H. Rottke

Evolution of angular distributions in two-colour, few-photon ionization of helium

Single ionization of helium by a superposition of selected XUV high harmonics and infrared radiation has been studied by a momentum imaging technique. The measured angular distributions of photoelectrons are compared to numerical time-dependent calculations, showing very good agreement after average. The calculated angular distributions appear to depend critically on the delay between harmonic and infrared pulses on the attosecond scale, and on the relative phases and intensities of the harmonics.

Multiphoton ionization and dissociation of

We investigate multiphoton ionization (MPI) of at 532 nm excitation wavelength in an intensity range of -. We find strong fragmentation of the molecule with formation of , , , and as charged dissociation products. Photoelectron spectra show that the charged products originate in photodissociation of . Ionic dissociation starts after excitation of to the state and probably high vibrational levels of the bending mode in . Symmetric is significantly less probable than asymmetric dissociation. MPI of mainly populates ionic X (000) and (100) vibronic states below light intensity. At high intensity also B vibronic states seem to become directly populated with rising probability. All lowest-order MPI electron groups are accompanied by at least one above-threshold ionization group. A sensitive dependence of the photoelectron spectra on light intensity may point to bond-angle softening of low-lying X vibronic states of which is expected to be brought about by a favourable resonant one-photon coupling between the X and A electronic states.

Two-photon dissociative photoionization and photodissociation of single rotational states

We report on an experimental study of dissociative photoionization and neutral dissociation of by two-photon excitation from single rotational states (photon energy ). The B-state rovibrational levels are excited in the same light pulse by three-photon absorption from . Angle resolved photoelectron and ion kinetic energy distributions allow us to discriminate between the different decay channels. Neutral decay is detected by one-photon ionization of excited atomic hydrogen product states. The kinetic energy distribution of photoelectrons from dissociative photoionization shows an oscillatory structure which depends on the initial B-state rotational quantum number J. An indication of a corresponding structure in the ion kinetic energy distributions is only found for J = 0. Similar to results reported by Verschuur et al we find neutral decay preferentially into high principal quantum number atomic states. The results indicate that the doubly excited states play a crucial role in the decay processes.

Non-sequential double ionization of Ne in intense laser pulses: a coincidence experiment

The dynamics of Neon double ionization by 25 fs, 1.0 PW/cm 2 laser pulses at 795 nm has been studied in a many particle coincidence experiment. The momentum vectors of all ejected atomic fragments (electrons and ions) have been measured using combined electron and recoil-ion momentum spectroscopy. Electron emission spectra for double and single ionization will be discussed. In both processes the mean electron energies differ considerably and high energetic electrons with energies of more than 120 eV have been observed for double ionization. The experimental results are in qualitative agreement with the rescattering model.

Reaction microscope study of near-threshold photo double ionization of xenon using high harmonics

Photo double ionization of xenon in the vicinity of the threshold is investigated using high harmonics and a reaction microscope. Electron momentum and energy distributions, β-asymmetry parameters and momentum correlation are measured at 32.4 nm. The results agree with previous measurements where available. The advantages and limits of the method are discussed.

photodissociation in an intense infrared light field

The first experiment on infrared (IR) multiphoton excitation of with single rotational levels as initial states is reported. These states are prepared by one-photon excitation from the molecular ground state. We investigate the and ion yield, the dissociation fraction and the ion kinetic energy distribution as a function of the IR light intensity in the range from up to . The dissociation fraction depends sensitively on the initial rotational state. Transient resonances induced by AC Stark shifting states seem to influence the branching ratio between photoionization and dissociation. The photoion kinetic energy distributions show that the molecule dissociates after three-photon absorption into . The atoms are then photoionized. The AC Stark shift of the final dissociation product states in the IR light field seems to be responsible for a small intensity-dependent shift of the kinetic energy distribution.

Photoionization in combined ultra short XUV and infrared laser pulses

Multi-photon-double ionization of xenon by Ti:Sapphire laser pulses combined with their 25th harmonic has been studied by means of a momentum imaging spectrometer. The determination of the momenta of the emitted photoelectron pair and of its energy and angular correlation gives insight into the various mechanisms leading to double ionization. Although the conditions for non sequential direct multi-photon double ionization are met in the experiment, it is found that two-step sequential processes prevail.

High Resolution Investigation of H2 + Exploding in a High Intensity Ultrashort Laser Pulse

We investigate for the first time high resolution H+, D+ ion kinetic energy distributions from dissociation of H2 + and D2 + ions. Dissociation is initiated by ultrashort light pulses (∼ 70 fsec) at 790 nm and light intensities up to 1015 W/cm2. High resolution is achieved in this experiment by using a translationally cold beam of H2/D2 produced in a supersonic jet expansion.