Condensed Matter Systems Exposed to Radiation: Multiscale Theory, Simulations, and Experiment.

This roadmap reviews the new, highly interdisciplinary research field studying the behavior of condensed matter systems exposed to radiation. The Review highlights several recent advances in the field and provides a roadmap for the development of the field over the next decade. Condensed matter systems exposed to radiation can be inorganic, organic, or biological, finite or infinite, composed of different molecular species or materials, exist in different phases, and operate under different thermodynamic conditions. Many of the key phenomena related to the behavior of irradiated systems are very similar and can be understood based on the same fundamental theoretical principles and computational approaches. The multiscale nature of such phenomena requires the quantitative description of the radiation-induced effects occurring at different spatial and temporal scales, ranging from the atomic to the macroscopic, and the interlinks between such descriptions. The multiscale nature of the effects and the similarity of their manifestation in systems of different origins necessarily bring together different disciplines, such as physics, chemistry, biology, materials science, nanoscience, and biomedical research, demonstrating the numerous interlinks and commonalities between them. This research field is highly relevant to many novel and emerging technologies and medical applications.

THz to far-infrared spectra of the known crystal polymorphs of phenylalanine.

There is renewed interest in the structure of the essential amino acid phenylalanine in the solid state. Three new polymorphs were found in the years 2012 to 2014. Here, we investigate the structure, stability, and energetical ordering of these phases using first-principles simulations at the level of density functional theory incorporating van der Waals interactions. Two of the distinct crystal forms are found to be structurally similar and energetically very close after vibrational free energy corrections have been taken into account. Infrared absorption spectra are likewise calculated and compared to experimental measurements. By combining measurements obtained with a commercial Fourier transform infra-red spectrometer and a homemade air-photonics-based THz time domain spectrometer, we could carry out this comparison in the vibrational frequency region from 1 to 40 THz. The excellent agreement of the line positions and the established energy ranking allow us to identify the most stable polymorph of phenylalanine.

Probing Photoionization Dynamics in Acetylene with Angle-Resolved Attosecond Interferometry.

Photoionization of acetylene by extreme ultraviolet light results in a stand-alone contribution from the outermost valence orbital, followed by well-separated photoelectron bands from deeper molecular orbitals. This makes acetylene an ideal candidate for probing the photoionization dynamics in polyatomic molecules free from the spectral congestion often arising after interaction with an attosecond pulse train. Here, using an angle-resolved attosecond interferometric technique, we extract the photoionization time delays for the outermost valence orbital in acetylene relative to an atomic target, namely argon. Compared to argon, the photoemission from the acetylene molecule is found to be advanced by almost 28 attoseconds. The strong variation of the relative photoionization time delays as a function of the photoemission angle was interpreted using an analytical model based on semiclassical approximations to be the interplay between different short-range potentials along and perpendicular to the molecular axis. Our results highlight the importance of using attosecond time-resolved measurements to probe the nonspherical nature of the molecular potential, even in the case of relatively small, linear systems.

GlAIcomics: a deep neural network classifier for spectroscopy-augmented mass spectrometric glycans data.

Carbohydrate sequencing is a formidable task identified as a strategic goal in modern biochemistry. It relies on identifying a large number of isomers and their connectivity with high accuracy. Recently, gas phase vibrational laser spectroscopy combined with mass spectrometry tools have been proposed as a very promising sequencing approach. However, its use as a generic analytical tool relies on the development of recognition techniques that can analyse complex vibrational fingerprints for a large number of monomers. In this study, we used a Bayesian deep neural network model to automatically identify and classify vibrational fingerprints of several monosaccharides. We report high performances of the obtained trained algorithm (GlAIcomics), that can be used to discriminate contamination and identify a molecule with a high degree of confidence. It opens the possibility to use artificial intelligence in combination with spectroscopy-augmented mass spectrometry for carbohydrates sequencing and glycomics applications.

On-the-fly investigation of XUV excited large molecular ions using a high harmonic generation light source.

We present experiments where extreme ultraviolet femtosecond light pulses are used to photoexcite large molecular ions at high internal energy. This is done by combining an electrospray ionization source and a mass spectrometer with a pulsed light source based on high harmonic generation. This allows one to study the interaction between high energy photons and mass selected ions in conditions that are accessible on large-scale facilities. We show that even without an ion trapping device, systems as large as a protein can be studied. We observe light induced dissociative ionization and proton migration in model systems such as reserpine, insulin and cytochrome c. These results offer new perspectives to perform time-resolved experiments with ultrashort pulses at the heart of the emerging field of attosecond chemistry.

Few-Femtosecond Isotope Effect in Polyatomic Molecules Ionized by Extreme Ultraviolet Attosecond Pulse Trains.

