W. Ho

Single-molecule chemistry

The ability to probe individual atoms and molecules have made it possible to reveal properties which otherwise would be hidden in the study of an ensemble of atoms and molecules. The scanning tunneling microscope (STM) with its unmatched spatial resolution and versatility literally allows us to touch atoms and molecules one at a time and to carry out experiments which previously were only imagined. One of the great attributes of the STM is that it provides a real space view of the individual molecules and the atomic landscape of their environment, thus removing many of the uncertainties surrounding the nature of the system under study. Combining its imaging, manipulation, spectroscopic characterization, and chemical modification capabilities, the STM has enabled direct visualization of chemistry by revealing the fundamental properties of atoms and molecules and their interactions with each other and the environment. While femtosecond lasers have made it possible to study chemistry at the temporal limit, t...

State resolved studies of photochemical dynamics at surfaces

Abstract Photochemical surface processes are initiated by the absorption of photons by an adsorbate or the substrate, and followed by various energy transfer and relaxation processes that lead to nuclear motion along the reaction coordinates. A powerful way to gain insight into the molecular dynamics of such processes involves the determination of final state distributions in the translational and internal degrees of freedom of desorbed product molecules. These distributions include the angular distribution, the translational, vibrational, and rotational energy distributions, the spatial alignment of angular momentum vectors, and populations of spin-orbit and lambda doublet states. Additional information is contained in correlations between two or more degrees of freedom, e.g., the velocity distribution as a function of desorption angle or internal state, or correlations between spin-orbit and rotational state populations. In this Report we review state resolved studies of surface photochemistry (hν ⩽ 6.4 eV) for which the final state distributions have contributed to our understanding of the microscopic dynamics, focusing on recent progress in our ability to explain the observed distributions with simple dynamical models. After briefly reviewing basic photochemical excitation and reaction mechanisms at surfaces, we discuss experimental techniques, including time-of-flight measurement by mass spectrometer and laser spectroscopic techniques. We then review vibrational distributions of photochemically desorbed molecules, interpreting them in terms of a simple quantum dynamics model. A section on translational distributions discusses dynamical models explaining observed velocity distributions of photochemically desorbed species for both weakly and strongly quenched systems. Next we review rotational distributions of photochemically desorbed molecules, and present a dynamical model that can explain many of the observed distributions, as well as the ubiquitous positive correlations between the rotational and translational degrees of freedom. After discussing recent measurements of the spatial alignment of angular momentum vectors of photodesorbed molecules, we review models explaining spin-orbit state propensities and spin-orbit-rotational correlations for photodesorption and surface photodissociation systems.

A variable-temperature scanning tunneling microscope capable of single-molecule vibrational spectroscopy

The design and performance of a variable-temperature scanning tunneling microscope (STM) is presented. The microscope operates from 8 to 350 K in ultrahigh vacuum. The thermally compensated STM is suspended by springs from the cold tip of a continuous flow cryostat and is completely surrounded by two radiation shields. The design allows for in situ dosing and irradiation of the sample as well as for the exchange of samples and STM tips. With the STM feedback loop off, the drift of the tip–sample spacing is approximately 0.001 A/min at 8 K. It is demonstrated that the STM is well-suited for the study of atomic-scale chemistry over a wide temperature range, for atomic-scale manipulation, and for single-molecule inelastic electron tunneling spectroscopy (IETS).

Nitric oxide adsorption, decomposition, and desorption on Rh(100)

Nitric oxide adsorption, decomposition, and desorption were studied on Rh(100) in the temperature range from 88 to 1100 K using electron energy loss spectroscopy (EELS) and temperature programmed desorption (TPD). The EEL spectrometer was equipped with a multichannel detector for fast data acquisition. There are two adsorption states of NO on Rh(100), designated α1NO and α2NO, characterized by vibrational modes at 114 and 196 meV, respectively, and assigned to a lying down or highly inclined species and a vertically adsorbed species. The populations of the two states as functions of the total NO coverage were measured on the clean surface and with coadsorbed oxygen and CO. These coadsorbed species, whether adsorbed before or after the NO, increase the α2 population at the expense of α1. A model that includes an adsorbate–adsorbate interaction (range≈7 A) which converts α1NO to α2NO and which permits adsorbing NO to diffuse so as to favor α1 adsorption fits the measured populations of the two species on th...

Thermally activated oxidation of NHs on Pt(111): intermediate species and reaction mechanisms

Abstract Results of a study of the thermally activated reactions of ammonia with atomic and molecular oxygen on Pt (111) are presented. Ultrahigh vacuum, surface sensitive techniques such as temperature programmed desorption and reaction spectroscopies (TPD, TPRS) , electron energy loss spectroscopy (EELS) , Auger electron spectroscopy (AES) , and low energy electron diffraction (LEED) were used in this study. H 2 O, NO, and N 2 are produced in various amounts depending on reactant concentrations. Stable intermediate species OH, NH, and NH 2 are identified by EELS. Assignment of reaction mechanisms based on liberation of hydrogen via stepwise N-H bond cleavage by atomic oxygen and OH is made through an EELS analysis of reaction intermediates and a TPRS analysis of NH 3 coverage dependences of product distributions. A forthcoming report will describe the photo-induced reaction of NH 3 and O 2 coadsorbed on Pt (111)

Atomistic studies of O2 dissociation on Pt(111) induced by photons, electrons, and by heating

The adsorption and subsequent dissociation of O2 on Pt(111) was studied by variable temperature scanning tunneling microscopy in the temperature range of 40 to 215 K. Tight clustering of bridge site molecules is observed on terraces between 40 and 70 K, indicating a highly mobile precursor to chemisorption. Coexistence of bridge and fcc hollow site molecules in fractal-shaped islands is observed after dosing between 70 and 95 K. Dissociation of these species was induced by uv radiation, inelastic tunneling electrons, and heating. In all three cases, two O atoms are found within two lattice constants of the original molecule and one to three lattice constants apart.

