Slow Predissociation Research Articles

Multiphoton ionization dissociation dynamics of iodoethane studied with velocity map imaging technique

Halogenated alkanes destroy the ozone layer, and iodoethane is one of the important representative halogenated alkanes. Time-of-flight mass spectrometry and velocity map imaging technique are used for investigating the photoionization dissociation dynamics of iodoethane, induced by 800 nm femtosecond laser. The dissociation mechanisms of iodoethane are obtained and discussed by analyzing the velocity distributions and angular distributions of the fragment ions generated in the dissociation. The measurements by time-of-flight mass spectrometry show that iodoethane cations generates C2H5+, I+, CH2I+, C2H2+, C2H3+ and C2H4+. The fragments related to CI bond fragmentation are C2H5+ ions and I+ ions, and the dissociation mechanisms are C2H5I+ C2H5++I and C2H5I+ C2H5+I+ respectively. Comparison between the configurations before and after ionization shows that the CI bond length is 0.2220 nm before ionization and turns longer and becomes 0.2329 nm after ionization. This indicates that the CI bond becomes more unstable after ionization and is more prone to dissociation. Moreover, the velocity map images of C2H5+ and I+ ions are acquired, from which the speed and angular distribution of C2H5+ and I+ are obtained. The analysis of speed distribution of the fragment ions shows that there are two channels, i.e. high energy channel and low energy channel in the dissociation process for producing C2H5+ and I+ ion. The difference between the ratios of the high energy channel and the low energy channel is small, indicating that the high energy channel and the low energy channel of the two dissociation processes are similar. According to the further analysis of the angular distribution of the fragment ions, it is found that the anisotropy parameter of C2H5+ is close to 0 (isotropic), the production channel of which may correspond to the slow vibration predissociation process. The anisotropy parameters of I+ ions are higher, which may be due to the rapid dissociation process on the repulsive potential energy surface. In addition, the density functional theory is used to calculate the configuration change of the iodoethane molecule before and after ionization, the energy level and oscillator strength for the ionic state in order to obtain more insights into the photodissociation dynamics.

Vacuum Ultraviolet Photodissociation Dynamics of Isocyanic Acid: The Hydrogen Elimination Channel

Photodissociation dynamics of the H-atom channel from HNCO photolysis between 124 and 137 nm have been studied using the H-atom Rydberg tagging time-of-flight technique. Product translational energy distributions and angular distributions have been determined. Two dissociation channels, H + NCO (X(2)Π) and H + NCO(A(2)Σ(+)), have been observed. The former channel involves two different dissociation pathways; one is a slow predissociation pathway through internal conversion from the excited state to the S0 state, and the other is a fast predissociation pathway through internal conversion from the excited state to the S1 state. The latter channel dominates by a prompt dissociation via coupling to the S2 state. As the photon energy increases, dissociation on the ground state S0 becomes dominant. Vibrational structures are observed in both the NCO(X) and NCO(A) channels, which can be assigned to the bending mode excitation with some stretching vibrational excitation.

Full-dimensional quantum state resolved predissociation dynamics of HCO 2 prepared by photodetaching HCO 2−

A full-dimensional quantum dynamics study of the photodetachment of HCO 2 − is reported using an exact wavepacket method. The calculated photodetachment spectrum identified several HCO 2 resonances observed in the experiment, but with quantitatively different intensities, indicating imperfection in the HCO 2 potential-energy surface. The narrow widths of the lowest resonances indicate slow tunneling-facilitated predissociation. The dissociation of these resonances yields CO 2 not only in its ground vibrational state, but also in excited ones, particularly at high energies. Both the symmetric stretching and bending modes, which are coupled via Fermi resonance, are found to be excited.

