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Journal of physics / B 55(1), 014001 (2022). doi:10.1088/1361-6455/ac489d, 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., Published by IOP Publ., Bristol

XUV multiphoton ionization of molecules is commonly used in free-electron laser experiments to study charge transfer dynamics. However, molecular dissociation and electron dynamics, such as multiple photon absorption, Auger decay, and charge transfer, often happen on competing time scales, and the contributions of individual processes can be difficult to unravel. We experimentally investigate the Coulomb explosion dynamics of methyl iodide upon core–hole ionization of the shallow inner-shell of iodine (4d) and classically simulate the fragmentation by phenomenologically introducing ionization dynamics and charge transfer. Under our experimental conditions with medium fluence and relatively long XUV pulses (∼75 fs), we find that fast Auger decay prior to charge transfer significantly contributes to the charging mechanism, leading to a yield enhancement of higher carbon charge states upon molecular dissociation. Furthermore, we argue for the existence of another charging mechanism for the weak fragmentation channels leading to triply charged carbon atoms. This study shows that classical simulations can be a useful tool to guide the quantum mechanical description of the femtosecond dynamics upon multiphoton absorption in molecular systems.

Physical review / A 103(2), 022832 (2021). doi:10.1103/PhysRevA.103.022832, We report angle-resolved measurements on photoelectrons emitted upon near-threshold two-photon double ionization (TPDI) of Kr irradiated by free-electron laser (FEL) pulses. These photoelectron angular distributions (PADs) are compared with the results of semirelativistic R-matrix calculations. As reported by Augustin et al. [Phys. Rev. A 98, 033408 (2018)], it is found that the presence of autoionizing resonances within the bandwidth of the exciting FEL pulse strongly influences the PADs. In contrast to measurements on lower-Z targets such as Ne and Ar, the larger spin-orbit interaction, inherent in 4p-subshell hole states of Kr, permits us to resolve and study PADs associated with some of the fine-structure components of the Kr$^+$ and Kr$^{2+}$ ions., Published by Inst., Woodbury, NY

Optica 9(4), 429 - (2022). doi:10.1364/OPTICA.454920, Ultrafast X-ray spectroscopies require flexible X-ray properties together with high temporal and spectral resolution. Here, we demonstrate simultaneous sub-20 fs and sub-eV resolutions for pump/probe experiments, without the need for additional photon arrival-time monitors., Published by OSA, Washington, DC

Rydberg excitations in the vacuum ultraviolet spectral range may open up molecular photoreaction pathways not accessible from lower-lying valence states. Here, single-shot UV/VUV pump–probe spectroscopy was used to study the photodissociation dynamics of iodomethane after 268 nm excitation in the A-band and excitation of the 6p (2E3/2) Rydberg state at 161 nm. By combining weak-field VUV single-photon ionization with sub-10 fs temporal resolution and the superior statistical accuracy of the single-shot technique, sub-30 fs wave packet dynamics upon excitation in the A-band by a UV pump pulse were disclosed. Population transfer from the Rydberg state to the 2 1A1 valence state leading to 100 fs dissociation dynamics was observed by utilizing the same methodology in a VUV-pump/UV-probe scheme.

The application of a novel fast optical-imaging camera, TimepixCam, to molecular photoionization experiments using the velocity-map imaging technique at a free-electron laser is described. TimepixCam is a 256 × 256 pixel CMOS camera that is able to detect and time-stamp ion hits with 20 ns timing resolution, thus making it possible to record ion momentum images for all fragment ions simultaneously and avoiding the need to gate the detector on a single fragment. This allows the recording of significantly more data within a given amount of beam time and is particularly useful for pump–probe experiments, where drifts, for example, in the timing and pulse energy of the free-electron laser, severely limit the comparability of pump–probe scans for different fragments taken consecutively. In principle, this also allows ion–ion covariance or coincidence techniques to be applied to determine angular correlations between fragments.

