Accurate ab initio theoretical/computational work on dynamics and spectroscopy begins with a potential energy surface (PES). My talk therefore begins with a brief review of progress we have made in developing accurate ab initio global PESs for reaction dynamics. “Roaming” is an unusual alternate pathway to reaction products that was found in the unimolecular dissociation of H₂CO by running roughly 100 000 trajectories on such a PES. The signatures of roaming were seen in the spectroscopic detection of the rotational states of CO correlated with translational energy distribution of the H₂. I will discuss roaming in NO₃ photodissociation to NO+O₂ and give a short history of the topic. In particular I will recount how poor Franck-Condon factors in pioneering LIF detection experiments in 1997 of the low-lying vibrational states of O₂ plus the assumption of a “prior” vibrational distribution led to the wrong conclusions about the O₂ vibrational-state distribution. Later more sophisticated experiments obtained the correct vibrational distribution, which led to the (correct) speculation about roaming in this system. I conclude with some comments about roaming wavefunctions and will wonder aloud about ways to detect wavefunctions spectroscopically. (Roaming wavefunctions have been reported by Hua Guo and co-workers for the MgH₂.)

Traditional techniques (e.g. REMPI, imaging, etc.) that are used to study reaction dynamics are able to provide a great deal of fundamental information about systems containing atoms and smaller molecules. However, as larger molecules and more complex systems are targeted, it becomes more of a challenge to determine isomer- and vibrational level-specific information and accurate branching ratios. In order to complement existing methods and obtain information about larger systems, a Ka-band (26-40 GHz) chirped-pulse Fourier transform microwave (CP-FTMW) spectrometer has been has been constructed. The system integrates a pulsed uniform supersonic flow (PUSF) source to ensure that experimental conditions, such as temperature and density, are well-known and constant. This PUSF system is based around a high-throughput piezoelectric stack valve, a Laval nozzle, and simple pumping scheme. This system is able to produce cold, uniform flows with densities on the order of 10¹⁶ cm⁻³ that persist for up to 20 cm from the nozzle exit. A description of this system and its characterization will be presented.

H₃⁺ + H₂→ H₅⁺→ H₃⁺ + H₂ is a prototypical proton transfer reaction of astrochemical significance. Developing a deeper understanding of its reaction mechanism will provide important physical insights into similar and more complicated proton transfer reactions. The large amplitude vibrational motions in the H₅⁺ collision complex lead to the scrambling of its five protons, which complicates theoretical descriptions of this molecular ion. Using minimized energy path diffusion Monte Carlo, the one-dimensional zero-point corrected potential energy surface is mapped out as a function of the chosen reaction coordinate.¹ The evolution of the wave function along this reaction coordinate is characterized by the energy and the probability amplitude associated to it. This methodology is also extended to allow for the investigation of excited states in selected vibrational modes that are important in the proton scrambling dynamics. In particular, the vibrations of interest are ones that evolve between large amplitude motions in H₅⁺ and rotations of the H₃⁺ and H₂ fragments as the reaction proceeds. In addition, as H₅⁺ has five protons we must account for the anti-symmetry of the total nuclear wave function, which places restrictions to the possible pathways involved in the proton scrambling dynamics.² The implications of these symmetry restrictions are also discussed.
-----
¹C. E. Hinkle and A. B. McCoy, J. Phys. Chem. Lett., 1, 562 (2010) ²M. Quack, Mol. Phys., 34, 477 (1977)

