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C H 3 + , a cornerstone intermediate in interstellar chemistry, has recently been detected for the first time by using the James Webb Space Telescope. The photodissociation of this ion is studied here. Accurate explicitly correlated multi-reference configuration interaction ab initio calculations are done, and full-dimensional potential energy surfaces are developed for the three lower electronic states, with a fundamental invariant neural network method. The photodissociation cross section is calculated using a full-dimensional quantum wave packet method in heliocentric Radau coordinates. The wave packet is represented in angular and radial grids, allowing us to reduce the number of points physically accessible, requiring to push up the spurious states appearing when evaluating the angular kinetic terms, through projection technique. The photodissociation spectra, when employed in astrochemical models to simulate the conditions of the Orion bar, result in a lesser destruction of C H 3 + compared to that obtained when utilizing the recommended values in the kinetic database for astrochemistry. [ABSTRACT FROM AUTHOR]

Photodissociation dynamics of the OH bond of phenol is studied with an optimally shaped laser pulse. The theoretical model consists of three electronic states (the ground electronic state, ππ* state, and πσ* state) in two nuclear coordinates (the OH stretching coordinate as a reaction coordinate, r, and the CCOH dihedral angle as a coupling coordinate, θ). The optimal UV laser pulse is designed using the genetic algorithm, which optimizes the total dissociative flux of the wave packet. The latter is calculated in the adiabatic asymptotes of the S0 and S1 electronic states of phenol. The initial state corresponds to the vibrational levels of the electronic ground state and is defined as |nr, nθ⟩, where nr and nθ represent the number of nodes along r and θ, respectively. The optimal UV field excites the system to the optically dark πσ* state predominantly over the optically bright ππ* state with the intensity borrowing effect for the |0, 0⟩ and |0, 1⟩ initial states. For the |0, 0⟩ initial condition, the photodissociation to the S1 asymptotic channel is favored slightly over the S0 asymptotic channel. Addition of one quantum of energy along the coupling coordinate increases the dissociation probability in the S1 channel. This is because the wave packet spreads along the coupling coordinate on the πσ* state and follows the adiabatic path. Hence, the S1 asymptotic channel gets more (∼ 11%) dissociative flux as compared to the S0 asymptotic channel for the |0, 1⟩ initial condition. The |1, 0⟩ and |1, 1⟩ states are initially excited to both the ππ* and πσ* states in the presence of the optimal UV pulse. For these initial conditions, the S1 channel gets more dissociative flux as compared to the S0 channel. This is because the high energy components of the wave packet readily reach the S1 channel. The central frequency of the optimal UV pulse for the |0, 0⟩ and |0, 1⟩ initial states has a higher value as compared to the |1, 0⟩ and |1, 1⟩ initial states. This is explained with the help of an excitation mechanism of a given initial state in relation to its energy. [ABSTRACT FROM AUTHOR]

Optical cavities, e.g., as used in organic polariton experiments, often employ low finesse mirrors or plasmonic structures. The photon lifetime in these setups is comparable to the timescale of the nuclear dynamics governing the photochemistry. This highlights the need for including the effect of dissipation in the molecular simulations. In this study, we perform wave packet dynamics with the Lindblad master equation to study the effect of a finite photon lifetime on the dissociation of the MgH+ molecule model system. Photon lifetimes of several different orders of magnitude are considered to encompass an ample range of effects inherent to lossy cavities. [ABSTRACT FROM AUTHOR]

The nonadiabatic photodissociation dynamics of alkali halide molecules excited by a femtosecond laser pulse in the gas phase are investigated theoretically, and it is shown that the population of the photoexcited molecules exhibits power-law decay with exponent −1/2, in contrast to exponential decay, which is often assumed in femtosecond spectroscopy and unimolecular reaction theory. To elucidate the mechanism of the power-law decay, a diagrammatic method that visualizes the structure of the nonadiabatic reaction dynamics as a pattern of occurrence of dynamical events, such as wavepacket bifurcation, turning, and dissociation, is developed. Using this diagrammatic method, an analytical formula for the power-law decay is derived, and the theoretical decay curve is compared with the corresponding numerical decay curve computed by a wavepacket dynamics simulation in the case of lithium fluoride. This study reveals that the cause of the power-law decay is the quantum interference arising from the wavepacket bifurcation and merging due to nonadiabatic transitions. [ABSTRACT FROM AUTHOR]

The H-atom photodissociation reaction from the pyridinyl radical (C5H5NH) via the low-lying πσ* excited electronic state is investigated by nonadiabatic time-dependent quantum wave-packet dynamics calculations. A model comprising three electronic states and three nuclear coordinates has been constructed using ab initio multi-configurational self-consistent-field and multi-reference perturbation theory methods. Two conical intersections among the three lowest electronic states have been characterized in the framework of the linear vibronic-coupling model. Time-dependent wave-packet simulations have been performed using the multi-configuration time-dependent Hartree method. The population dynamics of the diabatic and adiabatic electronic states and the time-dependent dissociation behavior are analyzed for various vibrational initial conditions. The results provide detailed mechanistic insight into the photoinduced H-atom dissociation process from a hypervalent aromatic radical and show that an efficient dissociation reaction through two conical intersections is possible. [ABSTRACT FROM AUTHOR]

