Internal Vibrational Energy Redistribution Research Articles

Singlet O2 Produced by Ultraviolet Dissociation of the β-ionone-O2 Complex.

We formed the gas-phase β-ionone-O2 complex in a supersonic expansion and then photodissociated the complex with light near 312 nm. Photodissociation resulted in the production of O2 in the a 1Δg state, which was ionized at 312 nm using (2 + 1) resonance-enhanced multiphoton ionization (REMPI). We recorded the 1O2 REMPI action spectrum and O2+ velocity map ion image following photodissociation of the complex. From the velocity map image, we determined the total recoil kinetic energy distribution from dissociation of the complex. Fitting the REMPI spectrum showed that the 1O2 product has an effective rotational temperature of about 50 K, while the recoil kinetic energy distribution was well fit with a statistical Boltzmann distribution having an effective translational temperature of 289 K. Using the average translational energy from the Boltzmann fit along with the complex dissociation energy from ab initio calculations, we determined that β-ionone was formed with an average of 2.87 eV of internal energy, which was 0.49 eV higher than previous measurements for the β-ionone triplet-state energy. Our own CCSD/cc-pVDZ//(U)MP2/cc-pVDZ calculations gave a minimum triplet-state energy of 2.04 eV. However, a large structural change occurs between the minimum singlet-ground-state geometry and the minimum triplet-excited-state geometry, and as a result, the calculated vertical energy for the triplet-state β-ionone was determined to be 3.30 eV. Comparing the ab initio and experimental results indicated that following excitation, β-ionone was formed in the triplet state but with significant internal vibrational energy. As such, complex dissociation likely proceeds following internal vibrational energy redistribution, which explains the statistical recoil kinetic energy distribution.

Ultrafast Photodissociation Dynamics of Highly Excited Iodobenzene on the C Band.

The photodissociation dynamics of highly excited iodobenzene from the C band absorption has been studied by femtosecond time-resolved ion yields techniques. Detailed photodissociation routes are discussed with the aid of high-level, spin-orbit resolved ab initio calculations of 1D potential energy curves. Upon 200 nm excitation within the C band, iodobenzene molecules on 7B2 and 7B1 states decay to 7A1 and 8B2 states through internal conversion in 75 fs, with electronic energy converted into high vibrational energy of 7A1 and 8B2 states. Subsequently, 7A1 and 8B2 states decay through internal vibrational energy redistribution in 540 fs, accompanied by the excited C-I mode and the resulting cleavage of the C-I bond. The overall time for the reaction starting from the phenyl-type modes and ending in final C-I fragmentation for I(2P3/2) production is 1.2 ps.

Unveiling Singlet Fission Mediating States in TIPS-pentacene and its Aza Derivatives.

Femtosecond pump-depletion-probe experiments were carried out in order to shed light on the ultrafast excited-state dynamics of triisopropylsilylethynyl (TIPS)-pentacene and two nitrogen-containing derivatives, namely, diaza-TIPS-pentacene and tetraaza-TIPS-pentacene. Measurements performed in the visible and near-infrared spectral range in combination with rate model simulations reveal that singlet fission proceeds via the extremely short-lived intermediate (1)TT state, which absorbs in the near-infrared spectral region only. The T1 → T3 transition probed in the visible region shows a rise time that comprises two components according to a consecutive reaction (S1 → (1)TT → T1). The incorporation of nitrogen atoms into the acene structure leads to shorter dynamics, but the overall triplet formation follows the same kinetic model. This is of particular importance, since experiments on tetraaza-TIPS-pentacene allow for investigation of the triplet state in the visible range without an overlapping singlet contribution. In addition, the pump-depletion-probe experiments show that the triplet absorption in the visible (T1 → T3) and near-infrared (T1 → T2) regions occurs from the same initial state, which was questioned in previous studies. Furthermore, an additional ultrafast transfer between the excited triplet states (T3 → T2) is identified, which is also in agreement with the rate model simulation. By applying depletion pulses, which are resonant with higher vibrational levels, we gain insight into internal vibrational energy redistribution processes within the triplet manifold. This additional information is of great relevance regarding the study of loss channels within these materials.

