy in the angular distribution of H⁺ fragments. Our results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses. The technique we describe is robust and provides a powerful tool for ultrafast science.
59 citations
TL;DR: The ionization of HeNe from below the He 1s3p excitation to the He ionization threshold is investigated and the interatomic Coulombic decay lifetime strongly increases with increasing vibration state.
Abstract: We investigate the ionization of HeNe from below the He $1s3p$ excitation to the He ionization threshold. We observe ${\mathrm{HeNe}}^{+}$ ions with an enhancement by more than a factor of 60 when the He side couples resonantly to the radiation field. These ions are an experimental proof of a two-center resonant photoionization mechanism predicted by Najjari et al. [Phys. Rev. Lett. 105, 153002 (2010)]. Furthermore, our data provide electronic and vibrational state resolved decay widths of interatomic Coulombic decay in HeNe dimers. We find that the interatomic Coulombic decay lifetime strongly increases with increasing vibrational state.
57 citations
TL;DR: This work investigates the single-photon double ionization of helium at photon energies of 440 and 800 eV and observes doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy.
Abstract: We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.
48 citations
TL;DR: Experimental observation of the energy sharing between electron and nuclei in above-threshold multiphoton dissociative ionization of H2 by strong laser fields results in multiple diagonal lines in their joint energy spectrum governed by the energy conservation of all fragment particles.
Abstract: We report experimental observation of the energy sharing between electron and nuclei in abovethreshold multiphoton dissociative ionization of H2 by strong laser fields. The absorbed photon energy is shared between the ejected electron and nuclei in a correlated fashion, resulting in multiple diagonal lines in their joint energy spectrum governed by the energy conservation of all fragment particles. Deposition of the photon energy to atoms and molecules is the primary step of the interactions of radiation with matter. The details of this deposition process, in particular how the photon energy is distributed among the subsystems and various internal degrees of freedom, determine all photon-induced chemical and physical dynamics. For the interaction with a strong laser field, this question of energy deposition gets even richer since it is well established that the energy of more photons than the minimal number required for ionization can be absorbed. For atoms in a strong field, this leads to discrete peaks in the photoelectron spectrum that are spaced by the photon energy and referred to as ‘‘above-threshold ionization’’ (ATI) [1]. For molecules the vibrational, rotational, and dissociative motions of the nuclei provide a sink for the photon energy in addition to the electrons. This has been observed in single-photon dissociative ionization of molecules exposed to synchrotron radiation [2–4], where the photon energy is shared by the freed electrons and the nuclear fragments. For the molecular multiphoton case, rich ATI spectra of the freed electron [5–9], bond-softening-induced molecular dissociative ionization [10–15], and the imaging of internuclear distance using nuclear kinetic energy release spectra [16–19] have been reported. The correlation between the fragment ion and the electron energy has most recently been studied in numerical simulations for H 2 þ [20,21]. A nontrivial sharingof the absorbedphoton energy between the electron and nuclei in multiphoton ionization of molecules was predicted and stimulated us to investigate this problem experimentally. Here, we report the experimental observation of the energy sharing between the emitted electron and nuclei from above-threshold multiphoton dissociative ionization of the simplest molecule H 2 by intense femtosecond laser pulses. Discrete numbers of absorbed photons can be identified by peaked diagonal lines in the joint energy spectrum (JES) of the coincidently measured electron and nuclei [20]. Since there is no direct coupling between the nuclei and the laser field for homonuclear diatomic molecules, the laser first couples to the electrons, and the electrons then couple to the nuclei. The energy taken by the nuclei therefore measures the correlation between the electrons and nuclei. Figure 1 shows a much simplified schematics of the process. By absorbing multiple photons (blue vertical arrows), the H2 molecule emits one electron and a nuclear wavepacket on the � þ (ground) state of H 2 þ is launched. It propagates on the � þ potential curve of H 2 þ .P art of this wavepacket already has sufficient energy to escape (direct pathway), while another part can be promoted to the dissociative � þ potential curve by resonant absorption of one additional photon (one-photon pathway). In the multiphoton picture, the sum of the kinetic energy of the proton (Ep), hydrogen atom (EH), and electron (Ee) after the end of the laser pulse is given by
47 citations
TL;DR: Comparing different tilted-pulse-front pumping schemes for single-cycle THz generation in LiNbO(3) crystals both theoretically and experimentally in terms of conversion efficiency and divergence has been found experimentally to depend on the pump imaging scheme employed.