Traditional second-order kinetic theories fail to describe sub-picosecond photochemical reactions when solvation and vibrational dephasing undermine the assumption of equilibrium initial conditions. Four-wave mixing spectroscopies may reveal insights into such non-equilibrium processes but are limited by the single "population time" available in these types of experiments. Here, we use two-dimensional resonance Raman (2DRR) spectroscopy to expose correlations between coherent nuclear motions of the reactant and product in the photodissociation reaction of triiodide. It is shown that the transition of a nuclear wavepacket from the reactant (triiodide) to product (diiodide) states gives rise to a unique pattern of 2DRR resonances. Peaks associated with this coherent reaction mechanism are readily assigned, because they are isolated in particular quadrants of the 2DRR spectrum. A theoretical model in which the chemical reaction is treated as a vibronic coherence transfer transition from triiodide to diiodide reproduces the patterns of 2DRR resonances detected in experiments. These signal components reveal correlation between the nonequilibrium geometry of triiodide and the vibrational coherence frequency of diiodide. The 2DRR signatures of coherent reaction mechanisms established in this work may generalize to studies of ultrafast energy and charge transfer processes. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of H2O in its first absorption band is investigated on an accurate potential energy surface based on a large number of high-level ab initio points. Several ro-vibrational states of the parent molecule are considered. Different from most previous theoretical studies, the spin-orbit and ?-doublet populations of the open-shell OH fragment are reported from full-dimensional wave packet calculations. The populations of the two spin-orbit manifolds are in most cases close to the statistical limit, but the ?-doublet is dominated by the A ? component, thanks largely to the fast in-plane dissociation of H2O( A1A'' ). Comparisons with experimental data and a Franck-Condon model are generally very good, although some discrepancies exist. [ABSTRACT FROM AUTHOR]

The photodissociation of vibrationally excited Cl2(v = 1) has been investigated experimentally using the velocity mapped ion imaging technique. The experimental measurements presented here are compared with the results of time-dependent wavepacket calculations performed on a set of ab initio potential energy curves. The high level calculations allow prediction of all the dynamical information regarding the dissociation, including electronic polarization effects. Using a combination of theory and experiment it was found that there was negligible cooling of the vibrational degree of freedom of the parent molecule in the molecular beam. The results presented are compared with those following the photodissociation of Cl2(v = 0). Although the same electronic states are found to be important for Cl2(v = 1) as for Cl2(v = 0), significant differences were found regarding many of the observables. The overall level of agreement between theory and experiment was found to be reasonable and confirms previous assignments of the photodissociation mechanism. [ABSTRACT FROM AUTHOR]

The role of triplet states in the UV photodissociation of N2O is investigated by means of quantum mechanical wave packet calculations. Global potential energy surfaces are calculated for the lowest two 3A′ and the lowest two 3A′′ states at the multi-reference configuration interaction level of electronic structure theory using the augmented valence quadruple zeta atomic basis set. Because of extremely small transition dipole moments with the ground electronic state, excitation of the triplet states has only a marginal effect on the far red tail of the absorption cross section. The calculations do not show any hint of an increased absorption around 280 nm as claimed by early experimental studies. The peak observed in several electron energy loss spectra at 5.4 eV is unambiguously attributed to the lowest triplet state 13A′. Excitation of the 21A′ state and subsequent transition to the repulsive branch of the 23A′′ state at intermediate NN-O separations, promoted by spin-orbit coupling, is identified as the main pathway to the N2(1Σg+)+O(3P) triplet channel. The yield, determined in two-state wave packet calculations employing calculated spin-orbit matrix elements, is 0.002 as compared to 0.005 ± 0.002 measured by Nishida et al. [J. Phys. Chem. A 108, 2451 (2004)]. [ABSTRACT FROM AUTHOR]

The time-resolved photodissociation dynamics of CH3I in the A-band has been studied theoretically using a wave packet model including four degrees of freedom, namely the C-I dissociation coordinate, the I-CH3 bending mode, the CH3 umbrella mode, and the C-H symmetric stretch mode. Clocking times and final product state distributions of the different dissociation (nonadiabatic) channels yielding spin-orbit ground and excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch ν1 and umbrella ν2 modes) CH3 fragments have been obtained and compared with the results of femtosecond velocity map imaging experiments. The wave packet calculations are able to reproduce with very good agreement the experimental reaction times for the CH3(ν1, ν2)+I*(2P1/2) dissociation channels with ν1 = 0 and ν2 = 0,1,2, and also for the channel CH3(ν1 = 0, ν2 = 0)+I(2P3/2). However, the model fails to predict the experimental clocking times for the CH3(ν1, ν2)+I(2P3/2) channels with (ν1, ν2) = (0, 1), (0, 2), and (1, 0), that is, when the CH3 fragment produced along with spin-orbit ground state I atoms is vibrationally excited. These results are similar to those previously obtained with a three-dimensional wave packet model, whose validity is discussed in the light of the results of the four-dimensional treatment. Possible explanations for the disagreements found between theory and experiment are also discussed. [ABSTRACT FROM AUTHOR]

The energy partitioning in the UV photodissociation of N2O is investigated by means of quantum mechanical wave packet and classical trajectory calculations using recently calculated potential energy surfaces. Vibrational excitation of N2 is weak at the onset of the absorption spectrum, but becomes stronger with increasing photon energy. Since the NNO equilibrium angles in the ground and the excited state differ by about 70°, the molecule experiences an extraordinarily large torque during fragmentation producing N2 in very high rotational states. The vibrational and rotational distributions obtained from the quantum mechanical and the classical calculations agree remarkably well. The shape of the rotational distributions is semi-quantitatively explained by a two-dimensional version of the reflection principle. The calculated rotational distribution for excitation with λ = 204 nm and the translational energy distribution for 193 nm agree well with experimental results, except for the tails of the experimental distributions corresponding to excitation of the highest rotational states. Inclusion of nonadiabatic transitions from the excited to the ground electronic state at relatively large N2-O separations, studied by trajectory surface hopping, improves the agreement at high j. [ABSTRACT FROM AUTHOR]