Methods developed by J. K. G. Watson for calculation of Franck-Condon Factors in systems undergoing linear ↔ bent electronic transitions in the harmonic normal mode basis have been extended to the acetylene ~A ¹A_u-~X ¹Σ_g⁺ transition in full dimension. Because the intensity of the overlap accumulates away from linear geometry, the Hamiltonian of the linear ~X state may be approximately separated into 3 rotations and 3N−6 vibrations, resulting in a one-to-one correspondence between the normal modes in the linear and bent geometries. The calculated results reproduce experimental intensities quantitatively only at low quanta of vibrational excitation due to the exclusion of anharmonic effects. However, the qualitative results explain a number of observations that were previously not understood. A change of basis to local bending modes of the ~X state has been performed to investigate Franck-Condon access to zero order bright states with extreme local bend excitation. These states are known to emerge above 12 quanta of bend excitation and are of interest because the local bending mode lies along the reaction coordinate in the acetylene \rightleftharpoons vinylidene isomerization. The results indicate that the best strategy for reaching extreme local benders involves Stimulated Emission Pumping from ~A-state levels with high excitation in ν₃′ and ν₄′, contrary to existing semi-classical arguments that ν₆′ grants the best access to local bend states.

Figure The [H,C,N] molecular system with the two linear isomers, HCN and HNC, is one of the most important model systems where it is possible to conduct eigenstate-resolved studies that encode the internal dynamics of strongly bound molecules at excitation energies relevant for chemical reactions. The geometric shape of a molecule in its vibrationless state is an intuitive and useful chemical concept. In highly excited vibrational states we expect a significant change of this molecular shape. For triatomic linear HAB molecules this change depends on the two stretching and the bending vibrational excitations. In highly excited bending states a fourth dynamical parameter becomes important: the vibrational angular momentum, l. The corresponding classical picture is the rotation of the hydrogen atom around the A-B core figure axis. The highest possible excitation for a given (v₂,l) bending state is the state with l=v₂. These are the states for which we expect the most significant dynamical effects due to the vibrational angular momentum. In this work we study the correlation between the molecular shape and the rovibrational eigenenergies of the [H,C,N] molecule in the v₁ v₂^v₂ v₃ states with the highest possible excitation of the vibrational angular momentum. The eigenergies we use have been measured using infrared HOTGAME (HOT GAs Molecular Emission) spectroscopy or come from the vibrational assignment of ab initio energy lists.

The ¹B₂ ~C STATE OF SO₂ has been the subject of extensive investigation because it is important in the atmospheric photodissociation of SO₂. The state has a double-minimum potential in the dissociation coordinate, ν₃, arising from vibronic interactions, leading to a staggering of vibrational levels with v₃ odd vs. even. We report the first direct observations of the v₃ fundamental and of other levels with b₂ vibrational symmetry (odd v₃). Our work has made use of LIF, IR-UV double resonance, and coherent MODR techniques. Implications of the precision measurement of v₃ staggerings to the determination of double-minimum potential barrier and to vibronic coupling will be discussed.

I will discuss the detection of sharp vibrational transitions with up to 40 quanta of total excitation above the dissociation limit of thiophosgene (SCCl₂). A 6-D CASSCF calculation and filtered diagonalization of eigenstates to compute dilution factors for bright states explains the 'survival' of these states and their expected number quantitatively.

The chemical kinetics of transient free radicals, such as HOCO and Criegee intermediates, play important roles in combustion and atmospheric processes. Establishing accurate kinetics models for these complex systems require knowledge of the reaction rates and lifetimes of all molecules along a particular reaction pathway. However, standard spectroscopic techniques lack a combination of sensitivity, frequency resolution, and adequate temporal resolution to survey these reactions on the μs timescale. To answer this challenge, we have developed time-resolved frequency comb spectroscopy (TRFCS). This novel technique allows for the detection of transient intermediates with high time-resolution and sensitivity while also permitting the direct determination of rotational state distributions of all relevant molecules. We demonstrate this technique in the mid-infrared spectral region, at 3.7 μm, by studying the photolysis of deuterated acrylic acid. We simultaneously observe the time-dependent concentrations of photoproducts trans-DOCO, HOD, and D₂O, identified through their unique rovibrational structure, with 5 ×10¹⁰ molecules cm⁻³ sensitivity, and with a time resolution of 25 μs. We aim to apply this technique to detect directly the formation of the DOCO intermediate in the OD + CO chemical reaction at atmospherically relevant pressures, in order to validate statistical rate models of this reaction.

