Ab initio vibrational predissociation dynamics of He-I2(B) complex

Three-dimensional quantum mechanical calculations on the vibrational predissociation dynamics of HeI2 B state complex are performed using a potential energy surface accurately fitted to unrestricted open-shell coupled cluster ab initio data, further enabling extrapolation for large I2 bond lengths. A Lanczos iterative method with an optimized complex absorbing potential is used to determine energies and lifetimes of the vibrationally predissociating He,I2(B,v') complex for v'相似文献   

Intramolecular vibrational redistribution following optical excitation

We study the time-dependence of vibrational redistribution following the excitation of a zero-order harmonic state. Two models are considered: (i) the (unphysical) case of a unique anharmonic coupling matrix element gives rise to very strong coherence effects, resulting in a very slow redistribution, at the time scale of the recurrence time 2 π?, characteristic of the manifold; (ii) on the contrary, a random distribution of the coupling matrix elements results into a redistribution rate; Δ_a = 2 π? ΔV²_a — where ΔV²_a is the variance of the distribution — with no influence of its average value. This last model is extended to the case of an initial excitation of mixed states which result from a selective coupling of a radiative state with some vibronic states of a non-radiative manifold. The experimental results available in the literature are reviewed; redistribution following an optical excitation of large molecules, with a vibrational excess energy as high as 5000 to 10000 cm^?1, is generally a slow process (≤ 10⁷ s^?1), in contrast with vibrational relaxation in the condensed phases.     

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH(3)(ν(2) = 3(+)) and NH(3)(ν(2) = 2(+)) have a vibrational ground-state partner NH(3)(ν = 0), but NH(3)(ν(2) = 1(+)) appears in partnership with another fragment in ν(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation.     

Intramolecular vibrational energy redistribution involving the torsion in CF3CH3: a molecular dynamics study

Classical trajectory calculations on intramolecular vibrational energy redistribution (IVR) involving the torsion in 1,1,1-trifluoroethane (TFE) are reported. Two potential energy functions (PEFs) are used to describe the potential energy surface. The "full" PEF gives excellent agreement with the experimental vibrational frequencies. The "simple" PEF omits nondiagonal interaction terms, but still gives very good agreement with the experimental frequencies. The "simple" PEF is intended to minimize mode-mode coupling. Neither PEF includes the HF elimination reaction. Calculations are carried out both with nominal microcanonical selection of initial coordinates and momenta, and with a modified selection method that places controlled amounts of energy in the torsion. Total (classical) vibrational energies from 0.005 to 140 kcal mol(-1) are investigated. The calculated time constants describing energy flow out of the torsional mode are <10 ps for classical vibrational energies near the classical reaction threshold energy (approximately 75 kcal mol(-1)) and greater. It is found that the rate of decay from the torsion largely depends on the amount of energy in the other vibrational modes. Analysis using power spectra shows that the torsional mode in TFE is strongly coupled to the other vibrational modes. These results strongly suggest that vibrational energy in TFE will not be sequestered in the torsion for time periods greater than a few tens of picoseconds when the molecule has enough energy to react via HF elimination.     

Intramolecular vibrational redistribution in Ne-Br2: the signature of intermediate resonances in the excitation spectrum

Quantum-mechanical simulations of the Ne-Br(2)(B,v') excitation spectra produced after vibrational predissociation in the v'=20-35 range are reported. The aim is to investigate the signature in the excitation spectra of intermediate resonances lying in the lower v相似文献   

Quantum dynamics of Ne-Br2 vibrational predissociation: the role of continuum resonances as doorway states

Wave-packet simulations of the Ne-Br2(B,upsilon') vibrational predissociation dynamics in the range upsilon' = 16-29 are reported. The aim is to interpret recent time-dependent pump-probe experiments [Cabrera et al., J. Chem. Phys. 123, 054311 (2005)]. Good agreement is found between the calculated and the experimental lifetimes corresponding to decay of the Ne-Br2(B,upsilon') initial state and to appearance of Br2(B,upsilon相似文献   

Calculation of Cl2 vibrational distributions after vibrational predissociation of He2-Cl2

An approximate model is tested in order to obtain the vibrational distributions of the Cl2 fragment produced after He2-Cl2(B) predissociation, when the dynamics is simulated by a full-dimensional wave-packet method. The model is intended to overcome the limitations imposed to a rigorous calculation of the vibrational distributions by the use of a finite grid size and absorbing boundary conditions. The good agreement found between the present results and the available experimental data indicates that the model is able to provide realistic estimates for the product vibrational distributions.     

