Shape Resonances as a Probe of an Evolving Nuclear and Electronic Structure in Molecules

Shape resonances are a ubiquitous phenomenon in electron–molecule scattering, in which the impinging electron is resonantly captured in a pseudo-bound state that is supported by the molecular potential. To study the electron scattering dynamics, we use time- and angle- resolved photoelectron spectroscopy here. With this technique, the transient evolution of the photoelectron angular distributions (PADs) from the ionization of an excited-state species can be measured. In the PADs, the electron–molecular-ion scattering dynamics are contained because the photoelectron necessarily interacts with the potential of the parent molecule as it escapes. The aim of this thesis is to investigate to what extent molecular dynamics, which are triggered by a pump laser pulse, are reflected in the PADs of the photoelectron spectra generated by an ionizing probe pulse, and how these effects can be rationalized in a photoelectron-scattering picture. Three experimental studies are covered in this thesis: In the first experiment, CF3I molecules are impulsively aligned in space by a short near-infrared pulse, which creates a rotational wave packet. During the revival of the rotational wave packet, PADs are measured for different molecular-axes distributions by photoionization with an ultrashort XUV pulse generated through high-order harmonic generation (HHG). Comparing the PADs thus obtained to the results of quantum-scattering calculations carried out with the ePolyScat suite of programs, we show that the alignment-dependent change in the PADs can be largely explained by two prominent shape resonances that contribute to the PADs in a distinctly different way geometrically. In the second experiment, we investigate the laser-assisted photoelectron recollisions that occur in strong-field ionization of atoms and molecules. We show how the differential scattering cross sections (DCSs) for the electron–molecular-ion collision process can be extracted from the resulting photoelectron spectrum. Then, we apply this approach to the investigation of the excited-state dynamics of I2 molecules that are prepared in the A or B state, leading to photodissociation and the creation of a vibrational wave packet, respectively. Again, by comparing to calculations carried out with ePolyScat, we conclude that the observed modulations in the DCSs of the rescattered electrons can be very well explained by considering two prominent shape resonances involved, the l=6 resonance of the diatomic molecular ion and the l=3 resonance of the free iodine atomic ion. In the third study, the time-resolved core-shell photoionization of dissociating halomethane molecules, namely CH3I and CH2ICl, is investigated employing ultrashort soft x-ray pulses provided by the free-electron laser FLASH in Hamburg, which are able to ionize the 4d shell of iodine close to the well-known “giant” photoionization resonance (again related to the l=3 shape resonance). We find that the dissociation clearly manifests as a shift of the 4d core-level binding energy, and that the time scale and temporal onset of this effect is distinctly different from that of the photoion measurements, which are commonly exploited to quantify the dissociation dynamics

Photoinduced Charge Carrier Dynamics and Electron Injection Efficiencies in Au Nanoparticle-Sensitized TiO2_2 Determined with Picosecond Time-Resolved X-ray Photoelectron Spectroscopy

Progress in the development of plasmon-enabled light-harvesting technologies requires a better understanding of their fundamental operating principles and current limitations. Here, we employ picosecond time-resolved X-ray photoemission spectroscopy to investigate photoinduced electron transfer in a plasmonic model system composed of 20 nm sized gold nanoparticles (NPs) attached to a nanoporous film of TiO2. The measurement provides direct, quantitative access to transient local charge distributions from the perspectives of the electron donor (AuNP) and the electron acceptor (TiO2). On average, approximately two electrons are injected per NP, corresponding to an electron injection yield per absorbed photon of 0.1%. Back electron transfer from the perspective of the electron donor is dominated by a fast recombination channel proceeding on a time scale of 60 ± 10 ps and a minor contribution that is completed after ∼1 ns. The findings provide a detailed picture of photoinduced charge carrier generation in this NP–semiconductor junction, with important implications for understanding achievable overall photon-to-charge conversion efficiencies

Imaging via Correlation of X-Ray Fluorescence Photons

We demonstrate that x-ray fluorescence emission, which cannot maintain a stationary interference pattern, can be used to obtain images of structures by recording photon-photon correlations in the manner of the stellar intensity interferometry of Hanbury Brown and Twiss. This is achieved utilizing femtosecond-duration pulses of a hard x-ray free-electron laser to generate the emission in exposures comparable to the coherence time of the fluorescence. Iterative phasing of the photon correlation map generated a model-free real-space image of the structure of the emitters. Since fluorescence can dominate coherent scattering, this may enable imaging uncrystallised macromolecules

Photodissociation of Aligned CH3I\mathrm{CH_3I} and C6H3F2I\mathrm{C_{6}H_{3}F_{2}I} Molecules Probed with Time-Resolved Coulomb Explosion Imaging by Site-Selective XUV Ionization

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon–iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules

Photodissociation of aligned CH3I and C6H3F2I molecules probed with time-resolved Coulomb explosion imaging by site-selective extreme ultraviolet ionization.

