David Ayuso

Ultrafast Electron Dynamics in Phenylalanine Initiated by Attosecond Pulses

In the past decade, attosecond technology has opened up the investigation of ultrafast electronic processes in atoms, simple molecules, and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub–4.5-femtosecond temporal scale, which is shorter than the vibrational response of the molecule. The ability to initiate and observe such electronic dynamics in polyatomic molecules represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems. Electronic dynamics in a complex polyatomic molecule are tracked faster than the time scale for vibrational motion. A very quick look at phenylalanine Over the past decade, laser technology has pushed back the fastest directly observable time scale from femtoseconds (quadrillionths of a second) to attoseconds (quintillionths of a second). For the most part, attosecond studies so far have probed very simple molecules such as H2 and O2. Calegari et al. now look at a more elaborate molecule—the amino acid phenylalanine. They tracked changes in the electronic structure of the compound after absorption of an ultrafast pulse, before the onset of conventional vibrational motion. Science, this issue p. 336

Charge migration induced by attosecond pulses in bio-relevant molecules

We acknowledge the support from the European Research Council under the ERC grants no. 637756 STARLIGHT, no. 227355 ELYCHE and no. 290853 XCHEM, LASERLABEUROPE (grant agreement no. 284464, European Commissions Seventh Framework Programme), European COST Action CM1204 XLIC, the Ministerio de Ciencia e Innovacion project FIS2013-42002-R, European grants MC-ITN CORINF and MC-RG ATTOTREND 268284, UKs Science and Technology Facilities Council Laser Loan Scheme, the Engineering and Physical Sciences Research Council (grant EP/J007048/ 1), the Leverhulme Trust (grant RPG-2012-735), and the Northern Ireland Department of Employment and Learning

Intramolecular photoelectron diffraction in the gas phase

We report unambiguous experimental and theoretical evidence of intramolecular photoelectron diffraction in the collective vibrational excitation that accompanies high-energy photoionization of gas-phase CF4, BF3, and CH4 from the 1s orbital of the central atom. We show that the ratios between vibrationally resolved photoionization cross sections (v-ratios) exhibit pronounced oscillations as a function of photon energy, which is the fingerprint of electron diffraction by the surrounding atomic centers. This interpretation is supported by the excellent agreement between first-principles static-exchange and time-dependent density functional theory calculations and high resolution measurements, as well as by qualitative agreement at high energies with a model in which atomic displacements are treated to first order of perturbation theory. The latter model allows us to rationalize the results for all the v-ratios in terms of a generalized v-ratio, which contains information on the structure of the above three molecules and the corresponding molecular cations. A fit of the measured v-ratios to a simple formula based on this model suggests that the method could be used to obtain structural information of both neutral and ionic molecular species.

Ultrafast Charge Dynamics in an Amino Acid Induced by Attosecond Pulses

In the past few years, attosecond techniques have been implemented for the investigation of ultrafast dynamics in molecules. The generation of isolated attosecond pulses characterized by a relatively high photon flux has opened up new possibilities in the study of molecular dynamics. In this paper, we report on experimental and theoretical results of ultrafast charge dynamics in a biochemically relevant molecule, namely, the amino acid phenylalanine. The data represent the first experimental demonstration of the generation and observation of a charge migration process in a complex molecule, where electron dynamics precede nuclear motion. The application of attosecond technology to the investigation of electron dynamics in biologically relevant molecules represents a multidisciplinary work, which can open new research frontiers: those in which few-femtosecond and even subfemtosecond electron processes determine the fate of biomolecules. It can also open new perspectives for the development of new technologies, for example, in molecular electronics, where electron processes on an ultrafast temporal scale are essential to trigger and control the electron current on the scale of the molecule.

Attosecond control of spin polarization in electron–ion recollision driven by intense tailored fields

Ionization of noble gases by strong infrared circularly-polarized laser pulses can produce electron currents with a controllable degree of spin polarization [1-4]. Spin polarization arises as a consequence of (1) entanglement between the liberated electron and the parent ion, and (2) sensitivity of ionization to the sense of electron rotation in the initial state. In this context, the use of two-color counter-rotating bicircular fields [5] opens new opportunities for introducing the spin degree of freedom into attosecond science [6], since the liberated electrons can be driven back towards the ionic core within one optical cycle.

