In this comment, we challenge the interpretation of ul-trafast optical pump X-ray probe diffraction experiments on gas phase I 2 put forth recently by Glownia et al. [1]. In that Letter, the x-ray diffraction from a sample per-turbatively prepared with excited state population a is given as ˜ S = N |af e (q) + (1 − a)f g (q)| 2 (1) where N is the number of molecules in the gas and f g/e (q) = g/e|ˆσe|ˆσ(q)|g/e is the ground/excited state elastic scattering amplitude (related to the Fourier transform of the electronic charge densityˆσdensityˆ densityˆσ operator). Reference [1] assumed "incoherent mixtures of ground and excited electronic states", neglecting electronic coherences from the onset. We thus consider only a diagonal electronic density matrix with elements a and 1 − a and restrict attention to elastic scattering. Importantly, the cross term (f g (q)f e (q)) resulting from the squaring in Eq. (1) amounts to heterodyne detection , the interference of a weak signal field (f e) with a strong reference (f g). Such holographic detection has been reported in transient X-ray diffraction in crystals [2]. For weak excitations, where only a small fraction of the molecules are excited (a ≪ 1), the ground-state signal serves as an in situ local oscillator for the weaker excited-state signal. In Ref. [1], it was argued that the linearity in the ex-citation fraction a of the cross term in Eq. (1) renders detection feasible in a heterodyne fashion, while the pure excited-state diffraction scales quadratically in a and is negligible. While we agree that "This signal is an incoherent sum of the coherent diffraction from each molecule", we point out that the correct expression [3, 4] for such a signal is S 1 = N ˆ σ * (q)ˆ σ(q) = N a|f g (q)| 2 + (1 − a)|f e (q)| 2 (2) where the expectation value. .. = Tr [.. . ρ], can be evaluated via a trace over the density matrix. The excited-state diffraction from a gas thus comes linear in the excitation fraction and the amplitude boost from heterodyne detection is neither necessary nor possible. Equation (2) and equivalents obtained from the independent atom approximation and rotational averaging have been known in the literature on time-resolved X-ray scattering for many years and appear also in electron diffrac-tion [3, 5, 6]. The possibility of heterodyne-detected diffraction in crystals (and other systems with long-range order) can be seen by partitioning the total charge density as a sum of molecular charge densitiesˆσdensitiesˆ densitiesˆσ gas = α ˆ σ α in S = ˆ σ * (q)ˆ σ(q). The diagonal terms in this double-sum generate Eq. (2) while the remaining, two-molecule terms are S 2 = α β =α e iq·(rα−r β) |af e (q) + (1 − a)f g (q)| 2 (3) where we have assumed identical molecules located at positions r α. This amounts to the observation that the electronic charge densities of distinct molecules are un-correlated so that, for α = β, we havê σ * β (q)ˆ σ α (q) = ˆ σ * β (q)ˆ σ α (q). The double-summation pre-factor in Eq. (3) encodes the long-range structure of the sample and, in cr...

No abstract

In order to probe the structure of reaction intermediates of photochemical reactions a new setup for laser-initiated time-resolved X-ray absorption (XAS) measurements has been developed. With this approach the arrival time of each photon in respect to the laser pulse is measured and therefore full kinetic information is obtained. All X-rays that reach the detector are used to measure this kinetic information and therefore the detection efficiency of this method is high. The newly developed setup is optimized for time-resolved experiments in the microsecond range for samples with relatively low metal concentration (∼1mM). This setup has been applied to study a multicomponent photocatalytic system with a Co(dmgBF(2))(2) catalyst (dmg(2-) = dimethylglyoximato dianion), [Ru(bpy)(3)](2+) chromophore (bpy = 2,2'-bipyridine) and methyl viologen as the electron relay. On the basis of the analysis of hundreds of Co K-edge XAS spectra corresponding to different delay times after the laser excitation of the chromophore, the presence of a Co(i) intermediate is confirmed. The calculated X-ray transient signal for a model of Co(i) state with a 0.14 Å displacement of Co out of the dmg ligand plane and with the closest solvent molecule at a distance of 2.06 Å gives reasonable agreement with the experimental data.

The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution. Photoexcitation of the prototype material KH 2 PO 4 induces coherent low-frequency motions of the PO 4 tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro-and paraelectric phase of KH 2 PO 4 .charge density maps | charge transfer | femtosecond dynamics | X-ray diffraction I onic crystals are characterized by a periodic arrangement of positive and negative ions with the electronic charges essentially localized at the ionic sites. For a unit cell geometry with a finite electric dipole moment, such materials are ferroelectric and display a macroscopic electric polarization (1). Ferroelectrics have received strong attention both from the viewpoint of their basic properties (1, 2) and for device applications (3). A prototype class of ionic ferroelectrics are hydrogen-bonded potassium dihydrogen phosphate KH 2 PO 4 (KDP) and its isomorphs in which the electric polarization originates from a diplacement between the H 2 PO − 4 units and the K þ cations along the c-axis of the orthorhombic crystal structure (4-9). At a critical temperature of T C ¼ 123 K, the macroscopic polarization disappears and KDP transforms into a paraelectric tetragonal phase (Fig. 1A). The microscopic mechanisms behind this phase transition have remained controversial (cf. Materials and Methods).A fundamental issue for understanding the relation between structure and (ferro)electric properties consists in the interplay of a change of ionic positions and/or hydrogen bond geometries on the one hand with relocations of electronic charge on the other (9, 10, 11). So far, most experimental and theoretical work has addressed this problem under quasi-static conditions, close to equilibrium, and/or on time scales much longer than ionic motions (12). Here, we introduce femtosecond X-ray powder diffraction (13, 14) as a real-time probe of coupled vibrational and charge motions occurring on a time scale between approximately 100 fs and a few picoseconds. As X-rays interact with the electronic charges in the KDP crystallites, a series of diffraction patterns taken with a time resolution of 100 fs allows for deriving the momentary ionic positions and the charge distributions simultaneously. Results and DiscussionThe experiments make use of a pump-probe scheme in which ionic motions and the concomitant charge relocations are induced by electronic excitation via tw...

We demonstrate a subpicosecond 1 kHz femtosecond x-ray source with a well-accessible quasi-point size (10 microm diameter) providing Cu K(alpha) emission with a maximum flux of 6.8 x 10(10) photons/s for continuous operation of 10 h. A new geometry that essentially facilitates the adjustment and diminishes the temporal jitter between the x-ray probe and the laser pump pulse is implemented for time-resolved diffraction experiments.