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Surface Dynamics Group Research:

Ultrafast Surface Chemical Dynamics

Overview

Our research in this area addresses ultrafast investigations of surface chemical dynamics as part of a larger program in Surface Chemical Dynamics. The overall goal of our component of this larger program is to establish links between vibrational, electronic and charge transfer dynamics and the chemistry at the molecule–surface interface. To understand the dynamics we conduct UHV surface chemistry experiments with subpicosecond laser pulses that allow us to follow the evolution of chemical species along the reaction coordinate in real time; i.e., to “clock” surface chemical reactions in the femtosecond time regime. Our ultimate aim is to use time-resolved surface probes to open up new windows onto the dynamical chemical processes involved in heterogeneous catalytic reactions.

 This research is a component of the Chemical Physics program in the Chemistry Department at Brookhaven and involves collaborations with Mike White (White Surface Dynamics Group), Ping Liu (Catalysis on the Nanoscale), and Alex Harris

Ultrafast Photoexcitation

It is well understood that short pulses of light can be used to superheat the electrons in a metal. The conditions prevailing at the surface under subpicosecond photoexcitation are reasonably approximated by the “two temperature model,” which describes the metal in terms of two separate but coupled heat baths, one associated with electronic excitation and the other with lattice (phonon) excitation. Numerical solution of a pair of coupled differential equations can be used to simulate the time and depth dependent response of the metal to ultrafast photoexcitation.

Simulated time-dependent response of the surface of palladium to ultrafast laser excitation using the “two-temperature model.” The deposited energy (shown in blue) is absorbed by the electronic degrees of freedom; the electrons are heated far in excess of the melting temperature of the metal and remain out of equilibrium with the lattice for ~2 ps. Once equilibrium is attained, the surface continues to cool on a timescale of 10’s to 100’s of picoseconds.

Because electron–phonon coupling times are of the order of 1–10 ps, subpicosecond light pulses create nonequilibrium conditions within the optical absorption depth of a metal. Because the electronic heat capacity is typically an order of magnitude smaller than the lattice heat capacity, the electrons are heated to temperatures approximately an order of magnitude greater than those achieved by the lattice. Electronic temperatures in range of several 1000’s of Kelvin can be routinely generated, while the lattice remains below the melting point of the metal.

 The “Empirical Friction Model”

In the early 1990’s, J. Misewich, T. Heinz and coworkers, and W. Ho and coworkers, found that these nonequilibrium electrons can effectively drive molecular chemistry at metal surfaces through diabatic excitation of adsorbates. Two regimes of excitation have been suggested: multiple electronic excitation of the adsorbate (understood in terms of a modified Menzel-Gomer-Redhead model) and “electronic friction” (which can be thought of as annihilation of electron hole pairs, or inelastic electron scattering events resulting in transfer of energy to the adsorbate). The former explicitly considers promotion of the adsorbate from its ground electronic state to an excited state, with coupling to the surface invoked only to describe the transition rate between the two states; the latter considers evolution of the adsorbate within a continuum of adsorbate–substrate states with coupling to the substrate being explicitly identified with the substrate–adsorbate energy transfer rate.

A useful phenomenological approach to modeling the dynamics that can capture both types of substrate–adsorbate energy transfer has been dubbed the “Empirical Friction Model” (see, for example, the work of M. Wolf, G. Ertl, M. Bonn and coworkers). In this model, a third temperature, the adsorbate temperature, is appended to the two-temperature model and the energy transfer is parameterized in terms of friction terms that couple the electronic heat bath and the lattice heat bath to the adsorbate temperature bath. This adsorbate temperature is understood to represent the degree of excitation along the relevant reaction coordinate in a simple 1-D model.

 

(Left) A schematic illustration of the empirical friction model. The adsorbate excitation is understood to be the result of frictional coupling between the electronic and lattice degrees of freedom of the metal and adsorbate. (Right) A schematic energy level diagram illustrating how generation of a hot electron distribution in the metal, resulting in population of energy states in resonance with the density of states of the adsorbate, can result in electron transfer from the metal to the adsorbate

Efficient Photoinduced Chemistry

Perhaps the simplest example of the chemistry that can be induced by ultrafast pulses is desorption: the metal surface is heated by the laser pulse, energy is coupled into the molecule–surface degrees of freedom (molecule–surface stretch, frustrated rotation, frustrated translation) and the adsorbate departs the surface as a neutral species. Such a photoinduced desorption process can be readily observed in ultrahigh vacuum by a quadrupole mass spectrometer as illustrated in the cartoon below.

