The kinetics of the reaction OH+CO→H+CO₂ has attracted experimental and theoretical studies for more than 40 years due to its importance in atmospheric and combustion environments. This reaction proceeds on a rich potential energy landscape, first by forming vibrationally excited HOCO*; subsequently, HOCO* either back reacts to OH+CO, dissociates to H+CO₂, or is stabilized to ground state HOCO by collisions with a third body. Due to the formation of the HOCO intermediate, the rate coefficient displays anomalous temperature and strong pressure dependences. Time-resolved Frequency Comb Spectroscopy (TRFCS) combines a mid-IR mode-locked femtosecond laser, a broadband optical enhancement cavity, and spatially dispersive detection system to simultaneously provide broad spectral bandwidth, high spectral resolution, high absorption sensitivity, and microsecond time resolution. We have applied this powerful technique to identify the deuterated analogues of HOCO isomers, trans-DOCO and cis-DOCO, for the first time in the reaction OD+CO under ambient conditions. By directly monitoring the concentrations of OD (reactant), trans-DOCO, cis-DOCO (intermediates), and CO₂(product), we unambiguously measure all pressure-dependent branching rates of the OD+CO reaction.

Chirped-pulsed (CP) Fourier transform rotational spectroscopy invented by Brooks Pate and coworkers a decade ago is an attractive tool for gas phase chemical dynamics and kinetics studies. A good reactor for such a purpose would have well-defined (and variable) temperature and pressure conditions to be amenable to accurate kinetic modeling. Furthermore, in low pressure samples with large enough number of molecular emitters, reaction dynamics can be observable directly, rather than mediated by supersonic expansion. In the present work, we are evaluating feasibility of in situ time-resolved CP spectroscopy in a room temperature flow tube reactor. Vinyl cyanide (CH₂CHCN), neat or mixed with inert gasses, flows through the reactor at pressures 1−50 μbar (0.76−38 mTorr) where it is photodissociated by a 193 nm laser. Millimeter-wave beam of the CP spectrometer co-propagates with the laser beam along the reactor tube and interacts with nascent photoproducts. Rotational transitions of HCN, HNC, and HCCCN are detected, with ≥ 10 μs time-steps for 500 ms following photolysis of CH₂CHCN. The post-photolysis evolution of the photoproducts’ rotational line intensities is investigated for the effects of rotational and vibrational thermalization of energized photoproducts. Possible contributions from bimolecular and wall-mediated chemistry are evaluated as well.

Water, an essential ingredient of life, is prevalent in space and various media. H₂O in the gas phase is the major polyatomic species in the interstellar medium (ISM) and a primary target of current studies of collisional dynamics. In recent years a number of theoretical and experimental studies have been devoted to H₂O-X (with X=He, H₂, D₂, Ar, …) elastic and inelastic collisions in an effort to understand rotational distributions of H₂O in molecular clouds. Although those studies treated several abundant species, no quantum mechanical calculation has been reported to date for a nonlinear polyatomic collider. We present in this talk the preliminary steps toward this goal, using the H₂O molecule itself as our collider, the very accurate MB-Pol surface to describe the intermolecular interaction and the MultiConfiguration Time Dependent (MCTDH) algorithm to study the dynamics. One main challenge in this effort is the need to express the Potential Energy Surface (PES) in a sum-of-products form optimal for MCTDH calculations. We will describe how this was done and present preliminary results of state-to-state probabilities.

