D. C. Jacobs

High‐resolution angle‐ and energy‐resolved photoelectron spectroscopy of NO: Partial wave decomposition of the ionization continuum

We report a two‐color high‐resolution energy‐ and angle‐resolved study of the photoelectrons produced in the (1+1’) REMPI of NO via rotational levels of the A 2Σ+ v=0 state. We find markedly different photoelectron angular distributions arising from production of ions in different rotational states (ΔN=0,±1,±2 transitions in the ionization step). We also observe that the ΔN=±2 angular distributions are very sensitive to the intermediate state alignment. A model is put forward in which experimental observables (angle‐ and energy‐resolved photoelectron spectra) are used to determine the attributes (relative amplitudes and phase shifts) of a small number of interfering continuum channels that contribute to the ionization step as well as the fraction of parallel character of the ionization step. Nearly 70% of the ejected photoelectrons are associated with the ΔN=0 ionization transition; the partial wave composition of these electrons is dominated by p character. The less important ΔN=±1 peaks have both s‐ and...

Reduction of 1+1 resonance enhanced MPI spectra to population distributions: Application to the NO A 2Σ+–X 2Π system

A two‐step methodology is presented for extracting ground state population distributions and alignment factors from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an isopower spectrum. In the second step populations and alignments are derived from the isopower spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0,0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.

Reduction of 1+1 resonance enhanced MPI spectra to populations and alignment factors

A theory is presented to reduce 1+1 resonance enhanced multiphoton ionization (REMPI) spectra to accurate rovibrational state population distributions. Classical and quantum mechanical treatments are developed to model the polarization dependence of the REMPI signal from an initially aligned ground state having cylindrical symmetry. The theory includes the effects of saturation and intermediate state alignment. It is demonstrated that, for favorable cases, 1+1 REMPI allows the determination of the relative population as well as the quadrupole and hexadecapole moments of the alignment for rovibrational levels of a linear molecule. The classical treatment differs from that of the quantum treatment by less than 5% for rotational quantum numbers greater than J=4, suggesting that the classical treatment suffices for 1+1 REMPI in most molecular systems.

Rotational population and alignment distributions for inelastic scattering and trapping/desorption of NO on Pt(111)

Rotationally resolved experiments on the NO/Pt(111) system explore the mechanisms of inelastic scattering and trapping/desorption. The rotational dynamics associated with these two regimes are markedly different. A neat supersonic NO beam is scattered at normal incidence from a Pt(111) crystal at 375–475 K. The non‐Boltzmann rotational population distribution of the scattered species exhibits considerable rotational excitation beyond the energy available from the incident beam. Thus, a surface vibration to rotational energy transfer mechanism must be operative. The accompanying rotational alignment data reveal that highly excited rotational states exhibit predominantly ‘‘cartwheel’’ motion. In contrast, rotationally excited molecules that desorb from a 553 K Pt(111) surface show a preference for ‘‘helicopter’’ motion. The opposite preferences for rotational alignment in the two dynamical regimes provide insight into the anisotropy of molecule–surface interactions.

Rotational alignment of NO desorbing from Pt(111)

The Letters to the Editor section is subdivided into four categories entitled Communications, Notes, Comments, and Errata. The textual material of each Letter is limited to 1200 words minus the following: (a) 200 words for a square figure one-column wide. Larger figures are scaled in proportion to their area; (b) 50 words for each displayed equation; (c) 7 words for each line of table including headings and horizontal rulings. Proof will be sent to authors. See the issue of 1 July 1987 for a fuller description of Letters to the Editor.

Simplified trajectory method for modeling gas–surface scattering: The NO/Pt(111) system

