State-resolved Dynamics Research Articles

Superatom State-Resolved Dynamics of the Au25(SC8H9)18(-) Cluster from Two-Dimensional Electronic Spectroscopy.

Superatom state-resolved dynamics of the Au25(SC8H9)18(-) monolayer-protected cluster (MPC) were examined using femtosecond two-dimensional electronic spectroscopy (2DES). The electronic ground state of the Au25(SC8H9)18(-) MPC is described by an eight-electron P-like superatom orbital. Hot electron relaxation (200 ± 15 fs) within the superatom D manifold of lowest-unoccupied molecular orbitals was resolved from hot hole relaxation (290 ± 20 fs) in the superatom P states by using 2DES in a partially collinear pump-probe geometry. Electronic relaxation dynamics mediated by specific superatom states were distinguished by examining the time-dependent cross-peak amplitudes for specific excitation and detection photon energy combinations. Quantification of the time-dependent amplitudes and energy positions of cross peaks in the 2.21/1.85 eV (excitation/detection) region confirmed that an apparent energetic blue shift observed for transient bleach signals results from rapid hot electron relaxation in the superatom D states. The combination of structurally precise MPCs and state-resolved 2DES can be used to examine directly the influence of nanoscale structural modifications on electronic carrier dynamics, which are critical for developing nanocluster-based photonic devices.

Rotational state-changing cold collisions of hydroxyl ions with helium

Understanding low-temperature molecular collisions is challenging, but using non-resonant photodetachment makes it possible to study the state-resolved dynamics of the inelastic collisions between hydroxyl ions and cold helium buffer gas.

State-Resolved Quantum Dynamics of Photodetachment of HCO2(-)/DCO2(-) on an Accurate Global Potential Energy Surface.

Full-dimensional quantum dynamics studies of the photodetachment of HCO2(-) and DCO2(-) are reported using a wave-packet method on an accurate global potential energy surface of the neutral HOCO/HCO2 system. The calculated photoelectron spectra reproduced both the positions and widths of the main HCO2 and DCO2 peaks observed in experiment. Specifically, both the (2)A1 and (2)B2 resonance peaks of the neutral radicals were identified in our simulations thanks to the adiabatic PES that captures both the (2)A1 and (2)B2 minima. The narrow widths and isotope effect of the lowest resonances are indicative of tunneling-facilitated predissociation. Furthermore, the dissociation product CO2 was found to be excited in both its symmetric stretching and bending modes, which are coupled via a strong Fermi resonance, but rotationally cold, in good agreement with the recent photoelectron-photodetachment coincidence experiments.

State-resolved ultrafast dynamics of impact ionization in InSb.

Impact ionization (IMP) is a fundamental process in semiconductors, which results in carrier multiplication through the decay of a hot electron into a low-energy state while generating an electron-hole pair. IMP is essentially a state selective process, which is triggered by electron-electron interaction involving four electronic states specified precisely by energy and momentum conservations. However, important state-selective features remain undetermined due to methodological limitations in identifying the energy and momentum of the states involved, at sufficient temporal resolution, to reveal the fundamental dynamics. Here we report state-resolved ultrafast hot electron dynamics of IMP in InSb, a semiconductor with the lowest band-gap energy. The ultrafast decay of state-resolved hot-electron populations and the corresponding population increase at the conduction band minimum are directly captured, and the rate of IMP is unambiguously determined. Our analysis, based on the direct knowledge of state-resolved hot electrons, provides far deeper insight into the physics of ultrafast electron correlation in semiconductors.

Complete state-resolved non-adiabatic dynamics of the O((3)P) + D2 → OD(X(2)Π) + D reaction.