The chemisorption and decomposition of ethylene and acetylene on Ni(110)

Abstract High resolution electron energy loss spectroscopy (HREELS), low energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) have been used to study the adsorption and decomposition of ethylene and acetylene on a Ni(110) surface. HREEL spectra are reported as a function of hydrocarbon exposure, temperature (80–500 K), and scattering angle for protonated and deuterated ethylene, and at 80 K for protonated and deuterated acetylene. Both ethylene and acetylene adsorb molecularly at 80 K, but both show rehybridization from the gas phase: ethylene to ~ sp 3 and acetylene to ~ sp 2.5 . All of the observable vibrational modes of ethylene are excited to different degrees by the dipole scattering mechanism, and its site group symmetry at 80 K is lower than C 2v . Ordered LEED patterns are formed on adsorption at low temperatures; a complex pattern for ethylene and a c(2 × 2) for acetylene. Ethylene begins to decompose above 200 K to form CCH intermediates with evolution of hydrogen. On additional heating the CCH species decompose to CH species. Finally by 500 K atomic carbon remains on the surface and forms a (4 × 5) ordered overlayer. The thermal decomposition of acetylene is more complex than that of ethylene, as evidenced by the TDS results. Possible bonding models of acetylene and the CCH species on Ni(110) are proposed.

Photochemistry of oriented molecules coadsorbed on solid surfaces: The formation of CO2+O from photodissociation of O2 coadsorbed with CO on Pt(111)

Measurement of a photoinduced reaction involving two types of molecular species coadsorbed with well‐defined configurations on a solid surface is reported. The photoinduced reaction, occurring on Pt(111) at 100 K, is O2+CO+hν→O+{O→CO}→O+CO2. A mechanism involving photochemically produced hot O atoms (with high translational energy, and possibly electronically excited) is proposed, in which the initial step involves selective photodissociation of O2 coadsorbed with CO. The O atom collides with a neighboring CO and forms CO2 which desorbs immediately from the surface. The nature of the adsorbed species was probed before and after irradiation by thermal desorption spectroscopy (TDS) and high resolution electron energy loss spectroscopy (EELS). It was found that the wavelength dependence of the CO2 production followed that for O2 photodissociation. At 338 nm the cross section for CO2 production is 3.3±0.5×10−20 cm2 and decreases to 2×10−21 cm2 at 443 nm. CO2 was not observed in EEL spectra following quenching...

Photodesorption of NO from Ag(111) and Cu(111)

The adsorption, thermal reactions, and photoreactions of NO on Ag(111) and Cu(111) at 80–85 K have been studied by thermal‐desorption spectroscopy (TDS), high‐resolution electron‐energy‐loss spectroscopy (HREELS), and photon‐induced desorption. Adsorption of NO on both surfaces is quite complicated. At saturation coverage, a number of chemical species are present, including atop and bridge‐bonded NO, atomic N and O, and N2O. Photodesorption of NO, N2, and N2O is observed simultaneously under low‐power photon irradiation in the wavelength range for 260–600 nm. From TD and HREEL spectra before and after photon irradiation, it is established that on both surfaces the atop NO is photoactive. Photon polarization, power‐, and wavelength‐dependences studies indicate that the mechanisms for photodesorption are nonthermal. A substrate‐mediated mechanism involving photogenerated carriers at low photon energies (<3 eV) and a direct excitation mechanism of the adsorbate‐surface complex at high photon energies are use...

Mechanisms of laser interaction with metal carbonyls adsorbed on Si(111)7×7: Thermal vs photoelectronic effects

Ultrahigh vacuum studies of the interaction of 514 nm radiation from a cw Ar ion laser and its second harmonic at 257 nm with mono‐ and multilayer coverages of Mo(CO)6, W(CO)6, and Fe(CO)5 adsorbed on Si(111)7×7 at 90 K using thermal desorption spectroscopy (TDS), laser induced desorption spectroscopy, high resolution electron energy loss spectroscopy (HREELS), and Auger electron spectroscopy were performed. A model for the temperature rise of the sample due to cw laser heating is developed. By directly measuring the substrate temperature, these experiments were able to distinguish between photoelectronic and thermal effects active in the decomposition and desorption mechanisms of the adsorbed carbonyls. Results from TDS and HREELS show that Mo(CO)6 and W(CO)6 are molecularly adsorbed, while Fe(CO)5 partially dissociates upon adsorption. The decomposition of adsorbed Mo(CO)6 is caused by electronic excitation due to direct absorption of the 257 nm radiation. Irradiation with 514 nm radiation results in no...