We present preliminary results describing the development of a new instrument that combines two powerful techniques: Chirped Pulse-Fourier Transform MicroWave (CP-FTMW) spectroscopy and pulsed uniform supersonic flows. It promises a nearly universal detection method that can deliver quantitative isomer, conformer, and vibrational level specific detection, characterization of unstable reaction products and intermediates and perform unique spectroscopic, kinetics and dynamics measurements. We have constructed a new high-power K_a-band, 26–40 GHz, chirped pulse spectrometer with sub-MHz resolution, analogous to the revolutionary CP-FTMW spectroscopic technique developed in the Pate group at University of Virginia. In order to study smaller molecules, the E-band, 60–90 GHz, CP capability was added to our spectrometer. A novel strategy for generating uniform supersonic flow through a Laval nozzle is introduced. High throughput pulsed piezo-valve is used to produce cold (30 K) uniform flow with large volumes of 150 cm³ and densities of 10¹⁴ molecules/cm³ with modest pumping facilities. The uniform flow conditions for a variety of noble gases extend as far as 20 cm from the Laval nozzle and a single compound turbo-molecular pump maintains the operating pressure. Two competing design considerations are critical to the performance of the system: a low temperature flow is needed to maximize the population difference between rotational levels, and high gas number densities are needed to ensure rapid cooling to achieve the uniform flow conditions. At the same time, collision times shorter than the chirp duration will give inaccurate intensities and reduced signal levels due to collisional dephasing of free induction decay. Details of the instrument and future directions and challenges will be discussed.

Carbenes (R₁-C-R₂) are a reactive class of compounds, usually characterized by an electron-deficient divalent carbon atom, found in applications ranging from organic synthesis to gas phase oxidation chemistry. Carbenes with 2- or 3-atom substituents often undergo rapid unimolecular isomerization, but may be stabilized if these substituents are electron-donating. Dihydroxycarbene (HO-\"C-OH) is one of the smallest singlet carbenes to be afforded this stability, due to its two electron-donating hydroxyl groups. We report the first gas-phase detection and structural characterization of this reactive species, using a combination of Fourier transform microwave spectroscopy and high level electronic structure calculations. Detection in the gas phase indicates that it is fairly stable relative to its isomers, formic acid (HCOOH) and the simplest Criegee intermediate (CH₂OO), the latter of which has recently received a great deal of attention for its role in the atmospheric ozonolysis of alkenes. Our experimental results yield a precise structure of HO-\"C-OH, and we comment on upcoming experiments investigating its stability and reactivity with other common atmospheric species.

We report on gas-phase experiments investigating the predissociation and possible IR-driven isomerization of the 3-aminophenol-ammonia complex (3-AP-NH₃). A molecular beam of 3-AP-NH₃ is vibrationally excited with pulsed IR light, initiating an intramolecular vibrational redistribution and subsequent dissociation. The 3-AP fragment is then probed state-selectively via multiphoton ionization (REMPI) and time-of-flight mass spectrometry. Of particular interest is an IR-driven feature which we associate tentatively with a trans-cis isomerization process. We see clear correlation between the excitation of specific vibrational modes (namely the NH₃ symmetric and OH stretches) and the presence of this feature, as evidenced by IR-action and IR-depletion spectra. The feature persists atop a broader signal which we assign to the predissociation of the complex and whose cutoff in REMPI-action experiments provides an upper bound on the dissociation energy for 3-AP-NH₃.