Photofragment angular momentum distribution beyond the axial-recoil approximation

The paper reviews recent advantages in the theory of the rapidly growing field of the angular momentum orientation and alignment in photolysis of small molecules. Particular emphasis is put on the photofragment orbital orientation and alignment in molecular photolysis beyond the axial-recoil approximation. One of the most important recent achievements is derivation of a universal expression for the recoil-angle dependence of the polarization cross section in terms of the anisotropy parameters (anisotropy transforming coefficients), which is valid for one photon photolysis reaction of any diatomic or polyatomic molecule irrespective of the reaction mechanism. The anisotropy parameters contain all information about the reaction dynamics in the frame of the quantum scattering theory, they can be either determined from experiment or calculated from ab initio theory. Expressions for the anisotropy parameters for the photolysis of rotating diatomic/linear molecules are analyzed, which contain information on radial and Coriolis nonadiabatic interactions as well as on the full range of interference effects. In the case of direct photodissociation, the molecular rotation is taken into consideration in the quasi-classical approximation by introducing the set of rotation factors, which depend on the molecular axis rotation angle γ and on the quantum numbers of the photon polarization matrix. In the case of slow predissociation, the molecular rotation is taken into consideration by introducing the set of reduction factors. As shown, in most cases molecular rotation decreases the angular momentum polarization of the photofragments.

The role of the Coriolis interaction on vector correlations in molecular predissociation: Excitation of isolated rotational lines

We present the full quantum mechanical expressions for the polarization differential cross sections of the photofragments produced in slow predissociation of a parent molecule via isolated rotational branches. Both radial and Coriolis nonadiabatic interactions between the molecular potential energy surfaces have been taken into account. The expressions describe the recoil angle distribution of the photofragments and the distributions of the photofragment angular momentum polarization (orientation and alignment) in terms of the anisotropy parameters of the ranks K=0,1,2. The explicit expressions for the anisotropy parameters are presented and analyzed which contain contributions from different possible photolysis mechanisms including incoherent, or coherent optical excitation of the parent molecule followed by the radial, or Coriolis nonadiabatic transitions to the dissociative states. The obtained expression for the zeroth-rank anisotropy parameter beta is valid for any molecule and for an arbitrary value of the molecular total angular momentum J. The expressions for the orientation (K=1) and alignment (K=2) anisotropy parameters are given in the high-J limit in terms of the generalized dynamical functions which were analyzed for the case of photolysis of linear/diatomic molecules. As shown, the Coriolis nonadiabatic interaction results in several new photolysis mechanisms which can play an important role in the predissociation dynamics.

The parity-adapted basis set in the formulation of the photofragment angular momentum polarization problem: The role of the Coriolis interaction

We present a theoretical framework for calculating the recoil-angle dependence of the photofragment angular momentum polarization taking into account both radial and Coriolis nonadiabatic interactions in the diatomic/linear photodissociating molecules. The parity-adapted representation of the total molecular wave function has been used throughout the paper. The obtained full quantum-mechanical expressions for the photofragment state multipoles have been simplified by using the semiclassical approximation in the high-J limit and then analyzed for the cases of direct photodissociation and slow predissociation in terms of the anisotropy parameters. In both cases, each anisotropy parameter can be presented as a linear combination of the generalized dynamical functions fK(q,q',q,q') of the rank K representing contribution from different dissociation mechanisms including possible radial and Coriolis nonadiabatic transitions, coherent effects, and the rotation of the recoil axis. In the absence of the Coriolis interactions, the obtained results are equivalent to the earlier published ones. The angle-recoil dependence of the photofragment state multipoles for an arbitrary photolysis reaction is derived. As shown, the polarization of the photofragments in the photolysis of a diatomic or a polyatomic molecule can be described in terms of the anisotropy parameters irrespective of the photodissociation mechanism.

Mass-Resolved Resonance Enhanced Ionization Study of Complicated Excited Electronic States of Rb2 near 430 nm and Their Predissociation Dynamics

We have investigated the Rb2 430 nm system by resonance enhanced two-photon ionization and photofragment yield spectroscopy. Four electronically excited states have been assigned, and their two-channel (fast and slow) predissociation has been observed. For the 3 1Piu, 3 1Sigmau+, 3 3Piu(0u+), and 3 3Piu(1u) states, electronic term values (Te) and vibrational constants (omegae and omegaexe) have been determined. For the predissociation dynamics, we have observed the fast predissociation threshold between the isotopically shifted 3 1Piu v' = 14 levels of 85Rb2 and 85Rb87Rb where the 4 2D5/2 + 5 2S1/2 atomic fine-structure limit is located. The 1 3Deltau state corresponding to the 4 2D5/2 + 5 2S1/2 atomic fine-structure limit has been assigned to the predissociating perturber responsible for the fast channel. Also, we have found that the fast channel branches out into two finer product channels due to long-range potential crossing.

