Oleg I. Tolstikhin

Measurement and laser control of attosecond charge migration in ionized iodoacetylene.

Electronic movement flashing into view Numerous chemical processes begin with ionization: the ejection of an electron from a molecule. What happens in the immediate aftermath of that event? Kraus et al. explored this question in iodoacetylene by detecting and analyzing the spectrum of emitted high harmonics (see the Perspective by Ueda). They traced the migration of the residual positively charged hole along the molecular axis on a time scale faster than a quadrillionth of a second. They thereby characterized the capacity of a laser field to steer the holes motion in appropriately oriented molecules. Science, this issue p. 790; see also p. 740 High harmonics reveal fine details of electronic rearrangement in a molecule in the first instants after ionization. [Also see Perspective by Ueda] The ultrafast motion of electrons and holes after light-matter interaction is fundamental to a broad range of chemical and biophysical processes. We advanced high-harmonic spectroscopy to resolve spatially and temporally the migration of an electron hole immediately after ionization of iodoacetylene while simultaneously demonstrating extensive control over the process. A multidimensional approach, based on the measurement and accurate theoretical description of both even and odd harmonic orders, enabled us to reconstruct both quantum amplitudes and phases of the electronic states with a resolution of ~100 attoseconds. We separately reconstructed quasi–field-free and laser-controlled charge migration as a function of the spatial orientation of the molecule and determined the shape of the hole created by ionization. Our technique opens the prospect of laser control over electronic primary processes.

`Slow' variable discretization: a novel approach for Hamiltonians allowing adiabatic separation of variables

We propose a novel approach for Hamiltonians allowing adiabatic separation of variables. Our method is based on the assumption of smoothness of the motion associated with the adiabatic variable instead of its slowness, which is assumed traditionally. Convergence in terms of the number of coupled channels in our method corresponds to the standard adiabatic expansion. However, neither laborious calculations of non-adiabatic couplings nor a priori information on locations of avoided crossings are required. The method is illustrated by calculating bound-state energies for several three-body Coulomb systems for states with zero total angular momentum.

Hyperspherical elliptic coordinates for the theory of light atom transfer reactions in atom-diatom collisions

We formulate and demonstrate a new method for quantum 3D calculations of light atom transfer reactions in atom-diatom collisions. The method follows a general scheme of the hyperspherical method, in common with other hyperspherical formulations in the field. The main novelty consists in the hyperspherical elliptic coordinates (ξ,η) used to parametrize the hypersphere. These coordinates have been introduced recently for studying three-body Coulomb systems, and here we apply them to study a system of three atoms. The coordinates are defined and their relation with the Smith-Whitten and Delves coordinates is explored. On account of a big difference between vibrational and rotational excitation energies in molecules, the hyperspherical adiabatic Hamiltonian allows adiabatic separation between ξ and η. This not only greatly facilitates solution of the hyperspherical adiabatic eigenvalue problem, but also provides an approximate classification of the states by a pair of indices (nξ,nη) representing vibrational ...

Quantum mechanical elucidation of reaction mechanisms of heavy-light-heavy systems: Role of potential ridge

A new idea to elucidate quantum reaction dynamics of heavy-light-heavy (HLH) systems is proposed on the basis of the hyperspherical elliptic coordinate approach. This coordinate system has a big advantage of nicely expressing good vibrational adiabaticity in the HLH systems. Taking this advantage, the concept of potential ridge is introduced, for the first time, in three-dimensional reactions. The potential ridge is proved to be very useful to extract some important avoided crossings which dominate the reaction dynamics. In fact, qualitative features of the reaction dynamics can be interpreted in terms of nonadiabatic transitions at those important avoided crossings near the potential ridge. Examples are: (i) onset of reaction for a specified initial rotational state, and (ii) major reactive transition for a specified initial rotational state. Avoided crossings to the left of the potential ridge are also useful to interpret certain aspects of reactions accompanying vibrational transitions. The new idea me...

Observation of laser-induced electronic structure in oriented polyatomic molecules

All attosecond time-resolved measurements have so far relied on the use of intense near-infrared laser pulses. In particular, attosecond streaking, laser-induced electron diffraction and high-harmonic generation all make use of non-perturbative light–matter interactions. Remarkably, the effect of the strong laser field on the studied sample has often been neglected in previous studies. Here we use high-harmonic spectroscopy to measure laser-induced modifications of the electronic structure of molecules. We study high-harmonic spectra of spatially oriented CH3F and CH3Br as generic examples of polar polyatomic molecules. We accurately measure intensity ratios of even and odd-harmonic orders, and of the emission from aligned and unaligned molecules. We show that these robust observables reveal a substantial modification of the molecular electronic structure by the external laser field. Our insights offer new challenges and opportunities for a range of emerging strong-field attosecond spectroscopies.

