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We report the first investigation of the fast excited-state intramolecular proton transfer of 3-hydroxyflavone. Picosecond stimulated emission and transient absorption spectroscopy demonstrate that the tautomer formation can be monitored by 620-nm transient absorption. The excited-state proton transfer was studied at 620 nm with femtosecond time resolution, and determined to be 240±50 fs in a nonpolar solvent environment. The fast excited-state proton transfer in methanol solution, however, was faster than the 125-fs instrument response

The first reported use of a pulsed rhodamine-dye laser to generate widely tunable nanosecond light pulses in the vibrational infrared by stimulated electronic Raman scattering is discussed. Narrow-band pulses of up to 120 microJ were generated from 3450 to 900 cm(-1) (2.9 to 11.1 microm), and broadband pulses up to 75 microJ were generated from 3390 to 1800 cm(-1) (2.9 to 5.5 microm) by scattering from the 6s-5d transition in cesium vapor. Cesium dimer effects on tuning-curve structure and efficiency are discussed.

A study of the photodissociation dynamics of Mn2(CO)10 and the vibrational relaxation of its subsequent photoproducts in solutions has been carried out using picosecond time‐resolved laser techniques. The molecule predissociates in less than 2–3 ps after excitation with a 295 nm photon. Two dissociation channels are open for this excitation wavelength, namely, the Mn–Mn bond breaking and the Mn–CO bond breaking, generating internally hot ⋅Mn(CO)5 and Mn2(CO)9, respectively. These two species have a different absorption spectrum in the visible region and are probed independently by varying the probe laser wavelength. The vibrational relaxation of these nascent photoproducts is observed for the first time. In cyclohexane the vibrationally hot Mn2(CO)9 reaches thermal equilibrium with the solvent through two distinct decay channels with time constants of 15 and 170 ps, respectively. The vibrationally cold Mn2(CO)9 then persists for many nanoseconds. The vibrational relaxation is found to be faster in the 2‐p...

A simple, classical stochastic model has been developed to study the vibrational energy relaxation of heteronuclear diatomic molecules in simple monatomic liquids. This work is an extension of an earlier study on homonuclear diatomic molecules [D. E. Smith and C. B. Harris, J. Chem. Phys. 92, 1312 (1990)]. The model is based on the generalized Langevin equation for generalized Brownian dynamics. The memory function of each atom in the diatomic is determined directly from classical molecular dynamics simulations in the solvent of interest and then scaled by a screening factor. The memory function is modeled using an autoregressive technique, which provides efficient evaluation of the friction integral. The effect of screening is accounted using a simple model based on the equilibrium structure of the diatomic in the Lennard‐Jones solvent, which is shown to provide very reasonable results. The model developed is applied to simulate the vibrational relaxation of iodine chloride, ICl, in its ground electronic...

A series of picosecond experiments and computer simulations will be presented that test collisional and hydrodynamic models for vibrational relaxation in liquids. The relationships between isolated binary collision models (IBC) and stochastic dynamics will be presented. The appropriateness of IBC theory in describing vibrational relaxation in liquids will also be discussed.

Angle-resolved two-photon photoemission was used to study the energy relaxation, population decay, and localization dynamics of image potential state (IPS) electrons in ultrathin films of dimethylsulfoxide (DMSO) on an Ag(111) substrate. Dynamic energy shifts of 50 ± 10 meV and 220 ± 10 meV were observed for n = 1 IPS electrons at one monolayer and two monolayer coverages of DMSO, respectively. The difference in energy shifts is attributed to rotational hindrance of the molecular dipole in the chemisorptive first monolayer. The finding confirms the proposed mechanism for the low differential capacitance of dimethylsulfoxide at noble metal interfaces in solution. A novel description of the IPS as a surface capacitance is presented to facilitate comparisons with electrochemical systems.

