Dynamics at conical intersections: the influence of O-H stretching vibrations on the photodissociation of phenol

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d(5) with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d(5) molecules with an initially excited O-H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d(5) radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation.     

Exploring molecular complexity: conical intersections and NH3 photodissociation

The role of conical intersections in the photodissociation of the A 1A2" state of NH3 is investigated using extended atomic basis sets and a configuration state function expansion of approximately 8.5 million terms. A previously unknown portion of the 1 1A-2 1A seam of conical intersections with only C(s) symmetry is located. This portion of the seam is readily accessible from the equilibrium geometry of the A 1A2" state. These conical intersections are expected to play a role in the competition between adiabatic and nonadiabatic pathways for NH3(A 1A2") photodissociation.     

Vibrationally mediated photodissociation of C2H4+

We report the vibrationally mediated photodissociation dynamics of C2H4+ excited through the B2Ag state. Vibrational state-selected ions were prepared by two-photon resonant, three-photon ionization of ethylene via (pi, 3s) and (pi, 3p) Rydberg intermediate states in the wavelength range 298-349 nm. Absorption of a fourth photon led to dissociation of the cation, and images of the product ions C2H3+ and C2H2+ were simultaneously recorded using reflectron multimass velocity map imaging. Analysis of the multimass images yielded, with high precision, both the total translational energy distributions for the two dissociation channels and the branching between them as a function of excitation energy. The dissociation of ions that were initially prepared with torsional excitation exceeding the barrier to planarity in the cation ground state consistently gave enhanced branching to the H elimination channel. The results are discussed in terms of the influence of the initial state preparation on the competition between the internal conversion to the ground state and to the first excited state.     

Theory of the photodissociation of ozone in the Hartley continuum: potential energy surfaces, conical intersections, and photodissociation dynamics

Ab initio potential energy and transition dipole moment surfaces are presented for the five lowest singlet even symmetry electronic states of ozone. The surfaces are calculated using the complete active space self consistent field method followed by contracted multireference configuration interaction (MRCI) calculations. A slightly reduced augmented correlation consistent valence triple-zeta orbital basis set is used. The ground and excited state energies of the molecule have been computed at 9282 separate nuclear geometries. Cuts through the potential energy surfaces, which pass through the geometry of the minimum of the ground electronic state, show several closely avoided crossings. Close examination, and higher level calculations, very strongly suggests that some of these seemingly avoided crossings are in fact associated with non-symmetry related conical intersections. Diabatic potential energy and transition dipole moment surfaces are created from the computed ab initio adiabatic MRCI energies and transition dipole moments. The transition dipole moment connecting the ground electronic state to the diabatic B state surface is by far the strongest. Vibrational-rotational wavefunctions and energies are computed using the ground electronic state. The energy level separations compare well with experimentally determined values. The ground vibrational state wavefunction is then used, together with the diabatic B<--X transition dipole moment surface, to form an initial wavepacket. The analysis of the time-dependent quantum dynamics of this wavepacket provides the total and partial photodissociation cross sections for the system. Both the total absorption cross section and the predicted product quantum state distributions compare well with experimental observations. A discussion is also given as to how the observed alternation in product diatom rotational state populations might be explained.     

Vibrationally mediated photodissociation of ethylene cation by reflectron multimass velocity map imaging

A new imaging technique, reflectron multimass velocity map ion imaging, is used to study the vibrationally mediated photodissociation dynamics in the ethylene cation. The cation ground electronic state is prepared in specific vibrational levels by two-photon resonant, three-photon ionization via vibronic bands of (pi, nf) Rydberg states in the vicinity of the ionization potential of ethylene, then photodissociated through the (B 2A(g)) excited state. We simultaneously record spatially resolved images of parent C2H4+ ions as well as photofragment C2H3+ and C2H2+ ions originating in dissociation from the vibronic excitations in two distinct bands, 7f 4(0)2 and 8f 0(0)0, at roughly the same total energy. By analyzing the images, we directly obtain the total translation energy distributions for the two dissociation channels and the branching between them. The results show that there exist differences for competitive dissociation pathways between H and H2 elimination from C2H4+ depending on the vibronic preparation used, i.e., on the vibrational excitation in the ground state of the cation prior to photodissociation. Our findings are discussed in terms of the possible influence of the torsional excitation on competition between direct dissociation, isomerization, and radiationless transitions through conical intersections among the numerous electronic states that participate in the dissociation.     

