A. L. Utz

Bond-selective control of a heterogeneously catalyzed reaction.

Energy redistribution, including the many phonon-assisted and electronically assisted energy-exchange processes at a gas-metal interface, can hamper vibrationally mediated selectivity in chemical reactions. We establish that these limitations do not prevent bond-selective control of a heterogeneously catalyzed reaction. State-resolved gas-surface scattering measurements show that the ν1 C-H stretch vibration in trideuteromethane (CHD3) selectively activates C-H bond cleavage on a Ni(111) surface. Isotope-resolved detection reveals a CD3:CHD2 product ratio > 30:1, which contrasts with the 1:3 ratio for an isoenergetic ensemble of CHD3 whose vibrations are statistically populated. Recent studies of vibrational energy redistribution in the gas and condensed phases suggest that other gas-surface reactions with similar vibrational energy flow dynamics might also be candidates for such bond-selective control.

The Chemistry of Bulk Hydrogen: Reaction of Hydrogen Embedded in Nickel with Adsorbed CH3

Studies in heterogeneous catalysis have long speculated on or have provided indirect evidence for the role of hydrogen embedded in the catalyst bulk as a primary reactant. This report describes experiments carried out under single-collision conditions that document the distinctive reactivity of hydrogen embedded in the bulk of the metal catalyst. Specifically, the bulk H atom is shown to be the reactive species in the hydrogenation of CH3 adsorbed on Ni(111) to form CH4, while the H atoms bound to the surface were unreactive. These results unambiguously demonstrate the importance of bulk species to heterogeneous catalytic chemistry.

The direct observation, assignment, and partial deperturbation of the ν4 and ν6 vibrational fundamentals in à 1Au acetylene (C2H2)

A pulsed‐laser double resonance technique provides previously unavailable spectroscopic data on the rovibrational structure of A 1Au acetylene (C2H2). Our assignment and analysis of transitions to the A state ν4’ (torsion) and ν6’ (antisymmetric in‐plane bend) vibrational fundamentals uncovers a strong Coriolis interaction between these two nearly degenerate modes and weaker Coriolis interactions between the ν4’/ν6’ pair and remote A state rovibrational levels. We deperturb the direct Coriolis interaction between ν4’ and ν6’ to obtain vibrational frequencies, Coriolis coupling constants and partially deperturbed rotational and centrifugal distortion constants for these previously unobserved fundamentals. Parity selection rules for the A←X band permit an unambiguous assignment of the vibrations (ν4’=764.9±0.1 cm−1 and ν6’=768.3±0.2 cm−1). We use these new experimental values to reassign several A state vibrations and to assign previously unidentified A state levels. We also identify two vibrational ...

Energy transfer in highly vibrationally excited acetylene: Relaxation for vibrational energies from 6500 to 13 000 cm−1

Vibrational overtone excitation of acetylene molecules to energies between 6500 and 13 000 cm−1 followed by interrogation of the excited states during collisional relaxation determines both the mechanism and rates of energy transfer. A pulsed visible or near‐infrared laser excites a single rotational state of C2H2 in the region of the first (2νCH), second (3νCH), or third (4νCH) overtone of the C–H stretching vibration, and an ultraviolet laser probes the excited molecules by laser‐induced fluorescence after a variable delay. The self‐relaxation rate constant of about 9×10−10 cm3 molecules−1 s−1 is almost twice the Lennard‐Jones collision rate constant and is nearly invariant with vibrational level. The energy‐transfer rate constants from these population transfer measurements agree with those extracted from pressure‐broadening data in both their size and insensitivity to vibrational state. Relaxation by the rare‐gas atoms He, Ar, and Xe is nearly half as efficient as self‐relaxation, suggesting that the ...

Collisional relaxation of single rotational states in highly vibrationally excited acetylene

The combination of vibrational overtone excitation with time‐resolved laser induced fluorescence detection of the excited molecule permits the study of collisional energy transfer in highly vibrationally excited molecules with single quantum state resolution. We apply this new technique to aceytlene excited in the region of the second CH stretching overtone transition (3νCH) (evib =9640 cm−1) and probed via the A electronic state. Our results show that self‐relaxation proceeds at essentially the gas kinetic collision rate while quenching by the rare gases He, Ar, and Xe is only about a factor of two slower. The insensitivity of the relaxation rate to the structure of the collision partner clearly points to rotational relaxation or intramolecular vibrational energy transfer as the mechanism for collisionally depopulating the initially prepared state.