Photofragment angular momentum distribution beyond the axial recoil approximation: Predissociation

We present the quantum mechanical expressions for the angular momentum distribution of the photofragments produced in slow predissociation. The paper is based on our recent theoretical treatment [J. Chem. Phys. 123, 034307 (2005)] of the recoil angle dependence of the photofragment multipole moments which explicitly treat the role of molecular axis rotation on the electronic angular momentum polarization of the fragments. The electronic wave function of the molecule was used in the adiabatic body frame representation. The rigorous expressions for the fragment state multipoles which have been explicitly derived from the scattering wave function formalism have been used for the case of slow predissociation where a molecule lives in the excited quasibound state much longer than a rotation period. Possible radial nonadiabatic interactions were taken into consideration. The optical excitation of a single rotational branch and the broadband incoherent excitation of all possible rotational branches have been analyzed in detail. The angular momentum polarization of the photofragments has been treated in the high-J limit. The polarization of the photofragment angular momenta predicted by the theory depends on photodissociation mechanism and can in many cases be significant.

Calculation of potential energy curves for Rb2 including relativistic effects

All-electron relativistic calculations have been performed on the Rb2 molecule. The molecular orbitals are optimized within a spin-free no-pair Hamiltonian formalism and spin-orbit coupling is treated using quasi-degenerate perturbation theory. Potential curves of the ground state and several excited states are calculated, and the spectroscopic constants T e, D e, R e and ωe are in good agreement with experimental values. The spin-orbit splittings at the 5p and 6p asymptotic limits are found to be underestimated by about 30%. Large perturbations in the spectra from the 11Σ+ u(A) state are predicted due to an avoided crossing with a 1 3Πub state caused by spin-orbit interaction. The predissociation dynamics of the 2 1ΠuC and 3 1ΠuD states is discussed. The calculations support the observation that a (1) 3 Δu state causes the fast predissociation of the 3 1ΠuD state but rule out the (2)3Σ+ u state as causing the slow predissociation at the lower part of the 3 1ΠuD potential energy curve.

Theoretical and experimental studies on the hydroquinone–argon 1:1 complex: A blueshifted O–H–Ar bond

Resonantly enhanced multiphoton ionization (REMPI), hole burning studies, as well as ab initio calculations on the hydroquinone–argon 1:1 complex, are presented in this paper. The REMPI spectrum displayed features due to the complex up to ∼1250 cm−1, which is unusually high for such complexes. The complex, like the monomer, was found to exist in both cis- and trans- conformational forms. Ab initio calculations carried out at the MP2/6-31+G* level for the S0 state of the complex yielded pi-bonded as well as H-bonded structures. The experimentally observed conformers are assigned pi-bonded structures. The high stability of the complex was attributed to slow vibrational predissociation rate constants due to the existence of multiple minima in the potential energy surface. Calculations on the S0 state of the H-bonded aminophenol–argon 1:1 complex are also reported and compared with the corresponding hydroquinone–argon structures. The H-bonded structures were found to possess blueshifted H bonds.

Photodissociation of NeBr2(B) below and above the dissociation limit of Br2(B)