A theoretical treatment of the photodissociation of the molecular ion Li2+ in one-color intense laser fields, using the time-dependent wave packet approach in a Floquet Born-Oppenheimer representation, is presented. Six electronic states 1,2 2Σg+, 1,2 2Σu+, 1 2Πg, and 1 2Πu are of relevance in this simulation and have been included. The dependences of the fragmental dissociation probabilities and kinetic energy release (KER) spectra on pulse width, peak intensity, polarization angle, wavelength, and initial vibrational level are analyzed to interpret the influence of control parameters of the external field. Three main dissociation channels, 1 2Σg+ (m = -1), 2 2Σg+ (m = -2), and 2 2Σu+ (m = -3), are seen to dominate the dissociation processes under a wide variety of laser conditions and give rise to well separated groups of KER features. Different dissociation mechanisms for the involved Floquet channels are discussed. [ABSTRACT FROM AUTHOR]

A laser scheme using a periodically changing frequency is used to induce oscillations of the internuclear motion, which are quantum analogs of classical vibrations in diatomic molecules. This is what we call the periodic laser adiabatic manipulation of the bond, or p-LAMB scheme. In p-LAMB, the carrier frequency of the laser must vary periodically from the blue to the red of a photodissociation band and backwards, following for instance a cosine-dependent frequency of period τc. In the adiabatic regime the dynamics is fully time-reversible. The amplitude of the internuclear oscillation is controlled by the pulse frequency ω(t), while τc determines the duration (or period) of the bond oscillation. In the presence of efficient dipole coupling, the bandwidth of the pulse is the main constraint to the maximum bond stretch that can be obtained. Before the onset of the adiabatic regime the dynamics are more complex, showing dispersion of the vibrational wave packet and anharmonic deformation of the bond. However, the nonadiabatic effects are mostly canceled and full revivals are observed at certain multiples of τc. [ABSTRACT FROM AUTHOR]

The photodissociation of N2O is studied by wave packet calculations using a global three-dimensional potential energy surface for the first excited 1A′ state. It is shown that the weak vibrational structures of the absorption cross section are caused by large-amplitude NN stretch motion, combined with strong excitation of the bend as well as the O–NN stretch. Weakening of the NN bond toward the N+NO channel is the necessary prerequisite. [ABSTRACT FROM AUTHOR]

The photodissociation of methyl iodide at different wavelengths in the red edge of the A-band (286–333 nm) has been studied using a combination of slice imaging and resonance enhanced multiphoton ionization detection of the methyl fragment in the vibrational ground state (ν=0). The kinetic energy distributions (KED) of the produced CH3(ν=0) fragments show a vibrational structure, both in the I(2P3/2) and I*(2P1/2) channels, due to the contribution to the overall process of initial vibrational excitation in the ν3(C–I) mode of the parent CH3I. The structures observed in the KEDs shift toward upper vibrational excited levels of CH3I when the photolysis wavelength is increased. The I(2P3/2)/I*(2P1/2) branching ratios, photofragment anisotropies, and the contribution of vibrational excitation of the parent CH3I are explained in terms of the contribution of the three excited surfaces involved in the photodissociation process, 3Q0, 1Q1, and 3Q1, as well as the probability of nonadiabatic curve crossing 1Q1←3Q0. The experimental results are compared with multisurface wave packet calculations carried out using the available ab initio potential energy surfaces, transition moments, and nonadiabatic couplings, employing a reduced dimensionality (pseudotriatomic) model. A general qualitative good agreement has been found between theory and experiment, the most important discrepancies being in the I(2P3/2)/[I(2P3/2)+I*(2P1/2)] branching ratios. Inaccuracies of the available potential energy surfaces are the main reason for the discrepancies. [ABSTRACT FROM AUTHOR]

We performed a theoretical study of the photodissociation dynamics of CH3Br in the à band using a wave packet propagation technique on coupled ab initio potential energy curves. The present model involves the 3Q1 and 1Q1 excited states which can be populated from the ground state by a perpendicular transition and which are correlated at large methyl-bromide distance to the ground bromide spin-orbit state, as well as the 3Q0 and 4E states which can be excited by a parallel and perpendicular transition (respectively) and both correlate to excited Br* spin-orbit state. The model provides absorption cross sections and branching ratios in excellent agreement with experimental results. Due to weak spin-orbit interaction, the 1Q1 state is the dominant contributor to the absorption cross section, except for the red wing of the band where 3Q0 and 3Q1 states have significant absorption. However, spin-orbit coupling is strong enough to induce nonadiabatic transitions between the 3Q1 and 1Q1 states during the dissociation process which should be experimentally detectable in the alignment properties of the fragments. Nonadiabatic transitions at the conical intersection between 3Q0 and 1Q1 are shown to play a minor role in this system. [ABSTRACT FROM AUTHOR]

We study the photodissociation of ICl- under moderately strong (TW/cm2) and short (below picosecond) laser pulses. Using a single resonant pump pulse, the photodissociation spectra shows two barely overlapping bands corresponding to Frank–Condon excitation and dissociation in two electronic states. By adding a nonresonant stronger control pulse we show that (1) the photodissociation bands can be blueshifted and (2) the asymptotic state of the fragments depends on the chosen pulse sequence. If the pump pulse precedes the control pulse or the control pulse straddles the pump pulse, the outgoing wave packet has components in the two dissociation channels, whereas if the control pulse precedes the pump pulse, the photodissociation proceeds selectively in a single channel. [ABSTRACT FROM AUTHOR]