The first quantum-state-resolved distributions over the full range of available product levels are reported for any isotopic variant of the elementary reaction of O((3)P) with molecular hydrogen. A laser-detonation source was used to produce a hyperthermal oxygen-atom beam, which allowed for sufficient collision energy to surmount the reaction barrier. This beam was crossed by a supersonic beam of D2. The nascent OD products were detected by laser-induced fluorescence. OD rotational distributions in vibrational states v' = 0, 1, and 2 at a collision energy of 25 kcal mol(-1) are reported, together with distributions for the dominant product vibrational level, v'= 0, at lower collision energies of 20 and 15 kcal mol(-1). The OD product is highly rotationally excited, to a degree that declines as expected for the higher vibrational levels or for reductions in the collision energy. The measured rovibrational distributions at the highest collision energy are in excellent agreement with previous theoretical predictions based on quantum scattering calculations on the triplet potential energy surfaces developed by Rogers et al. (J. Phys. Chem. A 2000, 104, 2308-2325). However, no significant OD spin-orbit preference was observed, in contrast to the predictions of most existing theoretical models of the non-adiabatic dynamics based on the widely used reduced-dimensional four-state model of Hoffmann and Schatz (J. Chem. Phys. 2000, 113, 9456-9465). Furthermore, a clear observed preference for OD Π(A') Λ-doublet levels is not consistent with a simple extrapolation of the calculated relative reaction cross sections on intermediate surfaces of (3)A' and (3)A″ symmetry.

State-resolved predissociation dynamics of the formyloxyl radical

Dissociation dynamics of the deuterated formyloxyl radical, DCO2, were studied by photoelectron–photofragment coincidence spectroscopy of DCO2-. Photodetachment at 4.27eV accesses the three lowest-lying electronic states (2A1, 2B2, and 2A2) of DCO2, leading to vibrational excitation dominated by the 2A1 state O−C−O bending mode. Predissociation leads to bending excitation in the CO2 products as observed in the product translational energy distribution. Gating the coincidence spectra by electron kinetic energy for each predissociated vibrational mode in the radical provides a direct measure of the CO2 bending vibration (ν2) product state distribution, revealing substantially more excitation than ab initio quantum-dynamics calculations predict.

Photodissociation of Methyl Iodide and Methyl Iodide Clusters at 193 nm

This study introduces the state resolved dynamics of CH3I when clustered in a supersonic expansion using He and Xe as seeding gases. Following excitation at 193 nm (6.4 eV), state specific alterations in photodissociation dynamics are observed. Four vibrational levels (ν2 = 0, 1, 2, 3) of the CH3 umbrella vibrational mode were probed, as were I(2P3/2) and I(2P1/2) atomic photofragments. For nascent CH3 fragments the measured kinetic energy releases and angular distributions show enormous sensitivity to the clustering. Manifestation of the clustering dynamics include: (a) shifting to higher energies (by up to 0.5 eV) and significant broadening of the kinetic energy distribution; (b) vibrational dependence of this increase in product kinetic energy; (c) angular distribution changes from a characteristic perpendicular character observed in free-standing CH3I into a parallel transition for the clustered moiety. Drastic effects of clustering are also observed when probing iodine-atom photofragments, namely the...

Stretching the Polanyi Rules

Chemistry Forty years ago, Polanyi laid out a framework to explain and predict the relative effectiveness of translational and vibrational energy in driving the reactions of atoms with diatomics. The current frontier in state-resolved molecular dynamics research encompasses the reactions of halogen atoms with methane and its various isotopologues—a six-atom system. Over the past several years, successive experimental and theoretical studies have sought to clarify the extent to which the Polanyi Rules apply (or fail to apply) to such systems. For the reaction of chlorine with CHD3, the Rules suggest that the product-like transition state ought to favor the efficacy of vibration over translation, although the results of recent studies have been ambiguous in this regard. Zhang et al. now report quantum dynamics simulations of this reaction and compare their results with those from previous experiments and quasiclassical trajectories. The simulations suggest that C–H stretch excitation does indeed promote the reaction, except at collision energies below 1 kcal/mol. J. Phys. Chem. Lett. 10.1021/jz301649w (2012).

Visualizing Reaction Progress and the Geometry and Instability of the Transition State

Transition state theory (TST), also called activated complex theory, is often discussed in general chemistry and physical chemistry courses when learning kinetics. This demonstration takes the abstract concepts of transition state, activation energy, and reaction progress (reaction coordinate) and makes them more concrete and accessible. The demonstration uses a children’s toy (Hoberman Switch Pitch) to model a chemical reaction that follows a reactant through a transition state before making a product. The reaction progress, activation energy, and specific geometry of the transition state are discussed as part of a question and answer dialogue. The complex, multidimensionality of a reaction coordinate and the inherent instability of a transition state become evident. The demonstration also shows that certain reactions may “go” more readily if the energy of a reactant is in a particular type of internal mode (vibrations or rotations). The preferential partition of a given total energy into internal modes of motion versus translational collision energy may allow a system to access the transition state more easily. The demonstration may also aid in the teaching of isomerization reactions, collision theory, and state-resolved reaction dynamics.

