Initial Vibrational Distribution Research Articles

Dynamical Study of the Dissociative Chemisorption of CHD3 on Pd(111).

The specific reaction parameter (SRP) approach to density functional theory has been shown to model reactions of polyatomic molecules with metal surfaces important for heterogeneous catalysis in the industry with chemical accuracy. However, transferability of the SRP functional among systems in which methane interacts with group 10 metals remains unclear for methane + Pd(111). Therefore, in this work, predictions have been made for the reaction of CHD3 on Pd(111) using Born–Oppenheimer molecular dynamics while also performing a rough comparison with experimental data for CH4 + Pd(111) obtained for lower incidence energies. Hopefully, future experiments can test the transferability of the SRP functional among group 10 metals also for Pd(111). We found that the reactivity of CHD3 on Pd(111) is intermediate between and similar to either Pt(111) or Ni(111), depending on the incidence energy and the initial vibrational state distribution. This is surprising because the barrier height and experiments performed at lower incidence energies than investigated here suggest that the reactivity of Pd(111) should be similar to that of Pt(111) only. The relative decrease in the reactivity of Pd(111) at high incidence energies is attributed to site specificity of the reaction and to dynamical effects such as the bobsled effect and energy transfer from methane to the surface.

Temperature perturbation related to the invisible ink vibrationally excited nitric oxide monitoring (VENOM) technique: a simulation study.

The limits of applicability of the invisible ink variant of the vibrationally excited nitric oxide monitoring (VENOM) technique for three distinct flow fields is reported in this work. This technique involves the generation of a grid of vibrationally excited NO (X,Π2) by exciting the NO A-X electronic transition at 226 nm, which subsequently relaxes via fluorescence and collisional quenching to produce vibrationally excited NO (X,Π2). This grid is then probed by two laser sheets tuned to distinct rotational states. The resulting images allow for the simultaneous measurement of temperature and velocity. The flow fields presented in this work provide a range of NO concentrations, vibrational lifetimes, pressures, temperatures, and collisional quenching, which explore the applicability of the invisible ink variant to a wide range of conditions. We have modelled the initial NO, O2, and N2 vibrational and rotational energy distribution resulting from the combination of fluorescence and quenching of electronically excited NO. The subsequent rethermalization of the sample, in particular the long-time vibrational relaxation, has been modelled using a forced harmonic oscillator model. The time-dependent temperature perturbation due to the invisible ink technique is evaluated for two distinct timescales: a short-timescale temperature rise resulting from collisional quenching and rotational/translational thermalization and a long-timescale temperature rise caused by vibrational thermalization. Under low pressures where fluorescence dominates quenching, there is minimal temperature perturbation of the flow field on the timescale of a VENOM measurement, and the short-timescale temperature perturbation only becomes significant at high NO seed concentrations. The predicted signal-to-noise ratio of the invisible ink method is unaffected for low-pressure, low-temperature flow fields. However, preserving signal-to-noise ratio for a high-temperature, high-pressure flow field could prove challenging due to the impact of quenching and self-absorption. Overall, we find that the invisible ink method is predicted to be a viable laser-based diagnostic for velocimetry and thermometry over a wide range of experimental conditions.

Spatially homogeneous relaxation of CO molecules with resonant VE transitions

In this paper, we study vibrational relaxation of CO molecules with excited electronic states. We consider three electronic terms and account for VV exchanges of vibrational energy within each electronic term, VT transitions of vibrational energy into a translational one, and VE exchange of vibrational energy between electronic terms. The initial vibrational state of the gas is strongly nonequilibrium. The effect of VE exchange on the vibrational relaxation of CO molecules is estimated for different kinds of initial vibrational distributions, in particular, the Treanor and Gordiets ones generalized for gases with electronically excited states. The set of equations of state-to-state vibrational kinetics, together with the gas dynamic equations, is solved numerically in the zero-order approximation of the Chapman–Enskog method for the case of spatially homogeneous relaxation. The following results are obtained: neglecting VE exchanges leads to an incorrect assessment of the number density for each electronic level; however, the error is small for the ground electronic state. It is shown that VE exchanges qualitatively affect the time dependence of the vibrational temperature.