Utilizing Ancillary Ligands to Optimize the Photophysical Properties of 4H‐Imidazole Ruthenium Dyes

Correlations between structural features and photophysical properties offer the possibility to design dyes with tailor-made properties. In this respect, the photophysical properties of a series of 4H-imidazole (4H-im) ruthenium dyes with varying chemical and electronic structures of the complex fragments [(tpy)Ru(4H-im)X] {tpy=4,4',4''-tri-tert-butyl-2,2':6',2''-terpyridine; X=Cl(-), NCS(-)} and [(bpy)2Ru(4H-im)](+) {bpy=4,4'-di-tert-butyl-2,2'-bipyridine} were investigated. Variation of the π-donor/acceptor properties of the ancillary ligands offers the possibility to tune the relative energies of the d donor and π* acceptor orbitals of 4H-im, which results in a shift in the Ru→4H-im (1)MLCT (MLCT=metal-to-ligand charge transfer) absorption band in the visible range. Further, the energies of the excited states and also the interactions and mixing of the metal d orbital with the π* 4H-im orbital are sensitive to the chemical and electronic structures of the complex fragment. This causes subtle changes in the photoinduced processes; for example, the rate of the interim population of a planarized state increases with increasing/decreasing π-acceptor/donor character of the ancillary ligand. Although ground-state repopulation in the tpy species follows the energy-gap law, local symmetry-related effects have to be considered to account for the very short lifetime of the excitation in the bpy complex. Additionally, the ultrafast intramolecular relaxation processes leading from the initially excited states to the relaxed triplet states localized on the 4H-im ligand (internal conversion, internal vibrational energy redistribution, intersystem crossing) depend on the nature of the complex fragment. The high excitation-wavelength-dependent rate for the population of the relaxed triplet states in the bpy complex points to additional interligand electron-transfer contributions upon excitation into the higher lying Ru→bpy (1)MLCT states.

Quantum-dynamical Modeling of the Rydberg to Valence Excited-State Internal Conversion in Cyclobutanone and Cyclopentanone

In this paper we present 4-state, 5-dimensional Vibronic Coupling Hamiltonians for cyclobutanone and cyclopentanone. Wave packet calculations using these Hamiltonians reveal that for cyclobutanone the (n,3s) to (n,π*) internal conversion involves direct motion in nuclear modes coupling the two states leading to fast population transfer. For cyclopentanone, internal vibrational energy redistribution is a bottleneck for activating reactive nuclear modes leading to slower population transfer.

Symmetry, vibrational energy redistribution and vibronic coupling: The internal conversion processes of cycloketones

In this paper, we discern two basic mechanisms of internal conversion processes; one direct, where immediate activation of coupling modes leads to fast population transfer and one indirect, where internal vibrational energy redistribution leads to equidistribution of energy, i.e., ergodicity, and slower population transfer follows. Using model vibronic coupling Hamiltonians parameterized on the basis of coupled-cluster calculations, we investigate the nature of the Rydberg to valence excited-state internal conversion in two cycloketones, cyclobutanone and cyclopentanone. The two basic mechanisms can amply explain the significantly different time scales for this process in the two molecules, a difference which has also been reported in recent experimental findings [T. S. Kuhlman, T. I. Sølling, and K. B. Møller, ChemPhysChem. 13, 820 (2012)].

