Using a combined theoretical and experimental approach, we investigate the non-adiabatic dynamics of the prototypical ethylene (C(2)H(4)) molecule upon π → π∗ excitation. In this first part of a two part series, we focus on the lifetime of the excited electronic state. The femtosecond time-resolved photoelectron spectrum (TRPES) of ethylene is simulated based on our recent molecular dynamics simulation using the ab initio multiple spawning method with multi-state second order perturbation theory [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A 113, 13656 (2009)]. We find excellent agreement between the TRPES calculation and the photoion signal observed in a pump-probe experiment using femtosecond vacuum ultraviolet (hν = 7.7 eV) pulses for both pump and probe. These results explain the apparent discrepancy over the excited state lifetime between theory and experiment that has existed for ten years, with experiments [e.g., P. Farmanara, V. Stert, and W. Radloff, Chem. Phys. Lett. 288, 518 (1998) and K. Kosma, S. A. Trushin, W. Fuss, and W. E. Schmid, J. Phys. Chem. A 112, 7514 (2008)] reporting much shorter lifetimes than predicted by theory. Investigation of the TRPES indicates that the fast decay of the photoion yield originates from both energetic and electronic factors, with the energetic factor playing a larger role in shaping the signal.

Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

Through a combined experimental and theoretical approach, we study the nonadiabatic dynamics of the prototypical ethylene (C2H4) molecule upon π → π* excitation with 161 nm light. Using a novel experimental apparatus, we combine femtosecond pulses of vacuum ultraviolet and extreme ultraviolet (XUV) radiation with variable delay to perform time resolved photo-ion fragment spectroscopy. In this second part of a two part series, the XUV (17 eV < hν < 23 eV) probe pulses are sufficiently energetic to break the C–C bond in photoionization, or to photoionize the dissociation products of the vibrationally hot ground state. The experimental data is directly compared to excited state ab initio molecular dynamics simulations explicitly accounting for the probe step. Enhancements of the CH2+ and CH3+ photo-ion fragment yields, corresponding to molecules photoionized in ethylene (CH2CH2) and ethylidene (CH3CH) like geometries are observed within 100 fs after π → π* excitation. Quantitative agreement between theory and experiment on the relative CH2+ and CH3+ yields provides experimental confirmation of the theoretical prediction of two distinct conical intersections and their branching ratio [H. Tao, B. G. Levine, and T. J. Martinez, J. Phys. Chem. A. 113, 13656 (2009)]. Evidence for fast, non-statistical, elimination of H2 molecules and H atoms is observed in the time resolved H2+ and H+ signals.

We have recently shown that homogeneous and heterogeneous kinetics can be distinguished by experiments that compare the evolution of the population of a state over two time intervals [E. van Veldhoven et al., ChemPhysChem 8, 1761 (2007)]. This paper elaborates on the analogy between these multiple population-period transient spectroscopy (MUPPETS) experiments and more familiar spectroscopies based on the evolution of coherences. Using a modified inverse-Laplace transform, a standard kinetics decay is re-expressed as a "rate spectrum." A nonexponential decay creates a linewidth in this spectrum. Mechanisms for line broadening in rate spectra are compared to those for line broadening in frequency-domain spectra. Homogeneous and heterogeneous kinetics are defined precisely and are shown to be the counterparts of homogeneous and inhomogeneous line broadenings in frequency-domain spectra. Homogeneous line broadening mechanisms are further divided into equilibrium and nonequilibrium mechanisms, with equilibrium mechanisms more prevalent in frequency spectra and nonequilibrium mechanisms more prevalent in rate spectra. Spectral representations of two-dimensional MUPPETS experiments are developed that are equivalent to two-dimensional coherence spectroscopies. In particular, spectra equivalent to hole-burning and to correlation spectra are defined. Frequency-domain spectra are often modeled as an inhomogeneous distribution of identical homogeneous line shapes. A parallel homogeneous-heterogeneous model for kinetics is defined. Within this model, MUPPETS has sufficient information to completely separate the homogeneous and heterogeneous contributions to a nonexponential decay, even when the homogeneous contribution is nonexponential.

The discovery of the spin echo by Hahn in 1950 [1] set the stage for the development of a multitude of "coherence" spectroscopies. The expansion, elaboration and application of coherence methods [2] continues to this day in NMR, [3] electronic, [4,5] infrared [6][7][8] and Raman [9][10][11] spectroscopy. The key feature in the original spin echo-and in all subsequent coherence spectroscopies as well-is a comparison of the evolution of a coherence over two different time periods. Herein we show that a parallel, but analogous, set of spectroscopies can be based on comparing the evolution of a population over different time periods. We refer to these spectroscopies as Multiple Population Period Transient Spectroscopies (MUPPETS). In their simplest form, coherence echoes separate homogeneous and inhomogeneous sources of line broadening. Analogous MUP-PETS experiments are a general method for separating homogeneous and heterogeneous sources of nonexponential kinetics. A novel six-beam generalization of heterodyne-detected transient-grating spectroscopy [12,13] is performed to validate the theoretical predictions.In general, any nonexponential relaxation can be attributed to one of two types of mechanism: differences in the rates of different molecules within the sample (dynamic heterogeneity) or a time varying rate for each individual molecule (dynamic homogeneity). Figure 1 illustrates this distinction in the context of electronic relaxation in a four-level optical system. The lefthand side of Figure 1 represents a mixture of two molecules, a and b, that have the same transition frequency, but that have different electronic relaxation rates, k a and k b . A standard, onedimensional (1D) ground-state recovery experiment on the mixture yields a nonexponential decay that is due to the heterogeneity of the rates. (We consider a multiexponential decay to be nonexponential.) Although we will use a permanent heterogeneity in this paper to demonstrate the principles of MUP-PETS, often the structures or local environments that produce different relaxation rates are short-lived and cannot be distinguished by standard methods.The right-hand side of Figure 1 represents a system whose electronic relaxation rate is affected by a conformational coordinate q. As a result of the evolution of q, every molecule experiences an identical nonexponential decay. This mechanism is an example of homogeneous dynamics. In general, sequences of intermediate states, dynamics triggered by excitation and hierarchical kinetics [14] are all capable of creating homogeneous nonexponential decays.The issue of homogeneous versus heterogeneous dynamics becomes important any time nonexponential kinetics are observed, and it plays a role in many diverse areas of chemistry and physics. It has been most deeply discussed in the context of supercooled liquids. The fragmentation of the liquid into mesoscopic domains with varying local viscosity has been observed very close to the glass transition.[15] Computer simulations suggest that this heterogeneity originat...