Abstract: We compare different tilted-pulse-front pumping schemes for single-cycle THz generation in LiNbO(3) crystals both theoretically and experimentally in terms of conversion efficiency. The conventional setup with a single lens as an imaging element has been found to be highly inefficient in the case of sub-50 fs pump pulses, mainly due to the resulting chromatic aberrations. These aberrations are avoided in the proposed new setup, which employs two concave mirrors in a Keplerian telescope arrangement as the imaging sequence. This partially compensates spherical aberrations and results in a ca. six times higher conversion efficiency in the case of 35-fs optical pump pulse duration compared to the single-lens setup. A THz field strength of 60 kV/cm was obtained using 0.5 mJ pump pulses. The divergence of the THz beam has been found experimentally to depend on the pump imaging scheme employed.
41 citations
TL;DR: In this paper, the phase between two colors of an elliptically polarized two-color pulse is unambiguously and straightforwardly assigned by tracing the momentum of the released electron, streaked by the rotating laser field.
Abstract: We study the directional dissociative ionization of diatomic molecules (H2, N2, Ar2, and CO) by intense phase-controlled elliptically polarized two-color pulses. The phase between the two colors of our elliptically polarized two-color pulse is unambiguously and straightforwardly assigned by tracing the momentum of the released electron, streaked by the rotating laser field, which is imprinted in the momentum of the correlated ion. The laser-driven electron motion, electron-localization-assisted enhanced multielectron ionization, and role of the orbital shape for the asymmetric dissociative ionizations of various molecules are discussed.
33 citations
TL;DR: In this paper, the authors investigated the contribution of interatomic Coulombic decay induced by ion impact in neon and argon dimers to the production of low-energy electrons.
Abstract: We investigate the contribution of interatomic Coulombic decay induced by ion impact in neon and argon dimers (Ne${}_{2}$ and Ar${}_{2}$) to the production of low-energy electrons Our experiments cover a broad range of perturbation strengths and reaction channels We use 1137 MeV/u S${}^{14+}$, 0125 MeV/u He${}^{1+}$, 01625 MeV/u He${}^{1+}$, and 0150 MeV/u He${}^{2+}$ as projectiles and study ionization, single and double electron transfer to the projectile, as well as projectile electron loss processes The application of a COLTRIMS reaction microscope enables us to retrieve the three-dimensional momentum vectors of the ion pairs of the fragmenting dimer into Ne${}^{q+}$-Ne${}^{1+}$ and Ar${}^{q+}$-Ar${}^{1+}$ ($q=$ 1, 2, 3) in coincidence with at least one emitted electron
32 citations
TL;DR: D discrete above-threshold ionization like peaks in the sum energy of both electrons, as predicted by all quantum mechanical calculations, are found; more surprisingly, however, is the observation of twoAbove-th threshold ionization combs in the energy distribution of the individual electrons.
Abstract: We report on the observation of discrete structures in the electron energy distribution for strong field double ionization of argon at 394 nm. The experimental conditions were chosen in order to ensure a nonsequential ejection of both electrons with an intermediate rescattering step. We have found discrete above-threshold ionization like peaks in the sum energy of both electrons, as predicted by all quantum mechanical calculations. More surprisingly, however, is the observation of two above-threshold ionization combs in the energy distribution of the individual electrons.
27 citations
TL;DR: The results reveal that a classical descriptions of the slits provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.
Abstract: We simultaneously measured the momentum transferred to a free-floating molecular double slit and the momentum change of the atom scattering from it. Our experimental results are compared to quantum mechanical and semiclassical models. The results reveal that a classical description of the slits, which was used by Einstein in his debate with Bohr, provides a surprisingly good description of the experimental results, even for a microscopic system, if momentum transfer is not ascribed to a specific pathway but shared coherently and simultaneously between both.
TL;DR: In this article, the symmetry of the valence electronic orbital of an anisotropic atomic-molecular complex of ArCO was investigated by tracing its three-body Coulomb breakup following triple ionization in a phase-controlled elliptically polarized two-color pulse.
Abstract: We simultaneously probe the molecular structure and the symmetry of valence electronic orbital of an anisotropic atomic-molecular complex of ArCO by tracing its three-body Coulomb breakup following triple ionization in a phase-controlled elliptically polarized two-color pulse. The geometry of ArCO is found to be tilted T shaped, where the angle between the covalent and van der Waals bonds is 65\ifmmode^\circ\else\textdegree\fi{} with oxygen pointing towards argon. The asymmetric profiles of the outmost orbitals from the CO site are probed by directional dissociation of the contained CO subunit as a function of the laser phase. Our results clearly image the asymmetric geometry of ArCO and thus reveal the anisotropic interaction between a rare-gas atom and a heteronuclear diatomic molecule.