The dynamics of the I 2 geminate recombination reaction in liquid Xe are monitored with transient picosecond absorption spectroscopy as a function of solvent density and temperature. Specifically, the I 2 X state vibrational relaxation and the nonradiative electronic curve crossing from the deeply bound A′ and A states to the X state are measured over a Xe density range which spans the entire liquid phase region of the phase diagram. The density dependence of the X state vibrational relaxation is shown to be consistent with the isolated binary collision (IBC) model of vibrational relaxation through a procedure which, unlike previous studies of IBC theory, requires no estimate of the collision frequency. This verification procedure is performed by ascertainingwhether vibrational energy versus time plots at different solvent density can be overlapped by a linear scaling of the time axis. The consistency with IBC theory demonstrated in this system is surprising because of the low I 2 vibrational frequency (170–214 cm −1 ) and the large amplitude oscillations involved in the relaxation process. Contrary to theoretical predictions of the A′/A state lifetime based upon Langevin modeling in the high density Kramers limit, the A′/A state lifetime shows very little dependence on solvent density although both viscosity and activation barrier increase substantially with solvent density. This finding suggests that a non-Markovian description of the solvent frictional effects and a model of nonadiabatic curve crossing must be included in order to correctly calculate the excited state lifetime.

Two-photon photoemission of image potential states above monolayers of p-xylene/Ag(111) shows that electrons with different momenta have very different rise and decay rates as a function of parallel momentum. The dynamics are due to energy and momentum loss (intraband relaxation), which we model as a stochastic process isomorphic to the overdamped motion of a harmonic oscillator. The method extracts a friction coefficient from the data which can be explained by electron−electron scattering in a formalism based on the Lindhard dielectric function. One-electron excitations (interband transistions) dominate the dissipation mechanism, with a smaller contribution from collective electronic excitations (plasmons).

The thickness-dependent interfacial band structure was determined for thin films of C(60) on Ag(111) by angle-resolved two-photon photoemission spectroscopy. Dispersions of molecular-orbital derived bands (HOMO, LUMO+1, and LUMO+2) were acquired, and limits were placed on their possible effective masses. A group theoretic approach is also incorporated to further understand the properties of these states. The HOMO, LUMO+1, and LUMO+2 bands possess (best-fit) effective masses of -7 m(e), -7 m(e), and -12 m(e), respectively. These values are consistent with theoretical calculations, averaged over the closely spaced subbands for each state, and provide practical limits on the effective fundamental charge-transport properties of C(60) films.

The photochemical reactions of [CpW(CO)3]2 with the Lewis base P(OMe)3 are examined on the nanosecond and microsecond time scales using step-scan FTIR spectroscopy. Photolysis at 532 nm produces the 17-electron (17e) radicals CpW(CO)3*, which are in equilibrium with the 19-electron (19e) radicals CpW(CO)3P(OMe)3* on the nanosecond time scale. The reactions of the 19e radical are directly observed for the first time; the major reaction pathway is spontaneous loss of a carbonyl to form the 17e species CpW(CO)2P(OMe)3*, with a barrier of 7.6 +/- 0.3 kcal/mol for this process. The minor reaction pathway (20%) at this concentration of P(OMe)3 (85 mM) is disproportionation to form the products CpW(CO)3P(OMe)3+ and CpW(CO)3-. On the microsecond time scale, the 17e radicals CpW(CO)2P(OMe)3* dimerize to form the ligand substitution product [CpW(CO)2P(OMe)3]2. These results indicate that the 19e species is a stable intermediate rather than transition state in the ligand substitution reaction, and this type of reactivity is likely to be typical of 17e organometallic radicals which undergo associative substitution mechanisms.

