How electronic superpositions drive nuclear motion following the creation of a localized hole in the glycine radical cation

In this work we have studied the nuclear and electron dynamics in the glycine cation starting from localized hole states, using the Quantum Ehrenfest (QuEh) method. The nuclear dynamics is controlled both by the initial gradient and by the instantaneous gradient that results from the oscillatory electron dynamics (charge migration). We have used the Fourier transform (FT) of the spin densities to identify the normal modes of the electron dynamics. We observe an isomorphic relationship between the electron dynamics normal modes (ED-NM) and the nuclear dynamics, seen in the vibrational normal modes (Vib-NM). The FT spectra obtained this way show bands that are characteristic of the energy differences between the adiabatic hole states. These bands contain individual peaks that are in one-to-one correspondence with atom pair (+ •) ↔(• +) resonances (APR), which in turn stimulate nuclear motion involving the atom pair. With such understanding we anticipate 'designer' coherent superpositions that can drive nuclear motion in a particular direction

Prediction of attosecond light pulses in the VUV range in a high-order-harmonic-generation regime

Attosecond light pulses within the vacuum ultraviolet (VUV) energy range are predicted by solving the time-dependent Schrödinger equation (TDSE) for a model neon atom in short laser pulses of different field polarization states. We compare high-order harmonic generation in linearly polarized laser pulses to the method of polarization gating and find attosecond pulses that approach the Fourier limit of 700 as given by an indium filter, spectrally centered at 15 eV. At such low energies, harmonic generation has low sensitivity to ellipticity, which enables the generation of elliptically polarized attosecond pulses. We also show that emission at the atomic transition energies is strongly damped by including intensity averaging. © 2013 American Physical Society.EPSRC/EP/I032517/1EU Marie Curie Initial Training Network/FASTQUASTDF

Carrier-envelope phase stability of hollow-fibers used for high-energy, few-cycle pulse generation

We investigated the carrier-envelope phase (CEP) stability of a hollow-fiber setup used for high-energy, few-cycle pulse generation. Saturation of the output pulse energy is observed at 0.6 mJ for a 260 um inner-diameter, 1 m long fiber, statically filled with neon, with the pressure adjusted to achieve an output spectrum capable of supporting sub-4fs pulses. The maximum output pulse energy can be increased to 0.8mJ by using either differential pumping, or circularly polarized input pulses. We observe the onset of an ionization-induced CEP instability, which does not increase beyond an input pulse energy of 1.25 mJ due to losses in the fiber caused by ionization. There is no significant difference in the CEP stability with differential pumping compared to static-fill, demonstrating that gas flow in differentially pumped fibers does not degrade the CEP stabilization.Comment: 4 pages, 4 figure

Electron population dynamics in resonant non-linear x-ray absorption in nickel at a free-electron laser

Free-electron lasers provide bright, ultrashort, and monochromatic x-ray pulses, enabling novel spectroscopic measurements not only with femtosecond temporal resolution: The high fluence of their x-ray pulses can also easily enter the regime of the non-linear x-ray–matter interaction. Entering this regime necessitates a rigorous analysis and reliable prediction of the relevant non-linear processes for future experiment designs. Here, we show non-linear changes in the L3-edge absorption of metallic nickel thin films, measured with fluences up to 60 J/cm2. We present a simple but predictive rate model that quantitatively describes spectral changes based on the evolution of electronic populations within the pulse duration. Despite its simplicity, the model reaches good agreement with experimental results over more than three orders of magnitude in fluence, while providing a straightforward understanding of the interplay of physical processes driving the non-linear changes. Our findings provide important insights for the design and evaluation of future high-fluence free-electron laser experiments and contribute to the understanding of non-linear electron dynamics in x-ray absorption processes in solids at the femtosecond timescale