Following ionization by an extreme ultraviolet (XUV) attosecond pulse train, a polyatomic molecule can be promoted to more-than-one excited states of the residual ion. The ensuing relaxation dynamics is often facilitated by several reaction coordinates, making them difficult to disentangle by the usual spectroscopic means. Here, we show that in atto-chemistry isotope labeling can be an efficient tool for unraveling the relaxation pathways in highly excited photoionized molecules. Employing an XUV pump pulse and a near-infrared probe pulse, we found the nuclear as well as coupled electron-nuclear dynamics in ethylene to be almost 40% faster compared to that of its deuterated counterpart. The findings, which are supported by advanced nonadiabatic dynamics calculations, led to the identification of the relevant nuclear coordinates controlling the relaxation. Our experiment highlights the relevance of ultrashort XUV pulses to capture the isotopic effect in few-femtosecond molecular photodynamics.

Anisotropic dynamics of two-photon ionization: An attosecond movie of photoemission.

Imaging in real time the complete dynamics of a process as fundamental as photoemission has long been out of reach because of the difficulty of combining attosecond temporal resolution with fine spectral and angular resolutions. Here, we achieve full decoding of the intricate angle-dependent dynamics of a photoemission process in helium, spectrally and anisotropically structured by two-photon transitions through intermediate bound states. Using spectrally and angularly resolved attosecond electron interferometry, we characterize the complex-valued transition probability amplitude toward the photoelectron quantum state. This allows reconstructing in space, time, and energy the complete formation of the photoionized wave packet.

Time-resolved relaxation and cage opening in diamondoids following XUV ultrafast ionization.

Unraveling ultrafast processes induced by energetic radiation is compulsory to understand the evolution of molecules under extreme excitation conditions. To describe these photo-induced processes, one needs to perform time-resolved experiments to follow in real time the dynamics induced by the absorption of light. Recent experiments have demonstrated that ultrafast dynamics on few tens of femtoseconds are expected in such situations and a very challenging task is to identify the role played by electronic and nuclear degrees of freedom, charge, energy flows and structural rearrangements. Here, we performed time-resolved XUV-IR experiments on diamondoids carbon cages, in order to decipher the processes following XUV ionization. We show that the dynamics is driven by two timescales, the first one is associated to electronic relaxation and the second one is identified as the redistribution of vibrational energy along the accessible modes, prior to the cage opening that is involved in all fragmentation mechanisms in this family of molecules.

Controlled ultrafast ππ*-πσ* dynamics in tryptophan-based peptides with tailored micro-environment.

Ultrafast charge, energy and structural dynamics in molecules are driven by the topology of the multidimensional potential energy surfaces that determines the coordinated electronic and nuclear motion. These processes are also strongly influenced by the interaction with the molecular environment, making very challenging a general understanding of these dynamics on a microscopic level. Here we use electrospray and mass spectrometry technologies to produce isolated molecular ions with a controlled micro-environment. We measure ultrafast photo-induced ππ*-πσ* dynamics in tryptophan species in the presence of a single, charged adduct. A striking increase of the timescale by more than one order of magnitude is observed when changing the added adduct atom. A model is proposed to rationalize the results, based on the localized and delocalized effects of the adduct on the electronic structure of the molecule. These results offer perspectives to control ultrafast molecular processes by designing the micro-environment on the Angström length scale.

Experimental Evidence of Sensitivity of the High Harmonic Generation to Hydrogen Bonding.

The influence of hydrogen bonding and the associated attosecond hole delocalization on the high-order harmonic generation (HHG) process is investigated with the help of hydrogen-bonded binary mixture of acetonitrile and chloroform solvent vapors. We observe a strong enhancement of the HH yield compared to the results obtained with pure samples. We propose that the observed increase of HHG efficiency is due to the presence of hydrogen-bonded binary mixture. Numerical simulations show evidence of the attosecond hole delocalization in the hydrogen-bonded complex of acetonitrile and chloroform. This attosecond hole delocalization contributes to the enhancement of the harmonic yield in the hydrogen-bonded complex. To the best of our knowledge, this is the first report on the sensitivity of the high harmonic generation process to the hydrogen bonding.

On-the-Fly Femtosecond Action Spectroscopy of Charged Cyanine Dyes: Electronic Structure versus Geometry.

Understanding optical properties of molecular dyes is required to drive progress in molecular photonics. This requires a fundamental comprehension of the role of electronic structure, geometry, and interactions with the environment in order to guide molecular engineering strategies. In this context, we studied charged cyanine dye molecules in the gas phase with a controlled microenvironment to unravel the origin of the spectral tuning of this class of molecules. This was performed using a new approach combining femtosecond multiple-photon action spectroscopy of on-the-fly mass-selected molecular ions and high-level quantum calculations. While arguments based on molecular geometry are often used to design new polymethine dyes, we provide experimental evidence that electronic structure is of primary importance and hence the decisive criterion as suggested by recent theoretical investigations.