A model is presented to describe the dynamical processes of trapping/desorption as well as direct and indirect inelastic scattering on single‐crystal surfaces. Newton’s equations of motion are integrated for a system consisting of a rigid rotor interacting with a slab of 19 surface atoms. The surface atom which is closest to the center of mass of the molecule is permitted to translate only along the surface normal. In turn, this mobile surface atom is harmonically coupled to a microcanonical heat bath consisting of three subsurface atoms. This method is much less computationally intensive than the typical generalized Langevin equation (GLE) approach. Direct comparison is made between the predictions of this model and experiment for the NO/Pt(111) system. In the case of trapping/desorption, the model accurately describes the observed dependence of rotational alignment on rotational quantum number. For the inelastic scattering regime, the model successfully reproduces the degree of rotational excitation and qualitatively accounts for the observed rotational alignment. In addition, the model predicts correlations between final state velocity and final state rotational angular momentum (both direction and magnitude), as well as the effect of molecular orientation and surface impact parameter on the overall trapping probability.A model is presented to describe the dynamical processes of trapping/desorption as well as direct and indirect inelastic scattering on single‐crystal surfaces. Newton’s equations of motion are integrated for a system consisting of a rigid rotor interacting with a slab of 19 surface atoms. The surface atom which is closest to the center of mass of the molecule is permitted to translate only along the surface normal. In turn, this mobile surface atom is harmonically coupled to a microcanonical heat bath consisting of three subsurface atoms. This method is much less computationally intensive than the typical generalized Langevin equation (GLE) approach. Direct comparison is made between the predictions of this model and experiment for the NO/Pt(111) system. In the case of trapping/desorption, the model accurately describes the observed dependence of rotational alignment on rotational quantum number. For the inelastic scattering regime, the model successfully reproduces the degree of rotational excitation and...

Rotational dynamics of desorption and inelastic scattering for the NO/Pt(111) system

The rotational population and alignment distributions of NO are measured subsequent to the molecule’s interaction with a clean Pt(111) surface. Two distinct dynamical regimes of trapping/desorption and inelastic scattering are studied over the temperature range 375–550 K. For the case of molecular desorption, molecules which desorb with a large amount of angular momentum (J>12.5) prefer to rotate in the plane of the surface. Those molecules which scatter from the surface show the opposite preference for rotational alignment, i.e., they preferentially rotate in a plane perpendicular to the surface. The non‐Boltzmann rotational distribution reveals that a large fraction of the scattered molecules contain more energy in rotation than initially exists as total energy in the beam. This observation indicates the important role of surface vibration to rotational energy transfer in this system. A quasiclassical stochastic trajectory model is able to account for much of the observed phenomena.

Rotational alignment of NO from Pt(111). Inelastic scattering and molecular desorption

The rotational alignment distribution of NO has been measured subsequent to the molecules interaction with a well characterized Pt(111) surface. Internal state distributions have been probed using 1 + 1 resonance-enhanced multiphoton ionization (REMPI) spectroscopy in which lines of the NO A 2Σ+–X2Π(0, 0) band constitute the resonant transition. NO/Pt(111) scattering has been studied in two distinct regimes: inelastic scattering and trapping/desorption. In both cases, there is relatively no preferential alignment of rotation for J < 12.5. However, molecules with higher rotational angular momentum show a marked increase in alignment. Inelastically scattered molecules prefer to rotate in a plane normal to the surface (‘cartwheel’ motion), whereas desorbing molecules prefer to rotate in a plane parallel to the surface (‘helicopter’ motion). These measurements provide new insight into momentum transfer at surfaces and the nature of the transition state which leads to molecular desorption.

Reduction of 1+1 REMPI spectra to population distributions: Saturation and intermediate state alignment effects

A two‐step methodology is presented for extracting ground state population distributions from 1+1 resonance enhanced multiphoton ionization (REMPI) spectra. In the first step the ion signal is corrected for variation with laser intensity as it is collected, generating an iso‐power spectrum. In the second step populations and alignments are derived from the iso‐power spectrum by correcting for the interdependent effects of saturation and intermediate state alignment. This procedure is applied to a room temperature thermal distribution of nitric oxide using the 1+1 REMPI process in which lines of the NO A 2Σ+–X 2Π (0, 0) band constitute the resonant transition. The present treatment is able to recover the known rovibrational population distribution, independent of branch choice, over a wide range of practical operating conditions.

Utilization of 1+1 REMPI as a probe of rotational dynamics in gas‐surface scattering

The spectroscopic technique of 1+1 resonance enhanced multiphoton ionization (REMPI) spectroscopy gives a quantitative measure of the rotational population and alignment distributions of molecules that have interacted with a surface. The two distinct dynamical regimes of trapping/desorption and inelastic scattering are studied for the NO/PT(111) system. In the case of molecular desorption, molecules which desorb with a large amount of angular momentum (J≳12.5) prefer to rotate in the plane of the surface. Those molecules which inelastically scatter from the surface show the opposite preference for rotational alignment, i.e., they preferentially rotate in a plane perpendicular to the surface. The rotational population distribution reveals that surface vibrational energy is efficiently transferred into rotational energy in the scattering process.