Vibrational analysis of excited and ground electronic states of all-trans retinal protonated Schiff-bases

We report on vibrational coherence dynamics in excited and ground electronic states of all-trans retinal protonated Schiff-bases (RPSB), investigated by time-resolved Degenerate Four-Wave-Mixing (DFWM). The results show that wave packet dynamics in the excited state of RPSB consist of only low-frequency (<800 cm(-1)) modes. Such low-frequency wave packet motion is observed over a broad range of detection wavelengths ranging from excited state absorption (∼500 nm) to stimulated emission (>600 nm). Our results indicate that low-frequency coherences in the excited state are not activated directly by laser excitation but rather by internal vibrational energy redistribution. This is supported by the observation that similar coherence dynamics are not observed in the electronic ground state. Challenging previous experimental results, we show that the formation of low-frequency coherence dynamics in RPSB does not require significant excess vibrational energy deposition in the excited state vibrational manifolds. Concerning ground state wave packet dynamics, we observe a set of high-frequency (>800 cm(-1)) modes, reflecting mainly single and double bond stretching motion in the retinal polyene-chain. Dephasing of these high-frequency coherences is mode-dependent and partially differs from analogous vibrational dephasing of the all-trans retinal chromophore in a protein environment (bacteriorhodopsin).

Exploring the Effects of Intramolecular Vibrational Energy Redistribution on the Operation of the Proton Wire in Green Fluorescent Protein

The operation of the proton wire in Green Fluorescent Protein has been simulated by quantum dynamics and considering the coupling to the protein environment by means of a bath of harmonic oscillators. The simulation consists of 36 explicit and fully quantum degrees of freedom: 6 degrees of freedom represent the configuration of the proton wire, which are coupled to 30 bath coordinates. Regimes of weak and strong coupling have been studied. It is found that presence of the bath induces a fast energy transfer from the proton wire to the bath, with characteristic times under 400 fs. This internal vibrational redistribution happens at the expense of the potential energy content of the proton wire, deformed through the interaction to the bath from its uncoupled state. Strong coupling induces a slowing-down of the operation of the wire because it hinders to some extent the approaching of donor and acceptor atoms to distances in which proton transfer can occur. Internal vibrational energy redistribution affects the dynamics, but from our simulations we conclude that it cannot be the only cause responsible for the experimentally reported fluorescence rise times.

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 O 2. This concept is illustrated by performing collision simulations between Ag 6 - and Au 6 - clusters and O 2 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 O 2 are responsible for considerably different sticking probabilities of O 2 to silver and gold clusters, respectively. In the case of Au 6 - + O 2 , resonant IVR between the cluster and the O 2 subunit activates the O–O bond and promotes the subsequent oxidation reaction. In contrast, in the case of Ag 6 - + O 2 fast dissipative IVR on the time scale of ∼1 ps leads to a higher sticking probability for O 2 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 O 2 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 Au 6 O 2 - 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.

Reactivity-promoting criterion based on internal vibrational energy redistribution

We propose to introduce intrinsic dynamical properties as a criterion for promoting reactivity of small size noble metal reactive centers relevant for heterogeneous catalysis. To illustrate the concept, collisions between Ag6- or Au6- clusters and molecular oxygen have been investigated with direct ab initio molecular dynamics by using DFT. We show that different nature and efficiency of internal vibrational energy redistribution (IVR) during reaction dynamics is responsible for significantly different sticking probabilities of O2 to gold and to silver clusters. In the case of Au6- and O2 collisions, resonant IVR occurs between two subunits activating O2 and promoting subsequent oxidation reactions. In contrast, a dissipative IVR in Ag6- and O2 molecule prevents O2 to react with other adsorbates. These findings allow us to introduce the nature of IVR as a criterion for promoting the reactivity of noble metal clusters: Resonant IVR between reactants promotes reactivity toward adsorbates.

Interplay of computational chemistry and transient absorption spectroscopy in the ultrafast studies

The primary physical and chemical processes in the photochemistry of 1-(trideuteromethyl)-2,3,4-trideutero (1) and 1-acetoxy-2-methoxy-(2) 9,10-anthraquinones were studied using femtosecond transient absorption spectroscopy and computational chemistry. Excitation of 1 and 2 at 270 nm results in the population of a set of highly excited singlet states which decay within the laser pulse by internal conversion and vibrational energy redistribution. The transient absorption spectra of the lowest singlet and triplet excited states of substituted anthraquinones 1 and 2 as well as the triplet excited and ground states of the products were detected. The assignments of the transient absorption spectra were performed on the basis of quantum chemical calculations of the electronic absorption spectra of the intermediates. Time-dependent density functional theory or CASSCF/CASPT2 procedure were used to calculate the spectroscopic properties of the intermediates.

