Greg O. Sitz

Rotational state-resolved sticking coefficients for H2 on Pd(111): Testing dynamical steering in dissociative adsorption

We have measured the sticking coefficient of H2 on Pd(111) as a function of incident translational energy, over the range 31–94 meV, and initial H2 rotational state J, for J=0 through J=5, at a surface temperature of 423 K. These measurements test recent theoretical predictions that the H2/Pd sticking coefficient should decrease with increasing J, for low J, because more quickly rotating molecules are less easily “steered” to dissociation sites by interaction with the surface as they approach the metal [A. Gross, S. Wilke, and M. Scheffler, Phys. Rev. Lett. 75, 2718 (1995)]. We find that the sticking coefficient is sensitive to the rotational quantum number, first decreasing as J is raised from 0 to 3, then increasing again for J=4 and 5. The greatest sensitivity found occurs for an incident translational energy of 73 meV, for which the sticking coefficient decreases from 0.76±0.05 for J=0 to 0.54±0.13 for J=3. In addition, we have found that a fraction of the H2 incident in states J=0 and J=1 is excited ...

Gas surface interactions studied with state-prepared molecules

The interaction of gas-phase molecules with solid surfaces is complicated by the many molecular and surface degrees-of-freedom (DOFs) which may affect the scattering process. This review covers experimental work in which the quantum state populations of molecules incident on a surface are prepared or selected in some manner, and the scattering of these prepared molecules is followed. Experiments are discussed in which essentially all (electronic, vibrational, rotational, and translational) molecular DOFs are controlled. Two main techniques to produce a flux of state-prepared molecules for surface studies have been developed; both are natural extensions of existing gas-phase scattering work. The first employs direct optical excitation and relies on the associated selection rules to generate population in a desired state. The second uses inhomogeneous electric or magnetic fields as a filter to transmit a particular state and reject others. Specific applications of each of these techniques are reviewed with emphasis placed on the insight gained from the quantum state specific nature of the incident molecules.

Rovibrationally inelastic scattering of (v=1, j=1) H-2 from Cu(100) experiment and theory

A comparison between experiment and theory is performed for the scattering of (v=1, j=1) H2 from Cu(100) at normal incidence. Experimentally, this system was studied using molecular beam techniques, with stimulated Raman pumping employed to overpopulate (v=1, j=1) in the incident beam, and resonance enhanced multi-photon ionization used to detect the H2 scattered in two (v=1, j) states, and two (v=0, j) states. Theoretically, six-dimensional wave packet calculations were performed, employing a new, extended potential energy surface that was computed with density functional theory, using the generalized gradient approximation and a slab representation of the metal surface. Theory and experiment are in good agreement for the survival probability, i.e., the probability for rovibrationally elastic scattering. However, the theory overestimates the probabilities for rotationally inelastic scattering (to v=1, j=3) and for rovibrationally inelastic scattering (to v=0, j=5 and 7) for channels that could be determi...

State-to-state scattering in a reactive system: H2(v=1,J=1) from Cu(100)

We have measured state-to-state scattering in the H2–Cu(100) and H2–Cu(110) systems for H2 initially prepared in the v=1,J=1 state, for several final (v,J) states. We observe survival in the initial state (v=1,J=1), rotational excitation within v=1 (into v=1,J=3), and rotational excitation plus vibrational relaxation into final states (v=0,J=5) and (v=0,J=7). For each of these final states, we measure the branching fraction (relative to the incident v=1,J=1 flux) and the mean final translational energy. In addition, for rotational excitation from J=1 to 3, the dependence on initial translational energy and surface temperature is measured within v=1 and compared with results within v=0. Our results are compared with published state-resolved associative desorption results, similar scattering experiments, and recent theoretical calculations.

Population and alignment of N2 scattered from Ag(111)

A well‐characterized Ag(111) surface in an ultrahigh vacuum system is bombarded with supersonically cooled N2 and the scattered molecules are detected near the specular angle using 2+2 resonance enhanced multiphoton ionization. The first three even spatial moments of the angular momentum distribution have been measured. The zeroth moment is the population of a given rotational level, while the second and fourth moments describe the quadrupole and hexadecapole alignment of the J vector with respect to the surface normal. The relative population distribution shows an excess at high J characteristic of rotational rainbow scattering. For J>7, the second and fourth moments closely approach their limiting values expected when the J vector lies in the plane of the surface. It is concluded that the N2/Ag(111) system closely approaches the ideal of a rigid rotor colliding with a flat surface.

