A Gaussian wavepacket propagation study of non-adiabatic dynamics

The Gaussian wave-packet propagation (GWP) approach in the coherent state algebraic formalism has been applied to calculate the dynamics of a few model single-mode systems and a model two-mode system. To circumvent the problems arising in defining the initial conditions and the potential surface for such systems in this formalism, we have constructed a new Hamiltonian which is derived by mapping the original Hamiltonian on to a single electronic surface. Good agreement with exact results have been obtained.     

Towards disentangling coupled electronic-vibrational dynamics in ultrafast non-adiabatic processes

Femtosecond time-resolved photoelectron spectroscopy is emerging as a new technique for investigating polyatomic excited state dynamics. Due to the sensitivity of photoelectron spectroscopy to both electronic configurations and vibrational dynamics, it is well suited to the study of non-adiabatic processes such as internal conversion, which often occur on sub-picosecond time scales. We discuss the technical requirements for such experiments, including lasers systems, energy- and angle-resolved photoelectron spectrometers and new detectors for coincidence experiments. We present a few examples of these methods applied to problems in diatomic wavepacket dynamics and ultrafast non-adiabatic processes in polyatomic molecules.     

Orthogonal control of dissociation dynamics relative to thermodynamics in a main-chain reversible polymer

A pair of homologous and soluble reversible polymers display nearly identical solution structures but dramatically different dissociation dynamics (1.0 vs ca. 100 s-1) along their main chains. The polymers are formed by organopalladium-pyridine coordination, and steric effects at the metal center control the dynamics. Importantly, the association constant is not significantly affected by the sterics. The resulting orthogonal control over dissociation kinetics provides a general tool for studying the effects of dynamics independent of thermodynamics in supramolecular systems, particularly main-chain reversible polymers.     

Local control of the quantum dynamics in multiple potential wells

The driven wave-packet dynamics in potentials exhibiting several potential wells is investigated. Therefore, local-control strategies are employed where the control field is constructed from the system's dynamics at any instant of time. It is shown that particles can be moved successively between various potential minima. Furthermore, results presented indicate that the intuitive local-control scheme allows for the initiation of a clockwise or counterclockwise rotational motion of a model molecular motor.     

Roaming dynamics in acetone dissociation

DC slice imaging has been employed to study the photodissociation dynamics of acetone at 230 nm, with detection of the CO photoproduct via the B (v' = 0) (1)Sigma(+) <-- X (v' = 0) (1)Sigma(+) transition. A bimodal translational energy distribution observed in the CO fragments points to two distinct dissociation pathways in the 230 nm photolysis of acetone. One pathway results in substantial translational energy release (E(ave) approximately 0.3 eV) along with rather high rotational excitation (up to J' = 50) of CO, and is attributed to the thoroughly investigated stepwise mechanism of bond cleavage in acetone. The other dissociation pathway leads to rotationally cold CO (J' = 0-20) with very little energy partitioned into translation (E(ave) approximately 0.04 eV) and in this way it is dynamically similar to the recently reported roaming mechanism found in formaldehyde and acetaldehyde dissociation. We ascribe the second dissociation pathway to an analogous roaming dissociation mechanism taking place on the ground electronic state following internal conversion. For acetone, this would imply highly vibrationally excited ethane as a coproduct of rotationally cold CO, with the ethane formed above the threshold for secondary decomposition. We estimate that about 15% of the total CO fragments are produced through the roaming pathway. Rotational populations were obtained using a new Doppler-free method that simply relies on externally masking the phosphor screen under velocity map conditions in such a way that only the products with no velocity component along the laser propagation direction are detected.     

Unravelling the details of vitamin D photosynthesis by non-adiabatic molecular dynamics simulations

We investigate the photodynamics of vitamin D derivatives by a fully analytical implementation of the linear response time-dependent density functional theory surface hopping method (LR-TDDFT-SH). Our study elucidates the dynamics of the processes involved in vitamin D formation at the molecular level and with femtosecond resolution. We explain the major experimental findings and provide new insights that cannot directly be obtained from experiments: firstly, we investigate the dynamics of the photoinduced ring-opening of provitamin D (Pro) and cyclohexadiene (CHD) and the subsequent rotational isomerization. In agreement with recent experiments and CC2 calculations, only the bright S(1) state is involved in the ring-opening reaction. Our calculations confirm the experimentally reported 5 : 1 ratio between the excited state lifetimes of Pro and CHD. The longer lifetimes of Pro are attributed to steric constraints of the steroid skeleton and to temperature effects, both emerging directly from our simulations. For CHD and Pro, we present an explanation of the biexponential decay recently reported by Sension and coworkers [Tang et al., J. Phys. Chem., 2011, 134, 104503]: our calculations suggest that the fast and slow components arise from a reactive and an unreactive reaction pathway, respectively. Secondly, we assess the wavelength dependent photochemistry of previtamin D (Pre). Using replica exchange molecular dynamics we sample the Pre conformers present at thermal equilibrium. Based on this ensemble we explain the conformation dependent absorption and the essential features of Pre photochemistry. Consistent with the experiments, we find ring-closure to occur mostly after excitation of the cZc conformers and at lower energies, whereas Z/E isomerization of the central double bond preferably occurs after excitation at higher energies. For the isomerization we provide the first theoretical evidence of the proposed hula-twist mechanism. Our results show that LR-TDDFT-SH is a highly valuable tool for studying the photochemistry of moderately large systems, even though challenges remain in the vicinity of conical intersections.     