Luminorefrigeration: vibrational cooling of NaCs

We demonstrate the use of optical pumping of kinetically ultracold NaCs to cool an initial vibrational distribution of electronic ground state molecules X(1)Σ(+)(v ≥ 4) into the vibrational ground state X(1)Σ(+)(v=0). Our approach is based on the use of simple, commercially available multimode diode lasers selected to optically pump population into X(1)Σ(+)(v=0). We investigate the impact of the cooling process on the rotational state distribution of the vibrational ground state, and observe that an initial distribution, J(initial)=0-2 is only moderately affected resulting in J(final)=0-4. This method provides an inexpensive approach to creation of vibrational ground state ultracold polar molecules.

Laser cooling of the vibrational motion of Na2combining the effects of zero-width resonances and exceptional points

We propose various scenarios for molecular vibrational cooling combining the effects of two kinds of resonance states occurring during the photodissociation of Na${}_{2}$ taken as an illustrative example. Such resonances result from an appropriate sampling of laser parameters (wavelength and intensity): (a) For particular choices of intensity and wavelength, two resonance energies can be brought to complete coalescence, with their positions and widths becoming equal and leading to a so-called exceptional point (EP) in the parameter plane. Advantage can be taken from such points for very selective laser-controlled vibrational transfer strategies. (b) For specific intensities, far beyond the perturbation regime, some resonances can have a zero width (infinite lifetime). They are referred to as a zero-width resonance (ZWR) and may be used for vibrational purification purposes. We show how appropriately shaped, experimentally reachable laser pulses, encircling EPs or inducing ZWRs, may be used for a thorough and comprehensive control aiming at population transfer or purification schemes, which, starting from an initial field-free vibrational distribution, ends up in the ground vibrational level.

Quasi-classical model of non-destructive wavepacket manipulation by intense ultrashort nonresonant laser pulses

A quasi-classical model (QCM) of nuclear wavepacket generation, modification and imaging by three intense ultrafast near-infrared laser pulses has been developed. Intensities in excess of 1013 W cm-2 are studied, the laser radiation is non-resonant and pulse durations are in the few-cycle regime, hence significantly removed from the conditions typical of coherent control and femtochemistry. The 1sσ ground state of the D2 precursor is projected onto the available electronic states in D2+ (1sσg ground and 2pσu dissociative) and D++D+ (Coulomb explosion) by tunnel ionization by an ultrashort ‘pump’ pulse, and relative populations are found numerically. A generalized non-adiabatic treatment allows the dependence of the initial vibrational population distribution on laser intensity to be calculated. The wavepacket is approximated as a classical ensemble of particles moving on the 1sσg potential energy surface (PES), and hence follow trajectories of different amplitudes and frequencies depending on the initial vibrational state. The ‘control’ pulse introduces a time-dependent polarization of the molecular orbital, causing the PES to be modified according to the dynamic Stark effect and the transition dipole. The trajectories adjust in amplitude, frequency and phase-offset as work is done on or by the resulting force; comparing the perturbed and unperturbed trajectories allows the final vibrational state populations and phases to be determined. The action of the ‘probe’ pulse is represented by a discrete internuclear boundary, such that elements of the ensemble at a larger internuclear separation are assumed to be photodissociated. The vibrational populations predicted by the QCM are compared to recent quantum simulations (Niederhausen and Thumm 2008 Phys. Rev. A 77 013404), and a remarkable agreement has been found. The applicability of this model to femtosecond and attosecond time-scale experiments is discussed and the relation to established femtochemistry and coherent control techniques are explored.

On the Statistical Nature of Collision and Surface-Induced Dissociation: A Theoretical Investigation of Aluminum Clusters

