Daniel Matsiev

Conversion of large-amplitude vibration to electron excitation at a metal surface

Gaining insight into the nature and dynamics of the transition state is the essence of mechanistic investigations of chemical reactions, yet the fleeting configuration when existing chemical bonds dissociate while new ones form is extremely difficult to examine directly. Adiabatic potential-energy surfaces—usually derived using quantum chemical methods that assume mutually independent nuclear and electronic motion—quantify the fundamental forces between atoms involved in reaction and thus provide accurate descriptions of a reacting system as it moves through its transition state. This approach, widely tested for gas-phase reactions, is now also commonly applied to chemical reactions at metal surfaces. There is, however, some evidence calling into question the correctness of this theoretical approach for surface reactions: electronic excitation upon highly exothermic chemisorption has been observed, and indirect evidence suggests that large-amplitude vibrations of reactant molecules can excite electrons at metal surfaces. Here we report the detection of ‘hot’ electrons leaving a metal surface as vibrationally highly excited NO molecules collide with it. Electron emission only occurs once the vibrational energy exceeds the surface work function, and is at least 10,000 times more efficient than the emissions seen in similar systems where large-amplitude vibrations were not involved. These observations unambiguously demonstrate the direct conversion of vibrational to electronic excitation, thus questioning one of the basic assumptions currently used in theoretical approaches to describing bond-dissociation at metal surfaces.

Vibrationally promoted electron emission from low work-function metal surfaces.

We observe electron emission when vibrationally excited NO molecules with vibrational state v, in the range of 9 < or = v < or =18, are scattered from a Cs-dosed Au surface. The quantum efficiency increases strongly with v, increasing up to 10(-2) electrons per NO (v) collision, a value several orders of magnitude larger than that observed in experiments with similar molecules in the ground vibrational state. The electron emission signal, as a function of v, has a threshold where the vibrational excitation energy slightly exceeds the surface work function. This threshold behavior strongly suggests that we are observing the direct conversion of NO vibrational energy into electron kinetic energy. Several potential mechanisms for the observed electron emission are explored, including (1) vibrational autodetachment, (2) an Auger-type two-electron process, and (3) vibrationally promoted dissociation. The results of this work provide direct evidence for nonadiabatic energy-transfer events associated with large amplitude vibrational motion at metal surfaces.

An advanced molecule-surface scattering instrument for study of vibrational energy transfer in gas-solid collisions

We describe an advanced and highly sensitive instrument for quantum state-resolved molecule-surface energy transfer studies under ultrahigh vacuum (UHV) conditions. The apparatus includes a beam source chamber, two differential pumping chambers, and a UHV chamber for surface preparation, surface characterization, and molecular beam scattering. Pulsed and collimated supersonic molecular beams are generated by expanding target molecule mixtures through a home-built pulsed nozzle, and excited quantum state-selected molecules were prepared via tunable, narrow-band laser overtone pumping. Detection systems have been designed to measure specific vibrational-rotational state, time-of-flight, angular and velocity distributions of molecular beams coming to and scattered off the surface. Facilities are provided to clean and characterize the surface under UHV conditions. Initial experiments on the scattering of HCl(v = 0) from Au(111) show many advantages of this new instrument for fundamental studies of the energy transfer at the gas-surface interface.

Generation of Tunable Narrow Bandwidth Nanosecond Pulses in the Deep Ultraviolet for Efficient Optical Pumping and High Resolution Spectroscopy

Nanosecond optical pulses with high power and spectral brightness in the deep ultraviolet (UV) region have been produced by sum frequency mixing of nearly transform-limited-bandwidth IR light originating from a home-built injection-seeded ring cavity KTiOPO(4) optical parametric oscillator (OPO) and the fourth harmonic beam of an injection-seeded Nd:YAG laser used simultaneously to pump the OPO with the second harmonic. We demonstrate UV output, tunable from 204 to 207 nm, which exhibits pulse energies up to 5 mJ with a bandwidth better than 0.01 cm(-1). We describe how the approach shown in this paper can be extended to wavelengths shorter than 185 nm. The injection-seeded OPO provides high conversion efficiency (>40% overall energy conversion) and superior beam quality required for highly efficient downstream mixing where sum frequencies are generated in the UV. The frequency stability of the system is excellent, making it highly suitable for optical pumping. We demonstrate high resolution spectroscopy as well as optical pumping using laser-induced fluorescence and stimulated emission pumping, respectively, in supersonic pulsed molecular beams of nitric oxide.

Vibrationally promoted electron emission at a metal surface: electron kinetic energy distributions

We report the first direct measurement of the kinetic energy of exoelectrons produced by collisions of vibrationally excited molecules with a low work function metal surface exhibiting electron excitations of 64% (most probable) and 95% (maximum) of the initial vibrational energy. This remarkable efficiency for vibrational-to-electronic energy transfer is in good agreement with previous results suggesting the coupling of multiple vibrational quanta to a single electron.

Vibrational overtone excitation in electron mediated energy transfer at metal surfaces.

Vibrational overtone excitation is, in general, inefficiently stimulated by photons, but can under some circumstances be efficiently stimulated by electrons. Here, we demonstrate electron mediated vibrational overtone excitation in molecular collisions with a metal surface. Specifically, we report absolute vibrational excitation probabilities to ν = 1 and 2 for collisions of NO(ν = 0) with a Au(111) surface as a function of surface temperature from 300 to 985 K. In all cases, the observed populations of vibrationally excited NO are near those expected for complete thermalization with the surface, despite the fact that the scattering occurs through a direct “single bounce” mechanism of sub-ps duration. We present a state-to-state kinetic model, which accurately describes the case of near complete thermalization (a regime we call the strong coupling case) and use this model to extract state-to-state rate constants. This analysis unambiguously shows that direct vibrational overtone excitation dominates the production of ν = 2 and that, within the context of our model, the intrinsic strength of the overtone transition is of the same order as the single quantum transition, suggesting a possible way to circumvent optical selection rules in vibrational pumping of molecules. This result also suggests that previous measurements of vibrational relaxation of highly vibrationally excited NO exhibiting highly efficient multi-quantum jumps (Δν ∼ −8) are mechanistically similar to vibrational excitation of NO(ν = 0).

Vibrationally promoted emission of electrons from low work function surfaces: Oxygen and Cs surface coverage dependence

We observe electron emission from low work function Cs covered Au surfaces due to the scattering of vibrationally excited NO molecules with 18 quanta of vibration in the ground electronic state. Additional experiments explore the influence of oxygen exposure on the electron emission. These results indicate a nondissociative mechanism for reactivity of vibrationally excited NO on these surfaces and provide evidence against an Auger-like mechanism. We note a remarkable similarity between trends of the vibrationally produced electron emission as the surface composition changes and reports on surface work function.

Transport and focusing of highly vibrationally excited NO molecules

We report experiments where hexapole focusing is combined with stimulated emission pumping in a molecular beam, providing control over the molecule’s rovibronic quantum numbers, its laboratory frame velocity and its transverse divergence. Hexapole focusing profiles can be quantitatively reproduced by classical trajectory simulations. These experiments provide new ways of manipulating beams of vibrationally excited molecules including: (1) transverse refocusing and concomitant improved efficiency for transport of the vibrationally excited molecules, (2) relative enrichment of the concentration of the vibrationally excited molecules with respect to the unexcited portion of the beam and, (3) orientation of vibrationally excited molecules.