Dynamics of photodissociation of ethylene and its isotopomers at 157 nm: Branching ratios and kinetic-energy distributions

Deuterium Isotope Effect on Internal Conversion of Ethylene Studied by Time-Resolved Photoelectron Spectroscopy

The effect of deuterium isotopes on the internal conversion of ethylene is studied by using extreme ultraviolet time-resolved photoelectron spectroscopy. For deuterium-labeled ethylene, the time scale for ultrafast internal conversion is increased by a factor of approximately √2, in agreement with the results of ab initio multiple spawning calculations, indicating the essential role played by hydrogen motion in the conversion process. Following internal conversion, a metastable species with an electron binding energy of ∼9 eV is produced, and it decays with a time constant similar to that for both isotopologues.

Quantum Quality with Classical Cost: Ab Initio Nonadiabatic Dynamics Simulations Using the Mapping Approach to Surface Hopping

Nonadiabatic dynamics methods are an essential tool for investigating photochemical processes. In the context of employing first-principles electronic structure techniques, such simulations can be carried out in a practical manner using semiclassical trajectory-based methods or wave packet approaches. While all approaches applicable to first-principles simulations are necessarily approximate, it is commonly thought that wave packet approaches offer inherent advantages over their semiclassical counterparts in terms of accuracy and that this trait simply comes at a higher computational cost. Here we demonstrate that the mapping approach to surface hopping (MASH), a recently introduced trajectory-based nonadiabatic dynamics method, can be efficiently applied in tandem with ab initio electronic structure. Our results even suggest that MASH may provide more accurate results than on-the-fly wave packet techniques, all at a much lower computational cost.

The Composition and Chemistry of Titan's Atmosphere

In this review I summarize the current state of knowledge about the composition of Titan's atmosphere and our current understanding of the suggested chemistry that leads to that observed composition. I begin with our present knowledge of the atmospheric composition, garnered from a variety of measurements including Cassini-Huygens, the Atacama Large Millimeter/submillimeter Array, and other ground- and space-based telescopes. This review focuses on the typical vertical profiles of gases at low latitudes rather than global and temporal variations. The main body of the review presents a chemical description of how complex molecules are believed to arise from simpler species, considering all known "stable" molecules-those that have been uniquely identified in the neutral atmosphere. The last section of the review is devoted to the gaps in our present knowledge of Titan's chemical composition and how further work may fill those gaps.

Nonadiabatic simulations of photoisomerization and dissociation in ethylene using ab initio classical trajectories

We simulate the nonadiabatic dynamics of photo-induced isomerization and dissociation in ethylene using ab initio classical trajectories in an extended phase space of nuclear and electronic variables. This is achieved by employing the linearized semiclassical initial value representation method for nonadiabatic dynamics, where discrete electronic states are mapped to continuous classical variables using either the Meyer-Miller-Stock-Thoss representation or a more recently introduced spin mapping approach. Trajectory initial conditions are sampled by constraining electronic state variables to a single initial excited state and by drawing nuclear phase space configurations from a Wigner distribution at a finite temperature. An ensemble of classical ab initio trajectories is then generated to compute thermal population correlation functions and analyze the mechanisms of isomerization and dissociation. Our results serve as a demonstration that this parameter-free semiclassical approach is computationally efficient and accurate, identifying mechanistic pathways in agreement with previous theoretical studies and also uncovering dissociation pathways observed experimentally.