Preparation and decay of alignment in N2 (v=1)

We report measurements of the collisional decay of the angular momentum alignment for N2 (v=1) at 298 K. Stimulated Raman pumping of S‐branch (v=1←0) transitions is used to prepare a selected rotational state of N2 with an anisotropic spatial J distribution in the v=1 state. After allowing an appropriate time interval for collisions to occur, 2+2 resonance‐enhanced multiphoton ionization is used (through the a 1Πg←X 1Σ+g transition) to detect the relative population and alignment of the pumped level and other levels to which rotational energy transfer has occurred. We have performed a series of measurements in which a selected even rotational level (Ji=0–14) is excited and the time‐dependent level population and alignment are measured at several delay times. We find the decay of alignment to be no faster than the decay of the population of the pumped level, indicating that pure m‐state changing collisions are slower that J‐state changing collisions. We have also observed substantial alignment of molecules...

Theoretical evidence for nonadiabatic vibrational deexcitation in H2(D2) state-to-state scattering from Cu(100)

Dynamical calculations are presented for electronically nonadiabatic vibrational deexcitation of H2 and D2 in scattering from Cu(111). Both the potential energy surface and the nonadiabatic coupling strength were obtained from density functional calculations. The theoretically predicted magnitude of the deexcitation and its dependence on incident energy and isotope are all in agreement with state-to-state scattering experiments [on Cu(100)], and this gives indirect evidence for a nonadiabatic mechanism of the observed deexcitation. Direct evidence could be obtained by measuring the chemicurrent associated with the deexcitation, and its properties have been predicted.

Surface temperature dependence of rotational excitation of H2 scattered from Pd(111)

We have measured rotational excitation into rotational states J=3, 4, and 5 for H2 scattered from Pd(111) as a function of surface temperature and incident translational energy. Excitation is found to occur even when the incident H2 translational energy is less than the energy level spacing between the initial and final rotational states. Thus, part of the excitation energy is coming from the surface, not from solely translational–rotational energy coupling. There is a strong surface temperature dependence to the rotational excitation that is well described by an Arrhenius-type expression. When fit to the Arrhenius equation, the apparent activation energy is less than the rotational energy level spacing and decreases as the translational energy of the incident molecules is increased. Based on inspection of the calculated H2/Pd(111) potential energy surface, we attribute this lowered activation energy to an extension of the bond length when the molecule interacts with the surface. The stretching of the mol...

Vibrational excitation of NO scattered from Cu(110)

The vibrational excitation of nitric oxide (NO) has been observed in molecular beam scattering from Cu(110). The degree of vibrational excitation increases exponentially with increasing surface temperature and depends linearly on the incident translational energy. The scattered rotational state distribution in both NO(v = 0) and NO(v = 1) depends strongly on the incident translational energy and weakly on the surface temperature which is indicative of direct inelastic scattering. If only mechanical forms of energy transfer between the molecule and the surface are considered, vibrational excitation of the molecules is unlikely. The exponential surface temperature dependence observed could be an indication of electronically mediated energy transfer.

OBSERVATION AND CHARACTERIZATION OF DIRECT INELASTIC AND TRAPPING-DESORPTION CHANNELS IN THE SCATTERING OF N2 FROM CU(110)

Resonantly enhanced multiphoton ionization (REMPI) and time resolved molecular beam techniques were used to study the rotational state and velocity distributions of N2 scattered from Cu(110). At a surface temperature of 85 K, two peaks were observed in time‐of‐flight (TOF) measurements. We interpret these as a direct inelastic channel at early time and a trapping desorption channel at later time. As the surface temperature is raised the direct inelastic scattering becomes stronger as the trapping desorption channel grows weaker. Rotational state distributions show that the early TOF peak has a large number of populated states characteristic of direct inelastic scattering; the later TOF peak has a rotational state distribution well characterized by the surface temperature. The rotational state distributions of the direct‐inelastic channel were found to depend linearly on both surface temperature, Ts and incident energy, Ei. The absence of rotational rainbows at low Ei and low Ts is attributed to multiple c...