The role of specific normal modes during non‐Born–Oppenheimer dynamics: the S1–S0 internal conversion of β‐carotene interrogated on a femtosecond time‐scale with coherent anti‐Stokes Raman scattering

AbstractThe population flow from the first excited singlet state (S1) to the electronic ground state (S0), facilitated through the S1–S0 internal conversion, and the subsequent internal vibrational energy redistribution and vibrational cooling processes are monitored selectively with respect to the different forms of nuclear motion in all‐trans‐β‐carotene. This is realized by exciting the molecule into the second excited singlet state (S2) with an auxiliary pump laser pulse and interrogating the population recovery into the electronic ground state by means of a time‐delayed, coherent anti‐Stokes Raman (CARS) process. This spectroscopic scheme is referred to as a pump‐CARS scheme in analogy to the classical pump–probe scheme in time‐resolved spectroscopy. Here, the profound enhancement of the CARS signal intensity in the case of a Raman resonance to a specific vibrational mode is utilized as an intensity filter that amplifies the signal from the vibrational modes of interest, making the contributions from other vibrational modes negligible. This filter allows for the population flow in a specific vibrational mode to be monitored as the radiationless electronic transition between the S1 and S0 state takes place. This spectroscopic scheme opens up the possibility of identifying the vibrational motion with a large‐amplitude motion in the CC double bond symmetric stretch as the primary acceptor of population from the S1 state. A mechanism with which the other normal modes are populated is postulated. Copyright © 2002 John Wiley &amp; Sons, Ltd.

Ultrafast decay of electronically excited singlet cytosine via a pi,pi* to n(O),pi* state switch.

Singlet fluorescence lifetimes of adenosine, cytidine, guanosine, and thymidine, determined by femtosecond pump-probe spectroscopy (Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2000, 122, 9348. Pecourt, J.-M. L.; Peon, J.; Kohler, B. J. Am. Chem. Soc. 2001, 123, 10370), show that the excited states produced by 263 nm light in these nucleosides decay in the subpicosecond range (290-720 fs). Ultrafast radiationless decay to the ground state greatly reduces the probability of photochemical damage. In this work we present a theoretical study of isolated cytosine, the chromophore of cytidine. The experimental lifetime of 720 fs indicates that there must be an ultrafast decay channel for this species. We have documented the possible decay channels and approximate energetics, using a valence-bond derived analysis to rationalize the structural details of the paths. The mechanism favored by our calculations and the experimental data involves (1) a two-mode decay coordinate composed of initial bond length inversion followed by internal vibrational energy redistribution (IVR) to populate a carbon pyramidalization mode, (2) a state switch between the pi,pi* and nO,pi* (excitation from oxygen lone pair) excited states, and (3) decay to the ground state through a conical intersention. A second decay path through the nN,pi* state (excitation from the nitrogen lone pair), with a higher barrier, involves out-of-plane bending of the amino substituent.

Ab initio nonadiabatic dynamics involving conical intersection combined with Wigner distribution approach to ultrafast spectroscopy illustrated on Na3F2 cluster

