Predissociation Process Research Articles

Electronic excitation dynamics of Li(H2)2: Dissociation mechanisms, lifetimes, and the validity of a hybrid quantum/classical approach

The dissociation dynamics of the cluster Li(H2)2, following the 2s→2p excitation of the Li atom, is studied in the framework of a collinear model. The process was investigated by exact quantum wave packet calculations, and the results were used to test a hybrid quantum/classical method, in which the highly quantum mechanical initial state of the cluster is described by a wave function, and the latter is used to sample initial positions and momenta for a classical treatment of the excited state dynamics. We found that the dynamics was dominated by two predissociation processes, Li*(H2)2→Li*–H2+H2 and Li*(H2)2→Li*+(H2)2, with the latter process having a higher yield. A relatively long dissociation lifetime, ∼10 ps, was found for the excited cluster. The slow vibrational predissociation rate was interpreted as due to the very low density of state involved. The hybrid quantum/classical approach was found to give product vibrational energy and velocity distributions in good accord with the distribution from exact calculation. However, the lifetimes from the hybrid approach were found to be much shorter than those from the exact quantum calculations. The hybrid approach is thus applicable even to photoexcitation of quantum clusters for studying certain selected properties.

E. Spectroscopy and Dynamics of Clusters and Nonrigid Molecules: Experiment and Theory. State‐Selective Analysis of Ground‐State Vibrational Predissociation Product of an Aromatic van der Waals Complex

AbstractThe method previously presented for mass‐selective ground‐state vibrational spectroscopy of aromatic van der Waals complexes by populating ground‐state levels via a pump/dump laser pulse sequence, followed by selective resonant two‐photon ionization (R2PI) of the vibrationally relaxed complexes [T. Bürgi et al., Chem. Phys. Lett., 225, 351 (1994)] is here extended to the detection of the aromatic molecular product from the S0 state vibrational predissociation (VP) process. Schemes and results are presented which allow (a) extended measurements of intra‐and intermolecular vibrational frequencies of carbazole·Ar in the S0 state, (b) detection of vibrationally hot, but rotationally cold carbazole VP product, and (c) vibrationally and rotationally hot carbazole VP products. A much improved determination of the van der Waals dissociation energy was made by combining results from various experiments, as D0(S0) = 530±1.5 cm−1.

The fragmentation of SH(A2?+): Ab initio calculations of spin-orbit and coriolis interactions

The electronic structure aspects of the fragmentation of the A2Σ+ state of SH via the predissociation and photodissociation processes, SH(A2Σ+, v, N, F1/F2) S(3PJ) + H(2S) and SH(X2Π, v, N, e/f) SH(A2Σ+, v, N, F1/F2) S(3PJ) + H(2S) are considered by ab initio configuration interaction (CI) calculations. All the relevant interstates A2Σ+, X2Π, 14Σ−, 12Σ−, and 14Π spin-orbit induced matrix elements, treated within the full microscopic Breit-Pauli approximation, and Coriolis (electronic orbital angular momentum) couplings are determined based on wavefunctions comprised of 680000-860000 configuration state functions. © 1995 John Wiley & Sons, Inc.

Spin–orbit predissociation dynamics of NeOH (X2Π)

A joint theoretical–experimental study has been carried out to examine the unimolecular dissociation dynamics of NeOH (X 2Π) complexes containing spin–orbit excited OH radicals. The spin–orbit predissociation process is a direct manifestation of the difference in the two potential-energy surfaces (PESs) required to describe the interaction of Ne (1S0) with the OH radical in its ground (X 2Π) electronic state. The theoretical investigation involved the determination of the ab initio PESs as well as the energies and widths of the NeOH resonances correlating with the spin–orbit excited state of OH (X 2Π1/2). In addition, the redistribution of predissociation flux is mapped out as a function of the fragment separation using the quantum flux method. Experimentally, the NeOH resonances derived from the spin–orbit excited state of OH have been observed by stimulated emission pumping (SEP). The energies and linewidths of the predissociative resonances are extracted from the SEP spectra and compared with theoretical predictions. A dramatic increase in the spin–orbit predissociation lifetime is observed upon intermolecular stretching excitation which principally reflects the reduced splitting between the two PESs with increasing intermolecular separation. Comparison of the experimentally measured and theoretically calculated observables provides a stringent test of the ab initio PESs and insight into the photodissociation mechanism.

Photodissociation and predissociation processes in OH: a time-dependent quantum mechanical study

Photodissociation from the ground vibrational and ground electronic state (X 2Π) and spin—orbit induced predissociation from the A 2∑ + state of OH have been studied using the time-dependent quantum mechanical approach. Photodissociation cross sections computed thus are in accord with the time-independent quantal results and are comparable to the experimental results. Predissociation linewidths (Γ) computed using the wavepacket golden rule treatment corresponding to v′ = 1−7 of A 2∑ + state are also in agreement with most of the earlier theoretical results. For v′ = 1−3, there is a general agreement between experiment and theory but for v′ = 4, the difference is substantial. For v′ = 5−7, there are no experimental results available. The results obtained from our calculations indicate that the inclusion of R-dependence of the spin-orbit coupling plays an important role in deciding the v′ dependence of Γ.