Intramolecular vibrational energy redistribution as state space diffusion: classical-quantum correspondence

We study the intramolecular vibrational energy redistribution (IVR) dynamics of an effective spectroscopic Hamiltonian describing the four coupled high frequency modes of CDBrClF. The IVR dynamics ensuing from nearly isoenergetic zeroth-order states, an edge (overtone) and an interior (combination) state, is studied from a state space diffusion perspective. A wavelet based time-frequency analysis reveals an inhomogeneous phase space due to the trapping of classical trajectories. Consequently the interior state has a smaller effective IVR dimension as compared to the edge state.     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI_2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of HeI2(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approxima tion.The total and partial decay widths,lifetimes,rates and their dependence on initial vibrational states were obtained for HeI2 at low initial vibrational excited levels.Our calculations show that the calculated tota decay widths,lifetimes and rates agree well with those extrapolated from experimental data available The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior.The very short propagation time (less.than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state inter-action between the fragments on the vibrationally deexcited adiabatic potential surface.The final state interaction between the fragments is shown to play an important role in determining the final rotational distri     

Three-dimensional quantum dynamics study of vibrational predissociation of HeI2 van der Waals molecule for low vibrational excitation using the time-dependent wave packet method

Three-dimensional quantum mechanical calculations for vibrational predissociation of He1₂(B) van der Waals molecules are presented using the time-dependent wave packet technique within the golden rule approximation. The total and partial decay widths, lifetimes, rates and their dependence on initial vibrational states were obtained for HeI₂ at low initial vibrational excited levels. Our calculations show that the calculated total decay widths, lifetimes and rates agree well with those extrapolated from experimental data available. The predicted total decay widths as a function of initial vibrational states exhibit highly nonlinear behavior. The very short propagation time (less than 1 ps) required in the golden rule wave packet calculation is determined by the duration time of the final state interaction between the fragments on the vibrationally deexcited adiabatic potential surface. The final state interaction between the fragments is shown to play an important role in determining the final rotational distribution. This interpretation clearly explains the dynamical effect that the final rotational distribution shifts to the lower rotational energy levels as the initial vibrational quantum numberu increases.     

Intramolecular vibrational energy redistribution in bridged azulene-anthracene compounds: ballistic energy transport through molecular chains

Intramolecular vibrational energy flow in excited bridged azulene-anthracene compounds is investigated by time-resolved pump-probe laser spectroscopy. The bridges consist of molecular chains and are of the type (CH(2))(m) with m up to 6 as well as (CH(2)OCH(2))(n) (n=1,2) and CH(2)SCH(2). After light absorption into the azulene S(1) band and subsequent fast internal conversion, excited molecules are formed where the vibrational energy is localized at the azulene side. The vibrational energy transfer through the molecular bridge to the anthracene side and, finally, to the surrounding medium is followed by probing the red edge of the azulene S(3) absorption band at 300 nm and/or the anthracene S(1) absorption band at 400 nm. In order to separate the time scales for intramolecular and intermolecular energy transfer, most of the experiments were performed in supercritical xenon where vibrational energy transfer to the bath is comparably slow. The intramolecular equilibration proceeds in two steps. About 15%-20% of the excitation energy leaves the azulene side within a short period of 300 fs. This component accompanies the intramolecular vibrational energy redistribution (IVR) within the azulene chromophore and it is caused by dephasing of normal modes contributing to the initial local excitation of the azulene side and extending over large parts of the molecule. Later, IVR in the whole molecule takes place transferring vibrational energy from the azulene through the bridge to the anthracene side and thereby leading to microcanonical equilibrium. The corresponding time constants tau(IVR) for short bridges increase with the chain length. For longer bridges consisting of more than three elements, however, tau(IVR) is constant at around 4-5 ps. Comparison with molecular dynamics simulations suggests that the coupling of these chains to the two chromophores limits the rate of intramolecular vibrational energy transfer. Inside the bridges the energy transport is essentially ballistic and, therefore, tau(IVR) is independent on the length.     

A full-dimensional quantum dynamical approach to the vibrational predissociation of Cl2-He2

A full-dimensional, fully coupled wave packet method is proposed and applied to investigate the vibrational predissociation dynamics of the Cl2(B,v')-He2 complex. Simulations are carried out for the resonance states associated with the v'=10-13 initial vibrational excitations of Cl2, and the results are compared with the available experimental data. A good agreement with experiment is achieved for the resonance lifetimes (typically within experimental error) and the Cl(2) fragment rotational distributions. The mechanism of dissociation of the two He atoms is found to be dominantly sequential, through the Deltav'= -2 channel. The probabilities obtained for the Deltav'= -1 dissociation channel are, however, overestimated due to the use of absorbing boundary conditions combined with finite grid effects. It is suggested that a mechanism of energy redistribution through the couplings between the van der Waals modes of the two weak bonds takes place in the Deltav'= -1 dissociation. This mechanism is consistent with the resonance lifetimes and Cl2 rotational distributions predicted. The favorable comparison with most of the experimental data supports the reliability of the potential used to model Cl2(B,v')-He2, at least in the present range of v' levels.     