The dynamics of allyl radical dissociation

Dissociation of the allyl radical, CH(2)CHCH(2), and its deuterated isotopolog, CH(2)CDCH(2), have been investigated using trajectory calculations on an ab initio ground-state potential energy surface calculated for 97,418 geometries at the coupled cluster single and double and perturbative treatment of triple excitations, with the augmented correlation consistent triple-ζ basis set level (CCSD(T)/AVTZ). At an excitation energy of 115 kcal/mol, corresponding to optical excitation at 248 nm, the primary channel is hydrogen loss with a quantum yield of 0.94 to give either allene or propyne in a ratio of 6.4:1. The total dissociation rate for CH(2)CHCH(2) is 6.3 × 10(10) s(-1), corresponding to a 1/e time of 16 ps. Methyl and C(2)H(2) are produced with a quantum yield of 0.06 by three different mechanisms: a 1,3 hydrogen shift followed by C-C cleavage to give methyl and acetylene, a double 1,2 shift followed by C-C cleavage to give methyl and acetylene, or a single 1,2 hydrogen shift followed by C-C cleavage to give methyl and vinylidene. In this last channel, the vinylidene eventually isomerizes to give internally excited acetylene, and the kinetic energy distribution is peaked at much lower energy (6.4 kcal/mol) than that for the other two channels (18 kcal/mol). The trajectory results also predict the v-J correlation, the anisotropy of dissociation, and distributions for the angular momentum of the fragments. The v-J correlation for the CH(3) + HCCH channel is strongest for high rotational levels of acetylene, where v is perpendicular to J. Methyl elimination is anisotropic, with β = 0.66, whereas hydrogen elimination is nearly isotropic. In the hydrogen elimination channel, allene is rotationally excited with a total angular momentum distribution peaked near J = 17. In the methyl elimination channel, the peak of the methyl rotational distribution is at J ≈ 12, whereas the peak of the acetylene rotational distribution is at J ≈ 28.     

State-resolved probes of methane dissociation dynamics

A new generation of experimental techniques quantifies the gas–surface reactivity of polyatomic reactants prepared in a single quantum state. These experiments eliminate internal state averaging and permit reactivity measurements on molecules with well-defined internal and translational energy. Varying the identity of the selected vibrational and rotational state and the molecule’s translational energy reveals how energy in specific energetic coordinates promotes reaction. When applied to methane’s dissociative chemisorption, which is rate-limiting in the industrial steam reforming reaction, these experiments reveal the molecular basis for activation, and they provide detailed insight into energy flow dynamics prior to reaction. This review will focus on experiments that quantify the reactivity of methane prepared in select rovibrational quantum states via optical excitation in a supersonic molecular beam. An overview will provide context, and a survey of experimental methods will emphasize features unique to these experiments. A presentation and discussion of state-resolved beam-surface scattering studies of methane activation on Ni(1 1 1), Ni(1 0 0), and Pt(1 1 1) will highlight the mechanistic and dynamical insights that such studies can provide. For example, while C–H stretching excitation best promotes transition state access on Ni(1 1 1) and Ni(1 0 0), bending excitation also activates dissociation, suggesting that many different energetic coordinates contribute to reactivity. Among those states studied, non-statistical behavior, including vibrational mode-specific and even bond-selective chemistry, is widespread, which indicates that the assumptions underlying statistical rate theories do not apply to this reaction. We examine the relevant timescales for energy exchange and reaction to provide a plausible explanation for the observation of non-statistical behavior. Finally, we suggest how these methods, and the results they have produced, might guide future work in the field.     