The diabatic photodissociation and photoionization processes of the NaI molecule are studied theoretically using the quantum wave packet method. A pump laser pulse is used to prepare a dissociation wave packet that propagates through both the ionic channel (NaI→Na++I-) and the covalent channel (NaI→Na+I). A Stark pulse is used to control the diabatic dissociation dynamics and a probe pulse is employed to ionize the products from the two channels. Based on the first order nonresonant nonperturbative dynamic Stark effect, the dissociation probabilities and the branching ratio of the products from the two channels can be controlled. Moreover the final photoelectron kinetic energy distribution can also be affected by the Stark pulse. The influences of the delay time, intensity, frequency, and carrier-envelope phase of the Stark pulse on the dissociation and ionization dynamics of the NaI molecule are discussed in detail. [ABSTRACT FROM AUTHOR]

The real time photodissociation dynamics of CH3I from the A band has been studied experimentally and theoretically. Femtosecond pump-probe experiments in combination with velocity map imaging have been carried out to measure the reaction times (clocking) of the different (nonadiabatic) channels of this photodissociation reaction yielding ground and spin-orbit excited states of the I fragment and vibrationless and vibrationally excited (symmetric stretch and umbrella modes) CH3 fragments. The measured reaction times have been rationalized by means of a wave packet calculation on the available ab initio potential energy surfaces for the system using a reduced dimensionality model. A 40 fs delay time has been found experimentally between the channels yielding vibrationless CH3(ν=0) and I(2P3/2) and I*(2P1/2) that is well reproduced by the calculations. However, the observed reduction in delay time between the I and I* channels when the CH3 fragment appears with one or two quanta of vibrational excitation in the umbrella mode is not well accounted for by the theoretical model. [ABSTRACT FROM AUTHOR]

Selective population transfer in electronic states of dissociative molecular systems is illustrated by adopting a control scheme based on Stark-chirped rapid adiabatic passage (SCRAP). In contrast to the discrete N-level system, dynamical Stark shift is induced in a more complex manner in the molecular electronic states. Wavepacket dynamics on the light-induced potentials, which are determined by the detuning of the pump pulse, can be controlled by additional Stark pulse in the SCRAP scheme. Complete population transfer can be achieved by either lowering the energy barrier along the adiabatic passage or placing the initial wavepacket on a well-defined dressed state suitable for the control. The determination of the pulse sequence is sufficient for controlling population transfer to the target state. [ABSTRACT FROM AUTHOR]

The nonadiabatic photodissociation dynamics of (HI)2 is simulated by applying a wave packet approach which starts from the I*–HI complex (where I* denotes the I(2P1/2) excited electronic state) produced after the photodissociation of the first HI moiety within (HI)2. In the model, two excited electronic potential surfaces corresponding to I*–HI(A 1Π1) and I–HI(A 1Π1), which interact through spin-rotation coupling, are considered. The simulations show that upon photodissociation of HI within I*–HI, the dissociating H fragment undergoes intracluster collisions with the I* atom. Some of these collisional events induce an electronically nonadiabatic transition which causes the deactivation of I* to the I ground electronic state. The probability of such nonadiabatic process is found to be 0.37%. Most of the photodissociation process takes place in the upper excited electronic surface [that of the I*–HI(A 1Π1) complex], where H dissociation is found to be mainly direct or involving weak H/I* intracluster collisions. These weak collisions with high collisional angular momentum, and therefore high collisional impact parameters associated, are responsible for most of the probability of nonadiabatic transitions found. The type of H/I* collisions leading to nonadiabatic transitions appears to be closely related to the nature of the spin-rotation coupling between the two excited electronic states involved. [ABSTRACT FROM AUTHOR]

The photodissociation dynamics of a HX (X=Cl,F) molecule adsorbed on a hexagonal ice surface at T=0 K is studied using time-dependent quantum wave packets and quasiclassical trajectories. The relevant potential energy surfaces are calculated using high-level ab initio methods. We present here two dimensional calculations for the dynamics of the hydrogen photofragment for both HCl and HF molecules. The purpose of this paper is to compare the photodissociation dynamics of the two molecules which are adsorbed on the ice surface with different equilibrium geometries. The total photodissociation cross section and the angular distribution are calculated. The comparison with classical trajectory calculations provides evidence for typical quantum effects and reveals rainbow structures. [ABSTRACT FROM AUTHOR]

The sequential photodissociation dynamics of (HI)2 is studied by means of a nonadiabatic wave packet treatment starting from the I*-HI complex. The model reproduces the main experimental findings for photolysis with 266 nm radiation. The results confirm that some of the H atoms dissociated from the I*-HI complex deactivate the I* atom through a H/I* intracluster collision which induces an I*→I electronically nonadiabatic transition. As a consequence, these H fragments become very fast by acquiring nearly all the I* excitation energy, equivalent to the I*/I spin-orbit splitting. A most interesting result is the high production of bound I2 fragments in highly excited rovibrational states in the photolysis, indicating that the H dissociation is mainly direct. [ABSTRACT FROM AUTHOR]

