Roadmap on dynamics of molecules and clusters in the gas phase

This roadmap article highlights recent advances, challenges and future prospects in studies of the dynamics of molecules and clusters in the gas phase. It comprises nineteen contributions by scientists with leading expertise in complementary experimental and theoretical techniques to probe the dynamics on timescales spanning twenty order of magnitudes, from attoseconds to minutes and beyond, and for systems ranging in complexity from the smallest (diatomic) molecules to clusters and nanoparticles. Combining some of these techniques opens up new avenues to unravel hitherto unexplored reaction pathways and mechanisms, and to establish their significance in, e.g. radiotherapy and radiation damage on the nanoscale, astrophysics, astrochemistry and atmospheric science

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr2IrO4.

We report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the [Formula: see text] states in Sr2IrO4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump-hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps-1.5 ns

Nonequilibrium lattice-driven dynamics of stripes in nickelates using time-resolved x-ray scattering

We investigate the lattice coupling to the spin and charge orders in the striped nickelate, La1.75Sr0.25NiO4, using time-resolved resonant x-ray scattering. Lattice-driven dynamics of both spin and charge orders are observed when the pump photon energy is tuned to that of an Eu bond- stretching phonon. We present a likely scenario for the behavior of the spin and charge order parameters and its implications using a Ginzburg-Landau theory

Decoupling spin-orbital correlations in a layered manganite amidst ultrafast hybridized charge-transfer band excitation

In the mixed-valence manganites, a near-infrared laser typically melts the orbital and spin order simultaneously, corresponding to the photoinduced d1d0→d0d1 excitations in the Mott-Hubbard bands of manganese. Here, we use ultrafast methods-both femtosecond resonant X-ray diffraction and optical reflectivity-to demonstrate that the orbital response in the layered manganite Nd1-xSr1+xMnO4(x=2/3) does not follow this scheme. At the photoexcitation saturation fluence, the orbital order is only diminished by a few percent in the transient state. Instead of the typical d1d0→d0d1 transition, a near-infrared pump in this compound promotes a fundamentally distinct mechanism of charge transfer, the d0→d1L, where L denotes a hole in the oxygen band. This finding may pave a different avenue for selectively manipulating specific types of order in complex materials of this class

Clocking femtosecond collisional dynamics via resonant X-ray spectroscopy

Electron-ion collisional dynamics is of fundamental importance in determining plasma transport properties, non-equilibrium plasma evolution and electron damage in diffraction imaging applications using bright x-ray free-electron lasers (FELs). Here we describe the first experimental measurements of ultra-fast electron impact collisional ionization dynamics using resonant core-hole spectroscopy in a solid-density magnesium plasma, created and diagnosed with the Linac Coherent Light Source x-ray FEL. By resonantly pumping the 1s ! 2p transition in highly-charged ions within an optically-thin plasma we have measured how off-resonance charge states are populated via collisional processes on femtosecond times scales. We present a collisional cross section model that matches our results and demonstrates how the cross sections are enhanced by dense-plasma effects including continuum lowering. Non-LTE (local thermodynamic equilibrium) collisional radiative simulations show excellent agreement with the experimental results, and provide new insight on collisional ionization and three-body-recombination processes in the dense plasma regime

Transient vibration and product formation of photoexcited CS2 measured by time-resolved x-ray scattering

We have observed details of the internal motion and dissociation channels in photoexcited carbon disulfide (CS<sub>2</sub>) using time-resolved x-ray scattering (TRXS). Photoexcitation of gas-phase CS<sub>2</sub> with a 200 nm laser pulse launches oscillatory bending and stretching motion, leading to dissociation of atomic sulfur in under a picosecond. During the first 300 fs following excitation, we observe significant changes in the vibrational frequency as well as some dissociation of the C-S bond, leading to atomic sulfur in the both <sup>1</sup>D and <sup>3</sup>P states. Beyond 1400 fs, the dissociation is consistent with primarily <sup>3</sup>P atomic sulfur dissociation. This channel-resolved measurement of the dissociation time is based on our analysis of the time-windowed dissociation radial velocity distribution, which is measured using the temporal Fourier transform of the TRXS data aided by a Hough transform that extracts the slopes of linear features in an image. The relative strength of the two dissociation channels reflects both their branching ratio and differences in the spread of their dissociation times. Measuring the time-resolved dissociation radial velocity distribution aids the resolution of discrepancies between models for dissociation proposed by prior photoelectron spectroscopy work