The photodissociation dynamics of the NeBr2 complex in the B electronic state is studied, for the first time, near the Br2(B) dissociation limit, below and above, when the complex is promoted from the ground T-shaped level in the X electronic state. A time-dependent treatment is used in which the initial wave packet is divided in two portions, one describing the slow predissociation dynamics below the Br2 dissociation threshold, and the second one, the fast complete dissociation in Ne+Br+Br fragments. Below that threshold, the absorption spectrum shows an increasing congestion as the vibrational energy content of Br2 increases, but narrow peaks appear again for the highest energy region of the spectrum. These peaks correspond to long lived resonances associated with “horseshoe” type states, as demonstrated by two-dimensional calculations. These resonances have a significant probability density for the linear geometry in which the Ne atom is inserted between the two bromine atoms. At this configuration the exchange of vibrational energy is rather inefficient which explains both why the spectrum is so sparse and resonances are so narrow. Above the Br2 dissociation threshold, the recombination of Br2 is found to be very inefficient, except for very low kinetic energies. The small recombination probabilities are due to vibrational couplings and not to any collisional caging effect. Since the complex remains essentially T-shaped during dissociation, extensive two-dimensional calculations are performed for longer times to better determine final vibrational distributions at low kinetic energies.

Spectroscopy of van der Waals molecules: Isomers and vibrational predissociation

The inert-gas-halogen complexes have been studied for several decades by jet spectroscopy. Much of the seemingly bizarre behavior has become understandable in terms of two virtually isoenergetic isomer forms. The recently recognized linear isomer of Ar–I2 has a virtually continuous B ¬ X excitation spectrum. It also undergoes a very rapid vibrational predissociation, and suffers no electronic quenching from the B state. The well-known T-shaped isomer shows slow vibrational predissociation, which is competitive with electronic quenching. The quenching distorts the vibrational distribution of the I2 B state photofragments, consequently leading to a false estimation of the T-shaped Ar–I2 (B) state dissociation energy. The binding energies for the T-shaped Ar–I2 (X) and Ar–I2 (B) are unambiguously determined from the recent dispersed fluorescence study, which are also in good accord with the ab initio calculation. We discuss aspects of pure vibrational laser-induced fluorescence of hydrogen fluoride complexes. We contrast the behavior of Ar–HF with Ne–HF and present new results for the vHF = 3 level of Ne–HF. PACS Nos.: 33.80Gj, 34.30th

Adiabatic and diabatic potential-energy surfaces of the CN(X 2Σ+,A 2Π)Ne complex and nonadiabatic predissociation dynamics

Adiabatic and diabatic potential-energy surfaces (PES’s) for the interaction of Ne(1S0) with the CN molecule in its ground (X 2Σ+) and first excited (A 2Π) electronic states were determined using multireference configuration-interaction calculations. The three electronically adiabatic potential-energy surfaces (PES’s, 2 A′ and 1 A″) are transformed to three diabatic PES’s plus one additional PES which describes the coupling between the two diabatic PES’s of A′ symmetry which correlate asymptotically with the X 2Σ+ and A 2Π states of CN. These four diabatic PES’s are then used in the study of the predissociation of the CN(A 2Π)Ne complex, subsequent to electronic excitation of the ground-state complex [CN(X)Ne]. Two channels: Fast spin–orbit predissociation [CN(A 2Π1/2,v=3)Ne→CN(A 2Π3/2,v=3)+Ne] and slow electronic predissociation [CN(A 2Π3/2,v=3)Ne→CN(X 2Σ+,v=7)+Ne] are investigated. The quantum flux method was used to determine the positions and widths of the metastable levels of the complex. For these predissociation processes, comparison is made with the experimental investigation by Fei, Lawrence, Chen, and Heaven.

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.

Coherence effects in the polarization of photofragments

The general quantum-mechanical treatment of coherence effects in the polarization of the fluorescence, as well as in the orientation of photofragments, is reviewed. They are the result of simultaneous excitation of dissociative electronic states with different symmetry correlating to the same limit. The coherent excitation produces interferences which depend both on the photoabsorption probability amplitudes as well as on the relative phases of the continuum vibrational wavefunctions at large distances. Oscillations in the polarization degree of photofragments fluorescence as a function of the photon energy are predicted. Examples are presented in the case of direct dissociation of diatomic molecules. The case of slow predissociation is also analysed. Finally, possible extensions to the photodissociation of polyatomic molecules are considered.

