The response functions needed for simulating and fitting two-dimensional infrared spectra are described including the distributions of vibrational frequency, anharmonicity, and coupling between vibrators. A simple method that does not involve explicit calculation is introduced to characterize the spectral line narrowing properties of each of the quantum paths contributing to 2D IR. It is shown that the 2D IR spectra need to be collected in both positive and negative frequency quadrants in order to optimize the information needed to evaluate these correlations. Two experimental examples of heterodyned 2D IR (acetone in ethylene glycol and acetylproline-ND2 in D2O) are described where the 2D IR spectra are obtained for both the conventional echo sequence of pulse delays and for the usually nonrephased signal. These two sets of spectra are quite different for both examples, as predicted. The latter exhibits an echo only when there is some anticorrelation between the relevant inhomogeneous distributions.

The power of two-dimensional (2D) IR spectroscopy as a structural method with unprecedented time resolution is greatly improved by the introduction of IR polarization conditions that completely eliminate diagonal peaks from the spectra and leave only the crosspeaks needed for structure determination. This approach represents a key step forward in the applications of 2D IR to proteins, peptides, and other complex molecules where crosspeaks are often obscured by diagonal peaks. The technique is verified on the model compound 1,3-cyclohexanedione and subsequently used to clarify the distribution of structures that the acetylproline-NH 2 dipeptide adopts in chloroform. In both cases, crosspeaks are revealed that were not observed before, which, in the case of the dipeptide, has led to additional information about the structure of the amino group end of the peptide.H eterodyned 2D IR experiments have recently been reported that are the IR analogues of pulsed correlated spectroscopy and nuclear Overhauser effect spectroscopy in 2D NMR (1-5). Like 2D NMR spectra that exhibit diagonal peaks at frequencies determined by one-dimensional spectra and crosspeaks between coupled nuclear spins, 2D IR spectra contain diagonal peaks and crosspeaks that represent coupled vibrational transitions. The 2D IR crosspeak intensities and splittings depend on the angles and distances between the vibrational modes and can be used to characterize the structures of peptides (3, 6, 7). One advantage (8) of 2D IR over other structure-determining methods is the time scale; 2D IR can monitor structures on a picosecond timescale, which allows transient protein dynamics to be followed. However, structural information is lost when the crosspeaks overlap with the more intense diagonal peaks. Methods have been developed in 2D NMR to minimize the overlap of diagonal and crosspeaks (9, 10). Here we report a technique based on quite different principles that completely eliminates the diagonal peaks from 2D IR spectra and allows the crosspeaks and hence structures to be more easily and better characterized.A useful characteristic of IR transitions is that the directions of their transition dipoles in the molecular frame are often predictable. For example, modes that are mainly localized on two atoms such as NOH, COH, or CAO stretches usually have transition dipole vectors near the bond axis, and the amide I modes of peptide units have transition dipoles whose directions are given by the positions of the amide atoms (11). The four polarized IR pulses in the 2D IR experiments successively interact with four transition dipoles of each molecule, and thus polarized 2D IR measurements can yield information on the geometric arrangements of the dipoles. The diagonal peaks are generated when the IR pulses all interact with the same mode and therefore interrogate only a single location in the structure. The crosspeaks are generated when the IR pulses interact with modes in different locations. The results are couplings and relative orientations that can be used t...

The dynamics of two-dimensional small-polaron formation at ultrathin alkane layers on a silver(111) surface have been studied with femtosecond time- and angle-resolved two-photon photoemission spectroscopy. Optical excitation creates interfacial electrons in quasi-free states for motion parallel to the interface. These initially delocalized electrons self-trap as small polarons in a localized state within a few hundred femtoseconds. The localized electrons then decay back to the metal within picoseconds by tunneling through the adlayer potential barrier. The energy dependence of the self-trapping rate has been measured and modeled with a theory analogous to electron transfer theory. This analysis determines the inter- and intramolecular vibrational modes of the overlayer responsible for self-trapping as well as the relaxation energy of the overlayer molecular lattice. These results for a model interface contribute to the fundamental picture of electron behavior in weakly bonded solids and can lead to better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

Two-photon photoemission is a promising new technique that has been developed for the study of electron dynamics at interfaces. A femtosecond laser is used to both create an excited electronic distribution at the surface and eject the distribution for subsequent energy analysis. Time- and momentum-resolved two-photon photoemission spectra as a function of layer thickness fully determine the conduction band dynamics at the interface. Earlier clean surface studies showed how excited electron lifetimes are affected by the crystal band structure and vacuum image potential. Recent studies of various insulator/metal interfaces show that the dynamics of excess electrons are largely determined by the electron affinity of the adsorbate. In general, electron dynamics at the interface are influenced by the substrate and adlayer band structures, dielectric screening, and polaron formation in the two-dimensional overlayer lattice.