Delayed Relaxation of Highly Excited Cationic States in Naphthalene.

Rapid energy transfer from electronic to nuclear degrees of freedom underlies many biological processes and astrophysical observations. The efficiency of this energy transfer depends strongly on the complex interplay between electronic and nuclear motions. In this study, we report two-color pump-probe experiments that probe the relaxation dynamics of highly excited cationic states of naphthalene, a prototypical polycyclic aromatic hydrocarbon molecule, which are produced using wavelength-selected, ultrashort extreme ultraviolet pulses. Surprisingly, the relaxation lifetimes increase with the cationic excitation energy. We postulate that the observed effect is the result of a population trapping that leads to delayed relaxation.

Ultrafast Nonadiabatic Cascade and Subsequent Photofragmentation of Extreme Ultraviolet Excited Caffeine Molecule.

Ultrafast XUV chemistry is offering new opportunities to decipher the complex dynamics taking place in highly excited molecular states and thus better understand fundamental natural phenomena as molecule formation in interstellar media. We used ultrashort XUV light pulses to perform XUV pump-IR probe experiments in caffeine as a model of prebiotic molecule. We observed a 40 fs decay of excited cationic states. Guided by quantum calculations, this time scale is interpreted in terms of a nonadiabatic cascade through a large number of highly correlated states. This shows that the correlation driven nonadiabatic relaxation seems to be a general process for highly excited states, which might impact our understanding of molecular processing in interstellar media.

A detailed-balance model for thermionic emission from polyanions: The case of fullerene dianions.

A detailed-balance model for thermionic emission from polyanions has been developed and applied to fullerene dianions. The specificity of this delayed decay process is electron tunneling through the repulsive Coulomb barrier (RCB). An analytical expression of the RCB is derived from electrostatic modeling of the fullerene cage. The reverse process, namely, electron attachment to the singly charged anion, is described by a hard sphere cross section weighted by the Wentzel-Kramers-Brillouin tunneling probability. This simple expression leads to a very good agreement with a measured time-resolved kinetic energy distribution of C842-. Electron binding energy is reduced when the fullerene cage size decreases, leading to an almost zero one for C702- and a negative one for C602-. Extension of the model to these systems of interest is discussed, and model outputs are compared with the experimental data from the literature.

An instrument combining an electrospray ionization source and a velocity-map imaging spectrometer for studying delayed electron emission of polyanions.

An instrument combining an electrospray ionization source and a velocity-map imaging (VMI) spectrometer has been developed in order to study the delayed electron emission of molecular anions and especially of polyanions. It operates at a high repetition rate (kHz) in order to increase the acquisition speed. The VMI spectrometer has been upgraded for nanosecond time resolution by gating the voltages applied on the position-sensitive detector. Kinetic energy release distribution of thermionic emission (without any contribution from direct detachment) can be recorded for well-defined delays after the nanosecond laser excitation. The capability of the instrument is demonstrated by recording photodetachment spectra of the benchmark C60(-) anion and C84(2-) dianion.

Multi-purpose two- and three-dimensional momentum imaging of charged particles for attosecond experiments at 1 kHz repetition rate.

We report on the versatile design and operation of a two-sided spectrometer for the imaging of charged-particle momenta in two dimensions (2D) and three dimensions (3D). The benefits of 3D detection are to discern particles of different mass and to study correlations between fragments from multi-ionization processes, while 2D detectors are more efficient for single-ionization applications. Combining these detector types in one instrument allows us to detect positive and negative particles simultaneously and to reduce acquisition times by using the 2D detector at a higher ionization rate when the third dimension is not required. The combined access to electronic and nuclear dynamics available when both sides are used together is important for studying photoreactions in samples of increasing complexity. The possibilities and limitations of 3D momentum imaging of electrons or ions in the same spectrometer geometry are investigated analytically and three different modes of operation demonstrated experimentally, with infrared or extreme ultraviolet light and an atomic/molecular beam.

Photoelectron spectroscopy of gramicidin polyanions: competition between delayed and direct emission.

We present the first photoelectron (PE) spectra of polypeptide polyanions. Combining PE spectroscopy and mass spectrometry provides a direct measurement of the stability of the polyanions with respect to electron detachment and of the repulsive energy between excess charges. The second electron affinity of gramicidin was found to amount to 2.35 +/- 0.15 eV, and the value of the repulsive Coulomb barrier was estimated to be 0.5 +/- 0.15 eV. The spectra are interpreted as resulting from a competition between delayed and direct emission.