The ligand rearrangement reaction of Cr(CO)6 is studied in a series of alcohol solutions using ultrafast infrared spectroscopy and Brownian dynamics simulations. Excitation with 266 nm light gives Cr(CO)5 which is quickly solvated by a ligand from the bath. In alcohol solutions, solvation by an alkyl or hydroxyl site can occur; all alkyl bound complexes eventually rearrange to hydroxyl bound complexes. This rearrangement has been described using both an intermolecular (stochastic) and intramolecular (chainwalk) mechanism. Experiments alone do not allow for characterization of the mechanism, and therefore, theoretical calculations were carried out for the first time by modeling the ligand rearrangement as a diffusive walk along a potential defined by the different interaction possibilities. Experiments and simulations were carried out for Cr(CO)6 in 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, 1-pentanol, 2-pentanol, 2-methylbutanol, and 3-methylbutanol. The trends in the theoretical and experimental rearrangement times are similar for all simulations carried out indicating that the two mechanisms have very similar ensemble behavior when bath effects are taken into account. The nature of the mechanism responsible for motion along the alcohol chain is not of primary importance in isolating the kinetic behavior because of the highly diffusive nature of the reaction. Future experimental and theoretical work will be directed at identifying a definitive assignment of the reaction mechanism.

The role of 19-electron intermediates in the photochemical disproportionation of [CpW(CO)3]2 (Cp = C5H5) with Lewis bases (PR3; R = OMe, Bu, Ph) is investigated on the ultrafast time scale using femtosecond VIS-pump, IR-probe spectroscopy. Formation of a 19-electron (19e) species CpW(CO)3PR3*by coordination of PR3 with photogenerated 17-electron (17e) radicals CpW(CO)3* is directly observed, and equilibrium is established between the 17e radicals and the 19e intermediates favoring 19e intermediates in the order: Bu > OMe >> Ph. Steric effects dominate the 17e/19e equilibrium when the cone-angle of the Lewis base exceeds a certain limiting value (between 132 degrees and 145 degrees ), but below this value electronic properties of the Lewis base control the 17e/19e dynamics. Disproportionation occurs in less than 200 picoseconds by electron transfer between a solvent caged 17e radical and 19e, highly reducing species. The rate and extent of ultrafast disproportionation depends on both the identity and concentration of the Lewis base. In low concentrations of PR3 (typically 1-2 M or less) or with Lewis bases whose equilibrium heavily favors 17e radicals (e.g., PPh3), disproportionation is rate-limited by breakdown of the solvent cage. Density functional theory calculations on vibrational frequencies and charge distributions of the various complexes support the experimental results.

A free program for the simulation of dynamic infrared (IR) spectra is presented. The program simulates the spectrum of two exchanging IR peaks based on simple input parameters. Larger systems can be simulated with minor modifications. The program is available as an executable program for PCs or can be run in MATLAB on any operating system. Source code is also provided to encourage computer coding projects based on the program. The program is useful for upper-division undergraduates and demonstrates several important concepts from physical chemistry. Example exercises based on the program are included.

The ability of time- and angle-resolved two-photon photoemission to estimate the size distribution of electron localization in the plane of a metal-adsorbate interface is discussed. It is shown that the width of angular distribution of the photoelectric current is inversely proportional to the electron localization size within the most common approximations in the description of image potential states. The localization of the n=1 image potential state for two monolayers of butyronitrile on Ag(111) is used as an example. For the delocalized n=1 state, the shape of the signal amplitude as a function of momentum parallel to the surface changes rapidly with time, indicating efficient intraband relaxation on a 100 fs time scale. For the localized state, little change was observed. The latter is related to the constant size distribution of electron localization, which is estimated to be a Gaussian with a 15+/-4 A full width at half maximum in the plane of the interface. A simple model was used to study the effect of a weak localization potential on the overall width of the angular distribution of the photoemitted electrons, which exhibited little sensitivity to the details of the potential. This substantiates the validity of the localization size estimate.