Vibrationally mediated photodissociation of ethene isotopic variants preexcited to the fourth C-H stretch overtone

H and D photofragments produced via vibrationally mediated photodissociation of jet-cooled normal ethene (C2H4), 1,2-trans-d2-ethene (HDCCDH), and 1,1-d2-ethene (CH2CD2), initially excited to the fourth C-H stretch overtone region, were studied for the first time. H and D vibrational action spectra and Doppler profiles were measured. The action spectra include partially resolved features due to rotational cooling, while the monitored room temperature photoacoustic spectra exhibit only a very broad feature in each species. Simulation of the spectral contours allowed determination of the band types and origins, limited precision rotational constants, and linewidths, providing time scales for energy redistribution. The H and D Doppler profiles correspond to low average translational energies and show slight preferential C-H over C-D bond cleavage in the deuterated variants. The propensities toward H photofragments emerge even though the energy flow out of the initially prepared C-H stretch is on a picosecond time scale and the photodissociation occurs following internal conversion, indicating a more effective release of the light H atoms.     

Beyond two-state conical intersections. Three-state conical intersections in low symmetry molecules: the allyl radical

Using multireference configuration interaction expansions comprised of over 7 million configuration state functions, three-state conical intersections are reported for the closely spaced, spectroscopically observed (tilde)B(2A1), (tilde)C(2B1), and (tilde)D(2B2) states (in C(2v) symmetry) of the allyl radical. These conical intersections of states which were previously assigned as the 3,4,5(2)A states and are here reassigned as the 4,5,6(2)A states, are expected to be accessible using optical probes. This conclusion is obtained from the structure of the minimum energy point on the 4,5,6(2)A three-state conical intersection seam which is similar to the equilibrium structure of the ground (tilde)X(2A2) state and only 1.1 eV above the (tilde)D(2B2) state at its equilibrium geometry. The seam of three-state degeneracies joins two two-state seams of conical intersection, the 4,5(2)A and 5,6(2)A conical intersection seams. The energy of the minimum energy point on the 4,5(2)A two-state seam is only 0.15 eV above that of the (tilde)D(2B2) state at its equilibrium structure. Three-state intersections are also reported for the 3,4,5(2)A states.     

Nonadiabatic decomposition of gas-phase RDX through conical intersections: an ONIOM-CASSCF study

Topographical exploration of nonadiabatically coupled ground- and excited-electronic-state potential energy surfaces (PESs) of the isolated RDX molecule was performed using the ONIOM methodology: Computational results were compared and contrasted with the previous experimental results for the decomposition of this nitramine energetic material following electronic excitation. One of the N-NO(2) moieties of the RDX molecule was considered to be an active site. Electronic excitation of RDX was assumed to be localized in the active site, which was treated with the CASSCF algorithm. The influence of the remainder of the molecule on the chosen active site was calculated by either a UFF MM or RHF QM method. Nitro-nitrite isomerization was predicted to be a major excited-electronic-state decomposition channel for the RDX molecule. This prediction directly corroborates previous experimental results obtained through photofragmentation-fragment detection techniques. Nitro-nitrite isomerization of RDX was found to occur through a series of conical intersections (CIs) and was finally predicted to produce rotationally cold but vibrationally hot distributions of NO products, also in good agreement with the experimental observation of rovibrational distributions of the NO product. The ONIOM (CASSCF:UFF) methodology predicts that the final step in the RDX dissociation occurs on its S(0) ground-electronic-state potential energy surface (PES). Thus, the present work clearly indicates that the ONIOM method, coupled with a suitable CASSCF method for the active site of the molecule, at which electronic excitation is assumed to be localized, can predict hitherto unexplored excited-electronic-state PESs of large energetic molecules such as RDX, HMX, and CL-20. A comparison of the decomposition mechanism for excited-electronic-state dimethylnitramine (DMNA), a simple analogue molecule of nitramine energetic materials, with that for RDX, an energetic material, was also performed. CASSCF pure QM calculations showed that, following electronic excitation of DMNA to its S(2) surface, decomposition of this molecule occurs on its S(1) surface through a nitro-nitrite isomerization producing rotationally hot and vibrationally cold distributions of the NO product.     

