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We study the decay of a helium/neon dimer after ionization and simultaneous excitation of either the neon or the helium atom using Cold Target Recoil Ion Momentum Spectroscopy (COLTRIMS). We find that, depending on the decaying state, either direct Interatomic Coulombic Decay (ICD) (i.e. mediated by a virtual photon exchange), exchange ICD (mediated by electron exchange) or radiative charge transfer occurs. The corresponding channels are identified.

Laser-driven electron recollision is at the heart of the rapidly growing field of attosecond science. The recollision wavepacket is qualitatively described within the strong-field approximation, which commonly assumes tunnelling ionization and plane-wave propagation of the liberated electron in the continuum. However, with increasing experimental sophistication, refinements to this simple model have become necessary. Through careful modelling and measurements of laser-induced recollision holography using aligned N 2 molecules, we demonstrate that the continuum electron wavepacket already carries a non-trivial spatial phase structure immediately following ionization. This effect is of rather general character: any molecule and any non-isotropic system that is ionized by a strong laser field will exhibit an offset in the phase of the continuum electron wavepacket. Specifically, this has important implications for any coherent scattering process in molecules, such as high-harmonic generation or laser-induced electron holography.

Analysis of the electrons ionized from water dimers suggests that the energy absorbed by one molecule is rapidly transmitted to the second molecule from which the electron is ejected. This process, referred to as intermolecular Coulombic decay, is a qualitatively different source of low-energy electrons to conventional direct ionization processes. At the transition from the gas to the liquid phase of water, a wealth of new phenomena emerge, which are absent for isolated H2O molecules. Many of those are important for the existence of life, for astrophysics and atmospheric science. In particular, the response to electronic excitation changes completely as more degrees of freedom become available. Here we report the direct observation of an ultrafast transfer of energy across the hydrogen bridge in (H2O)2 (a so-called water dimer). This intermolecular coulombic decay leads to an ejection of a low-energy electron from the molecular neighbour of the initially excited molecule. We observe that this decay is faster than the proton transfer that is usually a prominent pathway in the case of electronic excitation of small water clusters and leads to dissociation of the water dimer into two H2O+ ions. As electrons of low energy (∼0.7–20 eV) have recently been found to efficiently break-up DNA constituents1,2, the observed decay channel might contribute as a source of electrons that can cause radiation damage in biological matter.

We study electron correlation in sequential double ionization of noble gas atoms and HCl in intense, femtosecond laser pulses. We measure the photoelectron angular distributions of ${\mathrm{Ne}}^{+}$ relative to the first electron in a pump-probe experiment with 8 fs, 800 nm, circularly polarized laser pulses at a peak intensity of a few ${10}^{15}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. Using a linear-linear pump-probe setup, we further study He, Ar, and HCl. We find a clear angular correlation between the two ionization steps in the sequential double ionization intensity regime.

We coincidently measure the molecular frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role., This paper has been accepted for publication by Physical Review Letters

We measure the angular distribution of an electron emitted by a strong elliptically polarized two-color laser field from exploding doubly charged molecular nitrogen. This angular distribution is vastly different for emission of the electron from the up-field core of the molecule as compared to that from the down-field core. The emission from the down-field core leads to a slight rotation with respect to the internuclear axis in the direction expected by the Coulomb effect of the remaining ion, while, for the emission from the up-field core, this direction is inversed. Our semiclassical simulations suggest that this unexpected angular distribution is caused by an initial longitudinal momentum of the electron freed by over-the-barrier ionization above the inner barrier in the molecule. The initial kinetic energy is in the range of the potential energy of the Stark-shifted orbital above the barrier.

We observe multiply frustrated tunneling ionization-induced dissociation of the argon dimers by intense linearly polarized ultrashort laser pulses. By measuring the kinetic energy release and angular distribution of the Coulomb explosion of up to eightfold ionized argon dimers, we can trace the recapture of up to two electrons to Rydberg states of the highly charged compound at the end of the laser pulse. Upon dissociation of the dimer, the Rydberg electron prefers to localize at the atomic ion with the higher charge state. We probe the electron recapture dynamics by a time-delayed weak pulse.

We doubly ionize H(2)O by single photon absorption at 43 eV leading to H(+) + OH(+). A direct double ionization and a sequential process in which single ionization is followed by rapid dissociation into a proton and an autoionizing OH(*) are identified. The angular distribution of this delayed autoionization electron shows a preferred emission in the direction of the emitted proton. From this diffraction feature we obtain internuclear distances of 700 to 1100 a.u. at which the autoionization of the OH(*) occurs. The experimental findings are in line with calculations of the excited potential energy surfaces and their lifetimes.

We propose a new approach to obtain molecular frame photoelectron angular distributions from molecules ionized by intense laser pulses. With our method we study the angular tunnel ionization probability of H2 at a wavelength of 800 nm over an intensity range of 2-4.5 x 10(14) W/cm2. We find an anisotropy that is stronger than predicted by any existing model. To explain the observed anisotropy and its strong intensity dependence we develop an analytical model in the framework of the strong-field approximation. It expresses molecular ionization as a product of atomic ionization rate and a Fourier transform of the highest occupied molecular orbital filtered by the strong-field ionization process.

We have investigated the correlated momentum distribution of both electrons from nonsequential double ionization of helium in a 800 nm, 4.5×1014W/cm2 laser field. Using very high resolution coincidence techniques, we find a so-far unobserved fingerlike structure in the correlated electron momentum distribution. The structure can be interpreted as a signature of the microscopic dynamics in the recollision process. We identify features corresponding to the binary and recoil lobe in field-free (e,2e) collisions. This interpretation is supported by analyzing ab initio solutions of a fully correlated three-dimensional helium model. © 2007 The American Physical Society.

Using H2+ and D2+, we observe two-surface population dynamics by measuring the kinetic energy of the correlated ions that are created when H2+ (D2+) ionize in short (40-140 fs) and intense (10(14) W/cm2) infrared laser pulses. Experimentally, we find a modulation of the kinetic energy spectrum of the correlated fragments. The spectral progression arises from a hitherto unexpected spatial modulation on the excited state population, revealed by Coulomb explosion. By solving the two-level time-dependent Schrödinger equation, we show that an interference between the net-two-photon and the one-photon transition creates localized electrons which subsequently ionize.

We have investigated the full three-dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the recollision energy is of the order of the ionization potential. The momentum correlation in the direction perpendicular to the laser field depends on the time difference between the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize., Photonics North 2004: International Conference on Applications of Photonic Technology, ICAPT, September 26-29, 2004, Ottawa, ON, Canada, Series: Proceedings of SPIE