Session U06: X-ray, XFELs, and other Ultrafast Laser Sources

Time-resolved imaging of transient charge-transfer dynamics

The advent of free-electron lasers capable of producing intense pulses of short-wavelength light has enabled new time-resolved studies into photodynamics in a site-selective manner. By choosing a suitable photon energy, photoabsorption can be highly localized at an individual atomic orbital within a molecule. In the presented experiment, this is utilized to study charge between dissociating fragments in gas-phase 2-iodopropane molecules. The parent molecules are photoexcited by an ultraviolet (UV) pump pulse, initiating neutral photodissociation, prior to multiple ionization selectively at the I 4d site by an extreme-ultraviolet (XUV) pulse produced by the SACLA free-electron laser. By imaging the momenta of the resultant In+ ions, signatures of charge transfer from nascent highly charged iodine ions to the neutral alkyl radical cofragment are observed. At longer pump-probe delays, and therefore extended C-I bond distances, charge transfer is no longer possible, localizing charge at the dissociating iodine atom. Detailed analysis of charge transfer signal (which occurs at higher ion momentum due to mutual Coulombic repulsion between charged fragments) as a function of pump-probe delay, iodine charge state and momentum yields new insights into the physics underpinning charge rearrangement.   

Resonance-enhanced multiphoton ionization in the x-ray regime

Multiphoton ionization is one of the fundamental nonlinear processes when matter interacts with intense laser fields. In particular, Resonance-enhanced multiphoton ionization (REMPI) has been a widely-used spectroscopic technique but has yet been extended to the x-ray regime since it requires entirely different physical processes and interpretation. Conventional REMPI relies on the resonant excitation of a valence electron where the only relaxation pathway is radiative decay. On the other hand, a core-excited state after x-ray resonant excitation is subject to Auger-Meitner decay, which is orders of magnitude faster than radiative decay. Thus, the complex interplay between ultrafast decay processes and REMPI renders this process challenging to fully resolve in the x-ray regime. We present a first observation of REMPI in the x-ray regime. We observe nonlinear ionization to create Ar¹⁷⁺, where photon energies are insufficient to directly ionize a 1s electron. With the aid of state-of-the-art theoretical modeling, we attribute the ionization to a two-color REMPI-like process where the second harmonic creates a 1s → 2p transition and the fundamental pulse subsequently ionizes the system. The measured resonance profile of x-ray REMPI shows a broad, asymmetric, red-shifted distribution, which is a clear distinction from the conventional REMPI case. Moreover, theoretical results demonstrate a strong pulse length dependence of the resonance profile. Our analysis shows that the REMPI process occurs not only for Ar¹⁶⁺ but also for lower charge states, where multiple ionization competes with Auger lifetimes. We find the observed broadband nature and pulse-length dependence of the resonance profile to be due to overlapping resonances with lower Ar charge states.   

Coulomb explosion imaging of small polyatomic molecules with ultrashort x-ray pulses

Recent multi-ion coincidence experiments with high-repetition-rate x-ray free-electron laser pulses show promise to take the decades-old Coulomb explosion imaging technique to the next level. The coincident momentum measurement of the ion fragments from molecules irradiated by ultrashort x-ray pulses enables the reconstruction of the full 3D geometries of ground-state molecules including CH3I and CH2I2. By taking such measurements at varying time delays between the optical and x-ray pulses, we can directly capture the full 3D motion of molecules excited by optical pulses. The Fourier-decomposed components of the 3D motion match well with the expected vibrational modes for both molecules.   