Early on it was found that photoinduced processes driven by ultrafast laser pulses can be very efficient: a single pulse of on the order of ten millijoules per square centimeter can desorb on the order of ten percent of a monolayer. Furthermore, the yield is a highly non-linear function of the fluence; the yield, Y, goes as the fluence, F, to the power of n, where n is in the range of 4 to 9 or more. This behavior does not directly reflect a multiple-photon process, rather it reflects other non-linearities such as the exponential dependence of the desorption yield on the adsorbate temperature (see below). Furthermore, the non-linear behavior distinguishes this process from photoinduced desorption resulting from single electronic transitions driven by nanosecond ultraviolet pulses. Specifically, in the non-linear regime, desorption can be efficiently driven by visible and near-infrared photoexcitation where the photon energy is not high enough to result in electronic excitation of the adsorbate by a single-photon process, in contradistinction to excitation in the UV where a linear response is observed.

(Left) Illustration of the photoinduced desorption of CO from Pd and detection by a quadrupole mass spectrometer. (Right) An example measurement of the dependence of the desorption yield on the absorbed laser fluence, for CO desorption from Pd(111). The laser wavelength is ~800 nm.

Time-Resolved Measurements

Insight into the adsorbate–substrate energy coupling involved in surface chemical reactions can be derived from time-resolved surface dynamics measurements. A simple approach to time-domain observations is a two-pulse correlation measurement, where the excitation pulse is split into two pulses separated by a variable delay. The desorption (or reaction) yield is then measured as a function of the delay between the pulses. Because the response is non-linear (see above) there is a strong enhancement of the yield at short delays; the desorption yield is generally 5–10 times larger at zero delay than at long (~10–100 ps) delays.

The two-pulse correlation measurement measures the overall relaxation rate of the energy transferred from the metal surface into the reaction coordinate. In a simplified picture, this be understood by viewing the experiment as a “pump-probe” measurement where the first pulse “pre-heats” the adsorbate–substrate complex and the second pulse induces desorption. Thus, the amount of desorption is a measure of the level of excitation of the system when the second pulse arrives. A more refined understanding involves using a model, such as the empirical friction model, to extract energy transfer rates from the measurement.

A schematic illustration of the two pulse correlation setup. The results of such experiments can be viewed in the research highlights
 

Linking Ultrafast Photoinduced Chemistry to Heterogeneous Catalysis

We are interested in examining the connections and contrasts between ultrafast photoinduced chemistry and conventional surface chemistry. One way in which the two may be connected can be understood at a basic level by considering the mechanism for the photoinduced chemistry as a two-step process. In the first step, the energy deposited in the substrate is transferred to the adsorbate by coupling to the electronic temperature bath. In the second step, the hot adsorbate desorbs with a probability determined by the adsorbate–substrate potential energy surface.

The empirical friction model can be used to estimate the (time-dependent) rate of photoinduced reaction (e.g., simple desorption or associative desorption) by assuming an Arrhenius rate expression for the desorption rate:

Rd = q n exp[– Ea / kB Tads]

where the (time-dependent) temperature, Tads, is taken from the result of the empirical friction model simulation, Ea, is the activation energy for the desorption, kB is Boltzmann’s constant, n is the preexponential factor, and q is the coverage. Under the provisional assumption that the preexponential factor, n, and the activation energy for desorption, Ea, are equal to those that describe the thermal (non-photoinduced) process, this model can be understood as “pseudo-thermal”: the coupling of energy from the substrate to the adsorbate is a non-thermal, multiple electronic process, whereas the evolution of the adsorbate following the energy transfer proceeds on the same potential energy surface that governs thermal desorption.

Although this model is simple, it at least provides a means for us to begin to get a quantitative handle on the ultrafast photoinduced process and for comparing and contrasting ultrafast photoinduced chemistry and conventional surface chemistry. Some of our efforts have been directed toward testing the applicability of this model.

An illustration of the “pseudo-thermal” model for ultrafast photoinduced desorption. The mechanism is understood in terms of two steps: (i) coupling of energy from the substrate to the adsorbate occurs via a non-thermal, multiple electronic process, and (ii) evolution of the adsorbate proceeds on the same potential energy surface that governs thermal desorption.

 

 Last update on: March 09, 2009