Vibrational Predissociation Dynamics Research Articles

Probing the Dependence of Long-Range, Four-Atom Interactions on Intermolecular Orientation. 4. The Dissociation Dynamics of H2/D2···ICl(B,v'=3) and the Observation of Efficient Vibrational-Rotational Energy Transfer.

The vibrational predissociation dynamics of H2/D2···I35Cl(B,v'=3) complexes containing both para- and ortho-hydrogen prepared in different intermolecular vibrational levels were investigated. The Δv = -1 I35Cl(B,v = 2,j) rotational product-state distributions measured for excitation to the lowest-energy T-shaped levels of these complexes are mostly bimodal. The rotational distributions measured for excitation of the H2···I35Cl(B,v'=3) complexes are colder than those of the D2···I35Cl(B,v'=3) complexes, and there are only slight differences between those measured for the para- and ortho-hydrogen containing complexes. Excitation of the delocalized bending levels results in slightly colder rotational product-state distributions. The distributions suggest the dynamics result from more than impulsive dissociation off of the inner repulsive wall of the lower-energy H2/D2 + I35Cl(B,v = 2) potential surfaces of the products. The depths of these potentials and the energies available to these products also contribute to the dynamics. The formation of the Δv = -2, I35Cl(B,v = 1) product channel was only identified for excitation of levels within the ortho(j = 0)-D2 + I35Cl(B,v'=3) potential. The formation of this channel occurs via I35Cl(B,v'=3) vibrational to D2 rotational energy transfer forming the ortho(j = 2)-D2 + I35Cl(B,v = 1,j) products.

Effective Phase Space Representation of the Quantum Dynamics of Vibrational Predissociation of the ArBr2(B,ν =16···25) Complex.

We perform trajectory-based simulations of the vibrational predissociation of the ArBr2(B,ν=16···25) van der Waals triatomic complex, constrained to the T-shape geometry. To this aim, we employ a 2-fold mapping of the quantum dynamics into classical-like dynamics in an extended phase space. The effective phase space comprises two distinct sets of degrees of freedom, namely a collection of coupled harmonic oscillators and an ensemble of quantum trajectories. The time evolution of these variables represent bound and unbound motions of the quantum system, respectively. Quantum trajectories are propagated within the interacting trajectory representation. The comparison between the lifetimes of the predissociating complexes computed using the trajectory-based approach and the experimental results available for the target systems indicates that the present method is competitive with wavepacket propagation techniques. The competition between several simultaneous vibrational relaxation pathways was found to have a direct impact on the time scales of vibrational predissociation. Likewise, the analysis of the time evolution of the trajectories reveals the existence of regions in the effective phase space where transitions to vibrational states of higher energy are more likely to occur. The size and location of these regions influence the transient vibrational distributions and therefore the computed lifetimes. Furthermore, the mechanisms of energy redistribution along the dissociation coordinate are analyzed.

Mode-specific vibrational predissociation dynamics of (HCl)2 via the free and bound HCl stretch overtones.

Velocity-map ion imaging has been used to study the vibrational predissociation dynamics of the HCl dimer following infrared (IR) excitation in the HCl stretch overtone region near 1.77 Å. HCl monomer predissociation products were detected state-selectively using 2 + 1 resonance-enhanced multiphoton ionization spectroscopy. The IR action spectrum shows the free HCl stretch (2ν1), the bound HCl stretch (2ν2), and a combination band involving the intermolecular van der Waals stretching mode (2ν2 + ν4). Fragment speed distributions extracted from ion images obtained for a range of HCl(v = 0, 1; J) levels following vibrational excitation on the 2ν1 and 2ν2 bands yield the correlated product pair distributions. All product pairs comprise HCl(v = 1) + HCl(v = 0) and show a strong propensity to minimize the recoil kinetic energy. Highly non-statistical and mode-dependent HCl product rotational distributions are observed, in contrast to that observed following stretch fundamental excitation. Predissociation lifetimes are also mode-dependent: excitation of the free HCl leads to τVP = 13 ± 1 ns, while the bound stretch has a shorter lifetime τVP ≤ 6 ns. The dimer dissociation energy determined from energy conservation (D0 = 397 ± 7 cm-1) is slightly smaller than the previously reported values. The results are discussed in the context of previous observations for (HF)2 and (HCl)2 after excitation of HX stretch fundamentals and models for vibrational predissociation.