In our previous work [Rivera-Rivera et al. J. Chem. Phys. 142, 014303 (2015).] classical molecular dynamics simulations followed, in an Ar bath, the relaxation of nitromethane (CH₃NO₂) instantaneously excited by statistically distributing 50 kcal/mol among all its internal degrees of freedom. The 300 K Ar bath was at pressures of 10 to 400 atm. Both rotational and vibrational energies exhibited multi-exponential decay. This study explores mode-specific mechanisms at work in the decay process. With the separation of rotation and vibration developed by Rhee and Kim [J. Chem. Phys. 107, 1394 (1997).], one can show that the vibrational kinetic energy decomposes only into vibrational normal modes while the rotational and Coriolis energies decompose into both vibrational and rotational normal modes. Then the saved CH₃NO₂ positions and momenta can be converted into mode-specific energies whose decay over 1000 ps can be monitored. The results identify vibrational and rotational modes that promote/resist energy lost and drive multi-exponential behavior. In addition to mode-specificity, the results show disruption of IVR with increasing pressure.

Our fundamental understanding of synergistic hydrogen-bonding and proton-transfer phenomena has been advanced immensely by studies of model systems in which the coherent transduction of hydrons is mediated by two degenerate equilibrium configurations that are isolated from one another by a potential barrier of substantial height. This topography advantageously affords unambiguous signatures for the underlying state-resolved dynamics in the form of tunneling-induced spectral bifurcations, the magnitudes of which encode both the overall efficacy and the detailed mechanism of the unimolecular transformation. As a prototypical member of this class of compounds, 6-hydroxy-2-formylfulvene (HFF) supports an unusual quasi-linear O–H···O ↔ O···H–O reaction coordinate that presents a minimal impediment to proton migration – a situation commensurate with the concepts of low-barrier hydrogen bonding (which are characterized by great strength, short distance, and a vanishingly small barrier for hydron migration). A variety of fluorescence-based, laser-spectroscopic probes have been deployed in a cold supersonic free-jet expansion to explore the vibrational landscape and anomalously large tunneling-induced shifts that dominate the ~X¹A₁ potential-energy surface of HFF, thus revealing the most rapid proton tunneling ever reported for a molecular ground state (τ_pt ≤ 120fs). The surprising efficiency of such tunneling-mediated processes stems from proximity of the zero-point level to the barrier crest and produces a dramatic alteration in the canonical pattern of vibrational features that reflects, in part, the subtle transition from quantum-mechanical barrier penetration to classical over-the-barrier dynamics. The ultrafast proton-transfer regime that characterizes the ~X¹A₁ manifold will be juxtaposed against analogous findings for the lowest-lying singlet excited state ùB₂ (π*←π), where a marked change in the nature of the reaction coordinate leads to the near-complete quenching of proton transfer. Experimental results, as well as complementary quantum-chemical analyses, will be discussed and contrasted with those obtained for related hydron-migration systems in an effort to highlight the unique bonding motifs and reaction propensities evinced by HFF.

Quantum state resolved molecular beam scattering studies of small polyatomic molecules from metal surfaces present new challenges for experimentalists, but provide unprecedented new opportunities for detailed study of polyatomic molecular dynamics at surfaces. In the current work, we report preliminary characterization of the scattering of formaldehyde from the Au(111) surface. We report the measured desorption energy (0.31 eV), and characterize the distinct trapping-desorption and direct scattering channels, via the dependence of the scattered velocity and rotational distributions on surface temperature and incident molecular beam energy. Finally, we estimate the trapping probability as a function of incidence energy, which indicates the importance of molecular degrees of freedom in the mechanism for trapping.

The conversion of translational to rotational motion often plays a major role in the trapping of small molecules at surfaces, a crucial first step for a wide variety of chemical processes that occur at gas-surface interfaces. However, to date most quantum-state resolved surface scattering experiments have been performed on diatomic molecules, and very little detailed information is available about how the structure of non-linear polyatomic molecules influences the mechanisms for energy exchange with surfaces. In the current work, we employ a new rotationally-resolved 1+1′ resonance-enhanced multiphoton ionization (REMPI) scheme to measure rotational distribution in formaldehyde molecules directly scattered from the Au(111) surface at incident kinetic energies in the range 0.3-1.2 eV. The results indicate a pronounced propensity to excite a-axis rotation (twirling) rather than b- or c-axis rotation (tumbling or cartwheeling), and are consistent with a rotational rainbow scattering model. Classical trajectory calculations suggest that the effect arises-to zeroth order-from the three-dimensional shape of the molecule (steric effects). The results have broad implications for the enhanced trapping probability of prolate and near-prolate molecules at surfaces.

