Strong Angular Oscillation of Rotationally Resolved Differential Cross Section in the H + HD → H2 + D Reaction at the Collision Energy of 2.07 eV: Evidence of Geometric Phase Effects.

Geometric phase (GP) effects in chemical reactions are subtle quantum phenomena that are challenging to identify. In this work, we report a joint experimental and theoretical study of the H + HD → H2 + D reaction at a collision energy of 2.07 eV, which is far below the energy of the conical intersection of 2.53 eV. The rotationally state-resolved differential cross sections were measured by a crossed-beam experiment with the scheme of D-atom Rydberg tagging time-of-flight detection. Experimental angular distributions of three rotational states of H2 products exhibit notable variation near the backward scattering direction. Time-dependent quantum mechanics calculations (TDQMs) were carried out at the same collision energy, with and without the inclusion of GP. The experimental angular distributions are in good agreement with TDQM results with the inclusion of GP but do not agree with TDQM results without the inclusion of GP. This work demonstrates the existence of GP effects at energy far below the conical intersection.

Effects of C-H stretching excitation on the dynamics of the O(1D) + CHD3 → OH/OD + CD3/CHD2 reaction.

The vibrationally excited reaction O(1D) + CHD3(ν1 = 1) has been investigated by crossed-molecular-beam experiments with a time-sliced velocity map imaging technique. Detailed and quantitative information is extracted on the C-H stretching excitation effects on the reactivity and dynamics of the title reaction, with the help of preparation of C-H stretching excited CHD3 molecules by direct infrared excitation. Experimental results show that the vibrational stretching excitation of the C-H bond almost does not affect the relative contributions between different dynamical pathways for all product channels. For the OH + CD3 product channel, the vibrational energy of the C-H stretching excited CHD3 reagent is channeled exclusively into the vibrational energy of the OH products. The vibrational excitation of the CHD3 reactant changes the reactivities for the ground-state and umbrella-mode-excited CD3 channels very modestly, while it significantly suppresses the corresponding CHD2 channels. For the CHD2(ν1 = 1) channel, the stretching excited C-H bond of the CHD3 molecule acts almost as a pure spectator.

Spectroscopic snapshot for neutral water nonamer (H2O)9: Adding a H2O onto a hydrogen bond-unbroken edge of (H2O)8.

Structural characterization of neutral water clusters is crucial to understanding the structures and properties of water, but it has been proven to be a challenging experimental target due to the difficulty in size selection. Here, we report the size-specific infrared spectra of confinement-free neutral water nonamer (H2O)9 based on threshold photoionization, using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals in the 3200-3350 cm-1 region are observed, providing unique spectral signatures for the formation of an unprecedented (H2O)9 structure evolved by adding a ninth water molecule onto a hydrogen bond-unbroken edge of the (H2O)8 octamer with D2d symmetry. This nonamer structure coexists with the five previously identified structures that can be viewed as derived by inserting a ninth water molecule into a hydrogen bond-broken edge of the D2d/S4 octamer. These findings provide key microscopic information for systematic understanding of the formation and growth mechanism of dynamical hydrogen-bonding networks that are responsible for the structure and properties of condensed-phase water.

Reactivity oscillation in the heavy-light-heavy Cl + CH4 reaction.

It has long been predicted that oscillatory behavior exists in reactivity as a function of collision energy for heavy-light-heavy (HLH) chemical reactions in which a light atom is transferred between two heavy atoms or groups of atoms, but direct observation of such a behavior in bimolecular reactions remains a challenge. Here we report a joint theoretical and crossed-molecular-beam study on the Cl + CH4 → HCl + CH3 reaction. A distinctive peak at a collision energy of 0.15 eV for the CH3(v = 0) product was experimentally detected in the backward scattering direction. Detailed quantum-dynamics calculations on a highly accurate potential energy surface revealed that this feature originates from the reactivity oscillation in this HLH polyatomic reaction. We anticipate that such reactivity oscillations exist in many HLH reactions involving polyatomic reagents.

Infrared spectroscopy of neutral water clusters at finite temperature: Evidence for a noncyclic pentamer.