Trajectory-based modelling of the quantum dynamics of vibrational predissociation: Application to the Ar [formula omitted] complex

The quantum dynamics of vibrational predissociation of the Ar ⋯Br2 triatomic molecule is described within a trajectory-based framework. The Br2 stretching mode is mapped into a set of classical (coupled) harmonic oscillators, associated to each vibrational state of the diatomic molecule. The time evolution of the molecular wave packet along the dissociation coordinate is described within the hydrodynamical formulation of quantum mechanics, specifically using the interacting trajectory representation. The relatively small number of interacting trajectories required to attain numerical convergence (N=100), makes the present model very appealing in comparison with other trajectory-based methods. The underlying parameterisation of the density was found to represent accurately the evolution of the projection of the molecular wave packet along the van der Waals mode, from the ground vibrational state into the continuum. The computed lifetime of the predissociating level and the population dynamics are in very good agreement with those observed experimentally.

Vibrational predissociation dynamics of the nitric oxide dimer

We have studied the photofragmentation of NO dimer employing quasi-classical trajectories propagation using quantum probabilities distributions and Wigner distribution functions to determine the initial conditions of the system. The distributions of initial conditions in phase space, between Wigner distribution functions and quantum probabilities distributions, are not so different. The photofragmentation of NO dimer depends strongly on the excitation of the initial state, especially for the lowest of them. These results are in concordance with experimental values where we can see a similar tendency for the lifetimes different vibrational modes that have been studied. In this work we explore more intramolecular and intermolecular vibrational modes. We have increased the number of trajectories and the dissociation distance in the simulation as well.

Nuclear spin/parity dependent spectroscopy and predissociation dynamics in vOH = 2 ← 0 overtone excited Ne-H2O clusters: Theory and experiment.

Vibrationally state selective overtone spectroscopy and state- and nuclear spin-dependent predissociation dynamics of weakly bound ortho- and para-Ne-H2O complexes (D0(ortho) = 34.66 cm-1 and D0(para) = 31.67 cm-1) are reported, based on near-infrared excitation of van der Waals cluster bands correlating with vOH = 2 ← 0 overtone transitions (|02-〉 and |02+〉) out of the ortho (101) and para (000) internal rotor states of the H2O moiety. Quantum theoretical calculations for nuclear motion on a high level potential energy surface [CCSD(T)/VnZf12 (n = 3, 4)], corrected for basis set superposition error and extrapolated to the complete basis set (CBS) limit, are employed to successfully predict and assign Π-Σ, Σ-Σ, and Σ-Π infrared bands in the spectra, where Σ or Π represent approximate projections of the body-fixed H2O angular momentum along the Ne-H2O internuclear axis. IR-UV pump-probe experimental capabilities permit real-time measurements of the vibrational predissociation dynamics, which indicate facile intramolecular vibrational energy transfer from the H2O vOH = 2 overtone vibrations into the VdWs (van der Waals) dissociation coordinate on the τprediss = 15-25 ns time scale. Whereas all predicted strong transitions in the ortho-Ne-H2O complexes are readily detected and assigned, vibrationally mediated photolysis spectra for the corresponding para-Ne-H2O bands are surprisingly absent despite ab initio predictions of Q-branch intensities with S/N > 20-40. Such behavior signals the presence of highly selective nuclear spin ortho-para predissociation dynamics in the upper state, for which we offer a simple mechanism based on Ne-atom mediated intramolecular vibrational relaxation in the H2O subunit (i.e., |02±〉 → {|01±〉; v2 = 2}), which is confirmed by the ab initio energy level predictions and the nascent OH rotational (N), spin orbit (Π1/2,3/2), and lambda doublet product distributions.

Vibrational predissociation of aniline(water)n+ (n = 1–12)

Vibrational predissociation dynamics of aniline(water)n+ cluster ions formed via resonance two photon ionization followed by the absorption of single infrared photon was investigated. A linear tandem time of flight mass spectrometer designed for this research was tested by the dissociation of aniline+ at 266nm and observed decay constant matched well with the previous result. The rate of dissociation of aniline(water)n+ cluster increased with internal energy, whereas decreased with increasing the size of cluster, manifesting that the intramolecular vibrational energy redistribution precedes VP. The internal energies of the cluster ions were estimated from comparing the experimental and Rice–Ramsperger–Kassel–Marcus calculation results.

Vibrational predissociation and vibrationally induced isomerization of 3-aminophenol-ammonia.