Rotational analysis of the SiD2 Ã 1B1–X̃ 1A1 transition observed in a jet

The SiD2 radical was produced by ArF laser photolysis of C6H5SiD3 in a free-jet expansion, and the laser-induced fluorescence (LIF) excitation spectrum of the à 1B1–X̃ 1A1 transition of SiD2 was measured. The LIF excitation spectra of the five vibronic bands, (0,v2′,0)–(0,v2″,0), v2′–v2″= 0–0, 1–0, 2–0, 1–1, and 2–2, were obtained using a narrow-band dye laser with an intracavity étalon, the resolution of which attained to ∼0.03 cm−1. The rotational structures of the vibronic bands were well analyzed by a Hamiltonian including fourth-order terms, and the molecular constants were determined for the vibronic levels, v2=0, 1, and 2, of the à 1B1 and X̃ 1A1 states. By comparing the observed rotational line intensities with simulated ones, we found two kinds of intensity anomalies depending on the rotational quantum numbers J and Ka. We conclude that both the anomalies are caused by a predissociation process to the dissociation continuum, Si(3P)+D2, which was proposed in our previous paper [J. Chem Phys. 96, 44 (1992)]. The Ka dependent anomaly was explained by the interaction terms in the Fermi Golden Rule expression for the predissociation process, and the J dependence was interpreted by the final-state density.

Structural kinetics by stroboscopic gas electron diffraction 2. Time-dependent molecular intensities of predissociation processes

Various attempts have been reported in the literature [1–3] to develop stroboscopic gas electron diffraction (GED) for time-resolved studies of dynamic phenomena in laser-excited molecular species. This novel applications of GED requires the formation of new expressions for the scattering intensities, since conventional equations for GED data analysis are based on models that are restricted to equilibrium ensembles and molecules exhibiting small amplitude motions. As a first example, the time-dependent molecular intensities for species in adiabatic dissociative states have recently been reported [4]. In the current study the molecular intensity expression for the evolution of internuclear distances in predissociation processes on a diabatic potential energy surface is presented and some model radial distribution curves are shown. The structural kinetics of the photodissociation of NaI are modeled on a femtosecond time-scale. The time-dependent molecular intensities of NaI are directly parameterized in terms of the intramolecular potential energy surfaces and other parameters which characterize the transient state dynamics, as taken from experimental spectroscopic investigations reported in the literature. The acute dependence of the molecular intensities and radial distribution curves on the PES shows that solution of the inverse problem, i.e. determination of potential parameters from time-dependent electron diffraction data, is feasible in principle. As in the previous model study of adiabatic dissociative processes, it is found that details of the dynamics of photo-predissociation can be resolved on a time-scale shorter than that of the probing electron pulse. Furthermore, our analysis demonstrates the fundamental difference between the observables obtained from spectroscopy and electron diffraction: whereas no explicit structural information on the time-evolution of the nuclear configuration is obtained from spectroscopy, the molecular intensities of scattered electrons depend directly on the structural kinetics; i.e. the time-evolution of the internuclear distances.

Ab initio study of the predissociation processes of the ground and first excited states of N 2O +. Potential curves of the lowest states of N 2O +

The photoionisation dissociation study of the ground and first excited states of N 2O + has been carried out. The potential curves of different states of N 2O + and their spin-orbit coupling have been calculated. Predissociation phenomena are shown to be involved. The ground state X̃ 2Π is predissociated by a 2 4Σ − state; the coupling near the curve cross-over is about 60 cm −1. The first excited state à 2Σ + is predissociated (towards NO + + N dissociation) by a 1 4Π state on the one hand and by a 2 4Σ − state on the other hand. The couplings in the neighbourhood of the cross-over are weak (5 cm −1), due to the multiconfigurational wavefunctions of the states.

Comment: “Dynamics of the B and a 3Π(1u) States of Chlorine by Laser Induced Fluorescence”

The paper by Martinez et al.1 describes their study of the decay of the Cl2 B state by radiative and predissociation processes. The results of this work and the conclusions of their analysis are in strong disagreement with our results.2,3 We want to clarify these uncertainties in the present comment.

