Femtosecond dynamics of multielectron dissociative ionization by use of a picosecond laser

Abstract

The multiple ionization of molecular nitrogen has been studied by use of an intense picosecond laser (0.6 ps; 3x10'^ W/cm^; wavelength 600 nm). By measurement of the energies of the various fragment atomic ions (N"^, N^"^, and N^^) with a time-of-flight mass spectrometer it has proved possible to investigate the dynamics of the multielectron dissociative ionization process on a time scale of about 30 fs.PACS numbers: 33.80. Eh, 33.80.Rv We wish to report preliminary results of an experiment on the multiphoton ionization of molecular nitrogen. However, we do not wish to emphasize the multiphoton aspect of the multiple ionization processes observed, ^ but prefer to describe the laser in terms of a classical electromagnetic field. The mechanism involved is thought to be similar to that invoked in order to explain the field ionization of Rydberg atoms, ^ but in this case the laser field is so intense that it produces a potential difference along the molecular axis comparable with the binding energy of the outer electrons in the ground state.The idea behind the experiment is quite straightforward. A diatomic molecule is subjected to a large electric field created by a focused picosecond laser. As -^e molecule is ionized beyond the single-ion stage, the Tragment atomic ions mutually repel in the Coulomb field and the resulting energetic ions are detected by a timeof-flight (TOF) mass spectrometer. If additional electrons are stripped away as the molecule dissociates, the ions are now subjected to an even larger Coulomb repulsion. They will gain more energy and this will be reflected in the TOF spectrum. Thus, the detailed dynamics of multielectron dissociative ionization (MEDI) can be investigated on a time scale oi femtoseconds (the dissociation time scale), even though the laser pulse itself is of picosecond duration.The basic components of the experiment were the following. A dye laser was pumped by a mode-locked, frequency-doubled neodymium-doped yttrium-aluminum-garnet laser, producing 2.5-ps pulses (0.6-ps pulses with a saturable absorber in the dye). These individual pulses were amplified in a dye amplifier driven by a Qswitched frequency-doubled neodymium-doped yttriumaluminum-garnet laser operating at 10 Hz.

Ultrafast electronic spectroscopy for chemical analysis near liquid water interfaces: concepts and applications

“…Moiré deflectometry has been used to determine the density profile in the plasma channel and its lateral expansion [79]. Spectrometry of ions emitted from the plasma has yielded information on ion velocity and temperature [80].…”

Role of proton dynamics in efficient photoionization of hydrocarbon molecules

“…One of the most striking manifestations of this effect is enhanced ionization (EI) [27,28], where at a critical internuclear distance R c the ionization rate is strongly enhanced as compared to that at the equilibrium internuclear distance or that at an internuclear distance much larger than R c . By the development of the concept of enhanced ionization it became possible to explain the reason why the kinetic energy of fragment ions recorded by Coulomb explosion imaging was considerably lower than the kinetic energy expected, when the Coulomb explosion takes place at the equilibrium position of the ground state electronic potential energy surface of the neutral parent molecule [29][30][31][32][33]. Since then, the EI mechanism has been investigated extensively for diatomic molecules, e.g., [34][35][36][37], as well as for triatomic molecules [38][39][40][41].…”

Propensity of molecules to spatially align in intense light fields

“…(1), the first term causes reorientation while the second term, the so-called damping term, impedes the motion of the molecular axis towards the light field vector. In the field ionization Coulomb explosion model [4], the damping term arises due to the elongation of the molecular axis from R e to R c after the removal of one or two electrons by tunnel/over the barrier ionization.…”

“…In recent studies, it has been recognized that the molecular ionization rate depends on the angle that the internuclear axis makes with the light field vector, and that this also leads to anisotropic angular distributions [3]. Indeed, the ionization rate has to be computed using the field ionization Coulomb explosion model [4], whose angular dependence arises from the fact that the barrier suppression is given by E · r, where r denotes the molecular axis.…”