We investigate the vibrational predissociation dynamics of the hydrogen-bonded 3-aminophenol-ammonia cluster (3-AP-NH3) in the OH and NH stretching regions. Vibrational excitation provides enough energy to dissociate the cluster into its constituent 3-AP and NH3 monomers, and we detect the 3-AP fragments via (1 + 1) resonance-enhanced multiphoton ionization (REMPI). The distribution of vibrational states of the 3-AP fragment suggests the presence of two distinct dissociation pathways. The first dissociation channel produces a broad, unstructured feature in the REMPI-action spectrum after excitation of any of the OH or NH stretching vibrations, pointing to a nearly statistical dissociation pathway with extensive coupling among the vibrations in the cluster during the vibrational predissociation. The second dissociation channel produces distinct, resolved features on top of the broad feature but only following excitation of the OH or symmetric NH3 stretch in the cluster. This striking mode-specificity is consistent with strong coupling of these two modes to the dissociation coordinate (the O-H⋯N bond). The presence of clearly resolved transitions to the electronic origin and to the 10a(2) + 10b(2) state of the cis-3-AP isomer shows that vibrational excitation is driving the isomerization of the trans-3-AP-NH3 isomer to the cis-3-AP-NH3 isomer in the course of the dissociation.

Experiment and Theory Elucidate the Multichannel Predissociation Dynamics of the HCl Trimer: Breaking Up Is Hard To Do

The breaking of hydrogen bonds in molecular systems has profound effects on liquids, e.g., water, biomolecules, e.g., DNA, etc., and so it is no exaggeration to assert the importance of these bonds to living systems. However, despite years of extensive research on hydrogen bonds, many of the details of how these bonds break and the corresponding energy redistribution processes remain poorly understood. Here we report extensive experimental and theoretical insights into the breakup of two or three hydrogen bonds in the dissociation of a paradigm system of a hydrogen-bonded network, the ring HCl trimer. Experimental state-to-state vibrational predissociation dynamics of the trimer following vibrational excitation were studied by using velocity map imaging and resonance-enhanced multiphoton ionization, providing dissociation energies and product state distributions for the trimer's breakup into three separate monomers or into dimer + monomer. Accompanying the experiments are high-level calculations using diffusion Monte Carlo and quasiclassical simulations, whose results validate the experimental ones and further elucidate energy distributions in the products. The calculations make use of a new, highly accurate potential energy surface. Simulations indicate that the dissociation mechanism requires the excitation to first relax into low-frequency motions of the trimer, resulting in the breaking of a single hydrogen bond. This allows the system to explore a critical van der Waals minimum region from which dissociation occurs readily to monomer + dimer.

Imaging bond breaking and vibrational energy transfer in small water containing clusters

This letter presents a brief overview of our recent experimental studies of state-to-state vibrational predissociation (VP) dynamics of small hydrogen bonded (H-bonded) clusters following vibrational excitation. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) are used to determine accurate bond dissociation energies (D0) of (H2O)2, (H2O)3, HCl–H2O and NH3–H2O. Pair-correlated product energy distributions from the VP of these complexes are also presented and compared to theoretical models. Further insights into mechanisms are obtained from the recent quasi-classical trajectory (QCT) calculations of Bowman and coworkers. The D0 values for (H2O)2 and (H2O)3 are in very good agreement with recent calculated values, and the results are used to estimate the contributions of cooperative interactions to the H-bonding network.

Theoretical simulations of the vibrational predissociation spectra of H 5 + and D 5 + clusters

In the present study, the effect of the potential energy surface representation on the infrared spectra features of the H+ 5 and D+ 5 clusters is investigated. For the spectral simulations, we adopted a recently proposed (Sanz-Sanz et al. in Phys Rev A 84:060502-1–4, 2011) two-dimensional adiabatic quantum model to describe the proton-transfer motion between the two H2 or D2 units. The reported calculations make use of a reliable “on the fly” DFT-based potential surface and the corresponding new dipole moment surface. The results of the vibrational predissociation dynamics are compared with earlier and recent experimental data available from mass-selected photodissociation spectroscopy, as well as with previous theoretical calculations based on an analytical ab initio parameterized surfaces. The role of the potential topology on the spectral features is studied, and general trends are discussed.

A theoretical study on electronic predissociation in the NeBr2 van der Waals molecule

We present the first comprehensive ab initio study of the Ne–Br2 potential energy surfaces and the non-adiabatic couplings between the valence excited electronic states. These ab initio results are used to obtain 3-D approximate potentials for each electronic state, and these potentials are used in a wave packet calculation of the competing electronic predissociation and vibrational predissociation dynamics. The results of this calculation are in excellent agreement with both experimental results and a previous empirical fit to the experiments. The calculations allow us to observe not only the competition between vibrational and electronic dynamics for the dimer, but also the competition between two different electronic channels. Coupling to the 2g state dominates for the levels studied here, but coupling to the C state is progressively more important for low vibrational levels, and may dominate at levels below which the current results pertain. The ability of ab initio surfaces and couplings to so accurately reproduce experimental data raises the hope of a complete understanding of the VP and EP dynamics for other Rg-halogen dimers. Success in the case presented here is largely due to the fact that the VP dynamics for the vibrational levels in this study are in the simple, direct regime. Understanding the simple case so thoroughly provides new hope that the more complicated examples, such as ArI2 and NeCl2, for which experiment and theory are not currently in accord, may yet yield to analysis.

