Wave-particle duality is an inherent peculiarity of the quantum world. The double-slit experiment has been frequently used for understanding different aspects of this fundamental concept. The occurrence of interference rests on the lack of which-way information and on the absence of decoherence mechanisms, which could scramble the wave fronts. Here, we report on the observation of two-center interference in the molecular-frame photoelectron momentum distribution upon ionization of the neon dimer by a strong laser field. Postselection of ions, which are measured in coincidence with electrons, allows choosing the symmetry of the residual ion, leading to observation of both, gerade and ungerade, types of interference.

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Double-slit photoelectron interference in strong-field ionization of the neon dimer.

Organic light-emitting diodes (OLEDs) show promise for applications as high-quality self-emissive displays for portable devices such as cellular phones and personal organizers. Although monochrome operation is sufficient for some applications, the extension to multi-colour devices--such as RGB (red, green, blue) matrix displays--could greatly enhance their technological impact. Multi-colour OLEDs have been successfully fabricated by vacuum deposition of small electroluminescent molecules, but solution processing of larger molecules (electroluminescent polymers) would result in a cheaper and simpler manufacturing process. However, it has proved difficult to combine the solution processing approach with the high-resolution patterning techniques required to produce a pixelated display. Recent attempts have focused on the modification of standard printing techniques, such as screen printing and ink jetting, but those still have technical drawbacks. Here we report a class of electroluminescent polymers that can be patterned in a way similar to standard photoresist materials--soluble polymers with oxetane sidegroups that can be crosslinked photochemically to produce insoluble polymer networks in desired areas. The resolution of the process is sufficient to fabricate pixelated matrix displays. Consecutive deposition of polymers that are luminescent in each of the three RGB colours yielded a device with efficiencies comparable to state-of-the-art OLEDs and even slightly reduced onset voltages.

Multi-colour organic light-emitting displays by solution processing.

We review the phenonomena which occur in multiphoton physics when the electric field of the applied laser radiation becomes comparable with the Coulomb field strength seen by an electron in the ground state of atomic hydrogen. This field is reached at an irradiance of approximately . The normal perturbative photon-by-photon based picture of the interaction of individual electrons with the field is replaced by a tunnelling picture in which, in a time of the order of, or less than one optical cycle, atomic wavepackets are generated which escape the confining Coulomb potential. These wavepackets are strongly influenced by the laser, `quiver' and may be accelerated back to the parent ion in a recollision process. Phase-coherent effects locked to the laser field become important: high harmonics are generated from these recollisions. We discuss the theory of such effects, and review progress in understanding how this quiver motion can be coherently controlled. We discuss ionization dynamics and review mechanisms by which atoms may be stabilized in very strong fields. Finally, we discuss relativistic effects which occur at very high-intensities.

https://iopscience.iop.org/article/10.1088/0034-4885/60/4/001/pdf

Atomic physics with super-high intensity lasers

We demonstrated how the subcycle evolution of the electric field of light can be used to control the motion of bound electrons. Results are presented for the dissociative ionization of deuterium molecules (D2 ⇒ D+ + D), where asymmetric ejection of the ionic fragment reveals that light-driven intramolecular electronic motion before dissociation localizes the electron on one of the two D+ ions in a controlled way. The results extend subfemtosecond electron control to molecules and provide evidence of its usefulness in controlling reaction dynamics.

https://science.sciencemag.org/content/sci/312/5771/246.full.pdf

Control of Electron Localization in Molecular Dissociation

For the past several decades, we have been able to directly probe the motion of atoms that is associated with chemical transformations and which occurs on the femtosecond (10(-15)-s) timescale. However, studying the inner workings of atoms and molecules on the electronic timescale has become possible only with the recent development of isolated attosecond (10(-18)-s) laser pulses. Such pulses have been used to investigate atomic photoexcitation and photoionization and electron dynamics in solids, and in molecules could help explore the prompt charge redistribution and localization that accompany photoexcitation processes. In recent work, the dissociative ionization of H(2) and D(2) was monitored on femtosecond timescales and controlled using few-cycle near-infrared laser pulses. Here we report a molecular attosecond pump-probe experiment based on that work: H(2) and D(2) are dissociatively ionized by a sequence comprising an isolated attosecond ultraviolet pulse and an intense few-cycle infrared pulse, and a localization of the electronic charge distribution within the molecule is measured that depends-with attosecond time resolution-on the delay between the pump and probe pulses. The localization occurs by means of two mechanisms, where the infrared laser influences the photoionization or the dissociation of the molecular ion. In the first case, charge localization arises from quantum mechanical interference involving autoionizing states and the laser-altered wavefunction of the departing electron. In the second case, charge localization arises owing to laser-driven population transfer between different electronic states of the molecular ion. These results establish attosecond pump-probe strategies as a powerful tool for investigating the complex molecular dynamics that result from the coupling between electronic and nuclear motions beyond the usual Born-Oppenheimer approximation.

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Electron localization following attosecond molecular photoionization

We report a new kind of multiphoton dissociation in intense laser fields. ${\mathrm{H}}_{2}^{+}$ molecular ions were formed in above-threshold ionization of ${\mathrm{H}}_{2}$ gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/${\mathrm{cm}}^{2}$. The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of ${\mathrm{H}}_{2}^{+}$.

Softening of the H2+ molecular bond in intense laser fields.

We present a systematic experimental study of the dissociation and ionization of molecular hydrogen in intense laser fields at three different wavelengths: 1.064 \ensuremath{\mu}m, 532 nm, and 355 nm. The light intensity ranges from ${10}^{13}$ W/${\mathrm{cm}}^{2}$ to ${10}^{15}$ W/${\mathrm{cm}}^{2}$. Measurements include electron spectra and angular distributions, kinetic-energy spectra and angular distributions of ion fragments, and ion dissociation fractions. We report a number of interesting and novel phenomena, including the following: above-threshold ionization to various vibrational states in the molecular ion, production of very-high-energy electrons from ionization at 1.064 \ensuremath{\mu}m, enhanced dissociation of the ${\mathrm{H}}_{2}^{+}$ molecular ion via the mechanism of bond softening, above-threshold dissociation, and evidence for the alignment of the molecule in a high-intensity laser field.

Ionization and dissociation of H 2 in intense laser fields at 1.064 μm, 532 nm, and 355 nm

The authors report above-threshold ionisation in krypton irradiated by a two-colour radiation field ( lambda 1=1064 nm, lambda 2=532 nm), where the phase difference between the two colours can be precisely controlled. The ionisation rate depends not only on intensity but also on the relative phase between the two lasers, as do the relative intensities and widths of the electron spectral peaks.

https://iopscience.iop.org/article/10.1088/0953-4075/23/16/018/pdf

H. G. Muller

Papers

Softening of the H2+ molecular bond in intense laser fields.

Ionization and dissociation of H 2 in intense laser fields at 1.064 μm, 532 nm, and 355 nm

Above-threshold ionisation with a two-colour laser field