The unimolecular dissociation dynamics of aluminum clusters following collision with either a rare gas atom or a surface is investigated by classical trajectory simulations with model potentials. Two conformers of Al(6) with very distinct shapes, i.e., the spherical O(h) and planar C(2)(h) clusters, are considered in this work. The initial vibrational energy and angular momentum distributions resulting from collision, as well as the energy and angular momentum resolved lifetime distributions, of excited clusters were determined for both collision-induced dissociation (CID) and surface-induced dissociation (SID) processes. The partitioning of excitation energy acquired upon collision was found to depend on the excitation mechanism (CID or SID), as well as on the cluster molecular shape, especially in the case of CID. For both types of processes, the energy and angular momentum resolved excited cluster lifetime distributions were found to decay exponentially, in agreement with statistical theories of chemical reactions, suggesting intrinsic Rice-Ramsperger-Kassel-Marcus (RRKM) behavior. Moreover, the simulated microcanonical rate constants determined from the cluster lifetime distributions are in good agreement with the predictions of the orbiting transition state model of phase space theory (OTS/PST), which further supports the statistical character of cluster CID and SID. Thus, in the CID and SID of highly fluxional systems such as aluminum clusters, the rate of intramolecular vibrational energy redistribution (IVR) is much faster than the dissociation rate, which validates one of the key assumptions, i.e., post-collision statistical behavior, underlying the models that are routinely used to determine cluster binding energies from experimental CID/SID cross sections.

Ultrafast Photodissociation Dynamics of Acetone at 195 nm: I. Initial-state, Intermediate, and Product Temporal Evolutions by Femtosecond Mass-Selected Multiphoton Ionization Spectroscopy

The photodissociation dynamics of the acetone S2 (n, 3s) Rydberg state excited at 195 nm has been studied by using femtosecond pump-probe mass-selected multiphoton ionization spectroscopy. For the first time, the temporal evolutions of the initial state, intermediates, and methyl products were simultaneously measured and analyzed for this reaction to elucidate the complex dynamics. Two mechanisms were considered: (1) the commonly accepted mechanism in which the primary dissociation occurs on the first triplet-state surface, and (2) the recently proposed mechanism in which the primary dissociation takes place on the first singlet-excited-state surface. Our results and analyses supported the validity of the new mechanism. On the other hand, the conventional mechanism was found to be inadequate to describe the observed dynamics. The temporal evolution of methyl products arising from the secondary dissociation of hot acetyl intermediates exhibited a very complex behavior that can be ascribed to the combination of a nonuniform initial vibrational distribution and the competition between dissociation and slow intramolecular vibrational redistribution.

Measuring molecular wave functions using laser Coulomb explosion imaging with ultraviolet lasers

A high-frequency laser (λ<40 nm) can ionize the H2+ molecule via a one-photon transition to the electron–nuclei continua. From exact non-Born–Oppenheimer simulations of dynamics of H2+ in intense laser fields we show that it is possible to reconstruct the shape of the initial, stationary vibrational wave function |ψv(R)|2 by measuring kinetic energy spectra of either protons or electrons. The reconstruction procedure from proton spectra is based on a perturbative one-photon transition to the electron–proton continua and formally is similar to the previously discussed laser Coulomb explosion imaging technique. For sufficiently high explosion energy, proton spectra are directly proportional to |ψv(q2/EN)|2, where EN is the nuclear energy. Alternatively, one can measure kinetic energy of electrons, from which the proton energy can be deduced via the conservation of energy for a one-photon transition to the electron–proton continuum, and thus the shape of the first peak in the electron spectrum is also directly proportional to the initial vibrational probability distribution. We compare these two methods and also compare them with the previously proposed imaging methods based on Coulomb explosions induced by multiphoton transitions. We find that high-frequency lasers allow us to perform the imaging of a stationary wave function with longer and less intense pulses than pulses required in the previously discussed long-wavelength regime. Imaging based on electron spectra requires longer pulses, since the spread of photon energy in short pulses does not allow us to determine nuclear energy, in an accurate way, via conservation of energy. In general, the photo-electron spectra from H2+ are found to be very different from atomic spectra, the latter having much narrower peaks, while the former are greatly enlarged due to sharing the final energy with nuclei. The photo-electron and nuclear spectra show a very clear imprint of the nuclear wave function, e.g. if the molecule is prepared in the v-state that has v nodes, both spectra will also have v nodes.