Resonant Double-Inner-Shell Excitations in Nitrous Oxide with Ultrafast, Intense X-rays

The unprecedented intensities at few-to-sub-femtosecond timescales provided by X-ray free electron laser (XFEL) pulses can drive multiphoton interactions in molecules to generate new phenomena such as localized wavepackets of valence excited states. Understanding the additional fundamental processes occurring under such conditions is crucial to make sense of new experimental results. Under high-intensity, low probability processes can greatly influence the measurements; in resonant excitation with few-to-sub-femtosecond X-rays, sequential resonant excitations occurring prior to core-hole decay could significantly impact experiments inducing excited state dynamics. Thus, we theoretically investigate double nitrogen K-edge excitations of nitrous oxide (N₂O) with multiconfigurational electronic structure calculations. Advanced modelling techniques predict the experimental conditions and observables, including time-dependent simulations producing single and double core-excited states under intense, few-to-sub-femtosecond X-ray's, on-the-fly dynamics of the respective nuclear motion and two-step double core-excited Auger-Meitner decay spectra.   

Development of Attosecond Two-Color Pump/Probe Techniques with X-ray Free-Electron Lasers

The natural time scale of quantum mechanical motion of electrons in molecules and solids is on the order of hundreds of attoseconds to few femtoseconds. Probing dynamics on such timescales requires the generation of attosecond pulse pairs with variable delay and synchronization down to the sub-femtosecond level. We report the generation of GW-level attosecond pump/probe pulse pairs with tunable sub-femtosecond delays at the Linac Coherent Light Source (LCLS). Two pulses are generated in a harmonic configuration to access sub-femtosecond delays. We use the angular streaking technique to measure temporal delays between pulse pairs and demonstrate control of our pump/probe setup with attosecond accuracy. The measured minimal achievable delay is 200~300 attoseconds, with a corresponding delay jitter that is shorter than the duration of the pulse.    

Enhancements to ATOMIC to Enable More Options in Modeling XFEL-related Work

Isochoric heating by x-rays in the Kalpha – K edge range of mid-Z elements is of much interest as a method for the creation of large volumes of homogeneous Warm Dense Matter (WDM). WDM is ubiquitous in the Universe (eg planetary interiors) and in laboratory-produced plasmas. Ionization Potential Depression (IPD) is the lowering of the ionization threshold of an ion due to the electrostatic potential from surrounding electrons and neighboring ions. While the Atomic code has historically used the Stewart-Pyatt model in non-LTE calculations, the alternative Ecker-Kroll model has been implemented into the ATOMIC code. These two models will then be assessed by examining the IPD shift of Cu and Fe, with the emphasis on providing higher fidelity modeling capability for experiments that will eventually be executed on the European X-ray Free Electron Laser (XFEL). The planned measurements will record Cu and Fe emission spectra that can be directly compared to the ATOMIC predictions.

    

Generation of Electron Beams exceeding 100 keV by Direct Laser Acceleration using Longitudinal Electric Fields

We report on the generation of electron beams exceeding 100 keV by direct laser acceleration in a low-density gas using longitudinal electric fields. The utilization of radially polarized laser modes to generate longitudinal electric fields is a promising research area in the laser-based electron acceleration community. The Advanced Light Laser Source (ALLS) provided a 3 mJ, 12 fs pulse, centered at 1.8 µm with a repetition rate of 100 Hz. A TM₀₁ laser mode is created using a polarization state convertor and focused with an on-axis parabola (NA = 0.4) in a low-density molecular oxygen environment. The large longitudinal electric field component at the focus is theoretically capable of accelerating electrons up to relativistic energies with sub-femtosecond bunch durations. Our results in the gigawatt regime show the ability to create an electron beam up to a measured energy of 180 keV by scaling of the incident laser pulse energy. At these high intensities (> 10¹⁷ W/cm²), factors such as carrier envelop phase, non-optimal electron injection, nonlinear propagation effects and space charge can all play an increasing complex role in the acceleration mechanism. Further optimization coupled with increasing the laser pulse energy provides a viable pathway towards the ability to generate an energetic, ultrashort electron beam.   

