D. Feldmann

IR‐MPE/D dynamics studied by pulsed VUV laser photoionization

One‐photon ionization by a pulsed VUV laser has been employed to probe infrared multiphoton excitation and dissociation (IR‐MPE/D) dynamics in real time, in situ, and collision free during excitation by a CO2− laser pulse. In this first experiment acetic anhydride has been used as the parent molecule to demonstrate the feasibility and to study properties of the method. Using the 118.4 nm radiation from a VUV laser as source for photoionization, ions have been observed mass selectively with time resolution in the nanosecond range, determined by the laser pulse duration. By following production and decay rates of ions, the time dependence of the excitation and decay of parent molecules, and the evolution of neutral intermediates in primary and sequential steps could be observed, from which detailed information on excitation and dissociation pathways was deduced.

Multiphoton-ionization of hydrogen atoms in intense laserfields

The interaction of hydrogen atoms with strong laser fields at intensities up to some 1013 W cm−2 was studied experimentally at the wavelengths λ=355 nm, 532 nm and 1064 nm. The ion yield, the energy spectrum of the photoelectrons and their angular distributions were measured. The angular distributions at λ=355 nm and λ=532 nm provide a sensitive test for theoretical calculations. Comparison with the calculations available shows that perturbation theory with proper inclusion of atomic structure yields results which agree with experiment. Intensity dependent changes of angular distributions at λ=532 nm are observed, which indicate that at 1013 W cm−2 higher order processes become noticable. At λ=1064 nm the situation is more complicated, experimentally as well as theoretically. Intensities of some 1013 W cm−2 are necessary to observe ionization. Strong distortions of the atomic structure can be expected. Presently only qualitative aspects of the angular distributions can be discussed.

Resonances in the multiphoton ionisation spectrum of sodium atoms induced by a second strong laser field

The high-intensity (about 1010 W cm-2) multiphoton ionisation spectrum of sodium atoms was measured under atomic beam conditions in the presence of a second intense laser field (about 1010 W cm-2). In addition to the expected two-photon resonances in the three-photon ionisation spectrum, peaks are observed which can be explained as resonantly enhanced five-photon processes.

The energy spectrum of photoelectrons produced by multiphoton ionization of atomic hydrogen

The authors present a comparison of theoretical and experimental data for the first three above-threshold ionization channels of the photoelectron kinetic energy spectrum of atomic hydrogen when the atom is irradiated by a short pulse of linearly polarized light of wavelength 608 nm and peak intensity 6*1013 W cm-2 or 1.2*1014 W cm-2.

The angular distribution of photoelectrons from the three-photon ionisation of xenon in the region of autoionising resonances

Although the structure in the total three-photon ionisation cross section around an autoionising resonance at 13.134 eV predicted by a recent calculation has not been observed, the angular distribution of photoelectrons changes considerably at the corresponding wavelength.

Collisional redistribution of vibrational energy in CO2‐laser‐excited benzene

We have monitored changes in the population of benzene vibrational levels during and after irradiation with an intense CO2‐laser pulse. Our method was to induce fluorescence from the first excited singlet state (1B2u) by absorption of tunable ultraviolet radiation. By varying the delay time between the infrared and ultraviolet pulses and by employing ultraviolet frequencies within many bands of the 1A1g–1B2u system, we measured the temporal evolution of the vibrational energy distribution over 12 benzene levels. We also used the technique to assign bands of the ultraviolet absorption spectrum that involve vibrational levels pumped by the infrared laser.

Resonant multiphoton ionization of xenon in intense sub-ps-laser pulses

We observe that resonant ionization plays a dominant role in the multiphoton ionization (MPI) of xenon at wavelengths around 600 nm, irrespective of the detuning of multiples of the photon energy from excited states of the undisturbed atom. Angular distributions of energy resolved photoelectrons allow to identify excited states which are strongly ac-Stark shifted in the intense laser field and which serve as intermediate resonances in MPI processes. Angular distributions measured at different wavelengths show that the ac-Stark shift of excited states can be larger than the photon energyhω. Our results support the model proposed by Freeman et al.

Angular distribution of photo electrons from above-threshold-ionization (ATI) of xenon by 532 nm, 355 nm and 266 nm radiation

A time-of-flight electron energy spectrometer has been used to measure the angular distributions of photoelectrons emitted after the absorption of up to four excess photons above the ionization threshold of Xenon at 532 nm. For shorter wavelength less efficient ATI is observed. The shape of the angular distributions and the branching ratios for the two ionic fine structure states Xe+(2P3/2) and Xe+(2P1/2) may be plausibly attributed to the influence of excited states of the atom.

Multiphoton Ionization of Atomic Hydrogen in Intense Laser Pulses: The Role of Rydberg States

Multiphoton ionization (MPI) of atomic hydrogen by intense laser radiation can serve as the simplest test for comparison between experiment and theory. We wish to describe the present state of knowledge.

Avoided crossing of resonantly coupled excited states

The strong radiative single photon coupling between two excited states of sodium atoms (3p and 4d) has been studied at high intensities, exceeding 109 W cm-2. These excited states are probed by Raman-like resonant five-photon ionization processes. Using two tunable lasers the authors have observed that the doubly resonant process does not occur. In the model picture of photon-dressed atomic levels, this situation can be described as an avoided crossing, with a large minimum separation of 30 cm-1.