The ultrafast dynamics following metal-metal bond breaking in Fe3(CO)12 have been investigated. Following excitation at 590 nm, the ground state Fe3(CO)12 is converted to a coordinatively unsaturated isomer with bridging COs, Fe3(CO)12(unsat.), within the instrument response of 1.5 ps. The unsaturated isomer then returns to the ground state with an exponential time constant of 150 ps. This rate of isomerization back to the ground state implies a barrier of 4.3 kcal mol−1. This is in agreement with the results of Cotton and Hunter who put an upper limit of 6 kcal mol−1 on the isomerization of Fe3(CO)12 from an all terminal structure to a bridging structure. A portion (20%) of the excited molecules do not return to the ground state on the time scale of this experiment (

Two-photon photoemission of thiolate/Ag(111), nitrile/Ag(111), and alcohol/Ag(111) interfaces elucidates electron solvation and localization in two dimensions. For low coverages of thiolates on Ag(111), the occupied (HOMO) and unoccupied (LUMO) electronic states of the sulfer-silver bond are localized due to the lattice gas structure of the adsorbate. As the coverage saturates and the adsorbate-adsorbate nearest neighbor distance decreases, the HOMO and LUMO delocalize across many adsorbate molecules. Alcohol- and nitrile-covered Ag(111) surfaces solvate excess image potential state (IPS) electrons. In the case of alcohol-covered surfaces, this solvation is due to a shift in the local workfunction of the surface. For two-monolayer coverages of nitriles/Ag(111), localization accompanies solvation of the IPS. The size of the localized electron can be estimated by Fourier transformation of the wavefunction from momentum- to position-space. The IPS electron localizes to 15 +/- 4 angstroms full-width at half maximum in the plane of the surface, i.e., to a single lattice site.© (2003) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The photochemical C-H activation reactions of eta(3)-TpRh(CO)(2) (Tp = HB-Pz(3), Pz = 3,5-dimethylpyrazolyl) and CpRh(CO)(2) (Cp = C(5)H(5)) have been studied in a series of linear, cyclic, and aromatic hydrocarbon solvents on a femtosecond to microsecond time scale. These results have revealed that the structure of the hydrocarbon substrate affects the final C-H bond activation step, which is in accordance with the known preference of bond activation toward primary C-H sites. In the case of aromatic C-H activation, the reaction is divided into parallel channels involving sigma- and pi-solvated intermediates. Results for the analogous CpRh(CO)(2) molecule have shown that the coordination of the cyclopentadienyl ligand does not play a direct role in the dynamics of the reaction, in contrast to the C-H activation mechanism observed in eta(3)-TpRh(CO)(2) studies.

Electron localization in different systems has been a topic of study for over a half of a century; polaron formation in the solid state and electron solvation by liquids are just two examples. Structureless by itself but interacting strongly with the condensed phase environment, an electron is often utilized as a probe of its local microscopic structure. Despite that, there are few attempts to access the shape of a localized electronic wave function directly from an experiment. Here, we report an elegant way of estimating the electron localization size parallel to a two-dimensional metal/adsorbate interface by using time and angle resolved two-photon photoemission (TAR TPPE).

Angle- and time-resolved two-photon photoemission (TPPE) was used to investigate electronic states in the buffer layer of 4H-SiC(0001). An image potential state (IPS) series was observed on this strongly surface-bound buffer layer, and dispersion measurements indicated free-electron-like behavior for all states in this series. These results were compared with TPPE taken on bilayer graphene, which also show the existence of a free-electron-like IPS series. Lifetimes for the n = 2, and n = 3 states were obtained from time-resolved TPPE; slightly increased lifetimes were observed in the bilayer graphene sample for the n = 2 the n = 3 states. Despite the large band gap of graphene at the center of the Brillouin zone, the lifetime results demonstrate that the graphene layers do not behave as a simple tunneling barrier, suggesting that the buffer layer and graphene overlayers play a direct role in the decay of IPS electrons.