Effects of vibrational excitation of target N2 molecule in charge–transfer reaction of He+ with N2 at thermal energy

The effects of vibrational excitation of target N2 molecule in the charge-transfer (CT) reaction He++N2(X:v″)→N2+(C)+He has been studied at thermal energy using a flowing–afterglow method. The vibrational distribution of N2(X:v″⩾0), produced from a microwave discharge of Ar/N2 or He/N2 mixtures, was determined using N2+(B–X) emission resulting from the He(2 3S)/N2(X:v″) Penning ionization. Although the initial vibrational distribution of N2+(C) produced from the He+/N2(X:Nv″=0=100) reaction was Nv′=3:Nv′=4=100±5:28±2, it was Nv′=3:Nv′=4:Nv′=5:Nv′=6=100±5:60±3:41±2:49±2 in the He+/N2(X:Nv″=0:Nv″=1:Nv″=2:Nv″=3=91±5:100±5:100±5:18±1) reaction. The N2+(C:v′=3) level, which is the most favorable level in the thermal He+/N2(X:v″=0) reaction, is still favored in the He+/N2(X:v″⩾0) reaction. This indicates that the deviation from the energy-resonance rule becomes large by the vibrational excitation of N2(X:v″). The rotational distributions of v′=3 and 4 were similar between the He+/N2(X:v″=0) and He+/N2(X:v″⩾0) reactions, demonstrating that V→R transfer is insignificant for N2(X)→N2+(C) ionization in the He+/N2(X:v″⩾0) reaction.

Temperature-dependent photodissociation dynamics of ICN at 262 nm

Transient frequency-modulation (FM) Doppler spectroscopy has been used to measure scalar and vector correlations in state-selected CN fragments arising from the photodissociation of ICN at 262 nm at both 296 and 550 K. We find that the state-selected I */I branching ratio changes significantly with increased temperature, consistent with the increased width of the state-dependent CN rotational distribution. However, we find little change in the state-selected vector properties suggesting that thermal modification of the initial parent vibrational distribution is not a sensitive means to probe the nonadiabatic dynamics.

Translational Energy Distributions for Dissociation of the van der Waals Cation Species (C6H6···Arn)+ (n = 1,2) Measured by Velocity Map Imaging

The kinetic energy distributions associated with ejection of Ar from the cation van der Waals species (C6H6···Ar)+ and (C6H6···Ar2)+ have been measured by ion imaging with a velocity mapping configuration. The (C6H6···Ar2)+ dissociation was observed for the isomer with an Ar atom on each side of the benzene ring. The cations were created by (1+1) resonance-enhanced multiphoton ionization via their 610 transitions. The initial cation vibrational state population distributions were deduced from photoelectron spectra of benzene measured with the velocity map imaging spectrometer. The cations are produced with an average vibrational energy ∼1800−1900 cm-1. For dissociation of (C6H6···Ar2)+, on average enough energy remains in the (C6H6···Ar)+ fragment to eject the remaining Ar. The experiment views fragmentation of the subset of the (C6H6···Ar2)+ cations that lose a single Ar and for this reason the initial internal ion energy is significantly lower than 1800 cm-1 for the (C6H6···Ar2)+ dissociations monitored. The average initial energies above dissociation are estimated to be ∼1310 and ∼350 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The kinetic energy distributions are well fitted by the function P(E) = E1/2{c1 exp(−k1E) + c2 exp(−k2E)}. The average kinetic energies released were 92 ± 4 and 78 ± 5 cm-1 for (C6H6···Ar)+ and (C6H6···Ar2)+, respectively. The lower average kinetic energy released for (C6H6···Ar2)+ is attributed to its lower initial internal energy. For (C6H6···Ar)+ the kinetic energy released represents only a small fraction of the total energy that requires redistributing. A large proportion of the total energy is therefore taken up as rotational and vibrational energy of the benzene cation fragment.