Vibrationally resolved C 1s photoionization cross section of CF4

The differential photoionization cross section ratio (? = 1)/(? = 0) for the symmetric stretching mode in the C 1s photoionization of CF4 was studied both theoretically and experimentally. We observed this ratio to differ from the Franck?Condon ratio and to be strongly dependent on the photon energy, even far from the photoionization threshold. The density-functional theory computations show that the ratio is significantly modulated by the diffraction of the photoelectrons by the neighbouring atoms at high photon energies. At lower energies, the interpretation of the first very strong maximum observed about 60?eV above the photoionization threshold required detailed calculations of the absolute partial cross sections, which revealed that the absolute cross section has two maxima at lower energies, which turn into one maximum in the cross section ratio because the maxima appear at slightly different energies in ? = 1 and ? = 0 cross sections. These two strong, low-energy continuum resonances originate from the trapping of the continuum wavefunction in the molecular potential of the surrounding fluorine atoms and from the outgoing electron scattering by them.

Vibrationally Resolved B 1s Photoionization Cross Section of BF3.

Photoelectron diffraction is a well-established technique for structural characterization of solids, based on the interference of the native photoelectron wave with those scattered from the neighboring atoms. For isolated systems in the gas phase similar studies suffer from orders of magnitude lower signals due to the very small sample density. Here we present a detailed study of the vibrationally resolved B 1s photoionization cross section of BF3 molecule. A combination of high-resolution photoelectron spectroscopy measurements and of state-of-the-art static-exchange and time-dependent DFT calculations shows the evolution of the photon energy dependence of the cross section from a complete trapping of the photoelectron wave (low energies) to oscillations due to photoelectron diffraction phenomena. The diffraction pattern allows one to access structural information both for the ground neutral state of the molecule and for the core-ionized cation. Due to a significant change in geometry between the ground and the B 1s(-1) core-ionized state in the BF3 molecule, several vibrational final states of the cation are populated, allowing investigation of eight different relative vibrationally resolved photoionization cross sections. Effects due to recoil induced by the photoelectron emission are also discussed.

Strong-field control and enhancement of chiral response in bi-elliptical high-order harmonic generation: an analytical model

The generation of high-order harmonics in a medium of chiral molecules driven by intense bi-elliptical laser fields can lead to strong chiroptical response in a broad range of harmonic numbers and ellipticities [D. Ayuso et al, J. Phys. B 51, 06LT01 (2018)]. Here we present a comprehensive analytical model that can describe the most relevant features arising in the high-order harmonic spectra of chiral molecules driven by strong bi-elliptical fields. Our model recovers the physical picture underlying chiral high-order harmonic generation based on ultrafast chiral hole motion and identifies the rotationally invariant molecular pseudoscalars responsible for chiral dynamics. Using the chiral molecule propylene oxide as an example, we show that one can control and enhance the chiral response in bi-elliptical high-order harmonic generation by tailoring the driving field, in particular by tuning its frequency, intensity and ellipticity, exploiting a suppression mechanism of achiral background based on the linear Stark effect.

Attosecond Pump-Probe Spectroscopy of Charge Dynamics in Tryptophan

Attosecond pump-probe experiments performed in small molecules have allowed tracking charge dynamics in the natural time scale of electron motion. That this is also possible in biologically relevant molecules is still a matter of debate, because the large number of available nuclear degrees of freedom might destroy the coherent charge dynamics induced by the attosecond pulse. Here we investigate extreme ultraviolet-induced charge dynamics in the amino acid tryptophan. We find that, although nuclear motion and nonadiabatic effects introduce some decoherence in the moving electron wave packet, these do not significantly modify the coherence induced by the attosecond pulse during the early stages of the dynamics, at least for molecules in their equilibrium geometry. Our conclusions are based on elaborate theoretical calculations and the experimental observation of sub-4 fs dynamics, which can only be reasonably assigned to electronic motion. Hence, attosecond pump-probe spectroscopy appears as a promising approach to induce and image charge dynamics in complex molecules.

Attosecond control of spin polarization in electron-ion recollision driven by intense tailored fields

Ionization of noble gases by strong infrared circularly-polarized laser pulses can produce electron currents with a controllable degree of spin polarization [1-4]. Spin polarization arises as a consequence of (1) entanglement between the liberated electron and the parent ion, and (2) sensitivity of ionization to the sense of electron rotation in the initial state. In this context, the use of two-color counter-rotating bicircular fields [5] opens new opportunities for introducing the spin degree of freedom into attosecond science [6], since the liberated electrons can be driven back towards the ionic core within one optical cycle.