Nonadiabatic Photodissociation and Dehydrogenation Dynamics of n-Butyl Bromide Following p-Rydberg Excitation

Femtosecond time-resolved mass spectrometry, correlation mapping, and density functional theory calculations are employed to reveal the mechanism of C═C and C≡C formation (and related H₂ production) following excitation to the p-Rydberg states of n-butyl bromide. Ultrafast pump-probe mass spectrometry shows that nonadiabatic relaxation operates as a multistep process reaching an intermediate state within ∼500 fs followed by relaxation to a final state within 10 ps of photoexcitation. Absorption of three ultraviolet photons accesses the dense p-Rydberg state manifold, which is further excited by the probe beam for C─C bond dissociation and dehydrogenation reactions. Rapid internal conversion deactivates the dehydrogenation pathways, while activating carbon backbone dissociation pathways. Thus, unsaturated carbon fragments decay with the lifetime of p-Rydberg (∼500 fs), matching the growth recorded in saturated hydrocarbon fragments. The saturated hydrocarbon signals subsequently decay on the picosecond time scale as the molecule relaxes below the Rydberg states and into halogen release channels.

Ultrafast Photoisomerization of Ethylene Studied Using Time-Resolved Extreme Ultraviolet Photoelectron Spectroscopy

The photoisomerization of isolated ethylene (ethene) was observed in real time from the Franck-Condon region in the ¹ππ* state to ground-state products using time-resolved photoelectron spectroscopy with extreme ultraviolet (EUV, 21.7 eV) probe pulses. A combination of filamentation four-wave mixing and high-order harmonic generation was employed to obtain a temporal resolution of 31 ± 2 fs. The nuclear wave packet created by a 160 nm pump pulse accesses C═C twisted geometries within 10 fs, and the population transfer from the excited to the ground state occurs within the next 20-30 fs. Formation of vibrationally highly excited ground-state molecules was observed in less than 45 fs, and they decayed with two time constants of 0.87 and >5 ps. The interpretation of the photoelectron spectra is supported by vertical ionization energies calculated using XMS-CASPT2 along geodesically interpolated reaction paths from the Franck-Condon region to the products.

The 3s Rydberg state as a doorway state in the ultrafast dynamics of 1,1-difluoroethylene

The deactivation dynamics of 1,1-difluoroethylene after light excitation is studied within the surface hopping formalism in the presence of 3s and 3p Rydberg states using multi-state second order perturbation theory (MS-CASPT2).

Photoinduced C–H bond fission in prototypical organic molecules and radicals

We survey and assess current knowledge regarding the primary photochemistry of hydrocarbon molecules and radicals.

Femtosecond spectroscopy with vacuum ultraviolet pulse pairs

We combine different wavelengths from an intense high-order harmonics source with variable delay at the focus of a split-mirror interferometer to conduct pump-probe experiments on gas-phase molecules. We report measurements of the time resolution (<44 fs) and spatial profiles (4 μm × 12 μm) at the focus of the apparatus. We demonstrate the utility of this two-color, high-order-harmonic technique by time resolving molecular hydrogen elimination from C(2)H(4) excited into its absorption band at 161 nm.

Exploring the dynamics of reaction N+SiH(4) with crossed molecular-beam experiments and quantum-chemical calculations

We investigated the reaction N((4)S,(2)D,(2)P)+SiH(4) in crossed molecular beams at a collision energy of 4.7 kcal mol(-1) with a time-of-flight mass spectrometer and selective photoionization. Ion signals were observed at m/z=42-45, associated with two product channels, HSiNH/SiNH(2)+H+H and HSiN/HNSi+H(2)+H. The species producing the signal at m/z=43 is assigned to product HSiN/HNSi and that at m/z=44 to product HSiNH/SiNH(2). The signal observed at m/z=42 is attributed to daughter ions of those two products and that at m/z=45 to (29)Si and (30)Si isotopic variants. We report time-of-flight spectra as a function of laboratory angle and simulations for the two products, from which both kinetic-energy and angular distributions of products in the center-of-mass (c.m.) frame were derived. The dependence of release of kinetic energy on the c.m. scattering angle is weak. The average translational energy released is 7.7 kcal mol(-1) for product channel HSiNH/SiNH(2)+H+H and 30.3 kcal mol(-1) for product channel HSiN/HNSi+H(2)+H. Through consecutive triple fragmentation, the angular distribution is slightly anisotropic for product HSiNH/SiNH(2) but isotropic for product HSiN/HNSi. Assuming equal efficiencies of detection, we estimate the branching ratios of products HSiNH/SiNH(2) and HSiN/HNSi to be roughly 15:85. To facilitate an understanding of the reaction mechanisms, we calculated the potential-energy surface for reaction N((2)D)+SiH(4) with quantum-chemical methods. Reactions N((2)D)+SiH(4)-->SiNH(2)+H+H and N((2)D)+SiH(4)-->HNSi+H(2)+H account satisfactorily for the present experimental results. Isomeric products HSiNH and HSiN are minor in this work.