Infrared spectroscopic study of neutral water clusters is crucial to understanding of the hydrogen-bonding networks in liquid water and ice. Here we report infrared spectra of size-selected neutral water clusters, (H2O) n (n = 3-6), in the OH stretching vibration region, based on threshold photoionization using a tunable vacuum ultraviolet free-electron laser. Distinct OH stretch vibrational fundamentals observed in the 3,500-3,600-cm-1 region of (H2O)5 provide unique spectral signatures for the formation of a noncyclic pentamer, which coexists with the global-minimum cyclic structure previously identified in the gas phase. The main features of infrared spectra of the pentamer and hexamer, (H2O) n (n = 5 and 6), span the entire OH stretching band of liquid water, suggesting that they start to exhibit the richness and diversity of hydrogen-bonding networks in bulk water.

Ligand-Induced Tuning of the Electronic Structure of Rhombus Tetraboron Cluster.

A neutral boron carbonyl complex B4 (CO)3 is generated in the gas phase and is characterized by infrared plus vacuum ultraviolet (IR+VUV) two-color ionization spectroscopy and quantum chemical calculations. The complex is identified to have a planar C2v structure with three CO ligands terminally coordinated to a rhombus B4 core. It has a closed-shell singlet ground state that correlates to an excited state of B4 . Bonding analyses on B4 (CO)3 as well as the previously reported B4 and B4 (CO)2 indicate that the electronic structure of rhombus tetraboron cluster changes from a close-shell singlet to an open-shell singlet in B4 (CO)2 and to a close-shell singlet in B4 (CO)3 , demonstrating that the electronic structures of boron clusters can be effectively tuned via sequential CO ligand coordination.

State-to-state photodissociation dynamics of CO2 at 157 nm.

State-to-state photodissociation of CO2(v2 = 0 and 1) at 157 nm via the O(1D) + CO(X1Σ+) channel was studied by using the sliced velocity map imaging technique. Both the O(1D) and CO(X1Σ+) products were detected by (2 + 1) resonance enhanced multiphoton ionization (REMPI). Detection of CO via the B1Σ+ ←← X1Σ+ transition allowed ro-vibrational state-selective detection, and combined with imaging, the fragment energy and angular distributions have been derived. For CO(v = 0 and 1|j) products from the CO2(v2 = 0) molecule, the angular distributions of low-j CO display positive anisotropic parameters (about 0.8); with j increasing, the product anisotropic parameters gradually reduce to zero. While for CO(v = 0 and 1|j) products from the vibrational excited CO2(v2 = 1) molecule, the angular distributions of low-j CO also display positive anisotropic parameters; with j increasing, the product anisotropic parameters first decrease to zero and then become negative (about -0.5). Experimental results show that the observed variation of the product angular distribution with the rotational quantum number of CO is consistent with trends predicted by a classical model for non-axial fragment recoil. The results support advanced theoretical predictions of a predominantly parallel transition to the bent 21A' excited state of CO2, where bending introduces torque during the direct dissociation process.

Experimental and Theoretical Study of the Vibrationally Excited Reaction Cl + D2 (v = 1, j = 0) → DCl + D.

Vibrationally excited reaction of Cl + D2 (v = 1, j = 0) → DCl + D was investigated by a high-resolution crossed beam experiment, with D2 molecules in the vibrationally excited state prepared by the scheme of stimulated Raman pumping. Differential cross sections (DCSs) were obtained at three collision energies of 4.03, 4.93, and 5.68 kcal/mol. Backward scattering is dominant for both DCl (v' = 0) and DCl (v' = 1) products, and no forward scattering signal was observed at these three collision energies. Collision-energy-dependent DCS in the backward scattering direction was measured at collision energies between 3.62 and 5.97 kcal/mol. Comparing with the DCSs from the vibrational ground state, it is found that the vibrational excitation of D2 molecules significantly enhances the reactivity because of the later barrier nature of the reaction. No obvious oscillatory structure was found in the collision-energy-dependent DCS in the backward scattering direction, suggesting that the title reaction proceeds via a direct abstraction mechanism.

Acetaldehyde polymerization on Co(0001): the role of CO.