The ultrafast reaction dynamics following 295-nm photodissociation of Re2CO10 were studied experimentally with 300-fs time resolution in the reactive, strongly coordinating CCl4 solution and in the inert, weakly coordinating hexane solution. Density-functional theoretical (DFT) and ab initio calculations were used to further characterize the transient intermediates seen in the experiments. It was found that the quantum yield of the Re-Re bond dissociation is governed by geminate recombination on two time scales in CCl4, approximately 50 and approximately 500 ps. The recombination dynamics are discussed in terms of solvent caging in which the geminate Re(CO)5 pair has a low probability to escape the first solvent shell in the first few picoseconds after femtosecond photolysis. The other photofragmentation channel resulted in the equatorially solvated dirhenium nonacarbonyl eq-Re2(CO)9(solvent). Theoretical calculations indicated that a structural reorganization energy cost on the order of 6-7 kcal/mol might be required for the unsolvated nonacarbonyl to coordinate to a solvent molecule. These results suggest that for Re(CO)5 the solvent can be treated as a viscous continuum, whereas for the Re2(CO)9 the solvent is best described in molecular terms.

Time and angle resolved two photon photoemision has been used to study dynamic electron localization and solvation at methanol and n-heptane covered metal surfaces. Coverage dependant localization is observed for methylthiolate on Ag(111).

Femtosecond infrared spectroscopy has been applied to the study of ligand rearrangement dynamics of coordinatively unsaturated transition metal complexes. A new site-diffusion model based on the Langevin equation is proposed to describe the observed rearrangement dynamics.

In my essay on David Foster Wallace in Critique's special issue “An American Requiem: Elegies for Thirteen Novelists” (51.2), I briefly discuss Wallace's boyhood in Philo, Illinois, the tiny town h...

A delocalized electron at a metal-dielectric interface interacts with the adlayer and spatially localizes or self-traps on the femtosecond time scale into what is termed a small polaron. The dynamics can be studied by two-photon photoemission. Theoretical and experimental analyses reveal the interaction energy and the lattice vibrational mode that mediates electron localization. These results contribute to a fundamental understanding of electron behavior in weakly bonded solids and can lead to a better understanding of carrier dynamics in many different systems, including organic light-emitting diodes.

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.

The electronic structure and excited electron dynamics of the benzene/Ag(111) interface was studied with angle- and femtosecond time-resolved two-photon photoemission.

Using a recently developed electron transfer theory based on a path integral approach, we can model the femtosecond dynamics of electron self-trapping processes at interfaces.

The mechanism of alkane C-H bond activation by transition metal complexes such as CpM(CO)2 (M=Rh, Ir) has been intensely studied because it represents a first step in a catalytic process using unreactive hydrocarbons.[1] The bond activation reaction starts with the formation of monocarbonyl intermediates such as CpRh(CO). These species have been detected in the gas phase[2] and in liquefied rare Kr and Xe[3] by μs time resolved IR spectroscopy. Unfortunately, the subsequent oxidative insertion of CpRh(CO) into the C-H bond is not well understood due to its rapid rate and low quantum yield (~1%) for formation of the C-H activated product. These properties have hindered previous femtosecond and picosecond time-resolved studies of activation reaction in room temperature alkane solution. [4]

The response of solvent to the change of solute charge or dipole has been intensely studied in recent years[l]. In previous solvation experiments, time dependent fluorescence Stokes shifts of dye molecules in different solvents were measured, enabling the determination of solvation times[1,2]. Both molecular dynamics simulations and various theories have been used to relate these solvation times to solvent motions[3,4,5]. In this paper, we report the observation of solvent responses in the solvation of LDS 750 by directly monitoring the solvent vibrational spectral change, which can be more readily compared to theories and simulations. It was recently demonstrated[6] that a large change in a solute’s permanent dipole moment strongly perturbs the vibrational spectrum of surrounding solvent molecules.

Using angle-resolved two-photon photoemission with femtosecond time resolution, it is possible to measure many dynamical properties of electrons in 2-D systems, including tunneling, localization, and momentum scattering.

Femtosecond visible pump-IR probe studies of Cp2W2(CO)6 in P(OMe)3 and CH2Cl2 have allowed direct observation of a 19-electron intermediate and of disproportionation into CpW(CO)3- and CpW(CO)3P(OMe)3+ on the ultrafast time scale. A new disproportionation mechanism involving in-cage electron transfer between a 19-electron intermediate and a 17-electron radical has been proposed.