Dissociation energy and vibrational predissociation dynamics of the ammonia dimer

Experiments using infrared excitation of either the intramolecular symmetric N-H stretch (ν(NH,S)) or the intramolecular antisymmetric N-H stretch (ν(NH,A)) of the ammonia dimer ((NH(3))(2)) in combination with velocity-map ion imaging provide new information on the dissociation energy of the dimer and on the energy disposal in its dissociation. Ion imaging using resonance enhanced multiphoton ionization to probe individual rovibrational states of one of the ammonia monomer fragments provides recoil speed distributions. Analyzing these distributions for different product states gives a dissociation energy of D(0) = 660 ± 20 cm(-1) for the dimer. Fitting the distributions shows that rotations are excited up to their energetic limit and determines the correlation of the fragment vibrations. The fragments NH(3)(ν(2) = 3(+)) and NH(3)(ν(2) = 2(+)) have a vibrational ground-state partner NH(3)(ν = 0), but NH(3)(ν(2) = 1(+)) appears in partnership with another fragment in ν(2) = 1. This propensity is consistent with the idea of minimizing the momentum gap between the initial and final states by depositing a substantial fraction of the available energy into internal excitation.

Imaging H2O Photofragments in the Predissociation of the HCl−H2O Hydrogen-Bonded Dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded HCl-H(2)O dimer was studied following excitation of the dimer's HCl stretch by detecting the H(2)O fragment. Velocity map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, H(2)O fragments were detected by 2 + 1 REMPI via the C (1)B(1) (000) ← X (1)A(1) (000) transition. REMPI spectra clearly show H(2)O from dissociation produced in the ground vibrational state. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational states of H(2)O and were converted to rotational state distributions of the HCl cofragment. The distributions were consistent with the previously measured dissociation energy of D(0) = 1334 ± 10 cm(-1) and show a clear preference for rotational levels in the HCl fragment that minimize translational energy release. The usefulness of 2 + 1 REMPI detection of water fragments is discussed.

Imaging the State-Specific Vibrational Predissociation of the Hydrogen Chloride−Water Hydrogen-Bonded Dimer

The state-to-state vibrational predissociation dynamics of the hydrogen-bonded HCl-H(2)O dimer were studied following excitation of the HCl stretch of the dimer. Velocity-map imaging and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the HCl stretch of the dimer, HCl fragments were detected by 2 + 1 REMPI via the f (3)Delta(2) <-- X (1)Sigma(+) and V (1)Sigma(+) <-- X (1)Sigma(+) transitions. REMPI spectra clearly show HCl from dissociation produced in the ground vibrational state with J'' up to 11. The fragments' center-of-mass translational energy distributions were determined from images of selected rotational states of HCl and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well when using a dimer dissociation energy of D(0) = 1334 +/- 10 cm(-1). The rotational distributions in the water cofragment pair-correlated with specific rotational states of HCl appear nonstatistical when compared to predictions of the statistical phase space theory. A detailed analysis of pair-correlated state distributions was complicated by the large number of water rotational states available, but the data show that the water rotational populations increase with decreasing translational energy.

Competition between electronic and vibrational predissociation dynamics of the HeBr2 and NeBr2 van der Waals molecules

Direct measurements of the lifetimes of He(79)Br(2) and Ne(79)Br(2) B-state vibrational levels 10 < or = nu' < or = 20 have been performed using time-resolved optical pump-probe spectroscopy. The values do not obey the energy gap law for direct vibrational predissociation. For both molecules, the dissociation rate for nu'=11 is much faster than for nu'=12, and the nu'=13 rate is also faster than is consistent with the energy gap law. We attribute this unexpected behavior to an electronic predissociation channel. Based on Franck-Condon factors between the Br(2) B-state vibrational wave functions and the possible Br-Br product wave functions, we surmise that either the Br(2) (3)Pi(g)(1(g)) or (2(g)) state is responsible for the electronic predissociation. To our knowledge, this is the first time electronic predissociation and direct Deltanu=-1 vibrational predissociation have been observed to be in competition for a wide range of vibrational levels. As such, this problem deserves a detailed theoretical analysis.