We present a theoretical study of a femtosecond photo isomerization process due to a nonadiabatic radiationless decay from the first excited state through a conical intersection occurring in one of the nonstoichiometric halide-deficient clusters with one excess electron (Na3F2). This is an extension of the adiabatic dynamics study presented in the accompanying paper [J. Chem. Phys. 114, 2106 (2001)] for other members of the NanFn−1 series characterized by a strong ionic bonding for which the “frozen ionic bonds” approximation has been justified, allowing consideration of the optical response of the single excess electron in the effective field of the other electrons. In this contribution we outline the extension of the ab initio Wigner-distribution approach to nonadiabatic molecular dynamics which combines the Wigner–Moyal representation of the vibronic density matrix with the ab initio multistate molecular dynamics in the ground- and excited electronic states including the nonadiabatic coupling computed “on the fly” in connection with the fewest-switches hopping algorithm. This scheme allows accounting for temperature-dependent initial conditions, for the propagation in the excited state and in the ground state after the passage through the conical intersection, and for probing in the cationic ground state as well as for deriving analytic expressions for the pump–probe signals which utilize an ensemble of classical trajectories obtained at low computational demand. Our approach permits investigation of the photo isomerization through the conical intersection due to the long amplitude motion in the Na3F2 system in full complexity, taking into account all degrees of freedom. After breaking of one metallic and of one ionic bond the conical intersection occurs at the linear geometry and involves states of different symmetry which differ in the translocation of the one excess electron or positive charge localized at the Na atom from one end to the other of the system and separates two isomers with Cs and C2v structures. From the analysis of the nonadiabatic dynamics, the time scales for the metallic bond breaking of ∼90 fs and for the ionic bond breaking of ∼220 fs, for the passage through the conical intersection after ∼0.4 ps and for the internal vibrational energy redistribution (IVR) of more than 0.9 ps for the individual isomers, have been determined. The simulated fs pump–probe signals confirm the above results and provide the information about the experimental conditions such as laser frequencies and pulse duration under which bond breaking of different type as well as the population of each of the two isomers after the passage through the conical intersection can be identified. In this contribution we show that the mechanism of the photo isomerization at a conical intersection due to a long amplitude motion can occur in atomic clusters and is not necessarily limited to organic photochemistry.

On the role of nonergodicity and slow IVR in unimolecular reaction rate theory—A review and a view

A review of the history of unimolecular reaction rate theory reveals a strong reliance upon an assumption of ergodicity and rapid internal vibrational energy redistribution (IVR). The notable exception is the theory of N. B. Slater wherein fully separable harmonic dynamics was assumed. Recently realistic intermediate theories have been sought. A simple analysis will be presented here which systematizes the mechanism and the expected effects. The problem of nonergodicity and slow IVR is analyzed within the framework of the classical RRK theory. A phenomenological model, the RRK-S(n) model, previously applied to the study of the isomerization of trans-stilbene, is extended to account for an energy dependent internal energy transfer rate between a reactive oscillator and the remaining oscillators. The reaction dynamics is expected to show character transitions at three different energies. Below Eerg the dynamics will be predominantly nonergodic while above Eerg it will be predominantly ergodic. At the energy E0, the barrier height, the specific rate coefficient ka(E) becomes nonzero and generally rising with energy. Depending on the number of “reactive ” oscillators n, the rate of energy transfer between reactive and unreactive oscillators, the external collision frequency and the rate of decay of the reactive set of oscillators the specific rate coefficient shows a degree of IVR limitation which is in turn reflected in the total reaction rate coefficient k(ω). Illustrative calculations delineating expected behaviour within the confines of the model are presented.

Unimolecular dissociation of NO2. I. Classical trajectory and statistical calculations on a global potential energy surface

The title reaction is examined with classical and statistical (RRKM, SACM) mechanics on an ab initio three-dimensional global potential energy surface of the lowest adiabatic electronic state of NO2. The energy-resolved rate constants obtained within different theoretical frameworks are compared with each other and with the available experimental data. The question of the transition state definition in this barrierless reaction is analyzed and the applicability of the statistical hypothesis is discussed. Although the rate of internal vibrational energy redistribution is estimated to be larger than 5 ps−1 at the quantum dissociation threshold, which is much larger than the dissociation rate, a substantial fraction of classical trajectories remains regular. This causes statistical transition-state-like theories to overestimate the actual dissociation rate defined from the full classical dynamics calculations.