The adsorption and polymerization of acetaldehyde (CH3CHO) have been investigated on clean and CO pre-covered Co(0001) surfaces using the temperature programmed desorption (TPD) method. On the clean Co(0001) surface, CH3CHO molecules can polymerize to produce paraldehyde with very low efficiency. With pre-dosed CO molecules on Co(0001), the decomposition of CH3CHO is greatly inhibited. When the coverage of pre-dosed CO is <0.33 ML, no enhancement of CH3CHO polymerization is observed. However, when the pre-dosed CO coverage is >0.33 ML, the polymerization of CH3CHO is significantly enhanced during the TPD process. Further results suggest that CO molecules adsorbed at the bridge/hollow sites may initialize the polymerization by nucleophilic attack of CH3CHO molecules with their O atoms. Moreover, the polymerization product induced by CO molecules is not paraldehyde, but linear polymer chains of CH3CHO at low CH3CHO coverages and probably three dimensional polymer structures at high CH3CHO coverages.

Ultrafast decay dynamics of water molecules excited to electronic D[combining tilde]' and D[combining tilde]'' states: a time-resolved photoelectron spectroscopy study.

The ultrafast decay dynamics of water molecules excited to D[combining tilde]'1B1 and D[combining tilde]''1A2 states is studied by combining two-photon excitation and time-resolved photoelectron imaging methods. The lifetime of the D[combining tilde]'1B1(000) state of H2O (D2O) is determined to be 1.54 ± 0.1 (22.6 ± 1.6) ps, consistent with a previous high-resolution spectroscopic study. The H2O D[combining tilde]''1A2(000) state decays with a lifetime of 4.1 ± 0.2 ps, while in the D2O D[combining tilde]''1A2(000) state, two independent decay pathways are observed, with time constants of 0.55 ± 0.1 and 13 ± 1 ps, respectively. The former is proposed to be associated with a hitherto undocumented D[combining tilde]'' → C[combining tilde] pathway, induced by Coriolis interaction.

Infrared spectra of neutral dimethylamine clusters: An infrared-vacuum ultraviolet spectroscopic and anharmonic vibrational calculation study.

Infrared-vacuum ultraviolet (IR-VUV) spectra of neutral dimethylamine clusters, (DMA)n (n = 2-5), were measured in the spectral range of 2600-3700 cm-1. The experimental IR-VUV spectra show NH stretch modes gradually redshift to 3200-3250 cm-1 with the increase in the cluster size and complex Fermi Resonance (FR) pattern of the CH3 group in the 2800-3000 cm-1 region. Ab initio anharmonic vibrational calculations were performed on low-energy conformers of (DMA)2 and (DMA)3 to examine vibrational coupling among CH/NH and to understand the Fermi resonance pattern in the observed spectra features. We found that the redshift of NH stretching mode with the size of DMA cluster is moderate, and the overtone of NH bending modes is expected to overlap in frequency with the CH stretching fundamental modes. The FR in CH3 groups is originated from the strong coupling between CH stretching fundamental and bending overtone within a CH3 group. Well-resolved experimental spectra also enable us to compare the performance of ab initio anharmonic algorithms at different levels.

Infrared photodissociation spectroscopy of ion-radical networks in cationic dimethylamine complexes.

Infrared photodissociation spectroscopy was employed to establish the general trends in the stepwise growth motif of cationic dimethylamine (DMA)n+ (n = 4-13) complexes. Electronic structure calculations were performed to identify the structure of the low-lying isomers and to assign the observed spectral features. The results showed the preference of the formation of the proton-transferred (CH3)2NH2+ ion core. The (CH3)2NH2+-[(CH3)2N] ion-radical pair contact and the ion-radical separated pair could coexist at n = 4. The [(CH3)2N] radical is separated from the (CH3)2NH2+ ion core by one DMA molecule at n = 4-6 and by two or more DMA molecules in the larger clusters. This suggests that the (CH3)2NH2+-[(CH3)2N] ion-radical contact pair is not stable in the subsequent radiation-induced processes of DMA, and the [(CH3)2N] radical is released from the charged site in the cationic DMA networks.

Infrared photodissociation spectroscopy of cold cationic trimethylamine complexes.