Rotational and Vibrational State Distributions of HNC(0 0) from the Hot H Atom Reaction: H + (CN)2 → HNC + CN

The reaction dynamics of the five-atom system, H + (CN)2, was investigated by probing the minor product channel producing the transient HNC molecule. The complete initial energy disposition, translation, rotation, and vibration was determined for the HNC(0 0), = 00,11, product. The reaction was studied under bulk conditions and was initiated by energetic H atoms with a mean translational energy of 92 kJ mol-1. The HNC molecule was monitored by time- and frequency-resolved absorption spectroscopy with sub-Doppler resolution. The initial rotational state distribution of each HNC(0 0) vibrational level was measured and found to be well-described by a Boltzmann distribution. Only two vibrational levels were detected so that the initial HNC product vibrational level distribution was determined as well. The absolute reaction cross section for the title reaction was measured to be 2 × 10-18 cm2 for H atoms with a nominal translational energy of 113 kJ mol.-1

Phonon–photon translator

We study a two-level ion in a Paul trap coupled to a one mode cavity field. We show that an initial phonon vibrational distribution can be transferred to the cavity. In particular, we can generate Fock states in the cavity. We also study the experimental realization of the present scheme.

Dissociative recombination of vibrationally excitedHD+: State-selective experimental investigation

measured. The method is based on using merged electron and molecular ion beams in a heavy-ion storage ring together with molecular fragment imaging techniques which allow us to probe the vibrational-state population of the stored beam as a function of time as well as the final state of the dissociation. The initial vibrational distribution of the stored ion beam ~from a Penning ion source! is found to be in good agreement with a Franck-Condon model of electron impact ionization, apart from slightly larger experimental populations found for low vibrational states; its time evolution in the storage ring reflects the predicted vibrational level lifetimes. Dissociative recombination measurements were performed with the electron and ion beams at matched velocities ~corresponding to average collision energies of about 10 meV!, and at several well-defined collision energies in the range of 3‐11 eV. The obtained vibrational-state specific recombination rate coefficients are compared with theoretical calculations and show that, although an overall agreement exists between experiment and theory, large discrepancies occur for certain vibrational states at low electron energy. @S1050-2947~99!09411-1# PACS number~s!: 34.80.Gs

Initial vibrational level distribution of HCN[X̃ 1Σ+(v10v3)] from the CN(X 2Σ+)+H2→HCN+H reaction

The reaction of the cyano radical (CN) with hydrogen was studied by time-resolved infrared absorption spectroscopy of individual rovibrational states of HCN. The initial vibrational level distribution of HCN(v10v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. The experiments were carried out at two temperatures, 293 and 324 K, with similar results. It was estimated that about 50% of the available reaction exothermicity was deposited as vibrational excitation of the HCN product. Surprisingly, the HCN(101) vibrational level received a significant fraction of the observed vibrational population, implying that the CN vibration was not really a spectator bond in the reaction dynamics. Furthermore, the observed HCN(v10v3) vibrations only account for about 27% of the initial HCN population produced in the title reaction. A significant fraction of the product HCN molecules must have been produced with the bending mode excited, likely in combination with the H–C stretch vibrations.

Electron-vibration energy exchange models in nitrogen plasma flows

This work presents an examination of the validity of the simple linear Landau-Teller-type model proposed by Lee for the electron-vibration energy exchange term in nitrogen [in Thermal Design of Aeroassisted Orbital Transfer Vehicles, edited by H. F. Nelson (AIAA, New York, 1985), Vol. 96, p. 3; in Thermophysical Aspects of Re-entry Flows, edited by J. N. Moss and C. D. Scott (AIAA, New York, 1986), Vol. 103, p. 197]. Plasma flow conditions encountered in high enthalpy wind tunnels are considered. The time-dependent relaxation of the vibrational energy of nitrogen due to electron inelastic collisions is calculated. The influence of the anharmonicity of the molecule and of the initial vibrational temperature ${\mathrm{T}}_{\mathrm{v}}$ is studied. With a harmonic oscillator approximation, it is found that a linear Landau-Teller-type model is accurate to describe the vibrational energy relaxation rate for electron temperatures ${\mathrm{T}}_{\mathrm{e}}$ in the range 3000 K\ensuremath{\leqslant}${\mathrm{T}}_{\mathrm{e}}$\ensuremath{\leqslant}20 000 K. When ${\mathrm{T}}_{\mathrm{v}}$${\mathrm{T}}_{\mathrm{e}}$, our results show that the initial model proposed by Lee overestimates by a mean factor of 3 the electron-vibration (e-V) relaxation time. When ${\mathrm{T}}_{\mathrm{v}}$&gt;${\mathrm{T}}_{\mathrm{e}}$, the relaxation time appears to depend on the initial vibrational distribution. When the anharmonicity of nitrogen is taken into account, generally the relaxation rate of the vibrational energy deviates from a linear rate equation. In this case, it appears much more difficult to model accurately the e-V energy exchange term.