Journal of optics 24(2), 025502 (2022). doi:10.1088/2040-8986/ac4318, We present the results of an experiment investigating the generation of high-order harmonics by a femtosecond near-infrared (NIR) laser pulse in the presence of an extreme ultraviolet (XUV) field provided by a free-electron laser (FEL), a process referred to as XUV-assisted high-order harmonic generation (HHG). Our experimental findings show that the XUV field can lead to a small enhancement in the harmonic yield when the XUV and NIR pulses overlap in time, while a strong decrease of the HHG yield and a red shift of the HHG spectrum is observed when the XUV precedes the NIR pulse. The latter observations are in qualitative agreement with model calculations that consider the effect of a decreased number of neutral emitters but are at odds with the predicted effect of the correspondingly increased ionization fraction on the phase matching. Our study demonstrates the technical feasibility of XUV-assisted HHG experiments at FELs, which may provide new avenues to investigate correlation-driven electron dynamics as well as novel ways to study and control propagation effects and phase matching in HHG., Published by IOP Publ., Bristol

International Conference on Spectral Lines Shapes, Dublin, Ireland, 17 Jun 2018 - 22 Jun 2018; Journal of physics / Conference Series Conference Series 1289, 012030 - (2019). doi:10.1088/1742-6596/1289/1/012030, In this work, we present measurements of the intensity-dependent photoelectron spectrum of Kr irradiated by the FLASH FEL tuned to a photon energy of 25.8 eV. Intensity dependent photoelectron spectra were obtained with the aid of a Velocity Map Imaging (VMI) spectrometer. As the FEL photon energy is close to threshold, two photon sequential double ionization is favoured. The number of open channels is kept to a minimum and leading to a simple description of the process., Published by IOP Publ., Bristol

The photodissociation dynamics of CH

JoVE 140, e57055 (2018). doi:10.3791/57055, This protocol describes key steps in performing and analyzing femtosecond pump-probe experiments that combine a femtosecond optical laser with a free-electron laser. This includes methods to establish the spatial and temporal overlap between the optical and free-electron laser pulses during the experiment, as well as important aspects of the data analysis, such as corrections for arrival time jitter, which are necessary to obtain high-quality pump-probe data sets with the best possible temporal resolution. These methods are demonstrated for an exemplary experiment performed at the FLASH (Free-electron LASer Hamburg) free-electron laser in order to study ultrafast photochemistry in gas-phase molecules by means of velocity map ion imaging. However, most of the strategies are also applicable to similar pump-probe experiments using other targets or other experimental techniques., Published by JoVE, [S.l.]

International audience; 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 (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.

Extreme ultraviolet (XUV) and X-ray free-electron lasers enable new scientific opportunities. Their ultra-intense coherent femtosecond pulses give unprecedented access to the structure of undepositable nanoscale objects and to transient states of highly excited matter. In order to probe the ultrafast complex light-induced dynamics on the relevant time scales, the multi-purpose end-station CAMP at the free-electron laser FLASH has been complemented by the novel multilayer-mirror-based split-and-delay unit DESC (DElay Stage for CAMP) for time-resolved experiments. XUV double-pulses with delays adjustable from zero femtoseconds up to 650 picoseconds are generated by reflecting under near-normal incidence, exceeding the time range accessible with existing XUV split-and-delay units. Procedures to establish temporal and spatial overlap of the two pulses in CAMP are presented, with emphasis on the optimization of the spatial overlap at long time-delays via time-dependent features, for example in ion spectra of atomic clusters.

