Dynamical signatures from competing, nonadiabatic fragmentation pathways of S-nitrosothiophenol.

S-Nitrosothiols (RSNOs) are derived from the combination of sulfur and nitric oxide (NO) radicals in the Earth's atmosphere and fragment to products following photolysis. Extensive theoretical studies have focused on the thermodynamic and, to a lesser extent, photochemical properties of RSNOs. However, experimental studies of these compounds have been limited due to the inherent instability of RSNOs at room temperature. Using velocity map imaging (VMI), we explore the photodissociation dynamics of jet-cooled S-nitrosothiophenol (PhSNO) from 355 nm photolysis. We report the translational and internal energy distributions of the NO and thiophenoxy (PhS) co-fragments, which are determined by spatial detection of the ionized NO photofragments using 1+1 resonance-enhanced multiphoton ionization (REMPI). The velocity distributions indicate competing PhSNO nonadiabatic dissociation pathways, in which PhS is formed in the ground and first excited electronic states when probing high- and low-energy NO (X2Π1/2, v'', J'') rovibrational states, respectively. The results of multireference electronic structure calculations suggest that direct dissociation on the bright S2 state results in PhS formed in its excited electronic state, whereas intersystem crossing into the triplet manifold leads to population of PhS in its electronic ground state. The dynamical signatures from the dissociation processes are imprinted on the fragments' quantum states and relative translation, which we explore in rigorous detail using state-resolved imaging and high-level theoretical calculations.

Imaging the nonreactive collisional quenching dynamics of NO (A2Σ+) radicals with O2 (X3Σg -).

Nitric oxide (NO) radicals are ubiquitous chemical intermediates present in the atmosphere and in combustion processes, where laser-induced fluorescence is extensively used on the NO (A2Σ+ ← X2Π) band to report on fuel-burning properties. However, accurate fluorescence quantum yields and NO concentration measurements are impeded by electronic quenching of NO (A2Σ+) to NO (X2Π) with colliding atomic and molecular species. To improve predictive combustion models and develop a molecular-level understanding of NO (A2Σ+) quenching, we report the velocity map ion images and product state distributions of NO (X2Π, v″ = 0, J″, Fn, Λ) following nonreactive collisional quenching of NO (A2Σ+) with molecular oxygen, O2 (X3Σg -). A novel dual-flow pulse valve nozzle is constructed and implemented to carry out the NO (A2Σ+) electronic quenching studies and to limit NO2 formation. The isotropic ion images reveal that the NO-O2 system evolves through a long-lived NO3 collision complex prior to formation of products. Furthermore, the corresponding total kinetic energy release distributions support that O2 collision coproducts are formed primarily in the c1Σu - electronic state with NO (X2Π, v″ = 0, J″, Fn, Λ). The product state distributions also indicate that NO (X2Π) is generated with a propensity to occupy the Π(A″) Λ-doublet state, which is consistent with the NO π* orbital aligned perpendicular to nuclear rotation. The deviations between experimental results and statistical phase space theory simulations illustrate the key role that the conical intersection plays in the quenching dynamics to funnel population to product rovibronic levels.

Nonstatistical Dissociation Dynamics of Nitroaromatic Chromophores.

Organic carbon in the atmosphere is emitted from biogenic and anthropogenic sources and plays a key role in atmospheric chemistry, air quality, and climate. Recent studies have identified several of the major nitroaromatic chromophores embedded in organic "brown carbon" (BrC) aerosols. Indeed, nitroaromatic chromophores are responsible for the enhanced solar absorption of BrC aerosols, extending into the near UV (300-400 nm) and visible regions. Furthermore, BrC chromophores serve as temporary reservoirs of important oxidizing intermediates including hydroxyl (OH) and nitric oxide (NO) radicals that are released upon electronic excitation. The present work represents the first study of the 355 nm photolysis of known BrC chromophores ortho-nitrophenol and 2-nitroresorcinol, as well as the prototypical nitroaromatic, nitrobenzene. Experiments are carried out in a pulsed supersonic jet expansion with velocity map imaging of NO X2Π (ν″ = 0, J″) fragments to report on the photodissociation dynamics. The total kinetic energy release (TKER) distributions and the NO X2Π (ν″ = 0, J″) product state distributions deviate significantly from Prior simulations, indicating that energy is partitioned nonstatistically following dissociation. Experiments are conducted in tandem with complementary calculations using multireference Møller-Plesset second-order perturbation theory (MRMPT2) for stationary points obtained by using multiconfiguration self-consistent field (MCSCF) with an aug-cc-pVDZ basis on the ground and lowest energy triplet electronic states. Furthermore, insights into the partitioning of energy upon photodissociation are achieved by using relaxed scans at the MCSCF/aug-cc-pVDZ level of theory. As a whole, the results suggest that upon excitation to S1, all three nitroaromatics share a common overall mechanism for NO production involving isomerization of the nitro group, nonradiative relaxation to S0, and dissociation to form rotationally hot NO.

Packaging and Non-Hermetic Encapsulation Technology for Flip Chip on Implantable MEMS Devices.

We report here a successful demonstration of a flip-chip packaging approach for a microelectromechanical systems (MEMS) device with in-plane movable microelectrodes implanted in a rodent brain. The flip-chip processes were carried out using a custom-made apparatus that was capable of the following: 1) creating Ag epoxy microbumps for first-level interconnect; 2) aligning the die and the glass substrate; and 3) creating non-hermetic encapsulation (NHE). The completed flip-chip package had an assembled weight of only 0.5 g significantly less than the previously designed wire-bonded package of 4.5 g. The resistance of the Ag bumps was found to be negligible. The MEMS micro-electrodes were successfully tested for its mechanical movement with microactuators generating forces of 450 µN with a displacement resolution of 8.8 µm/step. An NHE on the front edge of the package was created by patterns of hydrophobic silicone microstructures to prevent contamination from cerebrospinal fluid while simultaneously allowing the microelectrodes to move in and out of the package boundary. The breakdown pressure of the NHE was found to be 80 cm of water, which is significantly (4.5-11 times) larger than normal human intracranial pressures. Bench top tests and in vivo tests of the MEMS flip-chip packages for up to 75 days showed reliable NHE for potential long-term implantation.