Dissociations after the à 1B1→B 1A1 photoexcitation of SiH2, SiHD, and SiD2 were studied to investigate excited-state dynamics and effects of the initial vibrational state. The cross section (σ) for the photodissociation relative to SiH2(B)→Si(1D)+H2 and the rovibrational population of the H2 fragment were computed using the wave packet propagation technique based on the three-dimensional potential energy surfaces (PESs) of the à and B electronic states and the transition dipole surfaces, which were reported in our previous paper [J. Chem. Phys. 122, 144307 (2005)]. The photodissociation spectrum consists of a broadband and a number of sharp peaks. For SiH2 and SiD2, the sharp peaks correspond to the resonance structure of the vibrational levels of the B state and the broadbands are nearly independent of the photon energy. The broadband for SiHD increases steeply with the photon energy above 30 000 cm-1. The flux leaving the computational grid for SiH2 and SiD2 consists of at least two components, whereas that for SiHD consists of only a faster component. These large isotope effects were discussed based on the valley to the dissociation channel on PES and the difference in the position of the initial wave packet for three isotopomers. [ABSTRACT FROM AUTHOR]

We present a novel quantum-dynamics approach suitable for computing direct dissociation processes, including electronic transitions. This approach combines quantum trajectories in the Lagrangian reference frame with standard fixed-grid wave packets in order to overcome the limitations and difficulties of both techniques. As a model application, we consider the ultrafast photodissociation of H2 excited by a femtosecond extreme UV laser pulse. [ABSTRACT FROM AUTHOR]

Exclusively selective OD bond dissociation of HOD has been demonstrated by the ultraviolet photodissociation at 243.1 nm through the fourth overtone state of the OD stretching mode (5νOD). Branching ratio between the OH and OD bond dissociation channels has been determined by detecting H and D atoms, utilizing a (2+1) resonance-enhanced multiphoton ionization (REMPI) process. The OD bond dissociation has been solely observed with the branching ratio [uppercase_phi_synonym](D+OH)/[uppercase_phi_synonym](H+OD)>12, which has been determined by the detection limit for the H atom. Time-dependent wave-packet calculations reveal two important features for the highly selective OD bond dissociation: (1) strong local-mode character of the 5νOD state and (2) limitation of the total excitation energy lower than the saddle point between the OH and OD dissociation channels in the à state. Additionally, the recoil velocity and angular distribution of the nascent D atom are roughly evaluated by analyzing the Doppler-resolved REMPI spectrum. Based on these results, the dynamics of the selective OD dissociation has been discussed in detail. [ABSTRACT FROM AUTHOR]

Pump–probe measurements of I2 in solid Ar are reported and analyzed to extract a description of cage response to impulsive excitation, from the gentle kick, up to the breaking point. The most informative data are obtained through wavepacket motion on cage-bound, but otherwise dissociative, potentials where the chromophore acts as a transducer to drive the cage and to report on the local dynamics. This general class of dynamics is identified and analyzed as a function of energy in Ar, Kr, and Xe. The overdriven cage rebounds with a characteristic period of 1.2 ps that shows little dependence on excitation amplitude in all hosts. After rebound, the cage rings as a local resonant mode in Ar, with a period of 1 ps and dephasing time of 3 ps. This mode remains at the Debye edge in Kr and Xe, with periods of 630 and 800 fs, and dephasing times of 8 and 6 ps, respectively. In the bound B-state, the cage fluctuates toward its dilated equilibrium structure on a time scale of 3 ps, which is extracted from the down-chirp in the molecular vibrational frequency. When kicked with excess energy of 4 eV, the Ar cage breaks with 50% probability, and the molecule dissociates. The kinetics of polarization selective, multiphoton dissociation with Gaussian laser intensity profiles is delineated and the ballistics of cage breakout is described: The photodissociation proceeds by destruction of the local lattice, by creating interstitials and vacancies. During large amplitude motion on cage-bound potentials, sudden, nonadiabatic spin–flip transitions can be observed and quantified in space and time. The spin–flip occurs with unit probability in Ar when the I*–I bond is stretched beyond 6 Å. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The competitive photodissociation of bromoacetyl chloride BrCH[sub 2]COCl in the first [sup 1]A[sup ″] state (S[sub 1]) by 248 nm photons is investigated by nonadiabatic wave packet simulations. We show that the preferential breaking of the stronger C–Cl bond (α to the excited carbonyl) over the weaker C–Br bond (β) could be explained by a diabatic trapping or nonadiabatic recrossing as previously proposed. Our energy resolved flux analysis agrees fairly well with the experimental branching ratio (C–Cl:C–Br=1.0:0.4). Even if this does not prove the mechanism, this at least prevents to discard it. A reduced dimensionality approach based on constrained Hamiltonian is used. The nonadiabatic dissociation is studied in the two C–O/C–X (X=Br, Cl) subspaces to emphasize the role of the C–O vibration upon [n[sub O]→π[sub CO][sup *]] excitation. The internal torsion and wagging dihedral angles are frozen at their Franck–Condon value, according to preliminary dynamical tests. The other inactive coordinates are optimized at the trans and C[sub s] constrained geometry in the first excited state. Corresponding 2D cuts in the potential energy surfaces have been computed at the CASSCF level. The nonadiabatic kinetic couplings are highly peaked along an avoided crossing seam in both cases. A two-state diabatic model with a constant potential coupling is proposed in the two C–O/C–X subspaces. The inclusion of the C–O stretching in the active coordinates improves the value of the branching ratio over our previous 1D computation. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report polarization spectroscopy of oriented gas phase nitroaromatic compounds, specifically nitrobenzene and the three isomers of nitrotoluene. Orientation of supersonically cooled molecules was achieved in a uniform electric field. The dependence of the yield of NO on the polarization direction of the excitation laser was used to derive the direction of the transition dipole. The transition to the S[SUB2] state centered at 250 nm demonstrated increased yield when the excitation laser was polarized perpendicular to the orientation field. The transition dipole was therefore characterized to be near perpendicular to the permanent dipole. The transition to S[SUB3] showed an opposite behavior, indicating that the transition dipole for S[SUB3] is primarily parallel to the permanent dipole. Quantitative analysis of the polarization dependence further revealed that in both cases, the transition dipole is not strictly parallel or perpendicular to the permanent dipole, and the tilt is about 20°-30°. This result demonstrates the sophisticated nature of electronically excited states of nitroaromatics. It is therefore not surprising that only two previous theoretical calculations agree with our results, while other calculations and experimental work in condensed phases do not. [ABSTRACT FROM AUTHOR]

Reports on the simulation of ultraviolet photolysis of argon-hydrochloric acid by means of an exact wave packet treatment in three dimensions. Mechanism of indirect dissociation of the hydrogen atom which leads to a total fragmentation into hydrogen, argon and chlorine; Probability of the photodissociation path.