Coherence in macroscopic high harmonic generation for different spatial focal phase distributions

To best take advantage of ultrafast laser sources from high harmonic generation (HHG) one needs to control properties of the spectrum such as the extent of the highest harmonics, the strength of the harmonic lines, and the cleanliness of the harmonics. To this end we study the impact of phase matching on the macroscopic HHG signal for the Gouy and Gaussian phase distributions as a function of the gas jet position [1]. The microscopic yields are obtained by interpolating results of the time-dependent Schrödinger equation and an individual-emitter approach is used to generate the approximate macroscopic signals [2]. Several measures are developed to qualitatively discuss the strengths and widths of the harmonic line and the relevance of off-harmonic radiation. For both the Gouy and Gaussian phase distributions the largest extent of the spectra is for positions of the gas jet after the focus. The relative strength of the harmonics is weaker for the Gaussian phase distribution due to the additional transverse dependence of the focal phase.   

Spectral Splitting of High Harmonic Generation in Extreme High-power Chirped Pulses

High harmonic generation (HHG) from the interaction of intense laser field with gas has been vastly studied over the past three decades. The microscopic physics behind this process is well explained by the semi-classical three-step model [1]: An electron is first ionized, then accelerates along a classical trajectory, and finally emits an extreme-ultraviolet (XUV) photon when re-collides with its parent atom. This process repeats itself every half optical cycle, which results in a comb-like spectrum of odd multiples of the laser’s central frequency.

Introducing a temporal chirp to the laser pulse shifts the harmonic spectrum in proportion to the amount of chirp and laser intensity [2–4]. Since chirped pulses are also stretched in time, the intensity of the driving laser pulse of a given energy is lowered. This sets an energy-dependent limit on the maximal possible chirp for which the intensity of the pulse is above the HHG threshold. Here, we present the advent of spectral splitting of individual harmonics when an extreme amount of temporal chirp is applied to the laser pulse.

We demonstrated the dependence of the HHG spectrum on the laser pulse’s temporal chirp using the NePTUN 20 TW laser system at Tel Aviv University [5]. This system delivers 25 fs long pulses with a central wavelength of 800 nm, and up to 600 mJ in energy. The pulses were focused using a 600 cm focal length lens into a variable-length cell, which in these experiments was set to be 6 cm long and was filled with Ar.

 

We reproduce the behavior of measured spectra numerically, and attribute the observed spectral splitting to temporal modulations of the harmonic generation efficiency with the evolving phase mismatch along the focal axis inside the gas cell.  

1. P. B. Corkum, Phys. Rev. Lett. 71 (1993).

2. D. G. Lee et al., Phys. Rev. Lett. 87 (2001).

3. J. Zhou, et al., Phys. Rev. Lett. 76 (1996).

4. S. J. Goh et al., Opt. express 24 (2016).

5. E. Porat et al. Proc. of SPIE 11036 110360I (2019)   

Demonstration of space-time wave packets localized in all dimensions

Space-time (ST) wave packets are pulsed beams endowed with tight spatio-temporal spectral correlations, resulting in propagation invariance at arbitrary group velocities. We previously introduced an experimental methodology to synthesize ST light sheets based on a 2D pulse shaper. Here, we introduce an experimental method to synthesize ST wave packets localized in all dimensions, including both transverse dimensions and time. This approach for synthesizing 3D ST relies on volume chirped-Bragg-gratings to spatially resolve the spectrum, followed by a Polar-to-Cartesian coordinate transformation to introduce a one-to-one correlation between wavelength and the radial transverse spatial frequency. Modulating this polar-to-Cartesian transformation enables dynamical tuning of the group velocity of the 3D ST wave packets over a wide range of values, whether subluminal or superluminal. We characterize the synthesized 3D ST wave packets in the Fourier domain to confirm the spatio-temporal structure and in the physical domain to verify the propagation invariance, and tune the group velocity of wave packets from 0.8c to 1.8c (c is the speed of light in vacuum). Finally, we assign a helical phase into the ST wave packets and observe diffraction-free ST-OAM wave packets traveling at 1.2c in free space.