The journal of chemical physics 149(20), 204313 (2018). doi:10.1063/1.5041381, The photodissociation dynamics of CH$_3$I and CH$_2$ClI at 272 nm were investigated by time-resolved Coulomb explosion imaging, with an intense non-resonant 815 nm probe pulse. Fragment ion momenta over a wide m/z range were recorded simultaneously by coupling a velocity map imaging spectrometer with a pixel imaging mass spectrometry camera. For both molecules, delay-dependent pump-probe features were assigned to ultraviolet-induced carbon-iodine bond cleavage followed by Coulomb explosion. Multi-mass imaging also allowed the sequential cleavage of both carbon-halogen bonds in CH$_2$ClI to be investigated. Furthermore, delay-dependent relative fragment momenta of a pair of ions were directly determined using recoil-frame covariance analysis. These results are complementary to conventional velocity map imaging experiments and demonstrate the application of time-resolved Coulomb explosion imaging to photoinduced real-time molecular motion., Published by American Institute of Physics, Melville, NY

The dynamics following laser-induced molecular photodissociation of gas-phase CH2BrI at 271.6 nm were investigated by time-resolved Coulomb-explosion imaging using intense near-IR femtosecond laser pulses. The observed delay-dependent photofragment momenta reveal that CH2BrI undergoes C-I cleavage, depositing 65.6% of the available energy into internal product states, and that absorption of a second UV photon breaks the C-Br bond of CH2Br. Simulations confirm that this mechanism is consistent with previous data recorded at 248 nm, demonstrating the sensitivity of Coulomb-explosion imaging as a real-time probe of chemical dynamics.

Physical review / A 96(1), 013428 (2017). doi:10.1103/PhysRevA.96.013428, We present a joint experimental and theoretical study of the VUV-induced dynamics of $H_2O$ and its deuterated isotopologues in the first excited state $(A^1B_1)$ utilizing a VUV-pump VUV-probe scheme combined with $ab$ initio classical trajectory calculations. 16-fs VUV pulses centered at 161 nm created by fifth-order harmonic generation are employed for single-shot pump-probe measurements. Combined with a precise determination of the VUV pulses' temporal profile, they provide the necessary temporal resolution to elucidate sub-10-fs dissociation dynamics in the 1+1 photon ionization time window. Ionization with a single VUV photon complements established strong-field ionization schemes by disclosing the molecular dynamics under perturbative conditions. Kinetic isotope effects derived from the pump-probe experiment are found to be in agreement with our by ab initio classical trajectory calculations, taking into account photoionization cross sections for the ground and first excited state of the water cation., Published by Inst., Woodbury, NY

Synopsis In the interstellar medium polycyclic aromatic hydrocarbon molecules (PAH) are exposed to strong ionizing radation leading to complex organic photochemistry. We investigated these ultrafast fragmentation reactions after ionization of the PAHs phenanthrene, fluorene and pyrene at a wavelength of 30.3 nm using pump probe spectroscopy at a free electron laser. We observe double ionization and afterwards hydrogen abstraction and acetylene loss with characteristic time scales for the reaction processes below one hundred femtoseconds.

Physical review / A covering atomic, molecular, and optical physics and quantum information 94(3), 033411(2016). doi:10.1103/PhysRevA.94.033411, Molecular wave-packet dynamics in oxygen are studied in the time domain, using a single-color VUVpump–VUV-probe scheme. 17-fs VUV pulses, centered at 161 nm are generated via high-order harmonicgeneration driven by an intense 800-nm pulse leading to VUV pulse energies that reach 1.1 μJ per pulse. Anall-reflective interferometric pump-probe scheme is used for studying the delay dependence of the molecularoxygen ion signal with simultaneous nonresonant photoionization of krypton as a precise timing-reference.Access to the excited dissociative state lifetime is provided by the resulting delay-dependent O$_{2}^{+}$ signal,ultimately limited by the molecular ionization window. The ability to use a two-photon VUV probe providesthe delay-dependent detection of O$^+$ as an additional observable, extending the dissociation observationwindow., Published by Inst., Woodbury, NY

We demonstrate the generation of microjoule level, sub-20-fs, Ti:Sa fifth-harmonic pulses utilizing a loose-focusing geometry in a long Ar gas cell. The VUV pulses centered at 161.8 nm reach pulse energies of 1.1 μJ per pulse, while the corresponding pulse duration is measured with a second-order, fringe-resolved autocorrelation scheme to be 18±1 fs. Nonresonant, two-photon ionization of Kr and three-photon ionization of Ne verify the fifth-harmonic pulse high-intensity content and indicate the feasibility of multi-photon VUV pump-VUV probe studies of ultrafast atomic and molecular dynamics.