TL;DR: Electron-localization-assisted enhanced ionization and molecular orbital profile-dominated, orientation-dependent ionization are discussed to reveal the order of electrons release from different sites of N2Ar.
Abstract: We study the order in which a strong laser field removes multiple electrons from a van der Waals (vdW) cluster. The N2Ar, with an equilibrium T-shaped geometry, contains both a covalent and a vdW bond and serves as a simple yet rich example. Interestingly, the fragmenting double and triple ionizations of N2Ar with vdW bond breaking are favored when the vdW bond is aligned along the laser field polarization vector. However, the orientation of the covalent bond with respect to the laser field rules the triple ionization when both the covalent and vdW bonds are simultaneously broken. Electron-localizationassisted enhanced ionization and molecular orbital profile-dominated, orientation-dependent ionization are discussed to reveal the order of electrons release from different sites of N2Ar. Strong laser fields effectively lead to multiple ionization of atoms, molecules, and clusters, which, in turn, results in multiple bond breakage, fragmentation, and Coulomb explosion. The microscopic mechanisms of this multiple laser matter interaction have mainly been studied in two size regimes: atoms and small linear molecules, for which the individual sequential ionization steps determine the physics, and extended systems, which are governed by collective effects. One key ingredient determining the order of ionization events is the binding energy, and a second is the geometry. As the size of the system increases, the ionization potentials for different orbitals and sites become more and more degenerate and the geometry plays an increasingly important role. Two elements of structure have been identified to govern strong field ionization. First, the shape of the orbital strongly influences the ionization probability. This is best established for molecules where the single ionization rate has been found to strongly depend on the angle between the polarization vector of the laser field and the molecular axis, as numerically predicted by the strong field approximation [1] and molecular Ammosov-Delone-Krainov tunneling rate [2], and experimentally demonstrated by probing the profiles [3–5] and the dipole moments [6 ]o f the ionizing orbitals. Second, the multiple ionization rate is found to be strongly enhanced for a molecule oriented along the laser field when the bond stretches to a critical length [7]. Under these conditions a localization of the charge on one site [8,9] and joint action of this charge center together with the light field boost the ionization probability, which is alternatively understood as the charge-resonance enhanced ionization [10,11]. This enhanced ionization scenario has been probed by the directional emission of the fragment ions [12], or the electron with respect to the field vector and molecular
TL;DR: It is demonstrated that the vibrational nuclear motion of singly ionized argon dimers can be controlled with two ultrashort laser pulses of different wavelengths, validating its interpretation as a pump-pulse-initiated population transfer between dipole-coupled Born-Oppenheimer electronic states of the dissociating Ar(2)(+) molecular ion.
Abstract: We demonstrate that the vibrational nuclear motion of singly ionized argon dimers can be controlled with two ultrashort laser pulses of different wavelengths. In particular, we observe a striking ‘‘gap’’ in the pump-probe-delay-dependent kinetic-energy release spectrum only if the probe-pulse wavelength exceeds the pump-pulse wavelength. This ‘‘frustrated dissociation effect’’ is reproduced by our two-state quantum mechanical model, validating its interpretation as a pump-pulse-initiated population transfer between dipole-coupled Born-Oppenheimer electronic states of the dissociating Ar 2 þ molecular ion. Our numerical results also reproduce the measured collapse and fractional revival of the oscillating Ar 2 þ nuclear wave packet, and, for single-pulse dissociation, the decrease of the kinetic-energy release with increasing laser wavelength.
TL;DR: In this paper, the authors present kinematically complete theoretical calculations and experiments for transfer ionization in H{}+}+\phantom{\rule{0.16em}{0ex}}$He collisions at 630 keV/u.
Abstract: We present kinematically complete theoretical calculations and experiments for transfer ionization in H${}^{+}+\phantom{\rule{0.16em}{0ex}}$He collisions at 630 keV/u. Experiment and theory are compared on the most detailed level of fully differential cross sections in the momentum space. This allows us to unambiguously identify contributions from the shake-off and binary encounter mechanisms of the reaction. It is shown that the simultaneous electron transfer and ionization is highly sensitive to the quality of a trial initial-state wave function.