Ultrafast reaction dynamics of electronically excited à state of ammonia clusters

Femtosecond pump–probe techniques combined with a reflectron time-of-flight mass spectrometer are utilized to study the ultrafast reaction dynamics of the electronically excited à state of ammonia clusters. All of the detected protonated cluster ions, (NH3)nH+ n=2−6, are observed to display two distinct features with respect to preselected pump–probe time delays; a fast decay, followed by a persistent ion signal leveling off to a finite nonzero value. The fast decay is attributed to a predissociation process; while an intracluster reaction, which leads to formation of long-lived intermediates (NH3)nNH4, is responsible for the nonzero falling off regime. The results provide conclusive experimental evidence that both an absorption–ionization–dissociation mechanism and an absorption–dissociation–ionization mechanism are operative in the à state of ammonia clusters.

State specific photodissociation of SO 2 and state selective detection of the SO fragment

In a pulsed molecular beam experiment, cold SO 2 molecules were generated in a seeded beam. Using the REMPI-TOF technique, detailed investigation of the predissociation process SO 2(C̃ 1B 2) → SO(X 3Σ −) + O( 3P) is possible. First experimental results show that predissociation from specified rovibrational levels of the parent molecule SO 2 and rovibrational detection of the SO fragment can be achieved. Rotational analysis of the vibrational band (142)—(000) of the C̃–X̃ system indicates the onset of predissociation in this band at the rotational quantum number J = 6, while for J ⩽ 5 no predissociation was observed.

Rotational state dependence of decay dynamics in the superexcited 7f Rydberg state (υ=1) of NO

The decay dynamics of the 7f Rydberg state (υ=1) of NO has been investigated with laser multiphoton excitation methods; not only NO+ ions generated by autoionization, but also fragment nitrogen atoms produced by predissociation have been directly probed. The fragment atoms have been found to populate both the 2p3 2DJ and 2p3 4S3/2 states. Population yield in the 4S state shows strong dependence on each 7f rotational level of NO, and this dependence is remarkably correlated with autoionization yield, while no such significant dependence exists for the 2D state. From the fact that only odd ℒ levels generate the 4S state, dissociative states causing this predissociation process have been identified as Σ+ states. Through detailed analysis, it has been shown that the predissociation rate of this channel is much larger than the autoionization rate, and that the N(2D)-generating predissociation is also mainly caused by dissociative Σ+ states. Moreover, it has been found that a major part of the total decay rate of each rotational level is strongly correlated with magnitude of its fσ character. From these facts, it has been concluded that the decay process in the 7f state is mainly governed by predissociation due to direct coupling with dissociative Σ+ valence states, which have been identified as A′ 2Σ+ and I 2Σ+ for the N(4S)- and N(2D)-generating predissociation channels, respectively.

Vibrational predissociation of the Ar⋅⋅⋅Cl2 van der Waals complex: The small molecule limit for intramolecular vibrational redistribution

A converged three-dimensional quantum treatment of vibrational predissociation in the Ar⋅⋅⋅Cl2(BΠ0u+3,υ′) van der Waals complex is presented. The potential energy surface used is a sum of pairwise Morse atom–atom interactions adjusted asymptotically to a C6/R6+C8/R8 anisotropic van der Waals form. Calculations have been performed in the energy region of Ar⋅⋅⋅Cl2(B,υ′=6, 10, and 11) excited levels. In agreement with the experimental findings, the final rotational distribution of Cl2 is found to be strongly dependent on the initial υ′ state being excited, as well as on the number of vibrational quanta lost in the vibrational predissociation process. The role of intramolecular vibrational redistribution for υ′=10 and 11 for which the Δυ=−1 channel is closed is also studied. It is found that the vibrational predissociation (VP) dynamics are dominated by the coupling of the zero-order ‘‘bright’’ state with a single ‘‘dark’’ state from the υ′−1 manifold of van der Waals vibrationally excited states which then decays to the continuum, and that the product state distribution is determined by the dissociation of the dark state. This is characteristic of the sparse limit for intramolecular vibrational redistribution. It also implies that the dissociation rate is not governed by a simple function of the initial quantum numbers such as the one given by the energy gap law. The golden rule approximation gives surprisingly accurate results for Ar⋅⋅⋅Cl2 dynamics. This will be very useful for fitting a potential energy surface to experimental results.

Wave packet dynamics of internal rotational predissociation in neon-hydrogen fluoride and neon-deuterium fluoride

We investigate the process of internal rotational predissociation in the NeHF and NeDF van der Waals complexes using time-dependent quantum mechanical calculations. The computations are performed in three degrees of freedom and for arbitrary values of the total angular momentum; they use a combination of the Fourier wave packet propagation method in the dissociative coordinate with a body-fixed basis set expansion in the intramolecular angular coordinates. Calculated values of the predissociation lifetimes of several states of these complexes are found to be in excellent agreement with the results of time-independent quantum calculations

