A.S. Alnaser

Momentum-imaging investigations of the dissociation of D2+ and the isomerization of acetylene to vinylidene by intense short laser pulses

We present momentum images of the ionic products from the ionization of D2 and C2H2 by short laser pulses. For D2, we use a pump–probe approach to investigate the dependence of the enhanced ionization on the internuclear distance. Evidence for two (not well separated) regions of enhancement is found near internuclear distances of 6 and 10 au. In the case of acetylene, we report clear evidence for the production of both acetylene and vinylidene dications with kinetic energy releases similar to those reported earlier by core electron removal. We also find very different angular distributions for the fragments in the two channels, consistent with a finite time for the isomerization.

Post ionization alignment of the fragmentation of molecules in an ultrashort intense laser field

We studied the angular distributions of the fragmented ions of diatomic molecules in an intense linearly polarized short laser pulse. In addition to the well-known dynamic alignment of the neutral molecules before ionization, we identified a more important post ionization alignment effect of the molecular ions. The latter is modelled quantum mechanically as resulting from the breakup of a rotating linear rotor. We showed that only for very short pulses are the two alignment mechanisms not important. In this case the angular distributions of the fragmented ions mimic the shape of the electronic density of the outermost molecular orbital.

High resolution kinetic energy release spectra and angular distributions from double ionization of nitrogen and oxygen by short laser pulses

We have used momentum imaging techniques to measure in high resolution the kinetic energy release spectra and angular distributions of coincident O + and N + ion pairs produced by short laser pulses (8–35 fs) on targets of N2 and O2 at peak intensities between 1 and 12 × 10 14 Wc m −2 . We record the full momentum vectors of both members of each pair and achieve a kinetic energy release resolution of less than 0.3 eV. We find that the process proceeds through well-defined electronic states of the excited molecular dications. Using linear and circularly polarized light, we identify two mechanisms for the production of these states, rescattering and sequential ionization. By using 8 fs pulses, we observe that the internuclear distance can be frozen during the pulse. For low intensities and 8 fs pulses, emission from N2 is strongly directed along the polarization vector, while that for O2 is not, a result we interpret as being due to the different symmetries of the outer orbitals of these molecules. For high intensities and longer pulses, the distributions increasingly fold towards the polarization vector, ultimately peaking at zero degrees for both molecules. For oxygen, a local peaking for molecules aligned at right angles to the polarization vector is seen. A discussion and interpretation of the results are presented. (Some figures in this article are in colour only in the electronic version)

Fragmentation pathways for selected electronic states of the acetylene dication

Coincident measurement of the Auger electron and fragment ion momenta emitted after carbon core-level photoionization of acetylene has yielded new understanding of how the dication fragments. Ab initio calculations and experimental data, including body-frame Auger angular distributions, are used to identify the parent electronic states and together yield a comprehensive map of the dissociation pathways which include surface crossings and barriers to direct dissociation. The Auger angular distributions for certain breakup channels show evidence of core–hole localization. (Some figures in this article are in colour only in the electronic version)

Wavelength dependence of momentum-space images of low-energy electrons generated by short intense laser pulses at high intensities

We have measured momentum-space images of low-energy electrons generated by the interaction of short intense laser pulses with argon atoms at high intensities. We have done this over a wavelength range from 400 to 800 nm. The spectra show considerable structure in both the energy and angular distributions of the electrons. Some, but not all, energy features can be identified as multi-photon resonances. The angular structure shows a regularity which transcends the resonant structure and may be due instead to diffraction. The complexity of the results defies easy model-dependent interpretations and invites full solutions to Schrodingers equation for these systems.

Multi-photon resonant effects in strong-field ionization: origin of the dip in experimental longitudinal momentum distributions

We studied ionization of neon and argon by intense linearly polarized femtosecond laser pulses of different wavelengths (400 nm and 800 nm) and peak intensities, and by measuring momentum distributions of singly charged positive ions in the direction parallel to laser polarization. For Ne the momentum distributions exhibited a characteristic dip at zero momentum at 800 nm and a complex multipeak structure at 400 nm. Similarly, for Ar the momentum distributions evolved from a complex multipeak structure with a pronounced dip in the centre at 400 nm, to a smooth distribution characteristic of pure tunneling ionization (800 nm, high intensities). In the intermediate regime (800 nm, medium to low intensities), for both atoms we observed recoil ion momentum distributions modulated by quasi-periodic structures usually seen in the photoelectron energy spectra in a multi-photon regime (ATI spectra). Ne did show a characteristic dip at low momentum, while the longitudinal momentum distribution for Ar exhibited a spike at zero momentum instead. The spectra did dramatically change at 400 nm, where both ions show the pronounced dip near zero momentum. Based on our results, we conclude that the structures observed in Ne and Ar momentum distributions reflect the specifics of atomic structure of the two targets and should not be attributed to effects of electron recollision, as was suggested earlier. Instead, as our results indicate, they are due to the effects of multi-photon resonant enhancement of strong-field ionization.

Photon-ion collisions and molecular clocks

The timing of molecular rearrangements can be followed in the time domain on a femtosecond scale by using momentum imaging techniques. Three examples are discussed in this paper: first, the diffraction of electrons ejected from the K-shell of one of the atomic constituents of the molecule takes a ‘picture’ of the molecule, and the correlation between the momentum vector of the photoelectron and the subsequent fragmentation pattern is used to estimate the time delay which accompanies the latter process. Second, the kinetic energy release of proton pairs from the double ionization of hydrogen by fast laser pulses is timed using the optical cycle as a clock. The mechanisms of rescattering, sequential and enhanced ionization are clearly identified in the momentum spectra. Third, the operation of rescattering double ionization in the case of nitrogen and oxygen molecules is discussed.