Full-dimensional quantum state resolved predissociation dynamics of HCO 2 prepared by photodetaching HCO 2−

A full-dimensional quantum dynamics study of the photodetachment of HCO 2 − is reported using an exact wavepacket method. The calculated photodetachment spectrum identified several HCO 2 resonances observed in the experiment, but with quantitatively different intensities, indicating imperfection in the HCO 2 potential-energy surface. The narrow widths of the lowest resonances indicate slow tunneling-facilitated predissociation. The dissociation of these resonances yields CO 2 not only in its ground vibrational state, but also in excited ones, particularly at high energies. Both the symmetric stretching and bending modes, which are coupled via Fermi resonance, are found to be excited.

Dissipative many-electron dynamics of ionizing systems

In this paper, we perform many-electron dynamics using the time-dependent configuration-interaction method in its reduced density matrix formulation (ρ-TDCI). Dissipation is treated implicitly using the Lindblad formalism. To include the effect of ionization on the state-resolved dynamics, we extend a recently introduced heuristic model for ionizing states to the ρ-TDCI method, which leads to a reduced density matrix evolution that is not norm-preserving. We apply the new method to the laser-driven excitation of H(2) in a strongly dissipative environment, for which the state-resolve lifetimes are tuned to a few femtoseconds, typical for dynamics of adsorbate at metallic surfaces. Further testing is made on the laser-induced intramolecular charge transfer in a quinone derivative as a model for a molecular switch. A modified scheme to treat ionizing states is proposed to reduce the computational burden associated with the density matrix propagation, and it is thoroughly tested and compared to the results obtained with the former model. The new approach scales favorably (∼N(2)) with the number of configurations N used to represent the reduced density matrix in the ρ-TDCI method, as compared to a N(3) scaling for the model in its original form.

State-resolved THz spectroscopy and dynamics of crystalline peptide–water systems

Vibrationally state-resolved THz spectra are obtained at cryogenic temperatures for three crystalline peptide-water systems that represent different structural motifs. The systems include two types of secondary structures and a hydrophobic peptide nanopore structure. Almost all of these systems are shown to undergo exchange with water at room temperature that alters the hydrogen bonding network in ways easily detectable in the THz region at cryogenic temperatures. Stark differences are observed in the spectra of model alpha-helical and beta-sheet structures upon water removal at hydrophilic binding sites. However, within the confined pore of a hydrophobic nanotube, water in the form of helical wires has a subtle but significant impact on the phonon modes of the tube. The THz spectra are shown to easily distinguish between the different hydration states of the system that have been independently characterized by mass change measurements. Spectral comparisons with quantum chemical predictions of fully relaxed crystal structures confirm the hydration states and give detailed information about the free energies associated with dehydration. The vibrational free energies are shown to make significant contributions to the overall energy balance of the dehydration processes.

Rotational state specific dissociation dynamics of D2O via the C̃ electronic state

The rotational state resolved photodissociation dynamics of D(2)O via the C state has been investigated using the D-atom Rydberg "tagging" time-of-flight technique, with a tunable vacuum ultraviolet photolysis light source. The photodissociation action spectrum of the D(2)O C<--X (3p(z)<--1b(1)) band has been recorded. The linewidths of rotational transitions have been determined by the Lorentzian profile simulation. Product kinetic energy distributions and angular distributions for individual rotational lines have been measured. From these distributions, the internal state distributions of the OD radical product as well as the state resolved angular anisotropy parameters for each rotational transition have been obtained. The dramatic variation of the OD product state distributions from different rotational excitations has been observed. These results suggest that there are two distinctive coupling channels from the C state to the lower electronic states: homogenous electronic coupling to the A((1)B(1)) state, resulting in a vibrationally hot OD(X (2)Pi) product and Coriolis-type coupling between the C((1)B(1)) state and the B((1)A(1)) state. Through the second mechanism, OD(X (2)Pi) as well as OD(A (2) summation operator(+)) products are produced. The OD(X (2)Pi) products are mainly populated in the vibrationally cold but extremely rotationally excited states. The comparison between the D(2)O and H(2)O results illustrates that the C((1)B(1))-->B((1)A(1)) pathway is relatively larger in the D(2)O photodissociation as a consequence of the isotopic effect.

