T. Jahnke

Understanding the role of phase in chemical bond breaking with coincidence angular streaking

Electron motion in chemical bonds occurs on an attosecond timescale. This ultrafast motion can be driven by strong laser fields. Ultrashort asymmetric laser pulses are known to direct electrons to a certain direction. But do symmetric laser pulses destroy symmetry in breaking chemical bonds? Here we answer this question in the affirmative by employing a two-particle coincidence technique to investigate the ionization and fragmentation of H₂ by a long circularly polarized multicycle femtosecond laser pulse. Angular streaking and the coincidence detection of electrons and ions are employed to recover the phase of the electric field, at the instant of ionization and in the molecular frame, revealing a phase-dependent anisotropy in the angular distribution of H⁺ fragments. Our results show that electron localization and asymmetrical breaking of molecular bonds are ubiquitous, even in symmetric laser pulses. The technique we describe is robust and provides a powerful tool for ultrafast science.

Characterization of a high-pressure microdischarge using diode laser atomic absorption spectroscopy

A high-pressure dc glow discharge based on micro-structured-electrode (MSE) arrays was investigated by diode laser atomic absorption spectroscopy. The microdischarge was studied at constant current in pure Ar for pressures ranging from 50 to 400 mbar. The measurements of the absolute population density of the excited 1s5 ,1 s 4 ,1 s 3, and 1s2 levels of Ar in the discharge revealed a population density of excited atoms in metastable and resonance states in the range 10 12 –10 15 cm −3 . The gas temperature and the electron number density were evaluated from the analysis of the absorption line profiles, taking into account significant broadening mechanisms. The gas temperature, derived from the Doppler broadening, was found to increase with pressure from 380 K at 50 mbar to 1100 K at 400 mbar. The electron number density was calculated from the Stark broadening and shift, and it ranges from 9 × 10 14 to 5 × 10 15 cm −3 . The MSE-sustained discharges are combining the non-equilibrium character with the advantage of high-pressure, which recommends them for non-thermal plasma processing e.g. surface treatment, plasma chemistry and generation of UV and VUV radiation.

Direct Determination of Absolute Molecular Stereochemistry in Gas Phase by Coulomb Explosion Imaging

Absolute Images Molecules are held together by a balance of charge between negative electrons and positive nuclei. When multiple electrons are expelled by laser irradiation, the remaining, mutually repulsive nuclei fly apart in a Coulomb explosion. Instead of traditional x-ray diffraction methods that require crystalline samples, Pitzer et al. (p. 1096) show that by tracking the fragment trajectories from laser-induced Coulomb explosions of relatively simple gas phase molecules, they can determine the absolute stereochemical configuration of enantiomers (mirror-image isomers). A simple molecule’s three-dimensional structure can be ascertained from the fragment trajectories when it is blown apart. Bijvoet’s method, which makes use of anomalous x-ray diffraction or dispersion, is the standard means of directly determining the absolute (stereochemical) configuration of molecules, but it requires crystalline samples and often proves challenging in structures exclusively comprising light atoms. Herein, we demonstrate a mass spectrometry approach that directly images the absolute configuration of individual molecules in the gas phase by cold target recoil ion momentum spectroscopy after laser ionization–induced Coulomb explosion. This technique is applied to the prototypical chiral molecule bromochlorofluoromethane and the isotopically chiral methane derivative bromodichloromethane.

Ultrafast Probing of Core Hole Localization in N2

Although valence electrons are clearly delocalized in molecular bonding frameworks, chemists and physicists have long debated the question of whether the core vacancy created in a homonuclear diatomic molecule by absorption of a single x-ray photon is localized on one atom or delocalized over both. We have been able to clarify this question with an experiment that uses Auger electron angular emission patterns from molecular nitrogen after inner-shell ionization as an ultrafast probe of hole localization. The experiment, along with the accompanying theory, shows that observation of symmetry breaking (localization) or preservation (delocalization) depends on how the quantum entangled Bell state created by Auger decay is detected by the measurement.

Interatomic and intermolecular Coulombic decay: the coming of age story

In pioneering work by Cederbaum et al an excitation mechanism was proposed that occurs only in loosely bound matter (Cederbaum et al 1997 Phys. Rev. Lett. 79 4778): it turned out, that (in particular) in cases where a local Auger decay is energetically forbidden, an excited atom or molecule is able to decay in a scheme which was termed ‘interatomic Coulombic decay’ (or ‘intermolecular Coulombic decay’) (ICD). As ICD occurs, the excitation energy is released by transferring it to an atomic or molecular neighbor of the initially excited particle. As a consequence the neighboring atom or molecule is ionized as it receives the energy. A few years later the existence of ICD was confirmed experimentally (Marburger et al 2003 Phys. Rev. Lett. 90 203401; Jahnke et al 2004 Phys. Rev. Lett. 93 163401; Ohrwall et al 2004 Phys. Rev. Lett. 93 173401) by different techniques. Since this time it has been found that ICD is not (as initially suspected) an exotic feature of van der Waals or hydrogen bonded systems, but that ICD is a very general and common feature occurring after a manifold of excitation schemes and in numerous weakly bound systems, as revealed by more than 200 publications. It was even demonstrated, that ICD can become more efficient than a local Auger decay in some system. This review will concentrate on recent experimental investigations on ICD. It will briefly introduce the phenomenon and give a short summary of the ‘early years’ of ICD (a detailed view on this episode of investigations can be found in the review article by U Hergenhahn with the same title (Hergenhahn 2011 J. Electron Spectrosc. Relat. Phenom. 184 78)). More recent articles will be presented that investigate the relevance of ICD in biological systems and possible radiation damage of such systems due to ICD. The occurrence of ICD and ICD-like processes after different excitation schemes and in different systems is covered in the middle section: in that context the helium dimer (He2) is a particularly interesting (and exotic) system in which ICD was detected. It was employed in several publications to elucidate the strong impact of nuclear motion on ICD and its longrange-character. The review will present these findings and their initial theoretical predictions and give insight into most recent time-resolved measurements of ICD.