Cryogenic ion-trap infrared photodissociation spectroscopy combined with a dielectric barrier discharge source was constructed to establish the general trends in the stepwise growth motif of trimethylamine (TMA)n+ complexes. The results showed a strong preference for the formation of a stable charge-shared NN type (TMA)2+ ion core over the proton-transferred CHN type ion core, evidencing that the source condition has a remarkable effect on the kinetic stability of isomers. A maximum of four TMA molecules are located perpendicularly to the NN axis of the charge-shared (TMA)2+ ion core. In the n = 7 and 8 clusters, the subsequent two TMA molecules are located at each end of the NN axis of the (TMA)2+ ion core, completing the first coordination shell. Starting at n = 9, the additional TMA molecules form a second solvation shell, and the cluster spectra show similarities to the solution phase spectrum of aqueous TMA.

Ultrafast excited-state dynamics of 2,5-dimethylpyrrole.

The ultrafast excited-state dynamics of 2,5-dimethylpyrrole following excitation at wavelengths in the range of 265.7-216.7 nm is studied using the time-resolved photoelectron imaging method. It is found that excitation at longer wavelengths (265.7-250.2 nm) results in the population of the S1(1πσ*) state, which decays out of the photoionization window in about 90 fs. At shorter pump wavelengths (242.1-216.7 nm), the assignments are less clear-cut. We tentatively assign the initially photoexcited state(s) to the 1π3p Rydberg state(s) which has lifetimes of 159 ± 20, 125 ± 15, 102 ± 10 and 88 ± 10 fs for the pump wavelengths of 242.1, 238.1, 232.6 and 216.7 nm, respectively. Internal conversion to the S1(1πσ*) state represents at most a minor decay channel. The methyl substitution effects on the decay dynamics of the excited states of pyrrole are also discussed. Methyl substitution on the pyrrole ring seems to enhance the direct internal conversion from the 1π3p Rydberg state to the ground state, while methyl substitution on the N atom has less influence and the internal conversion to the S1(πσ*) state represents a main channel.

Kinetics of the reaction of the simplest Criegee intermediate with ammonia: a combination of experiment and theory.

The kinetics of the reaction of the simplest Criegee intermediate (CH2OO) with ammonia has been measured under pseudo-first-order conditions with two different experimental methods. We investigated the rate coefficients at 283, 298, 308, and 318 K at a pressure of 50 Torr using an OH laser-induced fluorescence (LIF) method. Weak temperature dependence of the rate coefficient was observed, which is consistent with the theoretical activation energy of -0.53 kcal mol-1 predicted by quantum chemistry calculation at the QCISD(T)/CBS//B3LYP/6-311+G(2d,2p) level. At 298 K, the rate coefficient at 50 Torr from the OH LIF experiment was (5.64 ± 0.56) × 10-14 cm3 molecule-1 s-1 while at 100 Torr we obtained a slightly larger value of (8.1 ± 1.0) × 10-14 cm3 molecule-1 s-1 using the UV transient absorption method. These experimental values are within the theoretical error bars of the present as well as previous theoretical results. Our experimental results confirmed the previous conclusion that ammonia is negligible in the consumption of CH2OO in the atmosphere. We also note that CH2OO may compete with OH in the oxidation of ammonia under certain circumstances, such as at night-time, high altitude and winter time.

Temperature-Dependent Infrared Photodissociation Spectroscopy of (CO2)3+ Cation.

Infrared photodissociation spectra of He-buffer-gas-cooled (CO2)3+ were measured at ion trap temperatures of 15, 50, 150, and 280 K. Electronic structure calculations at the mPW2PLYPD/aug-cc-pVDZ level were performed to identify the structures of the low-lying isomers and to assign the observed spectral features. The experimental and calculated infrared spectra show that the (CO2)3+ cations formed in the source are primarily dominated by the charge partially delocalized C2O4+ motif, in which the positive charge is partially delocalized over the two CO2 molecules. Thermal heating at elevated internal temperature supplies sufficient energy to overcome the isomerization barriers and gives access to the charge completely delocalized (CO2) n+ ( n = 3) motif, in which the positive charge is almost completely delocalized over all of the constituent CO2 molecules.

Photodissociation dynamics of H2O at 111.5 nm by a vacuum ultraviolet free electron laser.