Vibrational predissociation in D2HF

Calculations are presented of linewidths for the vibrational predissociation of D2HF induced by excitation of the HF stretching vibration in the weakly bound molecule. A potential energy surface based on ab initio computations with the correct leading multipole terms has been used. The vibrational predissociation linewidths are obtained from accurate close-coupling computations of scattering resonances for the D2+HF collision. The calculated linewidths are in excellent agreement with those obtained from infrared absorption experiments and confirm that the mechanism for vibrational predissociation is D2HF(v=1) →D2(v1=1) +HF(v2=0). It is found that the vibrational predissociation linewidths associated with the Σ vibration in para-D2HF are about five times narrower than those for the Π vibration. This implies that it should be possible to observe the D2HF (v=0, Π→v=1,Σ) transition in infrared spectroscopy measurements. The rotational distributions of the D2+HF products of the vibrational predissociation process are predicted and should be observable.

Detection of nitrogen atoms produced by predissociation of superexcited Rydberg states of NO

Predissociation processes of superexcited Rydberg states (υ=1, n=8–17, l=s, p and f) of the NO molecule have been investigated by detecting the fragment nitrogen atom with a resonance-enhanced multiphoton ionization (REMPI) technique. As a result, the N( 2D) production predicted by theoretical calculations has been confirmed. Moreover, the unexpected generation of N( 4S) has also been observed. Yields of the fragment atoms are strongly dependent on both the principal and angular momentum quantum numbers of the Rydberg electron.

Predissociation of H3 near its ionization threshold induced by very weak electric fields.

We report the discovery of predissociation channels in H{sub 3} at energies within a few meV of the lowest ionization threshold. The predissociation affects selected {ital p}- and {ital d}-Rydberg states with high principal quantum numbers. For the {ital d} states, the predissociation process is induced by very weak electric fields, of order less than 1 V/cm. The predissociations are due to interaction with vibrationally excited states of low principal quantum number that exhibit significant coupling to the repulsive ground-state surface of H{sub 3}, 2{ital p}{sup 2}{ital E}{prime}. We discuss the possible significance of these predissociation channels for decay paths in dissociative recombination of H{sub 3}{sup +} with low-energy electrons.

Photoabsorption and photoionization cross sections of NH 3, PH 3, H 2S, C 2H 2, and C 2H 4 in the VUV region

Using synchrotron radiation as a continuum light source, the photoabsorption and photoionization cross sections of NH 3, PH 3, H 2S, C 2H 2, and C 2H 4 have been measured from their respective ionization thresholds to 1060 Å. The vibrational constants associated with the v 2 totally symmetric, out-of-plane bending vibration of the ground electronic state X̃ 2 A 1 of PH + 3 have been obtained. The cross sections and quantum yields for producing neutral products through photoexcitation of these molecules in the given spectral regions have also been determined. In the present work, autoionization processes were found to be less important than dissociation and predissociation processes in NH 3, Ph 3, and C 2H 4. Several experimental techniques have been employed in order to examine the various possible systematic errors critically.

A theoretical study of the vibrational predissociation of (NO) 2

A theoretical study of infrared photodissociation of (NO) 2 is reported assuming that potential coupling induces the vibrational predissociation (VP) process. It has been observed in literature that predissociation lifetime of vibrationally excited (NO) 2 shows distinctive vibrational mode specificity. We have calculated lifetimes and linewidths of the VP process. Our calculated results of lifetimes are 1.10 ns for symmetric NO stretching, and 56.3 ps for antisymmetric NO stretching. These values agree well with the corresponding experimental values (0.88 ns and 39 ps, respectively).

Photodissociation of CO−3: Product kinetic energy measurements as a probe of excited state potential surfaces and dissociation dynamics

The photodissociation process CO−3 +hν→O−+CO2 has been investigated at photon energies of 2.41, 2.50, 2.54, 2.60, and 2.71 eV. Experiments were conducted by crossing a mass-selected, 8 keV ion beam with a linearly polarized laser beam, and measuring the kinetic energy distributions of the charged photodissociation products. By varying the angle between the ion beam and laser polarization, angular distributions were obtained at photon energies of 2.41 and 2.54 eV. The photon energy dependence of the average photofragment kinetic energies shows conclusively that photodissociation at these photon energies does not proceed by a direct dissociation process on a repulsive potential surface, or by a statistical vibrational predissociation process on a bound surface. The photofragment angular distributions are isotropic, providing further evidence that precludes direct photodissociation on a repulsive potential surface. Ab initio calculations were performed using the gaussian86 programs. These calculations indicate that ground state CO−3 has a planar D3h geometry, and 2A′2 electronic symmetry. This ground state correlates adiabatically to the CO−2 +O dissociation asymptote, not the lower energy O−+CO2 asymptote. Taken together, these new experimental and theoretical results suggest that the photodissociation of CO−3 at these energies occurs via the interaction of bound and repulsive excited state potential surfaces. A new model of the potential surfaces of CO−3 is proposed.