Quantum dynamics through conical intersections: combining effective modes and quadratic couplings

We present a detailed study for the short-time dynamics through conical intersections in molecular systems related to the quadratic vibronic coupling (QVC) Hamiltonian [Müller, H.; K?ppel, H.; Cederbaum, L. S. New J. Chem. 1993, 17, 7-29] and the effective-mode formalism [Cederbaum, L. S.; Gindensperger, E.; Burghardt, I. Phys. Rev. Lett. 2005, 94, 113003]. Our approach is based on splitting the nuclear degrees of freedom of the whole system into system modes and environment modes. It was found that only three-effective environmental modes together with the system's modes are needed to describe the short-time dynamics of the complex system correctly. In addition, a detailed mathematical proof is given in the appendix to demonstrate that the exact cumulants are recovered up to the second order within the cumulant expansion of the autocorrelation function. The butatriene molecule is studied as an explicit showcase example to stress the viability of our proposed scheme and to compare with other systems.     

Influence of static and dynamical structural changes on ultrafast processes mediated by conical intersections

The technological needs imposed by the exponential miniaturization trend of conventional electronic devices has drawn attention towards the development of smaller and faster devices like ultrafast molecular switches. In recent years molecular switches emerge again in the focus of active and innovative research with state-of-the-art optical tools recording their dynamics in real time. Still many questions about the underlying microscopic mechanism are left open, including potential factors that effect the switching process in either way, improve or worsen it. Due to the complexity of such molecules it is difficult to obtain a global answer from experiment alone. On the other side molecular switches are generally too large for a complete quantum chemical and quantum dynamical calculation. In our group we therefore developed an ab initio based modular model to handle the laser induced quantum dynamics in molecular switches like fulgides. It enables us to study the effect of internal molecular coupling and of the molecular response to external fields. We can investigate the related wave packet dynamics, the switching efficiency and the controllability. Our results focus on the laser induced ring opening in fulgides, which equals one direction of the switching process. Presented are the influence of a conical intersection seam and of time-dependent potentials, mimicking the mean interaction with the environment. Furthermore the relation of controllability and the wave packet's momentum is studied and the influence of potential barriers on the switching dynamics is shown.     

Hierarchy of effective modes for the dynamics through conical intersections in macrosystems

An extension of the effective-mode theory for the short-time dynamics through conical intersections in macrosystems [L. S. Cederbaum et al., Phys. Rev. Lett. 94, 113003 (2005)] is proposed. The macrosystem, containing a vast number of nuclear degrees of freedom (modes), is decomposed into a system part and an environment part. Only three effective modes are needed-together with the system's modes-to accurately calculate low resolution spectra and the short-time dynamics of the entire macrosystem. Here, the authors propose an iterative scheme to construct a hierarchy of additional triplets of effective modes. This naturally extends the effective-mode formulation. By taking into account more and more triplets, the dynamics are accurately predicted for longer and longer times, and more resolved spectra can be calculated. Numerical examples are presented, computed using various numbers of additional effective modes.     

Ultrafast electron transfer in photosynthesis: reduced pheophytin and quinone interaction mediated by conical intersections

The mechanism of electron transfer (ET) from reduced pheophytin (Pheo(-)) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high-level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo(-) + Q) and (Pheo + Q(-)) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs). According to the present findings, an efficient ultrafast ET implies that the Pheo- and Q move toward each other in a given preferential parallel orientation, reaching the most effective arrangement for ET at intermolecular distances (R) around 5-3 Angstrom, where the lowest CIs are predicted. Favored donor/acceptor interactions are related to orientations with some overlap between the lowest occupied molecular orbitals (LUMO) of the two systems, and they lead to state-crossings at an earlier stage of the movement (larger R). Furthermore, when the topology of the interacting moieties does not make possible the LUMOs overlap, the corresponding diabatic potential energy curves do not intersect. Thus, it is anticipated that large scale motions, which are difficult to monitor experimentally, are actually occurring in the photosynthetic reaction centers of bacteria, algae, and higher plants, to fulfill the observed ultrafast ET processes.     