Fragmentation dynamics of the molecular ion Na+2 irradiated by an intense femtosecond pulse laser is studied using quantum wave packet propagations. It is demonstrated that the pulse duration (20–250 fs) can be used as a control parameter for both the total dissociation probability and the branching ratio between different dissociation channels. This pulse length effect is important when the duration of the pulse is shorter than the vibrational period of the molecular ion in the ground state. The effects of laser intensity (1011–3×1012 W/cm2), wavelength (680–780 nm) and initial vibrational level on the dissociation dynamics are also studied. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We calculate the alignment and orientation of the CN fragment of the photodissociation of ICN using a time-dependent method and the diabatic potentials and transition moments of Morokuma and co-workers [J. Chem. Phys. 100, 4894 (1994)]. The time-dependent Schrödinger equation is solved using a Chebyshev method evaluating sums required to do the matrix-vector products sequentially. To prevent the wave packet from drifting off the edge of our grid we propagate in successive steps. The ground state wave function, from which the molecule is dissociated, is computed using a three-dimensional variational method. We calculate alignment and orientation for a Ji=0→Jf=1 transition. We use parity-adapted angular basis functions. Our calculated alignment and orientation are qualitatively close to experimental results. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photodissociation of O3 in the Hartley band has been investigated by high-resolution photofragment translational spectroscopy (PTS). At λdiss=248 nm we determined the quantum yield of the dominant decay channel leading to O2(1Δg)+O(1D) and the fragment vibrational state distribution. The fragment recoil anisotropy (β=1.25±0.15) was found to be independent of the fragment vibrational states. Between λdiss=275 and 295 nm β assumes a value of 1.6±0.2, which exceeds the value expected for a simple impulsive process. Photofragment yield measurements carried out by PTS between 272 and 286 nm revealed a strong fluctuation of the vibrational state distribution with λdiss. Based on the small but distinct structure superimposed on the broad continuum of the Hartley band and the findings of recent 3D wave packet calculations, we propose this fluctuation, a manifestation of wavelength-dependent partial cross sections, to arise predominantly from an interference effect. The latter occurs between the part of the initially prepared wavepacket which propagates directly into the exit channel and a (minor) part which is temporarily trapped by the motion of the bending and symmetric stretching modes in the excited O3 molecule. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Rotational excitation and spatial alignment in moderate intensity radiation fields are studied numerically and analytically, using time-dependent quantum mechanics. Substantial rotational excitation is found under conditions typically used in time-resolved spectroscopy experiments. The broad rotational wave packet excited by the laser pulse is well defined in the conjugate angle space, peaking along the field polarization direction. Both the rotational excitation and the consequent spatial alignment can be controlled by the choice of field parameters. Fragment angular distributions following weak field photodissociation of the rotational wave packet are computed as a probe of the degree of alignment. In the limit of rapid photodissociation the angular distribution is peaked in the forward direction, reflecting the anisotropy of the aligned state. Potential applications of the effect demonstrated range from reaction dynamics of aligned molecules and laser-control to material deposition and laser-assisted isotope separation. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The fragmentation dynamics of the hydrogen molecular ion H2+ and of its isotopic derivate HD+subjected to an intense pulsed laser radiation are studied using quantum wave packet propagations. It is shown that bichromatic optical excitations are subject to a high degree of control through the variation of the relative phase between the two fields. A phase-locked (ω,2ω) laser pulse is used to induce asymmetry in the angular distribution of the emitted fragments. In addition, an appreciable isotope separation in the fragmentation of HD+ is predicted. The critical role of quantum molecular interferences in such phase-controllable processes is demonstrated. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The time-dependent self-consistent-field (TDSCF) approximation is used to study the photodissociation of the Ar–HCl cluster in a three-dimensional framework. The results are compared with numerically exact quantum calculations, and the properties and accuracy of the TDSCF approach are evaluated on this basis. The TDSCF approximation is used in Jacobi coordinates, and the total wave function is factorized into a wave packet for two coordinates associated with the H atom, and a wave packet for a single coordinate that describes the relative motion of the heavy particles. Quantitative agreement between the TDSCF and the exact results is found for most quantities calculated. The calculations show that photodissociation, and in particular the departure of the H atom is predominantly a direct process, but an appreciable amount of wave packet amplitude moving in excited state resonances is also found. This amplitude seems significantly larger than obtained in recent calculations by Schröder et al. [J. Chem. Phys. 100, 7239 (1994); Chem. Phys. Lett. 235, 316 (1995)]. The validity and computational efficiency of the TDSCF approach for realistic systems of this type is discussed. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In this work, we explore the applicability of a multiconfiguration version of the time-dependent Hartree approximation in photodissociation dynamics. A two-dimensional model system based on the A-band photodissociation of ICN is used. The propagation of the wave packet, which is expanded on grids, is carried out using pseudospectral methods and a predictor–corrector integrator. It is found that the quantum state resolved quantities, such as the final CN rotational distribution, require many more configurations than averaged quantities like the absorption spectrum. The calculated results from this work agree well with previous quantum calculations. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Advances in the time propagation of multidimensional wave packets are exploited to present the A-band photodissociation dynamics of methyl iodide for five active vibrational modes on the three relevant excited ab initio potential surfaces. The five modes considered represent all of the experimentally observed dynamical activity. The only modes neglected are the asymmetric C–H stretch and the asymmetric deformation of the methyl group. The kinetic energy operator corresponding to these five degrees of freedom is derived. The fully quantum mechanical calculation was implemented upon grids using 2880 distinct time-dependent configurations, determined by the multiconfigurational time-dependent Hartree algorithm, for each electronic state. All of the currently known experimental results regarding the umbrella vibration, symmetric C–H stretching vibration, perpendicular rotation, and parallel rotation of the photodissociated methyl radical fragment are well reproduced. The full wavelength dependence of all of these quantities is determined. The wavelength dependence of the energy deposited into translational, vibrational, and rotational motion is also given. The time evolution of the modes is presented in the context of correlated motion and its effect upon the dissociative process. Many of the details of the dynamics inherent to the conically intersecting nature of the excited surfaces is delineated. In particular it is shown that the Jahn–Teller distortion of the 1Q1 state is irrelevant in contributing to the perpendicular character of resonance Raman depolarization ratios. Results are compared and contrasted to previous calculations employing the collinear pseudotriatomic model with optimized empirical surfaces or the bent pseudotriatomic model with the same ab initio surfaces. [ABSTRACT FROM AUTHOR]