Resonant Auger decay driving intermolecular Coulombic decay in molecular dimers

In 1997, it was predicted that an electronically excited atom or molecule placed in a loosely bound chemical system (such as a hydrogen-bonded or van-der-Waals-bonded cluster) could efficiently decay by transferring its excess energy to a neighbouring species that would then emit a low-energy electron. This intermolecular Coulombic decay (ICD) process has since been shown to be a common phenomenon, raising questions about its role in DNA damage induced by ionizing radiation, in which low-energy electrons are known to play an important part. It was recently suggested that ICD can be triggered efficiently and site-selectively by resonantly core-exciting a target atom, which then transforms through Auger decay into an ionic species with sufficiently high excitation energy to permit ICD to occur. Here we show experimentally that resonant Auger decay can indeed trigger ICD in dimers of both molecular nitrogen and carbon monoxide. By using ion and electron momentum spectroscopy to measure simultaneously the charged species created in the resonant-Auger-driven ICD cascade, we find that ICD occurs in less time than the 20 femtoseconds it would take for individual molecules to undergo dissociation. Our experimental confirmation of this process and its efficiency may trigger renewed efforts to develop resonant X-ray excitation schemes for more localized and targeted cancer radiation therapy.

Fragmentation dynamics of CO(2)(3+) investigated by multiple electron capture in collisions with slow highly charged ions.

Fragmentation of highly charged molecular ions or clusters consisting of more than two atoms can proceed in a one step synchronous manner where all bonds break simultaneously or sequentially by emitting one ion after the other. We separated these decay channels for the fragmentation of CO(2)(3+) ions by measuring the momenta of the ionic fragments. We show that the total energy deposited in the molecular ion is a control parameter which switches between three distinct fragmentation pathways: the sequential fragmentation in which the emission of an O(+) ion leaves a rotating CO(2+) ion behind that fragments after a time delay, the Coulomb explosion and an in-between fragmentation--the asynchronous dissociation. These mechanisms are directly distinguishable in Dalitz plots and Newton diagrams of the fragment momenta. The CO(2)(3+) ions are produced by multiple electron capture in collisions with 3.2 keV/u Ar(8+) ions.

Multiorbital Tunneling Ionization of the CO Molecule

We coincidently measure the molecular-frame photoelectron angular distribution and the ion sum-momentum distribution of single and double ionization of CO molecules by using circularly and elliptically polarized femtosecond laser pulses, respectively. The orientation dependent ionization rates for various kinetic energy releases allow us to individually identify the ionizations of multiple orbitals, ranging from the highest occupied to the next two lower-lying molecular orbitals for various channels observed in our experiments. Not only the emission of a single electron, but also the sequential tunneling dynamics of two electrons from multiple orbitals are traced step by step. Our results confirm that the shape of the ionizing orbitals determine the strong laser field tunneling ionization in the CO molecule, whereas the linear Stark effect plays a minor role.

Interatomic Coulombic Decay following Photoionization of the Helium Dimer: Observation of Vibrational Structure

Using synchrotron radiation we simultaneously ionize and excite one helium atom of a helium dimer (He2) in a shakeup process. The populated states of the dimer ion [i.e., He(*+)(n = 2, 3) - He] are found to deexcite via interatomic Coulombic decay. This leads to the emission of a second electron from the neutral site and a subsequent Coulomb explosion. In this Letter we present a measurement of the momenta of fragments that are created during this reaction. The electron energy distribution and the kinetic energy release of the two He+ ions show pronounced oscillations which we attribute to the structure of the vibrational wave function of the dimer ion.

Time-resolved momentum imaging system for molecular dynamics studies using a tabletop ultrafast extreme-ultraviolet light source

We describe a momentum imaging setup for direct time-resolved studies of ionization-induced molecular dynamics. This system uses a tabletop ultrafast extreme-ultraviolet (EUV) light source based on high harmonic upconversion of a femtosecond laser. The high photon energy (around 42 eV) allows access to inner-valence states of a variety of small molecules via single photon excitation, while the sub--10-fs pulse duration makes it possible to follow the resulting dynamics in real time. To obtain a complete picture of molecular dynamics following EUV induced photofragmentation, we apply the versatile cold target recoil ion momentum spectroscopy reaction microscope technique, which makes use of coincident three-dimensional momentum imaging of fragments resulting from photoexcitation. This system is capable of pump-probe spectroscopy by using a combination of EUV and IR laser pulses with either beam as a pump or probe pulse. We report several experiments performed using this system.