Waveform control of orientation-dependent ionization of DCl in few-cycle laser fields.

Strong few-cycle light fields with stable electric field waveforms allow controlling electrons on time scales down to the attosecond domain. We have studied the dissociative ionization of randomly oriented DCl in 5 fs light fields at 720 nm in the tunneling regime. Momentum distributions of D(+) and Cl(+) fragments were recorded via velocity-map imaging. A waveform-dependent anti-correlated directional emission of D(+) and Cl(+) fragments is observed. Comparison of our results with calculations indicates that tailoring of the light field via the carrier envelope phase permits the control over the orientation of DCl(+) and in turn the directional emission of charged fragments upon the breakup of the molecular ion.

Phase control of molecular fragmentation with a pair of femtosecond-laser pulses.

We demonstrate the control of molecular fragmentation of o-xylene (C(8)H(10)) on a femtosecond time scale in two-pulse measurements with a pair of femtosecond-laser pulses. Parent and fragment-ion yields were recorded as a function of interpulse delays, i.e., different relative phases of the excitation pulses. The experiments revealed different fragmentation mechanisms in the temporal region of direct overlapping pulses and for separated pulses. For overlapping pulses all ion yields followed the excitation intensity which oscillated as a function of interpulse delay due to the change of constructive and destructive interference of the light fields. For larger delays, in particular, the oscillations of the C(+) and CH(3) (+) fragment-ion yield showed a significant deviation from each other. The results are interpreted as a manifestation of optical phase-dependent electronic excitations mapped onto the nuclear fragmentation dynamics.

Atomic dynamics in single and multi-photon double ionization: An experimental comparison.

We have used a multi-particle imaging technique (COLTRIMS) to observe the double ionization of rare gas atoms by multi-photon absorption of 800 nm (1.5 eV) femto-second laser pulses and by single photon absorption (synchrotron radiation). Both processes are mediated by electron correlation. We discuss similarities and differences in the three-body final state momentum distributions.

Recoil-Ion momentum distributions for single and double ionization of helium in strong laser fields

We have measured the momentum distributions of singly and doubly charged helium ions created in the focus of 220 fs, 800 nm laser pulses at intensities of (2.9-6.6)x10(14) W/cm(2). All ions are emitted strongly aligned along the direction of polarization of the light. We find the typical momenta of the He2+ ions to be 5-10 times larger than those of the He1+ ions and a two peak structure at the highest intensity.

Correlated electron emission in multiphoton double ionization

Electronic correlations govern the dynamics of many phenomena in nature, such as chemical reactions and solid state effects, including superconductivity. Such correlation effects can be most clearly investigated in processes involving single atoms. In particular, the emission of two electrons from an atom--induced by the impact of a single photon, a charged particle or by a short laser pulse--has become the standard process for studies of dynamical electron correlations. Atoms and molecules exposed to laser fields that are comparable in intensity to the nuclear fields have extremely high probabilities for double ionization; this has been attributed to electron-electron interaction. Here we report a strong correlation between the magnitude and the direction of the momentum of two electrons that are emitted from an argon atom, driven by a femtosecond laser pulse (at 38 TW cm(-2)). Increasing the laser intensity causes the momentum correlation between the electrons to be lost, implying that a transition in the laser-atom coupling mechanism takes place.

Polarized photoluminescence and spectral narrowing in an oriented polyfluorene thin film.

The oriented polyfluorene 1 is a promising candidate for a blue polarized emitter in organic optoelectronic devices. By examination of the anisotropic luminescence of thin films of 1, polarization-dependent spectral narrowing was found at high excitation densities, which is of particular interest in reducing the threshold for optically pumped polymer lasers. Femtosecond pump­probe experiments show optical gain at this spectral position.