We report fully converged autocorrelation functions governing the photodissociation of ozone in the Hartley band, by an exact solution of the time-dependent Schrödinger equation. A local-mode representation employing hyperspherical coordinates has been used for describing the dynamics. Two different potential energy surfaces (PESs) have been employed in the present investigation, and the results showed sensitivity to the choice of the PES. Our converged calculation for J=0 on the Sheppard–Walker PES near quantitatively reproduced the characteristic recurrence features in the autocorrelation function obtained by Johnson and Kinsey from the experimental spectrum of Freeman et al. This is in contrast to previously reported three-dimensional calculations which showed a factor of 10 higher recurrence intensities compared to the experimentally derived one. The absorption spectrum, obtained as the Fourier transform of the autocorrelation function, is in excellent agreement with the experimental spectrum and exhibits the unique features seen on top of the experimental spectrum, which has caused considerable theoretical interest over recent years. Calculations on the Yamashita–Morokuma surface showed a factor of 5 higher intensity for the recurrence features, in agreement with existing three-dimensional calculations. The effect of J on the autocorrelation function has also been investigated by carrying out a calculation for J=10 and invoking a planar approximation. The J effect became noticeable only at a later time (t>=150 fs) of the dissociation process and its effect on the absorption spectrum was found to be only marginal. [ABSTRACT FROM AUTHOR]

The dynamics of the molecular transition state, in a reaction or photodissociation process, may be analyzed by semiclassical methods. We investigate the classical dynamics of the transition state in the dissociation HgI2 (X 1Σ+g)→hνHgI(X 2Σ+)+I, and apply the semiclassical quantization methods based on periodic-orbit theory. A series of resonances is characterized in a low-energy regime, where the classical dynamics is regular, and at high energies, after a transition to chaos has occurred. In a complementary fashion, we analyze the quantum-mechanical propagation of wave packets. The spectrum which is derived from the quantum-mechanical autocorrelation function is compared with the semiclassical results. [ABSTRACT FROM AUTHOR]

We present quantitative results on photodissociation of CH2 (X 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD. [ABSTRACT FROM AUTHOR]

Two-dimensional wave packet calculations on the photodissociation of FNO→F+NO in the S1 state were carried out to analyze the asymmetric resonance line shapes (Fano profiles) in the absorption spectrum in terms of a continuum, a resonance, and an interference contribution. Vibrationally resolved partial cross sections were calculated which predict a strong variation of the vibrational state distribution in the NO fragment as a function of the energy in the vicinity of a resonance. This effect is due to a substantial interference contribution to the vibronic absorption cross section. The results were compared to recent experimental findings by Reisler and co-workers and to the Fano model applied to predissociation. Further, we simulated the excitation with a semi-infinite laser pulse to extract resonance energies from the time behavior of the calculated S1←S0 transition rates. [ABSTRACT FROM AUTHOR]

Using femtosecond absorption spectroscopy, the photodissociation dynamics of HgI2 in solution were investigated. Upon photolysis, the HgI fragment is formed in a coherent superposition of its vibrational states. Solvent dependence of the wave packet motion is evidenced by the different results for ethanol and DMSO and between solution and gas phase. [ABSTRACT FROM AUTHOR]

Quantitative results on photodissociation of CH2(X 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three-dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two-dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three-dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment. [ABSTRACT FROM AUTHOR]