Reaction dynamics on the formation of 1- and 3-cyanopropylene in the crossed beams reaction of ground-state cyano radicals (CN) with propylene (C3H6) and its deuterated isotopologues

Crossed molecular beams experiments were utilized to explore the chemical reaction dynamics of ground-state cyano radicals, CN(X(2)Sigma(+)), with propylene (CH3CHCH2) together with two d3-isotopologues (CD3CHCH2, CH3CDCD2) as potential pathways to form organic nitriles under single collision conditions in the atmosphere of Saturn's moon Titan and in the interstellar medium. On the basis of the center-of-mass translational and angular distributions, the reaction dynamics were deduced to be indirect and commenced via an addition of the electrophilic cyano radical with its radical center to the alpha-carbon atom of the propylene molecule yielding a doublet radical intermediate: CH3CHCH2CN. Crossed beam experiments with propylene-1,1,2-d3 (CH3CDCD2) and propylene-3,3,3-d3 (CD3CHCH2) indicated that the reaction intermediates CH3CDCD2CN (from propylene-1,1,2-d3) and CD3CHCH2CN (from propylene-3,3,3-d3) eject both atomic hydrogen through tight exit transition states located about 40-50 kJ mol(-1) above the separated products: 3-butenenitrile [H2CCDCD2CN] (25%), and cis/trans-2-butenenitrile [CD3CHCHCN] (75%), respectively, plus atomic hydrogen. Applications of our results to the chemical processing of cold molecular clouds like TMC-1 and OMC-1 are also presented.

Secondary Kinetics of Methanol Decomposition: Theoretical Rate Coefficients for 3CH2 + OH, 3CH2 + 3CH2, and 3CH2 + CH3

Direct variable reaction coordinate transition state theory (VRC-TST) rate coefficients are reported for the (3)CH(2) + OH, (3)CH(2) + (3)CH(2), and (3)CH(2) + CH(3) barrierless association reactions. The predicted rate coefficient for the (3)CH(2) + OH reaction (approximately 1.2 x 10(-10) cm(3) molecule(-1) s(-1) for 300-2500 K) is 4-5 times larger than previous estimates, indicating that this reaction may be an important sink for OH in many combustion systems. The predicted rate coefficients for the (3)CH(2) + CH(3) and (3)CH(2) + (3)CH(2) reactions are found to be in good agreement with the range of available experimental measurements. Product branching in the self-reaction of methylene is discussed, and the C(2)H(2) + 2H and C(2)H(2) + H2 products are predicted in a ratio of 4:1. The effect of the present set of rate coefficients on modeling the secondary kinetics of methanol decomposition is briefly considered. Finally, the present set of rate coefficients, along with previous VRC-TST determinations of the rate coefficients for the self-reactions of CH(3) and OH and for the CH(3) + OH reaction, are used to test the geometric mean rule for the CH(3), (3)CH(2), and OH fragments. The geometric mean rule is found to predict the cross-combination rate coefficients for the (3)CH(2) + OH and (3)CH(2) + CH(3) reactions to better than 20%, with a larger (up to 50%) error for the CH(3) + OH reaction.