Journal of optics 20(2), 024018 (2018). doi:10.1088/2040-8986/aaa326, The development of ultra-short intense laser sources in the visible and extreme ultraviolet (XUV) spectral range has led to fascinating studies in laser-matter interactions and attosecond science. In the majority of these studies, the system under investigation interacts with a focused light beam, which ionizes the system. The ionization products are usually measured by devices, which spatiotemporally integrate the ionization signal originating from the entire focal area, discarding in this way valuable information about the ionization dynamics that take place in the interaction volume. Here, we review a recently developed approach in measuring the spatially resolved photoionization yields resulting from the interaction of infrared (IR)/XUV ultra-short light pulses in gas phase media. We show how this approach enables (a) the in situ focus diagnostic, (b) quantitative studies of linear and non-linear ionization processes in the IR/XUV regime, (c) single-shot XUV-pump-XUV-probe studies and (d) single-shot 2nd-order XUV autocorrelation measurements., Published by IOP Publ., Bristol

Time-resolved spectroscopy in the vacuum/extreme ultraviolet (VUV/XUV) spectral range promises to disclose ultrafast dynamics in all states of matter. Completing a measurement within a single shot eliminates the influence of experimental parameter fluctuations and enhances its statistical significance. We demonstrate a single-shot temporal metrology scheme operating in the vacuum/extreme-ultraviolet spectral range, combining few-femtosecond resolution in a wide temporal window with high detection efficiency. An anticollinear geometry encodes temporal delay information on the beam propagation coordinate. The spatial distribution of ions created in the common focus is captured with a mass/charge-state-selective ion imaging spectrometer, resulting in a single-shot pump–probe measurement. We demonstrate a proof-of-principle single-shot VUV-pump/VUV-probe experiment by investigating ultrafast dissociation dynamics of O2 excited at 162 nm. The experimental determination of the finite instrument response in the same apparatus enables robust deconvolution of the investigated dynamics well beyond the instrument’s intrinsic temporal resolution.

We demonstrate second-order interferometric autocorrelation of a pulse in the vacuum-ultraviolet (VUV) spectral range using an optical arrangement equivalent to a Michelson interferometer. In an all-reflective design, wavefront splitting is realized with two moveable interdigitated reflective gratings forming a diffraction pattern with well separated orders and an intensity distribution depending on the precisely adjustable path-length difference. An imaging timeof-flight spectrometer is able to spatially select ions created by nonlinear twophoton absorption in the focus of the zeroth diffraction order. This arrangement is used to demonstrate interferometric autocorrelation in krypton with femtosecond VUV pulses at 160nm wavelength. In addition to the pulse duration, which is already accessible with non-collinear intensity autocorrelation, the full interferometric contrast of the presented approach enables us to extract also information on temporal phases.

We demonstrate the generation of a broadband coherent continuum extreme-ultraviolet (XUV) radiation produced by the interaction of gases with a many-cycle infrared (IR) laser field, utilizing a compact collinear many cycle-polarization gating (CMC-PG) device. The spectral width of the XUV radiation can support isolated pulses of 200 asec duration. The CMC-PG device forms a high energy content ultra-short temporal gate in a many-cycle laser pulse, within which the XUV emission is taking place. The gate width has been measured and is in agreement with the theoretical calculations. The simplicity, the compactness, the long term stability, and the high IR energy output within the gate, make the CMC-PG device an ideal tool for generating energetic isolated attosecond pulses and measure the carrier-envelope phase of a high-power many-cycle laser field.