Explorations of conical intersections and their ramifications for chemistry through the Jahn-Teller effect

Much recent progress has been made theoretically and computationally towards understanding the importance of conical intersections for chemical reactions. Nonetheless, experimental characterization of conical intersections has proven extremely difficult with one striking exception: the Jahn-Teller conical intersection. This article overviews the fundamental similarity of a variety of conical intersections and demonstrates how the spectroscopy of Jahn-Teller active molecules can be used to characterize them. Specific results are reviewed for four representative Jahn-Teller active molecules, C5H5, C6H6+, Ag3 and CH3O.     

Quantum dynamics through conical intersections in macrosystems: Combining effective modes and time-dependent Hartree

We address the nonadiabatic quantum dynamics of (macro)systems involving a vast number of nuclear degrees of freedom (modes) in the presence of conical intersections. The macrosystem is first decomposed into a system part carrying a few, strongly coupled modes, and an environment, comprising the remaining modes. By successively transforming the modes of the environment, a hierarchy of effective Hamiltonians for the environment can be constructed. Each effective Hamiltonian depends on a reduced number of effective modes, which carry cumulative effects. The environment is described by a few effective modes augmented by a residual environment. In practice, the effective modes can be added to the system’s modes and the quantum dynamics of the entire macrosystem can be accurately calculated on a limited time-interval. For longer times, however, the residual environment plays a role. We investigate the possibility to treat fully quantum mechanically the system plus a few effective environmental modes, augmented by the dynamics of the residual environment treated by the time-dependent Hartree (TDH) approximation. While the TDH approximation is known to fail to correctly reproduce the dynamics in the presence of conical intersections, it is shown that its use on top of the effective-mode formalism leads to much better results. Two numerical examples are presented and discussed; one of them is known to be a critical case for the TDH approximation.     

Short-time dynamics through conical intersections in macrosystems. I. Theory: effective-mode formulation

The short-time dynamics through a conical intersection of a macrosystem comprising a large number of nuclear degrees of freedom (modes) is investigated. The macrosystem is decomposed into a "system" part carrying a limited number of modes, and an "environment" part. An orthogonal transformation in the environment's space is introduced, as a result of which a subset of three effective modes can be identified which couple directly to the electronic subsystem. Together with the system's modes, these govern the short-time dynamics of the overall macrosystem. The remaining environmental modes couple, in turn, to the effective modes and become relevant at longer times. In this paper, we present the derivation of the effective Hamiltonian, first introduced by Cederbaum et al. [Phys. Rev. Lett. 94, 113003 (2005)], and analyze its properties in some detail. Several special cases and topological aspects are discussed.     

Substituent effects on dynamics at conical intersections: alpha,beta-enones

Femtosecond time-resolved photoelectron spectroscopy and high-level theoretical calculations were used to study the effects of methyl substitution on the electronic dynamics of the alpha,beta-enones acrolein (2-propenal), crotonaldehyde (2-butenal), methylvinylketone (3-buten-2-one), and methacrolein (2-methyl-2-propenal) following excitation to the S2(pipi*) state at 209 and 200 nm. We determine that following excitation the molecules move rapidly away from the Franck-Condon region, reaching a conical intersection promoting relaxation to the S1(npi*) state. Once on the S1 surface, the trajectories access another conical intersection, leading them to the ground state. Only small variations between molecules are seen in their S2 decay times. However, the position of methyl group substitution greatly affects the relaxation rate from the S1 surface and the branching ratios to the products. Ab initio calculations used to compare the geometries, energies, and topographies of the S1/S0 conical intersections of the molecules are not able to satisfactorily explain the variations in relaxation behavior. We propose that the S1 lifetime differences are caused by specific dynamical factors that affect the efficiency of passage through the S1/S0 conical intersection.     

Quadratic description of conical intersections: characterization of critical points on the extended seam

In this paper, we present a practical approach for the characterization of critical points on conical intersection seams as either local minima or saddle points using second-derivative technology. The utility of this methodology is illustrated by the analysis of seven S0/S1 (2Ag/1Ag) conical intersection points involved in the photochemistry of butadiene. The characterization of critical points on the crossing seam requires second derivatives computed in curvilinear coordinates. Using such coordinates, we can represent the branching space and the intersection space to second order. Although these curvilinear coordinates are conceptually important, they also give rise to two additional practical applications. First, such coordinates yield information on the nature of vibrational modes that are stimulated following radiationless decay at a crossing point. Second, the second-order force field is directly comparable to experimental spectroscopic data for Jahn-Teller systems. We will illustrate the latter idea for the cyclopentadienyl radical.