Arthur Suits and Oleg Vasyutinskii of Wayne State University are supported by the Experimental Physical Chemistry Program to carry out high-resolution ion imaging studies of photochemical processes in both ionic and neutral molecular systems. The objectives are to understand multi-surface dynamics and nonadiabatic processes in polyatomic systems; to examine coherent effects in photodissociation; to explore features of the ground and excited-state potential energy surfaces; to reveal unusual reaction mechanisms; to obtain accurate branching fractions for multi-channel processes and explore the dynamical issues controlling this branching; and to benchmark electronic structure and dynamical calculations for these challenging problems. These studies will exploit the recently developed reflectron multimass imaging apparatus to study conformationally- and vibrationally-mediated dissociation of ions, a new direction that promises insights into the light-driven dynamics of polyatomic systems. In parallel with the ion photodissociation studies, atomic orbital polarization in polyatomic systems will be used to investigate atomic and molecular interactions in unique detail. Detailed investigations of the complete angular momentum polarization, including contributions from higher moments, will be used to unravel the decay dynamics in cases were several excited electronic states play a role. Concurrently, new theoretical descriptions of these phenomena will be developed so that a direct connection can be made between the experiment and a complete quantum description of the polyatomic photodissociation process.
Outcomes from this research are anticipated to lead to major insights into how chemical reactions occur at the highest detail, and may compel substantial changes in how presumably statistical molecular dissociations are described. This project will provide challenging research opportunities for a diverse group of students and postdocs. As well, the technological and software advances that result will be shared freely with numerous research groups.
This program has two related aspects: imaging probes of dynamics in molecules and ions at high (i.e., VUV) energies, and imaging studies of atomic orbital polarization in photodissociation. In all this work our goals have been to use these phenomena to probe reactions involving multiple electronic states and the signatures of electronic rearrangement during reaction or dissociation; to reveal unusual reaction mechanisms with high resolution, state-resolved imaging methods; to obtain accurate branching fractions for multi-channel processes and explore the dynamical issues controlling this branching; and to benchmark electronic structure and dynamics calculations for these challenging problems. 1.1 Photoionization and photodissociation dynamics in polyatomic ions. Photoelectron imaging and conformationally-mediated dissociation of propanal cation: mode selectivity and detailed theoretical analysis. We reported surprising experimental evidence for conformationally selective dissociation of propanal cation (Fig. 1). The results showed production of two distinct product isomers, and the branching varied strongly depending on starting conformer despite negligible differences in energy and a very small barrier separating them. The experimental results were interpreted, on the basis of ab initio multiple spawning (AIMS) calculations from Tao and Martinez, as arising from distinct dynamics in the first excited ("dark") state of the cation, followed by conversion to the ground state via intersections in distinct configurations. We have since completed an extension of these initial studies of conformationally mediated dynamics in propanal cation, again in collaboration with Tao and Martinez. Experimentally, we have expanded the studies to include mode-selectivity and imaging results for the partially deuterated molecule. The results comparing mode-selectivity for the cis conformer as well as conformationally selective imaging are shown in Fig. 1 along with a plot of the dependence of the branching between the two product isomers. These have been augmented and interpreted based on calculations of the ultrafast excited state dynamics and surface intersections, combined with detailed characterization of the ground state surface with statistical calculations starting from the different geometries accessed by the different initial conformers. The deuterated results and the calculations embodied in Fig. 1 clearly confirm our explanation proposed in the original Science paper although the quantitative result is complicated by the fact that extensive secondary decomposition of the propanoyl product amplifies the observed effect. 1.2 High resolution imaging in neutral systems: orbital polarization and nonadiabatic dynamics. 1.2.1 Identification of the slow triplet component in the deep ultraviolet photodissociation of ozone. In an important paper in Science in the early 1990s, Houston, Wodtke and coworkers showed a bimodal distribution in the ground state oxygen atom from ozone dissociation at 226 nm as an additional, autocatalyic route to ozone formation in the stratosphere.In the ensuing decade or more, extensive theoretical and experimental work has failed to find a plausible pathway to formation of this highly vibrationally excited ground state O2 from deep UV photodissociation of ozone, and the source of this slow ground state O atom has remained a mystery. This has motivated our reinvestigation of this problem using the vast improvement in velocity resolution afforded by DC slice velocity map imaging supporting high-level theoretical calculations from Schinke and McBane. The results are shown in Fig. 2. Two peaks are resolved in the spectra, and these match closely with the predicted onsets for Herzberg states of O2. This work was published as a Communication to J. Chem. Phys. and selected as an "Editor’s Choice" article for 2009. 1.2.2. Nitric Acid Photodissociation: The O(1D)+HONO Channel. The ultraviolet photodissociation of nitric acid is of considerable importance in Earth’s atmosphere, motivating many studies of its dissociation processes, branching ratios and quantum yields. We have initiated an imaging study10 of nitric acid photodissociation through the S3 excited state with detection of O(1D), one of the major decomposition products in this region. Our images show structure reflecting the vibrational distribution of the HONO coproduct, and significant angular anisotropy that varies with recoil speed but is largely consistent with previous reports at 193 nm. The images also show substantial alignment of the O(1D), manifested as higher order Legendre moments to the angular distributions. The results provide additional insight into the dynamics of the dissociation of nitric acid through the S3 (2 1A´) excited state, helping to resolve some outstanding questions and pointing the way to future studies described below. 1.3 Technique Development. 1.3.1 Megapixel imaging. As a component of our NSF program, we developed a combined hardware and software system allowing us to achieve megapixel ion imaging using conventional, inexpensive CCD cameras. This effort was a natural evolution of the DC slice imaging approach developed in the previous grant period. The performance of the system meets or exceeds that of dedicated camera systems costing ten or twenty times as much. This software (programs IMACQ and IMAN) and guidance in its use is provided without charge to anyone interested, and we have distributed more than a dozen copies to-date.