The isotopic ratios for ozone observed in laboratory and atmospheric measurements, known as the ozone isotopic anomaly,[1,2] have been an open question in physical and atmospheric chemistry for the past 30 years. The biggest limitation in achieving agreement between theory and experiment has been the availability of a satisfactory[3-5] ground state potential energy surface (PES). The presence of a spurious reef feature in the asymptotic region of most PESs has been associated with large discrepancies between calculated and observed rates of formation especially at low temperature. We recently proposed a new global potential energy surface for ozone[6,7] possessing 4 features that make it suitable for kinetics and dynamics studies: excellent equilibrium parameters, good agreement with experimental vibrational levels, accurate dissociation energy and a transition region with accurate topography (without the reef artifact). This PES has been used recently to simulate the temperature dependent exchange reaction (16O+16O2) with a quantum statistical model[6,7], and, for the first time, a negative temperature dependence which agrees with experiments was obtained, indicating the good quality of this global surface. A quantum description of the ozone exchange and recombination reaction requires knowledge of the resonances but also the rovibrational levels just below the dissociation. We present results of global 3-well vibrational-state calculations up to the dissociation threshold and (J = 0) resonances up to 1000 cm⁻¹beyond. The calculations were done using a large DVR basis ( 24 million functions) with a symmetry-adapted Lanczos algorithm as well as MCTDH. Results indicate the presence of localized bound states at energies close to the dissociation threshold beyond which some long-lived resonances follow, contrasted with a few delocalized bound states with density at large values of the stretching coordinates. References: 1- K. Mauersberger et al., Adv. At. Mol. Opt. Phys. 50, 1 (2005) 2- R. Schinke et al., Ann. Rev. Phys. Chem. 57, 625 (2006) 3- R. Siebert et al., J. Chem. Phys. 116, 9749 (2002) 4- M. Ayouz and D. Babikov, J. Chem. Phys. 138, 164311 (2013) 5- V.G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013) 6- R. Dawes et al., J. Chem. Phys. 135, 081102 (2011) 7- R. Dawes et al., J. Chem. Phys. 139, 201103 (2013)

Using a pulsed plasma microjet to generate short-lived, electronically-excited diatomic molecules, and subsequently ejecting them into vacuum to cool via supersonic expansion, we are able to monitor the cooling of molecules having radiative lifetimes as low as 16 ns. Specifically, we report on the rotational cooling of He₂ molecules in the d³Σ_u⁺, e³Π_g, and f³Σ_u⁺ states, which have lifetimes of 25 ns, 67 ns, and 16 ns, respectively. The plasma microjet is driven with a 2.6 kV, 140 ns high-voltage pulse (risetime of 20 ns) which, when combined with a high-speed optical imaging system, allows the nonequilibrium rotational distribution for these molecular states to be monitored as they cool from  1200 K to below 250 K with spatial and temporal resolutions of below 10 μm and 10 ns, respectively. The spatial and temporal resolution afforded by this system also allows the observation of excitation transfer between the f³Σ_u⁺ state and the lower lying d³Σ_u⁺ and e³Π_g states. The extension of this method to other electronically excited diatomics with excitation energies > 5 eV will also be discussed.

We introduce cavity enhanced ultrafast transient absorption spectroscopy, which employs frequency combs and high-finesse optical cavities. Sub-100 fs pulses with a repetition rate of 90 MHz are generated by a home-built Ytterbium fiber laser. The amplified light has a power up to 10 W, which is used to pump an optical parametric oscillator, followed by second-harmonic generation(SHG) that converts the wavelength from near-IR to visible. A pump comb at 530 nm is separately generated by SHG. Both pump and probe combs are coupled into high-finesse cavities. Compared to the conventional transient absorption spectroscopy method, the detection sensitivity can be improved by a factor of ([(F)/(π)] )²  ∼ 10⁵, where F is the finesse of cavity. This ultrasensitive technology enables the direct all-optical dynamics study in molecular beams. We will apply the cavity enhanced ultrafast transient absorption spectroscopy to investigate the dynamics of visible chromophores and then extend the wavelength to mid-IR to study vibrational dynamics of small hydrogen-bonded clusters.