QM/MM non-adiabatic decay dynamics of formamide in polar and non-polar solvents

Non-adiabatic on-the-fly dynamics simulations of the photodynamics of formamide in water and n-hexane were performed using a QM/MM approach. It was shown that steric restrictions imposed by the solvent cage do not have an influence on the initial motion which leads to the lowest energy conical intersection seam. The initial deactivation in water is faster than in n-hexane and in the gas phase. However, most of the formamide molecules in water do not reach the ground state. The reason for the deactivation inefficiency in water is traced back to a decrease of close COHOH and NHOH(2) contacts which fall in the range of hydrogen bonds. The energy deposition into H-bond breaking events leaves molecules with less energy for surmounting the CN dissociation barrier. In both solvents, after hopping to the ground state, the solvent cage keeps the HCO and NH(2) fragments or CO and NH(3) products in close proximity. Consequently, the number of trajectories where fast recombination happens is augmented with delayed recombinations that start when the dissociation fragments hit the cage wall and return back. The hot ground state formamide is formed in an internal conversion process identical to the path leading to CN photodissociation. In the case of aqueous formamide, good agreement with experimental results is achieved by combining dynamics simulations starting from the S(1) and the S(2) excited states collecting high and low energy trajectories, respectively.     

Metastable dissociation dynamics of molecular cluster ions

Metastable uni-cluster dissociation for several hydrogen-bonded and van der Waals cluster ions are observed via resonance-enhanced two-photon ionization reflectron time-of-flight (TOF) mass spectrometry. All of the cluster ions studied show evaporation of a single molecule from the respective parent cluster ions as dominant metastable decay processes. Furthermore, the averaged metastable evaporation rate constants (k _evap) of these cluster ions in a fixed time domain of 0.2–50 µs are obtained by analyzing the relative intensity of metastable ion peaks due to evaporation in the acceleration and the field-free drift regions of the TOF mass spectrometer. An intensity anomaly in some of the observed metastable ion peaks, indicative of magic number stability of the cluster ion, is also presented.     

Communication: Partial linearized density matrix dynamics for dissipative, non-adiabatic quantum evolution

An approach for treating dissipative, non-adiabatic quantum dynamics in general model systems at finite temperature based on linearizing the density matrix evolution in the forward-backward path difference for the environment degrees of freedom is presented. We demonstrate that the approach can capture both short time coherent quantum dynamics and long time thermal equilibration in an application to excitation energy transfer in a model photosynthetic light harvesting complex. Results are also presented for some nonadiabatic scattering models which indicate that, even though the method is based on a "mean trajectory" like scheme, it can accurately capture electronic population branching through multiple avoided crossing regions and that the approach offers a robust and reliable way to treat quantum dynamical phenomena in a wide range of condensed phase applications.     

A non-adiabatic quantum-classical dynamics study of the intramolecular excited state hydrogen transfer in ortho-nitrobenzaldehyde

Ab initio surface-hopping dynamics calculations have been performed to simulate the intramolecular excited state hydrogen transfer dynamics of ortho-nitrobenzaldehyde (o-NBA) in the gas phase from the electronic S(1) excited state. Upon UV excitation, the hydrogen is transferred from the aldehyde substituent to the nitro group, generating o-nitrosobenzoic acid through a ketene intermediate. The semiclassical propagations show that the deactivation from the S(1) is ultrafast, in agreement with the experimental measurements, which detect the ketene in less than 400 fs. The trajectories show that the deactivation mechanism involves two different conical intersections. The first one, a planar configuration with the hydrogen partially transferred, is responsible for the branching between the formation of a biradical intermediate and the regeneration of the starting material. The conversion of the biradical to the ketene corresponds to the passage through a second intersection region in which the ketene group is formed.     

Intense-field modulation of NO2 multiphoton dissociation dynamics

We report on the dynamics of multiphoton excitation and dissociation of NO(2) at wavelengths between 395 and 420 nm and intensities between 4 and 10 TW cm(-2). The breakup of the molecule is monitored by NO A (2)Sigma(+)n(')=1,0-->X (2)Pi(r)n(")=0 fluorescence as a function of time delay between the driving field and a probe field which depletes the emission. It is found that generation of n(')=0 and 1 NO A (2)Sigma(+) results in different fluorescence modulation patterns due to the intense probe field. The dissociation dynamics are interpreted in terms of nuclear motions over light-induced potentials formed by coupling of NO(2) valence and Rydberg states to the applied field. Based on this model, it is argued that the time and intensity dependences of A (2)Sigma(+)n(')=0-->X (2)Pi(r)n(")=0 fluorescence are consistent with delayed generation of NO A (2)Sigma(+)n(')=0 via a light-induced bond-hardening brought about by the transient coupling of the dressed A (2)B(2) and Rydberg 3ssigma (2)Sigma(g) (+) states of the parent molecule. The increasingly prompt decay of A (2)Sigma(+)n(')=1-->X (2)Pi(r)n(")=0 fluorescence with increasing intensity, on the other hand, is consistent with a direct surface crossing between the X (2)A(1) and 3ssigma (2)Sigma(g) (+) dressed states to generate vibrationally excited products.     