Roaming Dynamics in Formaldehyde-d2 Dissociation

State-resolved photodissociation dynamics of formaldehyde-d(2), i.e., D(2)CO, at energies slightly above the deuterium atom elimination channel have been studied both experimentally and theoretically. The results showed a clear bimodal distribution of energy into molecular photofragments. Substantial translational excitation of products at high rotational levels of CO was observed together with the D(2) cofragment in moderately excited vibrational levels, whereas rather small translational energy release of CO in low rotational levels was matched by a large degree of vibrational excitation in the D(2) molecule. An analogous distribution of energy in two distinct channels has been recently observed under similar conditions in H(2)CO photolysis and attributed to two different dissociation pathways, namely, a pathway via the conventional transition state geometry and the previously unobserved pathway, deemed "roaming". Our experimental and theoretical data indicated that the same two dissociation pathways were responsible for the bimodal energy distribution into the molecular fragments resulting from the photolysis of D(2)CO. Energy partitioning into molecular products was compared between photolysis of H(2)CO and D(2)CO at energies slightly above the H/D atom abstraction threshold.

Toward Three-Dimensional Quantum State-Resolved Collision Dynamics at the Gas−Liquid Interface: Theoretical Investigation of Incident Angle

Quantum state-resolved energy transfer dynamics at the gas-liquid interface are explored through a comparison of classical molecular dynamics (MD) simulations and previously reported experimental studies (Perkins, B. G.; et al. J. Phys. Chem. A 2008, 112, 9234). Theoretically, large scale MD trajectory calculations have been performed for collisions of CO(2) with a model fluorinated self-assembled monolayer surface (F-SAMs), based on an explicit atom-atom interaction potential obtained from earlier theoretical studies (Martinez-Nunez, E.; et al. J. Phys. Chem. C 2007, 111, 354). Initial conditions for the simulations match those in the experimental studies where high-energy jet-cooled CO(2) molecules (E(inc) = 10.6(8) kcal/mol, <E(rot)> approximately 10 cm(-1)) are scattered from a 300 K perfluorinated liquid surface (PFPE) from a range of incident angles (theta(inc) = 0-60 degrees ). Nascent CO(2) rotational distributions prove to be remarkably well characterized by a simple two-temperature trapping-desorption (TD) and impulsive scattering (IS) model with nearly quantitative agreement between experimental and theoretical column integrated densities. Furthermore, three-dimensional (3D) quantum state resolved flux maps for glancing incident angles (theta(inc) approximately 60 degrees ) reveal broad, lobular distributions peaking strongly in the forward subspecular direction as cos(n)(theta(scat) - theta') with n approximately 5.6(1.2) and theta' approximately 49(2) degrees .

Multiphoton dissociation dynamics of CH3Br

The state-resolved photodissociation dynamics of CH3Br+ cations are studied using a combination of slice imaging and velocity mapping. Parent ions are prepared by (2 + 1) two-photon resonant three-photon ionization of CH3Br through the 5s Rydberg state in the energy region 29,069.8-29,976.0 cm(-1). Analysis of the photoelectron spectra reveals that CH3Br+ ions are produced in the spin-orbit ground electronic state with a wide distribution of vibrational excitation. The photodissociation of CH3Br+ produces CH3+ (X) + Br(2P(1/2))/Br(2P(3/2)) whose product state distribution shows substantial excitation in the umbrella mode of the CH3+ photofragment, especially when concurrently producing the Br(2P(1/2)) channel. The shapes of the photofragment angular distributions clearly show the different dissociation mechanisms in the Br(2P(1/2)) and Br(2P(3/2)) channels. The dissociation dynamics of the 5s Rydberg state of CH3Br are also presented and analyzed.