The role of dynamics in the function of proteins is well appreciated, but not precisely understood due to the difficulty in their measurement. Two-dimensional infrared (2D IR) spectroscopy is a powerful approach for the study of protein dynamics with high spatial and temporal resolution. This approach has led to the development of spectrally resolved IR probes that can be applied towards the measurement of dynamics at specific sites in a protein. However, the experimental time scale is limited by the vibrational lifetime of the probe, as such their remains a need for extended time scale probes. Towards the development of better 2D IR probes for the study of protein dynamics the spectroscopic characterization of p-cyano-seleno-phenylalanine (CNSePhe), isotopically labeled p-(¹³C¹⁵N-cyano)phenylalanine (¹³C¹⁵NPhe) and the site-specific incorporation of ¹³C¹⁵NPhe in the protein plastocyanin is discussed. The incorporation of the heavy Se atom and the isotopic labeling are shown to increase the vibrational lifetime of the probe which results in collection of 2D IR spectra for analysis of dynamics on longer timescales.

Plasmonic nanostructures provide unique environments for non-resonant excitation and switching of photochromic compounds. In this study, photochromic diarylethene molecules were deposited on top of a periodically ordered array of gold nanorods (170 x 80 nm) and then irradiated with < 100 fs laser pulses. Irradiation at 800 nm drives the plasmon resonance of the nanoparticle array and induces the photochromic conversion of molecules via non-resonant two-photon excitation. Transmission measurements using broadband continuum laser pulses probe the progress of the photochemical cycloreversion reaction as molecules switch from a visible-absorbing closed-ring structure to a transparent open-ring structure. The spatial dependence of the two-photon conversion of molecules in the plasmonic near field of the array is modeled using calculated field enhancements, and compared with similar measurements for a film of molecules on a glass substrate. Wavelength-dependent polarization effects in the near field of the array lead to interesting anisotropy results in the transmission signal. The results emphasize the importance of both the spatial dependence and anisotropy of the enhanced electric fields in driving non-resonant photochromic reactions.

Diode Pumped Alkali Lasers (DPAL) are continuous wave lasers, potentially capable of megawatt average powers. These lasers exploit the D1 and D2 lines of alkali metals resulting in a 3-level laser with the lasing transition in the near infrared region of the electromagnetic spectrum. Energy pooling processes involving collisions between excited alkali metals cause a fraction of the gain media to be highly excited and eventually ionized. These high energy cesium atoms and ions chemically react with small hydrocarbons utilized as buffer gasses for the system, depleting the gain media. A kinetic model supported by experimental data is introduced to explain the cumulative effects of optical trapping, energy pooling, and chemical reactivity in heavy alkali metal (Rb, Cs) systems. Spectroscopic studies demonstrating metal hydride formation will also be presented.

Decay rates of 335 nm emission from Hg₂^* and 485 nm emission from Hg₃^* were recorded under 266 nm photoexciation. A previously unobserved turning point in the decay rates with respect to Hg number density curve was recorded. A new rate equation model was built to reveal the three-body formation rates coefficients of Hg^*, Hg₂^*, and Hg₃^* by matching the simulated decay rates with Hg number density curves with experimental recorded ones: Hg^* + Hg + Hg → Hg₂^* + Hg Hg₂^* + Hg + Hg → Hg₃^* + Hg Hg₃^* + Hg + Hg → Hg₄^* + Hg Pump and probe experiments with 266 nm and tunable blue laser were also conducted and suppression of both 335 nm and 485 nm emission at different probe laser wavelength were recorded. The delay between occurring time of 335 nm and 485 nm was observed. The suppression intensity of the two cases were also analyzed and compared.