We investigate the photodissociation of CH3OH in the first absorption band (S0→S1) by a two dimensional wave packet study employing the associated internuclear potential energy surfaces obtained from ab initio calculations. The quantum chemical calculations are performed in the complete active space self-consistent field approach including the CH3O and OH bond distances with the CH3–O–H bond angle being fixed. The methyl group is considered as a structureless particle. The nuclear wave functions in the ground and in the excited electronic state are then calculated by using a novel expression for the kinetic energy operator in terms of bond coordinates for fixed bending angle and fixed total angular momentum J=0. The photodissociation of methanol is a very fast process and, in agreement with experiment, leads to a broad and structureless absorption spectrum. Dissociation of the vibrational ground state yields, also in qualitative agreement with experiment, exclusively the products CH3O+H. The main emphasis of the present study, however, is the influence of initial vibrational excitation in the electronic ground state on the fragmentation process. For example, the branching ratio for the chemical channels CH3+OH and CH3O+H is shown to exhibit a drastic dependence on the initial vibrational state of CH3OH(S0) as well as on the energy of the dissociating photon. Thus proper selection of the initial vibrational state and of the dissociating photon frequency can, in principle, be used to exert control of products to obtain branching ratios spanning several orders of magnitude. [ABSTRACT FROM AUTHOR]

The multiconfigurational time-dependent Hartree (MCTDH) approach is applied to an example showing very complex dynamics: the wave-packet dynamics on the three-dimensional B2 potential-energy surface of NO2. The ability of the MCTDH scheme to describe accurately the severe splitting of the wave packet on a saddle-shaped surface is demonstrated. Internal checks of the MCTDH calculation enable us to assess the degree of convergence without the need to resort to a numerically exact wave-packet calculation. As a representative observable the photodissociation spectrum is calculated and discussed. The A1/B2 vibronic coupling is neglected in our study, but the dynamics on the diabatic B2 surface is treated in its full three dimensionality. [ABSTRACT FROM AUTHOR]

The photodissociation of HCl in the cluster Ar...HCl by an extremely short pulse was studied using a hybrid quantum mechanical/classical approach. In this method, the H atom is treated quantum mechanically, the heavy atoms classically, and the time-dependent self-consistent-field (TDSCF) approximation is used to couple the quantum with the classical modes. The results are compared with those of classical trajectory calculations. On the whole, good qualitative agreement is found between the results of the (partly quantum) hybrid method and the pure classical ones. However, quantum interference effects of quantitative significance are found both in the angular and in the kinetic energy distribution of the H atom product. These effects, and resonances that contribute to the process, are analyzed in terms of wave packets obtained for the H atom in the hybrid method. The usefulness and applicability of the hybrid method are discussed in the light of the results. [ABSTRACT FROM AUTHOR]

In this paper, we analyze in detail the results of a quantum mechanical, time-dependent wave packet calculation of ICN photodissociation in the A continuum. Using the model potentials of Goldfield et al., the CN(X 2∑+) rotational final product distributions are calculated for all excitation frequencies. In a previous paper [J. Chem. Phys. 95, 1721 (1991)], we presented the details of the methodology of the calculation. The results are in good agreement with the time-independent close-coupling calculation by Guo and Schatz [J. Chem. Phys. 92, 2397 (1990)]. In the present paper, the connection between the final product state distribution and the wave packet reaction dynamics is discussed. In particular, the bimodal rotational distribution in the I(2P3/2) final products channel, with its superimposed oscillatory structure, is analyzed in detail. By focusing on the time evolution of the wave packet in k–j space, i.e., the conjugate space in the coordinate of I–CN relative translation and CN rotational motion, respectively, it is possible to determine completely the origin of these features. The bimodal distribution in the I(2P3/2) final products’ channel arises from the abrupt angle dependence of the coupling of the two surfaces. The high j distribution is enhanced relative to the low j distribution by selective recrossing of the low j amplitude to the I(2P1/2) surface at later times. The superimposed oscillations are the signature of a rotational rainbow which arises from the angular well of the bent potential surface. They are associated unambiguously with the high j portion of the distribution and are not due to the interference of the two modes of the ‘‘doorway state.’’ These observations allow for some speculation on the dynamical origin of the experimental distributions. [ABSTRACT FROM AUTHOR]

The wave packet dynamics of the photodissociation of H2+ under excitation by laser pulses of short durations at 329.7 nm are studied. The photodissociation process involves essentially two coupled channels, and the detailed mechanism for the formation of fragment kinetic energy spectra is examined by following the evolution of structures in the coupled-channel wave functions in momentum space. These structures appear in the channels’ momentum wave functions at P≠0, as the v=0 ground vibrational state is promoted to the dissociative channel then accelerated. The variations of these structures reflect the interplay between local laser-induced transitions and the accelerating–decelerating action of intrinsic molecular forces. The wave packet dynamics are studied for rectangular and Gaussian pulses of varying durations and peak intensities. In addition, two forms of channel couplings were considered corresponding to two different choices of the gauge: the electric-field (EF) gauge, in which the matter–field interaction is of the length form and the radiation-field (RF) gauge, in which it is of the velocity form. [ABSTRACT FROM AUTHOR]

The multiconfigurational time-dependent Hartree (MCTDH) approximation to the time-dependent Schrödinger equation is tested for a realistic three-dimensional example, the photodissociation of NOCl. The working equations of the MCTDH scheme introduced earlier are discussed in some detail. A computational scheme is presented which allows for efficient numerical MCTDH calculations. This scheme is applied to the photodissociation of NOCl after excitation to the S1 surface. The results are compared to the results of an exact wave-packet dynamics calculation. Fast convergence of the MCTDH results toward the exact one is found as the number of configurations is increased. The computation times of the MCTDH calculations are found to be much shorter than those of the exact calculation. Even MCTDH calculations including sufficiently many configurations for a fully converged (quasiexact) description require over two orders of magnitude less CPU time than an exact calculation. The so-called ‘‘natural populations’’ that are computed along with the MCTDH wave packet serve as a check of the degree of convergence without the need to resort to an exact (or more accurate) calculation. [ABSTRACT FROM AUTHOR]