Ultrafast extreme ultraviolet induced isomerization of acetylene cations.

Ultrafast isomerization of acetylene cations ([HC=CH](+)) in the low-lying excited A(2)Σ(g)(+) state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments C(+) + CH2(+) as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of 52±15 fs in a kinetic energy release (KER) window of 5.8

Three-dimensional momentum imaging of electron wave packet interference in few-cycle laser pulses.

Using a reaction microscope, three-dimensional (3D) electron (and ion) momentum (P) spectra have been recorded for carrier-envelope-phase (CEP) stabilized few-cycle ( approximately 5 fs), intense ( approximately 4 x 10(14) W/cm2) laser pulses (740 nm) impinging on He. Preferential emission of low-energy electrons (E(e)<15 eV) to either hemisphere is observed as a function of the CEP. Clear interference patterns emerge in P space at CEPs with maximum asymmetry, interpreted as attosecond interferences of rescattered and directly emitted electron wave packets by means of a simple model.

Attosecond real-time observation of electron tunnelling in atoms.

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

Pulse compression with time-domain optimized chirped mirrors.

Dispersive optical interference coatings (chirped mirrors - CMs) are designed by computer optimization of an analytically calculated initial multilayer. Traditionally, the relevant properties of the CM (reflectance and the frequency-dependence of the phase shift upon reflection) are optimized to match frequency-domain targets. We propose a novel target function that quantifies directly the capability of a multilayer to control the temporal shape of the reflected optical pulse. Employing this time-domain analysis/optimization one can design dispersive multilayers having air as medium of incidence and supporting the generation of pulses with durations in the sub-5-fs-range, as demonstrated in a proof-of-principle compression experiment.

Gouy phase shift for few-cycle laser pulses.

We measured for the first time the influence of the Gouy effect on focused few-cycle laser pulses. The carrier-envelope phase is shown to undergo a smooth variation over a few Rayleigh distances. This result is of critical importance for any application of ultrashort laser pulses, including high-harmonic and attosecond pulse generation, as well as phase-dependent effects.

Nonsequential double ionization at the single-optical-cycle limit.

We report differential measurements of Ar++ ion momentum distributions from nonsequential double ionization in phase-stabilized few-cycle laser pulses. The distributions depend strongly on the carrier-envelope (CE) phase. Via control over the CE phase one is able to direct the nonsequential double-ionization dynamics. Data analysis through a classical model calculation reveals that the influence of the optical phase enters via (i) the cycle dependent electric field ionization rate, (ii) the electron recollision time, and (iii) the accessible phase space for inelastic collisions. Our model indicates that the combination of these effects allows a look into single cycle dynamics already for few-cycle pulses.

Measurement of the phase of few-cycle laser pulses.

For the shortest pulses generated to date, the amplitude of the electromagnetic wave changes almost as rapidly as the field oscillates. The temporal variation of the field, which directly governs strong-field interactions, therefore depends on whether the maximum of the pulse amplitude coincides with that of the wave cycle or not, i.e., on the phase of the field with respect to the pulse envelope. It is demonstrated that the direction of electron emission from photoionized atoms can be controlled by varying the phase of the field, providing for the first time a tool for its accurate determination. Directing fast electron emission to the right or to the left with the light phase constitutes a new kind of coherent control.

A B-TOF mass spectrometer for the analysis of ions with extreme high start-up energies.

Weak magnetic deflection is combined with two acceleration stage time-of-flight mass spectrometry and subsequent position-sensitive ion detection. The experimental method, called B-TOF mass spectrometry, is described with respect to its theoretical background and some experimental results. It is demonstrated that the technique has distinct advantages over other approaches, with special respect to the identification and analysis of very highly energetic ions with an initially large energy broadening (up to 1 MeV) and with high charge states (up to 30+). Similar energetic targets are a common case in intense laser-matter interaction processes found during laser ablation, laser-cluster and laser-molecule interaction and fast particle and x-ray generation from laser-heated plasma.

Total, partial, and electron-capture cross sections for ionization of water vapor by 20-150 keV protons.

We present experimental results for proton ionization of water molecules based on a novel event by event analysis of the different ions produced (and lost). We are able to obtain mass analyzed product ion signals (e.g., H2O+, OH+, O+, O++, H+) in coincidence with the projectile analyzed after the collision, i.e., either being H+, neutral H after single electron capture during the ionization event, or H- after double electron capture. After proper calibration we are thus able to determine a complete set of cross sections for the ionization of a molecular target by protons including the total and the partial cross sections and in addition also the direct ionization and the electron capture cross sections.

Operating principle of an electron monochromator in an axial magnetic field.

Electron monochromators which are operated within an axial magnetic (guiding) field are especially suitable for the production of monochromatic electrons at low energies. Although in principle the technology of such devices has an appreciable historic background, we have discovered experimentally important new features, which cannot be understood using the previously published theories of operation. An in-depth study of the electron trajectories in a crossed electric and magnetic field using Simion1 showed a number of possible pitfalls, which have to be avoided in construction and operation. From our simulations we derived a novel design and operational method, which is currently under evaluation. We have already demonstrated that using this novel design an electron energy resolution of about 50 meV is realistic.

Nonadiabatic Multielectron Dynamics in Strong Field Molecular Ionization.

We report the observation of a general strong field ionization mechanism due to highly nonadiabatic multielectron excitation dynamics in polyatomic molecules. We observe that such excitation mechanisms greatly affect molecular ionization, fragmentation, and energetics. We characterized this phenomenon as a function of optical frequency, intensity, and molecular properties.