Multiple Roles of Highly Vibrationally Excited Molecules in the Reaction Zones of Detonation Waves

Recent experimental and theoretical advances in the understanding of high-pressure, high-temperature chemical kinetics are used to extend the nonequilibrium Zeldovich−von Neumann−Doring (NEZND) theory of self-sustaining detonation in liquid and solid explosives. The attainment of vibrational equilibrium behind the leading shock front by multiphonon up-pumping and internal vibrational energy redistribution establishes a high-temperature, high-density transition state or series of transition states through which the chemical decomposition proceeds. The reaction rate constants for the initial unimolecular decomposition steps are accurately calculated using high-temperature, high-density transition-state theory. These early reactions are endothermic or weakly exothermic, but they channel most of the available energy into excited vibrational states of intermediate product species. The intermediate products transfer some of their vibrational energy back into the transition states, accelerating the overall reaction rates. As the decomposition progresses, the highly vibrationally excited diatomic and triatomic molecules formed in very exothermic chain reactions are rapidly vibrationally equilibrated by “supercollisions”, which transfer large amounts of vibrational energy between these molecules. Along with vibrational−rotational and vibrational−translational energy transfer, these excited vibrational modes relax to thermal equilibrium by amplifying pressure wavelets of certain frequencies. These wavelets then propagate to the leading shock front and reinforce it. This is the physical mechanism by which the leading shock front is sustained by the chemical energy release.

Beats in internal vibrational energy redistribution in trans-stilbene — A study of the quantum and classical correspondence

Abstract Starting from recently observed beating patterns in trans-stilbene we consider the correspondence of classical molecular dynamics and quantum dynamics for the description of the internal vibrational energy redistribution in molecules. The beat frequency of two modes in 2:1 resonance is investigated with respect to quantum effects, sensitivity to anharmonic coupling and indirect coupling via a third oscillator. We find a robust correspondence between classical and quantum beat frequencies but a large sensitivity to the coupling terms in the potential. Refined anharmonic coupling appears to be needed to resolve the very low frequency beating patterns observed in trans-stilbene.

The unimolecular dissociation of HCO: I. Oscillations of pure CO stretching resonance widths

The unimolecular dissociation of the formyl radical HCO in the electronic ground state is investigated using a completely new ab initio potential energy surface. The dynamics calculations are performed in the time-independent picture by employing a variant of the log-derivative Kohn variational principle. The full resonance spectrum up to energies more than 2 eV above the vibrational ground state is explored. The three fundamental frequencies (in cm−1) for the H–CO and CO stretches, and the bending mode are 2446 (2435), 1844 (1868), and 1081 (1087), where the numbers in parentheses are the measured values of Sappey and Crosley obtained from dispersed fluorescence excitation spectra [J. Chem. Phys. 93, 7601 (1990)]. In the present work we primarily emphasize the dissociation of the pure CO stretching resonances (0v20) and their decay mechanisms. The excitation energies, dissociation rates, and final vibrational–rotational state distributions of CO agree well with recent experimental data obtained from stimulated emission pumping. Similarities with and differences from previous time-independent and time-dependent calculations employing the widely used Bowman–Bittman–Harding potential energy surface are also discussed. Most intriguing are the pronounced oscillations of the dissociation rates for vibrational states v2≥7 which are discussed in the framework of internal vibrational energy redistribution.

The conservation of quantum zero-point energies in classical trajectory simulations

We propose a computational technique to constrain the vibrational modes of a classical molecule to have energy greater than the quantum zero-point energy (ZPE). The trajectory of any mode with energy less than ZPE is projected to a neighboring point in phase space where the energy is equal to the ZPE and the phase angle of the mode is unchanged. All other modes are then perturbed in such a way as to conserve the total energy of the system. This technique is similar in principle to the method of holonomic constraints. We apply this ‘‘semiholonomic’’ TRAPZ (trajectory projection onto ZPE orbit) scheme to the two mode Hénon–Heiles system and find that it results in a decrease of ergodicity. Periodic limit cycle internal vibrational energy redistribution is observed. Implications of this method for the conservation of ZPE in quasiclassical trajectory simulations are discussed.