Photodissociation dynamics of vinyl fluoride (CH2CHF) at 157 and 193nm: Distributions of kinetic energy and branching ratios

Using photofragment translational spectroscopy and tunable vacuum-ultraviolet ionization, we measured the time-of-flight spectra of fragments upon photodissociation of vinyl fluoride (CH2CHF) at 157 and 193 nm. Four primary dissociation pathways--elimination of atomic F, atomic H, molecular HF, and molecular H2--are identified at 157 nm. Dissociation to C2H3 + F is first observed in the present work. Decomposition of internally hot C2H3 and C2H2F occurs spontaneously. The barrier heights of CH2CH --> CHCH + H and cis-CHCHF --> CHCH + F are evaluated to be 40+/-2 and 44+/-2 kcal mol(-1), respectively. The photoionization yield spectra indicate that the C2H3 and C2H2F radicals have ionization energies of 8.4+/-0.1 and 8.8+/-0.1 eV, respectively. Universal detection of photoproducts allowed us to determine the total branching ratios, distributions of kinetic energy, average kinetic energies, and fractions of translational energy release for all dissociation pathways of vinyl fluoride. In contrast, on optical excitation at 193 nm the C2H2 + HF channel dominates whereas the C2H3 + F channel is inactive. This reaction C2H3F --> C2H2 + HF occurs on the ground surface of potential energy after excitation at both wavelengths of 193 and 157 nm, indicating that internal conversion from the photoexcited state to the electronic ground state of vinyl fluoride is efficient. We computed the electronic energies of products and the ionization energies of fluorovinyl radicals.

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C–H stretch overtone

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.

Site and Isotopic Effects on the Angular Anisotropy of Products in the Photodissociation of Ethene at 157 nm

We measured angular-anisotropy parameters beta(E(t)) of fragments from photolysis of ethene and four isotopic variants at 157 nm using photo-fragment translational spectroscopy and selective photoionization. The averaged beta value of products ranges from -0.17 to 0.10, depending on dissociation pathways. Angular distributions of atomic hydrogen produced from C(2)H(4) and C(2)D(4) are isotropic. For dissociation into C(2)H(2) + H(2), beta has a small negative value whereas dissociation into C(2)D(2) + D(2) has an isotropic angular distribution. The photolysis of dideuterated ethene reveals site and isotopic effects on the angular distributions of products; products H(2), HD, and D(2) from photolysis of 1,1-CH(2)CD(2) have negative, nearly zero, and positive values of beta, respectively. Molecular hydrogen from photolysis of 1,2-cis-CHDCHD has a negative beta value and the anisotropy has a trend D(2) > H(2) > HD. Photolysis of 1,2-trans-CHDCDH produced a result similar to photolysis of 1,2-cis-CHDCHD for the angular anisotropy of molecular hydrogen except slightly more isotropic. A calculation of optimized geometries of ethene in the ground electronic state and pertinent transition structures enables a qualitative interpretation of the site and isotopic effects on the angular anisotropy of products. We deduce that the photoexcited state of ethene at 157 nm has a major character (1)B(1u) that produces a transition dipolar moment parallel to the C=C bond.

Distributions of angular anisotropy and kinetic energy of products from the photodissociation of methanol at 157 nm

We investigated distributions of angular-anisotropy parameter beta and kinetic energy of fragments after photodissociation of methanol using time-of-flight (TOF) mass spectrometry. Fragments, in particular CH(3)O and CO, were successfully detected using tunable radiation from a synchrotron for photoionization. Following O-H bond fission, a CH(3)O fragment with internal energy greater than 104 kJ mol(-1) dissociates to CH(2)O+H. Elimination of two H(2) accompanies formation of CO. The beta value of hydroxyl hydrogen is -0.26 whereas that of methyl hydrogen is zero. H(2) has two distinct components in TOF spectra; these rapid and slow components have beta values -0.30 and -0.18, respectively. The CH(3)+OH dissociation exhibits a highly anisotropic angular distribution with beta= -0.75. The beta values of fragments from CD(3)OH photolysis are addressed. From measurements of angular-anisotropy parameters of various fragments, we surmise that the transition dipole moment mu is almost perpendicular to the C-O-H plane and that n-3p(x) (2 (1)A") is the major photoexcited state at 157 nm.