We explore time-resolved Coulomb explosion induced by intense, extreme ultraviolet (XUV) femtosecond pulses from a free-electron laser as a method to image photo-induced molecular dynamics in two molecules, iodomethane and 2,6-difluoroiodobenzene. At an excitation wavelength of 267 nm, the dominant reaction pathway in both molecules is neutral dissociation via cleavage of the carbon-iodine bond. This allows investigating the influence of the molecular environment on the absorption of an intense, femtosecond XUV pulse and the subsequent Coulomb explosion process. We find that the XUV probe pulse induces local inner-shell ionization of atomic iodine in dissociating iodomethane, in contrast to non-selective ionization of all photofragments in difluoroiodobenzene. The results reveal evidence of electron transfer from methyl and phenyl moieties to a multiply charged iodine ion. In addition, indications for ultrafast charge rearrangement on the phenyl radical are found, suggesting that time-resolved Coulomb explosion imaging is sensitive to the localization of charge in extended molecules.peerReviewe

UV-induced dissociation of CH2_2BrI probed by intense femtosecond XUV pulses

The ultraviolet (UV)-induced dissociation and photofragmentation of gas-phase CH$_2$BrI molecules induced by intense femtosecond extreme ultraviolet (XUV) pulses at three different photon energies are studied by multi-mass ion imaging. Using a UV-pump–XUV-probe scheme, charge transfer between highly charged iodine ions and neutral CH$_2$Br radicals produced by C–I bond cleavage is investigated. In earlier charge-transfer studies, the center of mass of the molecules was located along the axis of the bond cleaved by the pump pulse. In the present case of CH$_2$BrI, this is not the case, thus inducing a rotation of the fragment. We discuss the influence of the rotation on the charge transfer process using a classical over-the-barrier model. Our modeling suggests that, despite the fact that the dissociation is slower due to the rotational excitation, the critical interatomic distance for charge transfer is reached faster. Furthermore, we suggest that charge transfer during molecular fragmentation may be modulated in a complex way

Time-resolved inner-shell photoelectron spectroscopy: From a bound molecule to an isolated atom

Due to its element and site specificity, inner-shell photoelectron spectroscopy is a widely used technique to probe the chemical structure of matter. Here, we show that time-resolved inner-shell photoelectron spectroscopy can be employed to observe ultrafast chemical reactions and the electronic response to the nuclear motion with high sensitivity. The ultraviolet dissociation of iodomethane (CH$_3$I) is investigated by ionization above the iodine $4d$ edge, using time-resolved inner-shell photoelectron and photoion spectroscopy. The dynamics observed in the photoelectron spectra appear earlier and are faster than those seen in the iodine fragments. The experimental results are interpreted using crystal-field and spin-orbit configuration interaction calculations, and demonstrate that time-resolved inner-shell photoelectron spectroscopy is a powerful tool to directly track ultrafast structural and electronic transformations in gas-phase molecules

Time-resolved inner-shell photoelectron spectroscopy: From a bound molecule to an isolated atom

Citation: Brauße, F., Goldsztejn, G., Amini, K., Boll, R., Bari, S., Bomme, C., … Rolles, D. (2018). Time-resolved inner-shell photoelectron spectroscopy: From a bound molecule to an isolated atom. Physical Review A, 97(4). https://doi.org/10.1103/PhysRevA.97.043429Due to its element and site specificity, inner-shell photoelectron spectroscopy is a widely used technique to probe the chemical structure of matter. Here, we show that time-resolved inner-shell photoelectron spectroscopy can be employed to observe ultrafast chemical reactions and the electronic response to the nuclear motion with high sensitivity. The ultraviolet dissociation of iodomethane (CH3I) is investigated by ionization above the iodine 4d edge, using time-resolved inner-shell photoelectron and photoion spectroscopy. The dynamics observed in the photoelectron spectra appear earlier and are faster than those seen in the iodine fragments. The experimental results are interpreted using crystal-field and spin-orbit configuration interaction calculations, and demonstrate that time-resolved inner-shell photoelectron spectroscopy is a powerful tool to directly track ultrafast structural and electronic transformations in gas-phase molecules