Normal modes analysis of Ã‐state acetylene based on directly observed fundamental vibrations

Recent experimental results permit a detailed normal modes analysis of A‐state acetylene (C2H2) and its isotopomers (C2HD and C2D2). Using only experimentally determined frequencies and measured or estimated anharmonicities, we determine harmonic frequencies for the 11 directly observed and unambiguously assigned vibrational fundamentals. The normal modes calculation varies force constants to fit the 11 harmonic frequencies and yields a complete set of harmonic frequencies, force constants, and Coriolis coefficients for the three isotopomers. A complete set of fundamental frequencies calculated from the set of harmonic frequencies allows a comparison to and, in some cases, suggests a reassessment of frequencies for tentatively assigned fundamental vibrations.

State‐to‐state rotational energy transfer in highly vibrationally excited acetylene

Vibrational overtone excitation of single rovibrational eigenstates in acetylene, followed by state‐resolved, laser‐induced fluorescence (LIF) interrogation of the collisionally populated quantum states, permits a direct determination of both the pathways and rates of state‐to‐state rotational energy transfer in a polyatomic molecule containing about 10 000 cm−1 of internal energy. The data, which we acquire under single‐collision conditions, demonstrate the importance of rotational energy transfer, even at high levels of vibrational excitation. The observed state‐to‐state rotational energy transfer pathways populate a wide range of angular momentum states and account for about 70% of the total relaxation rate. About one‐third of the total relaxation occurs by ‖ΔJ‖=2 transitions, which are the smallest allowed, but there are also single‐collision energy transfer pathways with ‖ΔJ‖ as large as 20 and ‖ΔE‖ as large as 600 cm−1 (≊3kT). The state‐resolved rate constants for rotational energy transfer decrease...

The direct observation, assignment, and partial deperturbation of ν5 and ν3+ν5 in à 1Au acetylene (C2H2)

A pulsed‐laser double resonance technique (vibrational overtone excitation combined with laser‐induced fluorescence detection) provides previously unavailable spectroscopic data on the rovibrational structure of A 1Au acetylene (C2H2). We collect fluorescence excitation spectra of transitions to vibronic levels lying between 2800 and 4300 cm−1 above the A state origin. In this region, we observe only two vibronic levels that are relatively unperturbed, which we assign to the A state antisymmetric C–H stretching fundamental vibration ν’5 and its combination with the trans‐bending vibration, ν’3 + ν’5. Parity and symmetry selection rules for the A←X band, ab initio predictions for the ν’5 fundamental frequency, and the known frequencies of other A state vibrations permit an unambiguous assignment of the vibrations. The fit of ν’5 and ν’3 + ν’5 to a near‐prolate asymmetric top Hamiltonian yields the observed vibrational frequencies (ν’5= 2857.4 ± 0.2 cm−1 and ν’3 + ν’5 = 3894.4 ± 0.1 cm−1) and rotation...

DIRECT OBSERVATION OF WEAK STATE MIXING IN HIGHLY VIBRATIONALLY EXCITED ACETYLENE

Abstract A pulsed-laser double-resonance technique probes the mixing of zero-order states in the 3 ν CH vibrational overtone ( ϵ vib ≈ 9640 cm −1 ) of X 1 Σ g + acetylene (C 2 H 2 ), where the calculated vibrational state density is about three states/cm −1 . Vibrational overtone excitation populates and laser induced fluorescence via the A 1 A u electronic state detects the molecular eigenstates, which have slightly mixed vibrational character because of weak interactions between the zero-order optically bright CH stretching state and optically dark background states. Observing the interacting states at low state density in the weak perturber limit dramatically simplifies the assignment and interpretation of the spectra. A two-state model recovers the important features of the experimental data including our prior observations of surprisingly intense A « X electronic transitions originating from 3 ν CH , the anomalous rotational-level dependence of the electronic absorption cross sections, and small perturbations in the 3 ν CH line positions. A multi-state deperturbation analysis gives coupling matrix elements of 0.01–0.05 cm −1 that are consistent with those measured for weak interactions in other polyatomic molecules at higher state densities.

Chemically Accurate Simulation of a Polyatomic Molecule-Metal Surface Reaction

Although important to heterogeneous catalysis, the ability to accurately model reactions of polyatomic molecules with metal surfaces has not kept pace with developments in gas phase dynamics. Partnering the specific reaction parameter (SRP) approach to density functional theory with ab initio molecular dynamics (AIMD) extends our ability to model reactions with metals with quantitative accuracy from only the lightest reactant, H2, to essentially all molecules. This is demonstrated with AIMD calculations on CHD3 + Ni(111) in which the SRP functional is fitted to supersonic beam experiments, and validated by showing that AIMD with the resulting functional reproduces initial-state selected sticking measurements with chemical accuracy (4.2 kJ/mol ≈ 1 kcal/mol). The need for only semilocal exchange makes our scheme computationally tractable for dissociation on transition metals.