Role of Multi-Electron Effects in the Asymmetry of Strong-Field Ionization and Fragmentation of Polar Molecules: The Methyl Halide Series.

We report angle- and momentum-resolved measurements of the dissociative ionization and Coulomb explosion of methyl halides (CH3F, CH3Cl, CH3Br, and CH3I) in intense phase-controlled two-color laser fields. At moderate laser intensities, we find that the emission asymmetry of low-energy CH3(+) fragments from the CH3(+) + X(+) (X = F, Cl, Br, or I) channel reflects the asymmetry of the highest occupied molecular orbital of the neutral molecule with important contributions from the Stark effect. This asymmetry is correctly predicted by the weak-field asymptotic theory, provided that the Stark effect on the ionization potentials is calculated using a nonperturbative multielectron approach. In the case of high laser intensities, we observe a reversal of the emission asymmetries for high-energy CH3(+) fragments, originating from the dissociation of CH3X(q+) with q ≥ 2. We propose ionization to electronically excited states to be at the origin of the reversed asymmetries. We also report the measurements of the emission asymmetry of H3(+), which is found to be identical to that of the low-energy CH3(+) fragments measured at moderate laser intensities. All observed fragmentation channels are assigned with the help of CCSD(T) calculations. Our results provide a benchmark for theories of strong-field processes and demonstrate the importance of multielectron effects in new aspects of the molecular response to intense laser fields.

Atomic Siegert states in an electric field: Transverse momentum distribution of the ionized electrons

The Siegert states of atoms in a static uniform electric field, defined as the solutions to the stationary Schroedinger equation satisfying the regularity and outgoing-wave boundary conditions, are discussed. An efficient method to calculate not only the complex energy eigenvalue, but also the eigenfunction for a general class of one-electron atomic potentials is introduced. An exact expression for the transverse momentum distribution of the ionized electrons in terms of the Siegert eigenfunction in the asymptotic region is derived. The method is illustrated by calculations of the energy, ionization width, and transverse momentum distribution as functions of the electric field for several lowest states of H, outer p shells of Ne, Ar, Kr, and Xe, and the active electron in H{sup -}. We also discuss the ionization of Ar by the pulse of a unidirectional time-dependent electric field, which illustrates the role of the Siegert states in the recently developed adiabatic theory of ionization of atoms by intense laser pulses [O. I. Tolstikhin et al., Phys. Rev. A 81, 033415 (2010)].

Theory of tunneling ionization of molecules: Weak-field asymptotics including dipole effects

The formulation of the parabolic adiabatic expansion approach to the problem of ionization of atomic systems in a static electric field, originally developed for the axially symmetric case [Phys. Rev. A 82, 023416 (2010)], is generalized to arbitrary potentials. This approach is used to rederive the asymptotic theory of tunneling ionization in the weak-field limit. In the atomic case, the resulting formulas for the ionization rate coincide with previously known results. In addition, the present theory accounts for the possible existence of a permanent dipole moment of the unperturbed system and, hence, applies to polar molecules. Accounting for dipole effects constitutes an important difference of the present theory from the so-called molecular Ammosov-Delone-Krainov theory. The theory is illustrated by comparing exact and asymptotic results for a set of model polar molecules and a realistic molecular ion HeH{sup 2+} in the 2p{sigma} state.

Extraction of electron?ion differential scattering cross sections for C2H4 by laser-induced rescattering photoelectron spectroscopy

We have measured angle-resolved rescattering electron momentum distributions for C2H4 generated by intense infrared laser pulses and extracted large-angle elastic differential cross sections (DCSs) for electrons scattering from C2H4+. The angle-dependent ionization rate describing the initial single-ionization step favours ionization from molecules that have their molecular plane aligned perpendicular to the laser polarization direction. The extracted DCSs are well reproduced by the theoretical calculation. We demonstrate that DCSs for electron–ion scattering of the nonlinear polyatomic molecule can be extracted from the laser-induced rescattering electron spectra.

Weak-field asymptotic theory of tunneling ionization: benchmark analytical results for two-electron atoms

The recently developed many-electron weak-field asymptotic theory of tunneling ionization of atoms and molecules in an external static electric field (Tolstikhin et al 2014, Phys. Rev. A 89, 013421) is extended to the first-order terms in the asymptotic expansion in field. To highlight the results, here we present a simple analytical formula giving the rate of tunneling ionization of two-electron atoms H− and He. Comparison with fully-correlated ab initio calculations available for these systems shows that the first-order theory works quantitatively in a wide range of fields up to the onset of over-the-barrier ionization and hence is expected to find numerous applications in strong-field physics.