Direct measurement of the vibrational predissociation lifetime of NeBr2 in the ground electronic state

The vibrational predissociation lifetime of NeBr₂ in the ground electronic state with one vibrational quantum in the halogen stretch was measured directly in a free jet expansion. NeBr₂(X, ν = 1) was detected by optical-optical double resonance to monitor the population as a function of distance from the nozzle to the laser interaction region. The vibrational predissociation lifetime for NeBr₂(X, ν = 1) was determined to be 8 ± 3 μs.     

Imaging the state-specific vibrational predissociation of the C2H2-NH3 hydrogen-bonded dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-acetylene dimer were studied following excitation in the asymmetric CH stretch. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the asymmetric CH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E' <-- X1A1' and C'1A1' <-- X1A1' transitions. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with one or two quanta in the symmetric bend (nu2 umbrella mode) and were converted to rotational-state distributions of the acetylene co-fragment. The latter is always generated with one or two quanta of bending excitation. All the distributions could be fit well when using a dimer dissociation energy of D0 = 900 +/- 10 cm(-1). Only channels with maximum translational energy <150 cm(-1) are observed. The rotational excitation in the ammonia fragments is modest and can be fit by temperatures of 150 +/- 50 and 50 +/- 20 K for 1nu2 and 2nu2, respectively. The rotational distributions in the acetylene co-fragment pair-correlated with specific rovibrational states of ammonia appear statistical as well. The vibrational-state distributions, however, show distinct state specificity among channels with low translational energy release. The predominant channel is NH3(1nu2) + C2H2(2nu4 or 1nu4 + 1nu5), where nu4 and nu5 are the trans- and cis-bend vibrations of acetylene, respectively. A second observed channel, with much lower population, is NH3(2nu2) + C2H2(1nu4). No products are generated in which the ammonia is in the vibrational ground state or the asymmetric bend (1nu4) state, nor is acetylene ever generated in the ground vibrational state or with CC stretch excitation. The angular momentum (AM) model of McCaffery and Marsh is used to estimate impact parameters in the internal collisions that give rise to the observed rotational distributions. These calculations show that dissociation takes place from bent geometries, which can also explain the propensity to excite fragment bending levels. The low recoil velocities associated with the observed channels facilitate energy exchange in the exit channel, which results in statistical-like fragment rotational distributions.     

Ultrafast dynamics through conical intersections and intramolecular vibrational energy redistribution in styrene

We report a femtosecond time-resolved photoelectron spectroscopy (TRPES) investigation of internal conversion in the first two excited singlet electronic states of styrene. We find that radiationless decay through an S(1)/S(0) conical intersection occurs on a timescale of ～4 ps following direct excitation to S(1) with 0.6 eV excess energy, but that the same process is significantly slower (～20 ps) if it follows internal conversion from S(2) to S(1) after excitation to S(2) with 0.3 eV excess energy (0.9 eV excess energy in S(1)).     

Picosecond IR-UV pump-probe spectroscopic study of the dynamics of the vibrational relaxation of jet-cooled phenol. I. Intramolecular vibrational energy redistribution of the OH and CH stretching vibrations of bare phenol

The intramolecular vibrational energy redistribution (IVR) of the OH stretching vibration of jet-cooled phenol-h6 (C6H8OH) and phenol-d8 (C6D8OH) in the electronic ground state has been investigated by picosecond time-resolved IR-UV pump-probe spectroscopy. The OH stretching vibration of phenol was excited with a picosecond IR laser pulse, and the subsequent temporal evolutions of the initially excited level and the redistributed ones due to the IVR were observed by multiphoton ionization detection with a picosecond UV pulse. The IVR lifetime for the OH stretch vibration of phenol-h6 was determined to be 14 ps, while that of the OH stretch for phenol-d8 was found to be 80 ps. This remarkable change of the IVR rate constant upon the dueteration of the CH groups strongly suggests that the "doorway states" for the IVR from the OH level would be the vibrational states involving the CH stretching modes. We also investigated the IVR rate of the CH stretching vibration for phenol-h6. It was found that the IVR lifetime of the CH stretch is less than 5 ps. The fast IVR is described by the strong anharmonic resonance of the CH stretch with many other combinations or overtone bands.     