NeCl2 and ArCl2: Transition from Direct Vibrational Predissociation to Intramolecular Vibrational Relaxation and Electronic Nonadiabatic Effects

Pump-probe results are reported for NeCl(2) excited to the Cl(2) B state, undergoing vibrational predissociation, and then probed via E <-- B transitions. Intensities, lifetimes and product vibrational branching ratios are reported for 16 < or = v' < or = 19 Cl(2) stretching quanta. The intensity of the signal rapidly decreases above v' = 17. Detailed wave packet calculations of the vibrational predissociation dynamics are performed to determine if the experimental results can be explained by the onset of IVR dynamics. The calculations and the experiment are in close accord for low vibrational levels. For higher levels, some, but not all, of the loss of experimental signal can be attributed to IVR. To test whether electronic relaxation dynamics are important for NeCl(2) and ArCl(2), excited state potential surfaces that incorporate spin orbit coupling effects are calculated. These surfaces are then used in a wave packet calculation that includes both vibrational predissociation and electronic predissociation dynamics. The results show that electronic predissociation is important for ArCl(2) levels above v' = 12. For NeCl(2) the calculation suggests that the onset of electronic predissociation should occur for levels as low as v' = 13 but may not contribute markedly to the observed loss of signal above v' = 17. Suggestions are made for further studies of this puzzling problem.

Imaging the State-Specific Vibrational Predissociation of the Ammonia−Water Hydrogen-Bonded Dimer

The state-to-state vibrational predissociation (VP) dynamics of the hydrogen-bonded ammonia-water dimer were studied following excitation of the bound OH stretch. Velocity-map imaging (VMI) and resonance-enhanced multiphoton ionization (REMPI) were used to determine pair-correlated product energy distributions. Following vibrational excitation of the bound OH stretch fundamental, ammonia fragments were detected by 2 + 1 REMPI via the B1E" <-- X1A1' transition. The REMPI spectra show that NH3 is produced with one and two quanta of the symmetric bend (nu2 umbrella mode) excitation, as well as in the ground vibrational state. Each band is quite congested, indicating population in a large number of rotational states. The fragments' center-of-mass (c.m.) translational energy distributions were determined from images of selected rotational levels of ammonia with zero, one, or two quanta in nu2 and were converted to rotational state distributions of the water cofragment. All the distributions could be fit well by using a dimer dissociation energy of D0 = 1538 +/- 10 cm(-1). The rotational state distributions in the water cofragment pair-correlated with specific rovibrational states of ammonia are broad and include all the J(KaKc) states allowed by energy conservation. The rotational populations increase with decreasing c.m. translational energy. There is no evidence for ammonia products with significant excitation of the asymmetric bend (nu4) or water products with bend (nu2) excitation. The results show that only restricted pathways lead to predissociation, and these do not always give rise to the smallest possible translational energy release, as favored by momentum gap models.

Vibrational Predissociation Dynamics of He−I2(B) Mediated by Orbiting Resonances

The He-I(2)(B, v) resonances embedded in the continuum of the v = 60 and 59 vibrational manifolds are investigated. Such states manifest a nature of overlapping and long-lived orbiting resonances, which are supported by centrifugal barriers originated in internal rotational excitation of I(2) and He within the complex. The same number of orbiting resonances, six, is found for v = 60 and for v = 59, with similar energy positions and wave functions, indicating that the spectrum of orbiting resonances changes slowly with the vibrational state v. Three of the orbiting resonances appear at nearly the same energies as the peaks observed experimentally in the vibrational relaxation cross section of I(2)(B, v = 21) through low temperature collisions with He. Such finding suggests that the cross section peaks have their origin in orbiting resonances of the He-I(2)(B, v = 21) complex formed upon the collision.

Vibrational predissociation in weakly-bound clusters: A dispersed fluorescence study of toluene rare-gas complexes

The vibrational predissociation (VP) dynamics of toluene–Ar and toluene–Ne complexes have been investigated using single vibronic level fluorescence spectroscopy. For several levels, comparison of the dispersed fluorescence spectra with those from monomer levels permits the identification of the final monomer vibrational level. Pumping of the low-lying S 1 13 0 1 / 24 0 1 25 0 1 ¯ or 37 0 1 ¯ transitions in toluene–Ar, for example, gives rise to efficient VP leaving the resulting toluene monomer in its vibrationless S 1 state. By contrast, in the case of toluene–Ne, VP following 37 1 ¯ level pumping leaves the toluene molecule in an excited 16 1 level which is optically inaccessible from the ground state.