Evaluation of non-adiabatic coupling integrals

The theoretical treatment of collisions of atoms and ions by means of a coupled state expansion requires the evaluation of matrix elements for the coupling of electronic and nuclear motion. It is shown that a stationary orbital expansion as opposed to the usual adiabatic expansion reduces most of the matrix elements to standard one-electron molecular integrals.     

Quantum study of electronically non-adiabatic collinear reactions. I. Hyperspherical description of the electronuclear dynamics

The hyperspherical close-coupling formalism is applied to the study of triatomic collinear chemical reactions involving electronically non-adiabatic transitions. We propose the use of hyperspherical coordinates in the complex zone and of locally adapted Jacobi coordinates in the fragmentation zones. Matching procedures, allowing the switch from one representation to the other, are described. The possibility of long-range electrovibrational couplings is considered. Possible simplifications in the practical implementation of the formalism are examined.     

Ultrafast dissociation dynamics of ketones at 195 nm

The photodissociation dynamics of the 3s Rydberg state of three ketones (CH₃CO–R, R=C₂H₅, C₃H₇, and iso-C₄H₉) and the ensuing dissociation of the nascent acetyl radical following 195 nm excitation were investigated by ultrafast photoionization spectroscopy. The 3s state the lifetimes of these ketones are similar (2.5–2.9 ps), though lifetimes of the acetyl radical range from 8.6 ps for CH₃CO–C₂H₅, 15 ps for CH₃CO–C₃H₇, to 23 ps for CH₃CO–(iso-C₄H₉), which suggests that for larger R more vibrational degrees of freedom compete for the excess energy so that less energy is partitioned into the internal energy of the acetyl radical.     

Molecular dynamics study of carbon dioxide hydrate dissociation

We present results from a molecular dynamics study of the dissociation behavior of carbon dioxide (CO(2)) hydrates. We explore the effects of hydrate occupancy and temperature on the rate of hydrate dissociation. We quantify the rate of dissociation by tracking CO(2) release into the liquid water phase as well as the velocity of the hydrate-liquid water interface. Our results show that the rate of dissociation is dependent on the fractional occupancy of each cage type and cannot be described simply in terms of overall hydrate occupancy. Specifically, we find that hydrates with similar overall occupancy differ in their dissociation behavior depending on whether the small or large cages are empty. In addition, individual cages behave differently depending on their surrounding environment. For the same overall occupancy, filled small and large cages dissociate faster in the presence of empty large cages than when empty small cages are present. Therefore, hydrate dissociation is a collective phenomenon that cannot be described by focusing solely on individual cage behavior.     

Local exchange-correlation approximations and first-row molecular dissociation energies

Recent Xα calculations of bond energies and other related properties of first-row diatomic molecules show very encouraging agreement with experiment. In the worst cases, however, the Xα dissociation energies overestimate the experimental values by almost 2 eV. Therefore, we have examined several refinements of the Xα theory and their effects on molecular bond lengths, bond energies, and vibrational frequencies. Among them, gradient corrections to the Xα exchange energy and also some variations of the local spin-density correlation energy approximation are considered. We find that a local exchange-correlation functional with gradient corrections gives dissociation energies in significantly better agreement with experiment than the Xα approximation.     

Reduced-diabatic vibrational close coupled treatment of molecular dissociation dynamics

A close coupled treatment in a vibrational adiabatic representation is applied to the study of molecular photodissociation dynamics. The procedure which is developed here involves three steps: transformation from a diabatic to an adiabatic basis set, truncation of the adiabatic basis set, back transformation to a reduced–diabatic basis set. In the two model cases which are studied, dissociation spectra show complicated peaks and dips, patterns interpreted in terms of shape and Feshbach resonances associated to vibrational predissociation with a relatively high potential barrier in the excited state. An important reduction in the number of channels required for a given final accuracy can be reached by using the reduced–diabatic basis set instead of the usual diabatic one. This is very promising for studying energy partitioning in molecular systems with several internal degrees of freedom taking part in the dynamics.     

Classical dynamics of multiphoton dissociation for a model system

A classical trajectory analysis of multiphoton dissociation on a model two-dimensional anharmonic potential surface is presented. Six categories of trajectory motion were found, and the degree of “instability” within each type was analyzed in terms of power spectra and exponential separation of neighboring phase points. The dissociation probability was studied as a function of laser intensity and frequency. In addition, the translational energy spectrum of the photofragments was analyzed on the basis of several statistical theories. Good agreement with RRKM theory was found for the final partitioning between translational and vibrational energy.