Anti-Stokes resonant x-ray Raman scattering for atom specific and excited state selective dynamics

Abstract

Ultrafast electronic and structural dynamics of matter govern rate and selectivity of chemical reactions, as well as phase transitions and efficient switching in functional materials. Since x-rays determine electronic and structural properties with elemental, chemical, orbital and magnetic selectivity, short pulse x-ray sources have become central enablers of ultrafast science. Despite of these strengths, ultrafast x-rays have been poor at picking up excited state moieties from the unexcited ones. With time-resolved anti-Stokes resonant x-ray Raman scattering (AS-RXRS) performed at the LCLS, and ab initio theory we establish background free excited state selectivity in addition to the elemental, chemical, orbital and magnetic selectivity of x-rays. This unparalleled selectivity extracts low concentration excited state species along the pathway of photo induced ligand exchange of Fe(CO) 5 in ethanol. Conceptually a full theoretical treatment of all accessible insights to excited state dynamics with AS-RXRS with transform-limited x-ray pulses is given-which will be covered experimentally by upcoming transform-limited x-ray sources.With the rapid evolution of sub-picosecond and femtosecond x-ray sources and particularly with the emergence x-ray free-electron lasers, first steps to join the unique electronic structure information of x-ray spectroscopy with the ultrafast time scale of dynamics in matter have been taken [1][2][3][4][5][6][7][8]. Often though, time resolved x-ray spectroscopy at these dilute transient species with x-ray absorption, x-ray fluorescence or electron spectroscopy suffer often from the difficult separation between the overlapping spectral signatures of the dilute excited state species and the ground state. To reach improved chemical selectivity with x-ray lasers nonlinear and OPEN ACCESS RECEIVED

Local Maps of Potential Energy Surfaces and Chemical Pathways

“…Usually, when investigating a subset of excited state species in a surrounding of non-excited species, it is extremely challenging to disentangle spectral contributions from either species so as to obtain selective information. Background-free information on the low concentration excited states is possible by utilizing the anti-Stokes channel of RIXS (AS-RIXS), where RIXS of the optically pumped molecule result in spectra features appearing on the negative energy transfer side of the RIXS spectrum [8]. The AS-RIXS process is depicted in the right panel in Figure 5.…”

“…The molecule is optically excited into the metal to ligand charge transfer (MLCT) state. Its decay and the MLCT rate are then monitored using time-resolved RIXS [8,22,23].…”

“…From the temporal evolution of the intensity in AS-RIXS regions, we observe excited-state dynamics of the transient-state 1 B 2 (LF)Fe(CO) 4 (panel R3) and the final photoproduct 3 B 2 (LF)Fe(CO) 4 (panel R4), which are clearly separated in energy from the signal of the neutral species. In R3, the anti-Stokes intensity disappears after ~400 fs with a deconvoluted exponential time constant of ~100 fs [8], whereas the photoproduct in region R4 retains significant intensity. RASSCF calculations for all excited states and the kinetic rate model of the photochemical pathway are in good agreement with the observed spectral features.…”

“…With this approach of subnatural linewidth RIXS, direct experimental access to potential energy surfaces becomes a reality; in particular, at the active centers and for distortions far from equilibrium that are close to transition and bifurcation points in reactions. Applying this approach of sub-natural line width RIXS to electronically or spin-excitated states will lead to Anti-Stokes spectral signatures [8], which additionally give insight into the potential energy surfaces of excited states. Here, sub-natural line width RIXS with transform-limited pulses will open a completely new approach to excited-state dynamics and reaction pathways.…”

Time-resolved electron(e,2e)momentum spectroscopy: Application to laser-driven electron population transfer in atoms

“…Moreover, various aspects of electronic motions can be retrieved using different probe schemes. For example, attosecond transient absorption spectroscopy tracks the frequencies, phases, and lifetimes of excited oscillating dipoles [17,18], time-resolved x-ray absorption and x-ray Raman spectroscopy provide element-specific probes of evolving electronic motion and chemical bonding [19,20], and x-ray and electron diffraction are able to directly image electronic spatial motions with (sub)angstrom resolution [21][22][23].…”