frontier-orbital interactions with atom specificity. We anticipate that the method will be broadly applicable in the chemical sciences, and complement approaches that probe structural dynamics in ultrafast processes.In our experimental set-up (Figure 1a), the valence electronic structure of Fe(CO) 5 is probed with femtosecond-resolution resonant inelastic x-ray scattering (RIXS) at the Fe L 3 -edge (Fe experiments. This triplet arises from a singlet state with a time constant of 300 fs, consolidating the notion 6 that sub-ps intersystem crossing appears to be common in the excited-state dynamics of transition-metal complexes 7,[22][23][24] . The persistence of the triplet Fe(CO) 4 ( 3 B 2 ) up to our maximum time delay of 3 ps is consistent with it undergoing a slow, spin-forbidden reaction with intersystem crossing to a solvent-complexed singlet state on the 50-100 ps time scale 4,5, 25 . However, the observed branching on a sub-ps time scale into the competing and simultaneous reaction channels of spin crossover and ligation to form coordinatively saturated species introduces an efficient pathway circumventing this spin barrier. It also supports the idea that the high density of electronic excited states and the relatively large amount of excess energy available in the system determine the course of the excited-state dynamics, rather than spin selection rules alone 5,6 . Fast ligation could be facilitated along the singlet pathway, confirming the general notion that solvent-stabilized metal centers form fast 3, 4, 11 and consistent with the observation of unsaturated carbonyl Cr(CO) 5 forming a solvent complex in alcohol solution within 1.6 ps 26 . An alternative proposal 20 for Fe(CO) 5 involves concerted exchange of CO and EtOH on the time scale of ligand dissociation of 100-150 fs. This would also proceed along a singlet pathway and in agreement with our results, as the temporal resolution of our measurements is not sufficient to distinguish between this concerted and the alternative sequential process. Revealing in detail 8 the influence of solvent-solute interactions will have to be the subject of future studies, which could also explore whether the structure of the solute prior to dissociation 20 influences the excited-state branching ratio between the different pathways.We find that the ligation capability of Fe(CO) 4 is mostly determined by its d σ * LUMO, which receives σ donation from occupied CO or ethanol ligand orbitals. Population of the antibonding d σ * orbital in excited singlet ( 1 B 2 ) and triplet ( 3 B 2 ) Fe(CO) 4 impedes σ donation from ligands (see sketches in Figure 3), explaining the inertness of these species against ligation; this problem is absent in the ligation channel that produces coordinately saturated species. Establishing this correlation of orbital symmetry with spin multiplicity and reactivity 27 is enabled by the atom specificity with which x-ray laser based femtosecondresolution spectroscopy can explore frontier-orbital interactions. This ability gives unique access t...

Modified SERS (surface-enhanced Raman scattering) substrates are based on self-assembled monolayers (SAMs) formed from compounds such as alkanethiols. Chain lengths ranging from ethanethiol to octadecanethiol were used to investigate properties of modified SERS substrates. These properties include determining the magnitude of the SERS electromagnetic enhancement, developing a sensitivity factor for detecting aromatic compounds, and evaluating SERS substrates for performance characteristics such as stability and solvent effects. A SERS electromagnetic enhancement was determined to be 2.1 × 103 for detecting aromatic compounds. The effects of experimental conditions on the SERS detection process were addressed. SERS sensitivity was shown to be highly dependent upon a correlation between the SERS interfacial distance dependence and a hydrophobic effect exhibited by the alkyl chain of the thiol. The distance dependence was more significant than the hydrophobic effect for detecting aromatic compounds. For comparison to the electromagnetic enhancement, a SERS sensitivity factor was determined for detecting benzene in water and in organic solvents. Experimental stability issues, such as solvent and laser exposure, were shown to affect the sensitivity of modified SERS substrates. 1-Propanethiol exhibited the strongest sensitivity and stability under all solvent and experimental conditions.