Fine-structure electronic predissociation in van der Waals molecules. I. Theory

A quantum-mechanical model for fine-structure predissociation in van der Waals molecules is presented. The process corresponds to: A(2S+1LJA )⋅⋅⋅M→A(2S+1LJ′A) +M where A is an atom and M an atom or a diatomic molecule A(2S+1LJA) ⋅⋅⋅M represents the van der Waals complex in a state which correlates to the 2S+1LJA electronic manifold of A. The general formalism based on close-coupling equations is presented first. The limiting case of slow predissociation is then considered and treated by perturbation theory. Finally, the application to the atom–atom and the atom–diatom van der Waals complexes for L=1 and S=1/2 or S=1 is discussed. This simple analysis allows predictions to be made for predissociation rates and lifetimes in cases where the spin–orbit interaction is larger than the van der Waals binding energy.

Dissociative decay of n=3 levels in H2. II. Excited by laser from the metastable c 3 Pi u- state of H2.

Fast-beam photofragment spectroscopy has been applied to study the photodissociation of metastable H/sub 2/ molecules in the c /sup 3/Pi/sub u//sup -/ state, formed by charge exchange of H/sub 2/ /sup +/ with Cs vapor. Transitions leading to radiatively dissociated or predissociated levels of the g, h, i, and j triplet gerade states of H/sub 2/ are observed. The decay mechanisms, level positions, and, in the case of predissociations, the lifetimes are established for (ro-)vibrational levels (vgreater than or equal to4). The transitions to vgreater than or equal to5 levels of the j /sup 3/..delta../sub g/ state appear as asymmetric peaks with widths of around 50 cm/sup -1/ (N = 2). A calculation of the rotational coupling of the j state to the vibrational continuum of the i /sup 3/Pi/sub g/ state confirms this very rapid predissociation (tauapprox. =10/sup -13/ s). For v = 4 of the j state, which couples to the continuum via the quasibound v = 5 level of the i state, a relatively slow predissociation (tauapprox. =10/sup -8/ s) is observed.

Measurement of the fragment kinetic energy distribution in laser photopredissociation of N 2O +

Using an electrostatic energy amlyzer together with a fast-ion-beam laser spectrometer, we have measured the kinetic energy spectrum of NO + ions produced in the slow predissociation of N 2O + prepared in specific vibrational and rotational levels of the à excited electronic state. For either the à 2 (100) or à 2Π(b) (110) states the resulting distribution is remarkably narrow, with almost all of the NO + ions produced in the ν″ = 4 vibrational level.

High-resolution spectroscopy of N 2O + in the near ultraviolet using fiblas (fast-ion-beam laser spectroscopy)

Fast-ion-beam laser spectroscopy (FIBLAS) is applied to the study of the first excited vibrational level of the Ā 2Σ + state of N 2O +. Detection of the molecular photofragments resulting from a slow predissociation of the excited state leads to high spectral resolution of the molecular fine and hyperfine structure. The large size of the hyperfine interaction is attributed to a strong contribution of the 2s atomic orbital of the outer nitrogen nucleus to the unpaired electron molecular orbital. Preliminary results concerning a new band identified as (110) 2Π(b)-(010) 2Σ (−) are also given. They suggest a correction of previously known values for both the ω 2 vibrational quantum and the Renner—Teller constant ε of the ground state X̄ 2 ???. The N 2 + example clearly demonstrates the power of an experimental approach which combines high-resolution laser spectroscopy and tandem mass analysis of both parent and photoproduct molecular-ion beams.

Lifetime studies of NO(A 2Σ +, υ′ = 4), NO(B 2Π, υ′ = 9) and NO(D 2 Σ +, υ′ = 0)

Lifetimes of NO(A 2Σ +, υ′=4), NO(B 2Π, υ′ = 9) and NO(D 2Σ +, υ = 0) have been studied using a single photon counting techniques combined with nanosecond flash excitation. The zero-pressure lifetimes and quenching rate constants have been obtained. These values and our earlier reports show that both NO(B 2Π, υ′ = 9) and NO(C 2Σ, υ′ = 0) predissociate with large quantum yields of 0.9 or more. On the contrary, NO(A 2Σ +, υ′ = 4) and NO(D 2Σ +, υ′ = 0) undergo very slow predissociation, though these levels are more energetic than NO(C 2Π, υ′ = 0). Possible dissociation mechanisms are discussed.