Photodissociation dynamics of H2O via the F̃ state at 111.5 nm were investigated using the high resolution H-atom Rydberg "tagging" time-of-flight (TOF) technique, in combination with the tunable vacuum ultraviolet free electron laser at the Dalian Coherent Light Source. The product translational energy distributions and angular distributions in both parallel and perpendicular directions were derived from the recorded TOF spectra. Based on these distributions, the quantum state distributions and angular anisotropy parameters of OH (X) and OH (A) products have been determined. For the OH (A) + H channel, highly rotationally excited OH (A) products have been observed. These products are ascribed to a fast direct dissociation on the B̃1A1 state surface after multi-step internal conversions from the initial excited F̃ state to the B̃ state. While for the OH (X) + H channel, very highly rotationally excited OH (X) products with moderate vibrational excitation are revealed and attributed to the dissociation via a nonadiabatic pathway through the well-known two conical intersections between the B̃-state and the X̃-state surfaces.

A kinetic study of the CH2OO Criegee intermediate reaction with SO2, (H2O)2, CH2I2 and I atoms using OH laser induced fluorescence.

The OH laser induced fluorescence method was used to study the kinetics of CH2OO reacting with SO2, (H2O)2, CH2I2 and I atoms. Decay of CH2OO is not strictly first-order since its self-reaction is rapid. With this consideration, we derived the rate coefficient of CH2OO + SO2/(H2O)2/CH2I2/I taking into account the contribution of the CH2OO self-reaction. For the CH2OO + SO2 reaction, the rate coefficient is measured to be (3.88 ± 0.13) × 10-11 cm3 molecule-1 s-1 at 10 Torr, which agrees very well with a previously reported value obtained by directly monitoring CH2OO using the UV absorption method with the CH2OO self-reaction considered. We did not observe obvious evidence for SO2 catalysed CH2OO isomerization or the intersystem crossing effect in this reaction. CH2OO + (H2O)2 is supposed to account for the major sink of CH2OO in the atmosphere, but previous rate coefficient measurements were not in good agreement. We have revisited this reaction including the self-reaction of CH2OO and obtained the rate coefficient to be (7.53 ± 0.38) × 10-12 cm3 molecule-1 s-1 at 60 Torr and 300 K. The rate coefficients of CH2OO + CH2I2 and CH2OO + I were measured to be (5.2 ± 2.6) × 10-14 and (2.2 ± 1.1) × 10-12 cm3 molecule-1 s-1 respectively.

An accidental resonance mediated predissociation pathway of water molecules excited to the electronic C[combining tilde] state.

The predissociation dynamics of water molecules in the electronic C[combining tilde] state were studied using the time-resolved photoelectron imaging method. Both vibrationless and vibrationally excited states in the C[combining tilde] electronic state were studied, with an emphasis on the vibrational excitation effects on the predissociation dynamics of the C[combining tilde] state. Besides the well-known rotationally and non-rotationally mediated predissociation pathways (Proc. Natl. Acad. Sci. U. S. A., 2008, 105, 19148), an accidental resonance mediated predissociation pathway for the first bending mode excited state in the C[combining tilde] electronic state of H2O is revealed, providing an excellent example of competition between non-adiabatic decay pathways involving at least five electronic states.

Ultrafast excited-state dynamics of 2,4-dimethylpyrrole.

The ultrafast excited-state dynamics of 2,4-dimethylpyrrole following excitation at wavelengths in the range of 255.8-199.7 nm are studied using the time-resolved photoelectron imaging method. It is found that excitation at longer wavelengths (255.8, 250.0, 246.0 and 242.0 nm) results in population of the S1(1πσ*) state, which decays out of the photoionization window in less than 30 fs. At 237.7 nm, the second 1πσ* state is excited, which decays in about 130 fs. At shorter pump wavelengths (231.8, 224.8, 217.5 and 199.7 nm), the assignments are less clear-cut. We tentatively assign the initially photoexcited states to the 1π3p Rydberg states, which decay in about 60 fs, with internal conversion to the S1(1πσ*) state as one of the decay channels. The lifetimes of these 1π3p Rydberg states vary little with the pump wavelengths in this wavelength range.