Mapping chemical bonding of reaction intermediates with femtosecond X-ray laser spectroscopy

. We determine the pathways in the photo-dissociation reactions of Fe(CO) 5 both in the gas phase and in solution by mapping the valence electronic structure of the reaction intermediates with femtosecond X-ray laser spectroscopy.


Introduction
Molecular structure and bonding determine the dynamic pathways of molecules in their multidimensional landscapes and thus define the outcome of chemical reactions. Characterizing chemical bonding in short-lived reaction intermediates is hence the key to understanding chemical selectivity.
Here we present our time-resolved X-ray view of a molecular dissociation reaction. We address photo-dissociation of the prototypical transition-metal carbonyl molecule Fe(CO)5 both in the gas phase and in solution ( Figure 1) [1]. How chemical bonding evolves during these reactions is largely unknown and the reaction pathway is debated in solution [2][3][4]. We use X-ray laser spectroscopy to map the electronic structure during dissociation with femtosecond time resolution [5]. This gives direct access to chemical bonding, spin and oxidation state of the metal center in the reaction intermediates and complements structural probes [6]. As we probe the same observables in gas phase and in solution our data enable a critical assessment of the role of the solvent in Fe(CO) 5

Experimental
Fe(CO) 5 molecules were pumped at 266 nm and probed at defined time delays with X-ray pulses from the X-ray free electron laser sources FLASH in Hamburg, Germany, for the gas-phase reaction and from the Linac Coherent Light Source (LCLS) in Stanford, USA, for the solution reaction (concentration 1 mol/l in ethanol). In the gas phase, valence band ( Figure 2, left), core 3p and resonant core-to-valence 3p3d photoelectron spectra (not shown) were measured. In solution, RIXS at the Fe L-edge was used as a probe of the valence electronic structure (Figure 2, right). The temporal resolution in both experiments amounted to ~300 fs given by the jitter between optical and X-ray lasers. Multi-configurational ab initio calculations of the X-ray spectra were performed to identify and characterize in detail the reaction intermediates.

Results
A selection of our femtosecond time-resolved X-ray laser spectroscopy results is shown in Figure 2. The differential gas-phase valence-band photoelectron spectra (Figure 2A) display positive intensities arising from the appearances of CO and photoproducts and negative intensities due to the depletion of Fe(CO) 5 . By accounting for Fe(CO) 5 depletion we deduce the valence-band photoelectron spectrum of Fe(CO) 4 at a delay of 1 ps. This shows how, compared to Fe(CO) 5 , the Fe 3d levels are shifted and the shape of the CO molecular bands are modified by the loss of one CO ligand. This information on chemical bonding complements structural information from electron diffraction [6]. The CO peak ( Figure 2B) at short delays rises in a step-like fashion due to CO dissociation from Fe(CO) 5 (3' in Figure 2B) and it increases exponentially for later delays (4' in Figure 2B) due to thermal CO desorption from vibrationally hot Fe(CO) 4 . This is direct evidence for the sequential CO loss postulated in [2]. With our RIXS approach we probe for the first time a chemical reaction in solution with femtosecond RIXS. The data shown in Figure 2C display the valence excitations probed locally and in an element-and site-selective way at the Fe center of Fe(CO) 4 . Comparison to theory (not shown) shows that Fe(CO) 4 is in a singlet state in contrast to [3] and in agreement with [4]. According to our data ( Figure 2D and calculations) Fe(CO) 5 is excited to a 1MLCT state (12) and relaxes within our time resolution to a 1LF state (23) with subsequent dissociation to singlet Fe(CO) 4 . In contrast to the gas phase the excess energy in Fe(CO) 4 is dissipated to the solvent and no further CO is lost. Details on chemical bonding, spin and oxidation state as inferred from a comparison to our calculations will be presented in addition.

Fig. 2.
Femtosecond time-resolved X-ray laser spectra mapping Fe(CO) 5 dissociation in the gasphase (A, B) and in solution (C-D). A: Gas-phase valence-band photoelectron spectra of, from top to bottom, CO, Fe(CO) 5 for various pump-probe delays (differential spectra), Fe(CO) 4 (calculated from the differential spectrum at 1 ps), Fe(CO) 5 with the Fe 3d and the C and O σ and π molecular orbitals. B: Intensity of the CO peaks vs. delay (circles) with a fit curve (line, step function plus exponential increase). C: RIXS spectra (energy loss vs. incident photon energy tuned to the Fe Labsorption edge) of solvated Fe(CO) 5 and Fe(CO) 4 (taken at 2.1 ps), respectively. D: Intensity of the marked region vs. delay (red) and a fit curve (blue, Gaussian plus step function).