103 research outputs found
Femtosecond-Terawatt Hard X Ray Pulse Generation with Chirped Pulse Amplification on a Free Electron Laser
Advances of high intensity lasers have opened up the field of strong field
physics and led to a broad range of technological applications. Recent x ray
laser sources and optics development makes it possible to obtain extremely high
intensity and brightness at x ray wavelengths. In this paper, we present a
system design that implements chirped pulse amplification for hard x ray free
electron lasers. Numerical modeling with realistic experimental parameters show
that near-transform-limit single-femtosecond hard x ray laser pulses with peak
power exceeding 1 TW and brightness exceeding
smmmrad0.1\%bandwdith can be
consistently generated. Realization of such beam qualities is essential for
establishing systematic and quantitative understanding of strong field x-ray
physics and nonlinear x ray optics phenomena.Comment: 23 pages, 7 figures, Accepted by PR
Imaging ultrafast excited state pathways in transition metal complexes by X-ray transient absorption and scattering using X-ray free electron laser source
This report will describe our recent studies of transition metal complex structural dynamics on the fs and ps time scales using an X-ray free electron laser source, Linac Coherent Light Source (LCLS). Ultrafast XANES spectra at the Ni K-edge of nickel(II) tetramesitylporphyrin (NiTMP) were successfully measured for optically excited state at a timescale from 100 fs to 50 ps, providing insight into its sub-ps electronic and structural relaxation processes. Importantly, a transient reduced state Ni(I) (π, 3d(x2−y2)) electronic state is captured through the interpretation of a short-lived excited state absorption on the low-energy shoulder of the edge, which is aided by the computation of X-ray transitions for postulated excited electronic states. The observed and computed inner shell to valence orbital transition energies demonstrate and quantify the influence of electronic configuration on specific metal orbital energies. A strong influence of the valence orbital occupation on the inner shell orbital energies indicates that one should not use the transition energy from 1s to other orbitals to draw conclusions about the d-orbital energies. For photocatalysis, a transient electronic configuration could influence d-orbital energies up to a few eV and any attempt to steer the reaction pathway should account for this to ensure that external energies can be used optimally in driving desirable processes. NiTMP structural evolution and the influence of the porphyrin macrocycle conformation on relaxation kinetics can be likewise inferred from this study
Determination of nonthermal bonding origin of a novel photoexcited lattice instability in SnSe
Interatomic forces that bind materials are largely determined by an often
complex interplay between the electronic band-structure and the atomic
arrangements to form its equilibrium structure and dynamics. As these forces
also determine the phonon dispersion, lattice dynamics measurements are often
crucial tools for understanding how materials transform between different
structures. This is the case for the mono-chalcogenides which feature a number
of lattice instabilities associated with their network of resonant bonds and a
large tunability in their functional properties. SnSe hosts a novel lattice
instability upon above-bandgap photoexcitation that is distinct from the
distortions associated with its high temperature phase transition,
demonstrating that photoexcitation can alter the interatomic forces
significantly different than thermal excitation. Here we report decisive
time-resolved X-ray scattering-based measurements of the nonequlibrium lattice
dynamics in SnSe. By fitting interatomic force models to the excited-state
dispersion, we determine this instability as being primarily due to changes in
the fourth-nearest neighbor bonds that connect bilayers, with relatively little
change to the intralayer resonant bonds. In addition to providing critical
insight into the nonthermal bonding origin of the instability in SnSe, such
measurements will be crucial for understanding and controlling materials
properties under non-equilibrium conditions
Ultrafast Excited State Relaxation of a Metalloporphyrin Revealed by Femtosecond X-ray Absorption Spectroscopy
Photoexcited NickelÂ(II) tetramesitylporphyrin
(NiTMP), like many
open-shell metalloporphyrins, relaxes rapidly through multiple electronic
states following an initial porphyrin-based excitation, some involving
metal centered electronic configuration changes that could be harnessed
catalytically before excited state relaxation. While a NiTMP excited
state present at 100 ps was previously identified by X-ray transient
absorption (XTA) spectroscopy at a synchrotron source as a relaxed
(d,d) state, the lowest energy excited state (<i>J. Am. Chem.
Soc.</i>, <b>2007</b>, <i>129</i>, 9616 and <i>Chem. Sci.</i>, <b>2010</b>, <i>1</i>, 642),
structural dynamics before thermalization were not resolved due to
the ∼100 ps duration of the available X-ray probe pulse. Using
the femtosecond (fs) X-ray pulses of the Linac Coherent Light Source
(LCLS), the Ni center electronic configuration from the initial excited
state to the relaxed (d,d) state has been obtained via ultrafast Ni
K-edge XANES (X-ray absorption near edge structure) on a time scale
from hundreds of femtoseconds to 100 ps. This enabled the identification
of a short-lived NiÂ(I) species aided by time-dependent density functional
theory (TDDFT) methods. Computed electronic and nuclear structure
for critical excited electronic states in the relaxation pathway characterize
the dependence of the complex’s geometry on the electron occupation
of the 3d orbitals. Calculated XANES transitions for these excited
states assign a short-lived transient signal to the spectroscopic
signature of the NiÂ(I) species, resulting from intramolecular charge
transfer on a time scale that has eluded previous synchrotron studies.
These combined results enable us to examine the excited state structural
dynamics of NiTMP prior to thermal relaxation and to capture intermediates
of potential photocatalytic significance
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