A new instrument for angle-resolved two-photon photoemission with exceptional sensitivity and energy resolution has allowed a detailed examination of the interaction of image-state electrons with adsorbates. In addition to measuring the electrostatic properties of molecular-thickness films, the technique serves as a probe of adsorbate growth modes, and provides new opportunities to explore the dynamics of electrons in well-controlled 2D systems.

One of the central themes of modern physical chemistry is to elucidate the elementary steps associated with a chemical reaction. While understanding of gas phase reactions has reached a high level of sophistication, many open questions remain concerning reactivity in the liquid phase. Reactant species suffer 1013 collisions per second in a room temperature solvent. These collisions can cause energy exchange between the solvent and the reactant, perturb or mix the reactant potential energy surfaces and spatially localize or separate reacting species, all on picosecond or subpicosecond time scales. Thus, ultrafast laser technology provides an outstanding tool for the study of condensed phase reaction dynamics. With ultrafast laser techniques, it will be possible to unveil the fundamental principles of condensed phase reactivity, including a detailed picture of reactive energy flow and the microscopic basis for dissociation quantum yields.

The states of image-potential electrons at the metal-dielectric interface were studied using angle-resolved two-photon photoemission in ultrahigh vacuum. The binding energy and dispersion of an electron in an image state were studied as a function of layer thickness for various metal-dielectric interfaces. Features representative of both delocalized and localized states were found. As best we can determine, this is the first observation of localized states at an interface with the type of nondispersive photoemission feature reported here. The propensity of layers of straight chain alkanes for localizing excess electrons at the metal-insulator interface correlates with the low electron mobilities observed for the corresponding molecular liquids. Excess electrons become localized as layers of aspherical hydrocarbons on Ag(111) are increased in thickness in a layer-by-layer fashion. However, layers of the nearly spherical neopentane molecule continue to primarily support a delocalized state at a coverage of three layers.

We report ultrafast measurements of the dynamic thermal expansion of a surface and the temperature dependent surface thermal diffusivity using a two-color reflection transient grating technique. Studies were performed on p-type, n-type, and undoped GaAs(100) samples at several temperatures. Using a 75 fs ultraviolet probe with visible excitation beams, the electronic effects that dominate single color experiments become negligible; thus surface expansion due to heating and the subsequent contraction caused by cooling provide the dominant influence on the diffracted probe. The diffracted signal was composed of two components, thermal expansion of the surface and heat flow away from the surface, allowing the determination of the rate of expansion as well as the surface thermal diffusivity. At room temperature a signal rise due to thermal expansion was observed, corresponding to a maximum average displacement of approximately equals 1 angstroms at 32 ps. Large fringe spacings were used, thus the dominant contributions to the signal were expansion and diffusion perpendicular to the surface. Values for the surface thermal diffusivity of GaAs were measured and found to be in reasonable agreement with bulk values above 50 degree(s)K. Below 50 degree(s)K, the diffusivity at the surface was more than an order of magnitude slower than in the bulk due to increased phonon boundary scattering. Comparison of the results with a straightforward thermal model yields good agreement over a range of temperatures (12 - 300 degree(s)K). The applicability and advantages of the transient grating technique for studying photothermal and photoacoustic phenomena are discussed.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

One of the most widely studied systems exhibiting proton transfer (PT) has been 3-hydroxyflavone (3HF). Proton transfer in 3HF was first proposed [1] to explain the observation of two distinct fluorescence bands in the blue and green spectral regions following the absorption of a UV photon. The blue, or near UV fluorescence is thought to arise from the normal form of the molecule, whereas emission from the tautomer is the more strongly Stokes shifted green fluorescence (Fig. 1). Careful studies have shown that the blue, normal emission appears only when 3HF is in hydrogen bonding solvents or in the presence of hydrogen bonding impurities. [2] In non-polar solvents, even at low temperatures, only the green tautomer emission is observed.