Theories of reactive scattering

This paper is an overview of the theory of reactive scattering, with emphasis on fully quantum mechanical theories that have been developed to describe simple chemical reactions, especially atom-diatom reactions. We also describe related quasiclassical trajectory applications, and in all of this review the emphasis is on methods and applications concerned with state-resolved reaction dynamics. The review first provides an overview of the development of the theory, including a discussion of computational methods based on coupled channel calculations, variational methods, and wave packet methods. Choices of coordinates, including the use of hyperspherical coordinates are discussed, as are basis set and discrete variational representations. The review also summarizes a number of applications that have been performed, especially the two most comprehensively studied systems, H+H2 and F+H2, along with brief discussions of a large number of other systems, including other hydrogen atom transfer reactions, insertion reactions, electronically nonadiabatic reactions, and reactions involving four or more atoms. For each reaction we describe the method used and important new physical insight extracted from the results.

State-Resolved Dynamics of the CH(A2Δ) Channels from Single and Multiple Photon Dissociation of Bromoform in the 10−20 eV Energy Range

Single photon dissociation of bromoform using synchrotron radiation has been investigated by Fourier transform visible fluorescence spectroscopy (FTVIS). The photodissociation of bromoform in the 12-18 eV energy range results in several products, among which are the CH(A2Delta) and CH(B2Sigma) radicals. Vibrational and rotational state distributions of the CH(A2Delta) are determined from their fluorescence spectra. From the threshold photon energy above which emission from the CH(A2Delta) radicals is observed, the most likely process leading to CH(A) formation is CHBr3 --> CH + 3Br rather than CHBr3 --> CH + Br + Br2. The rotational Boltzmann temperatures in the CH(A --> X) emission spectra for v' = 0 and v' = 1 range between 1570 and 3650 K, depending on the excitation photon energy. From the high rotational excitation, the results suggest that the mechanism for the loss of three bromine atoms is most likely sequential. A small negative emission anisotropy of the CH(A) radicals [(Ipar - Iper)/(Ipar + 2Iper) = -0.024 +/- 0.005] is constant across the action spectrum; a small net absorption dipole of CHBr3 in the vacuum ultraviolet is parallel to the 3-fold symmetry axis of the CHBr3 molecule. The state distributions of the CH(A2Delta) radicals from multiphoton dissociation of bromoform using the 266 nm output (three photons) of a femtosecond laser (Boltzmann temperatures: T(v'=0)(rot)= 4250 +/- 300 K; T(v'=1)(rot)= 3100 +/- 550 K) are compared to those from the single photon dissociation results (Boltzmann temperatures: T(v'=0)(rot)= 3650 +/- 150 K; T(v'=1)(rot)= 2400 +/- 200 K) at the same total excitation energy under collision free conditions. The analysis of the CH(A) rotational populations shows hotter rotational populations for the femtosecond experiment, also suggesting sequential dissociation of the bromoform in the femtosecond experiment. The duration of the femtosecond laser pulse is approximately 180 fs, setting a limit on the time scales for the multiple dissociations.

Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance

A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111).

Product spin–orbit state resolved dynamics of the H+H2O and H+D2O abstraction reactions

The product state-resolved dynamics of the reactions H+H(2)O/D(2)O-->OH/OD((2)Pi(Omega);v',N',f )+H(2)/HD have been explored at center-of-mass collision energies around 1.2, 1.4, and 2.5 eV. The experiments employ pulsed laser photolysis coupled with polarized Doppler-resolved laser induced fluorescence detection of the OH/OD radical products. The populations in the OH spin-orbit states at a collision energy of 1.2 eV have been determined for the H+H(2)O reaction, and for low rotational levels they are shown to deviate from the statistical limit. For the H+D(2)O reaction at the highest collision energy studied the OD((2)Pi(3/2),v'=0,N'=1,A') angular distributions show scattering over a wide range of angles with a preference towards the forward direction. The kinetic energy release distributions obtained at 2.5 eV also indicate that the HD coproducts are born with significantly more internal excitation than at 1.4 eV. The OD((2)Pi(3/2),v'=0,N'=1,A') angular and kinetic energy release distributions are almost identical to those of their spin-orbit excited OD((2)Pi(1/2),v'=0,N'=1,A') counterpart. The data are compared with previous experimental measurements at similar collision energies, and with the results of previously published quasiclassical trajectory and quantum mechanical calculations employing the most recently developed potential energy surface. Product OH/OD spin-orbit effects in the reaction are discussed with reference to simple models.