Ultrafast dynamics in noble metal clusters: The role of internal vibrational redistribution

We present a theoretical study of the ultrafast dynamics in noble metal clusters interacting with molecular oxygen which is of fundamental importance for the understanding and design of cluster-based heterogenous nanocatalysts. We demonstrate that intrinsic dynamical properties can significantly promote the reactivity of small noble metal clusters towards O₂. This concept is illustrated by performing collision simulations between and clusters and O₂ in the framework of the ab initio molecular dynamics (MD) using density functional theory (DFT). We show that different nature and efficiency of the internal vibrational energy redistribution (IVR) during the collisions with O₂ are responsible for considerably different sticking probabilities of O₂ to silver and gold clusters, respectively. In the case of , resonant IVR between the cluster and the O₂ subunit activates the O–O bond and promotes the subsequent oxidation reaction. In contrast, in the case of fast dissipative IVR on the time scale of 1 ps leads to a higher sticking probability for O₂ but the O–O bond is very rapidly deactivated and cannot participate in further oxidation processes. These findings allow us to introduce the nature of IVR as a criterion for promoting the reactivity of noble metal clusters. Such different behaviour of silver and gold clusters colliding with O₂ originates from difference in relativistic effects which are considerably more pronounced in the case of gold clusters causing more directional rigid bonding in contrast to silver clusters with more s-metallic floppy character. Moreover, we demonstrate that breaking of O–O bond can be induced in by a selective excitation of the O–O bond with an ultrashort pulse in the infrared spectral range. This opens the perspective to control the action of nanocatalysts by employing shaped laser pulses and thus bridges the fields of femtochemistry and cluster nanocatalysis.     

Quasi-classical trajectories study of Ne79Br2(B) vibrational predissociation

A full-dimensional quasi-classical trajectories study on the vibrational predissociation (VP) of the Ne79Br2(B) complex is presented. Following the most recent experiments, the Br2(B) vibrational levels v'=16-29 were explored. The total angular momentum, J, was taken to be zero, and a semiclassical Franck-Condon model to compute initial conditions from quantum distributions was employed. Predissociation lifetimes were extracted from Ne79Br2 population decay by using two different exponential laws. Predicted lifetimes are in excellent agreement with the last experimental results [J. A. Cabrera, C. R. Bieler, B. C. Olbricht, W. E. van der Veer and K. C. Janda, J. Chem. Phys., 2005, 123, 054311]. The Br2 fragment ro-vibrational distributions resulting from the VP of the molecule were obtained from the statistics of classical magnitudes using the standard binning procedure. Computed rotational distributions (for the Deltav'=-1, -2 channels) are also in very good agreement with the experimental results [M. Nejad-Sattari and T. A. Stephenson, J. Chem. Phys., 1997, 106 5454]. The influence of two quantum effects-the closing of the Deltav'=-1 dissociation channel and the intramolecular vibrational relaxation (IVR) mechanism-on the agreement with experimental rotational distributions, is discussed. Due to the classical character of our calculations and the binning procedure we used, the agreement of computed vibrational distributions with experimental and quantum theoretical is qualitative. For instance, for v'=28-for which the Deltav'=-1 channel is experimentally found to be closed-the Deltav'=-2 channel becomes statistically more significant. A discussion on the viability of similar quasi-classical methods to model the VP dynamics of analogous clusters is presented.     

Progress in understanding the intramolecular vibrational redistribution dynamics in the S(1) state of para-fluorotoluene

We employ zero-kinetic-energy (ZEKE) photoelectron spectroscopy with nanosecond laser pulses to study intramolecular vibrational redistribution (IVR) in S(1) para-fluorotoluene. The frequency resolution of the probe step is superior to that obtained in any studies on this molecule to date. We focus on the behavior of the 13(1) (C-CH(3) stretch) and 7a(1) (C-F stretch) vibrational states whose dynamics have previously received significant attention, but with contradictory results. We show conclusively that, under our experimental conditions, the 7a(1) vibrational state undergoes significantly more efficient IVR than does the 13(1) state. Indeed, under the experimental conditions used here, the 13(1) state undergoes very little IVR. These two states are especially interesting because their energies are only 36 cm(-1) apart, and the two vibrational modes have the same symmetry. We discuss the role of experimental conditions in observations of IVR in some detail, and thereby suggest explanations for the discrepancies reported to date.