Dissociative-recombination and excitation measurements with &lt;emb&gt;&lt;base&gt;H&lt;/base&gt;&lt;sup&gt;n+&gt;&lt;/sup&gt;&lt;sub&gt;2&lt;/sub&gt;&lt;/emb&gt; and &lt;emb&gt;&lt;base&gt;HD&lt;/base&gt;&lt;sup&gt;+&lt;/sup&gt;&lt;/emb&gt;s

The cross sections for dissociative recombination (DR) and dissociative excitation (DE) have been measured for the Hn+&gt;2 and HD+ molecular ions in the energy range from 0 to \ensuremath{\sim}30 eV. An imaging technique that provides a measure of the kinetic energy release in the DR process was used to yield information about the initial vibrational distribution of the molecular ions and the final atomic states of the DR reaction. The results for DR with HD+ in the vibrational ground level, \ensuremath{\nu}=0, are compared with other measurements and recent multichannel quantum defect theory calculations. Significant deviations that appear at high energy \ensuremath{\sim}5--10 eV are discussed. The present work reports on an absolute DE measurement with HD+ . The vibrational distribution of Hn+&gt;2 was manipulated by photodissociation of ions in vibrationally excited levels. We obtained DE and DR cross sections for Hn+&gt;2 ions populating primarily the two lowest vibrational levels \ensuremath{\nu}=0,1 and Hn+&gt;2 ions in a broad vibrational distribution. It is found that DR of Hn+&gt;2 with zero energy electrons results in the H(n=1)+H(n=2) final channel, open for all vibrational levels, and in the H(n=1)+H(n=3) channel, which is energetically open for initial vibrational levels \ensuremath{\nu}\ensuremath{\geqslant}5.

The initial vibrational level distribution and relaxation of HCN[X̃ 1Σ+(v1,0,v3)] in the CN(X 2Σ+)+CH4→HCN+CH3 reaction system

The reaction of the cyano radical (CN) with methane was studied by time-resolved infrared absorption spectroscopy by monitoring individual rovibrational states of the HCN and CH3 products. The initial vibrational level distribution of the bendless vibrational levels of HCN(v1,0,v3) was determined by plotting the time dependence of the fractional population of a vibrational level and extrapolating these curves to the origin of time. About 20% of the HCN products were observed to be initially produced in the HCN(v1,0,v3) vibrational levels, with v1 and v3=0,1,2. The CN radical was created by laser photolysis of three different precursors. Each photolyte provided a different initial vibrational level distribution of CN; however, similar initial HCN(v1,0,v3) vibrational level distributions were obtained independent of the CN radical precursor. This may indicate that the CN radical does not act as a spectator bond during the course of a reactive encounter for this system. The time dependence of the CH3 (00000) ground state was also followed using time-resolved infrared absorption spectroscopy. Preliminary data indicates that a large fraction, if not all, the CH3 radicals are produced in their ground state in the title reaction.

A study of oxygen 6300 Å airglow production through chemical modification of the nighttime ionosphere

The Release Experiments to Derive Airglow Inducing Reactions (RED AIR) conducted on April 3, 1989, and December 6, 1991, offer a unique set of observations for studying the specific processes associated with the production of the O(3P–1D) emission at 6300 Å. In these experiments, sounding rockets were used to place equal quantities of CO2 above and below hmax of the nocturnal F region. CO2 leads to 6300 Å emission by a three‐step process: (1) CO2 + O+ → O2+ + CO, (2) O2+ + e− → O* + O, (3) O* → O + hv6300. Direct measurements of plasma parameters and indirect measurements of the neutral atmosphere densities were used in conjunction with the Fluid Element Simulation (FES) computer code to model the temporal and spatial evolution of the observed 6300 Å airglow enhancement and accompanying plasma depletion. Using the currently accepted set of reaction rates relevant to F region chemistry, the quantum yield of O(1D) from reaction (2) was found to have a mild altitude dependence, decreasing by 16% from 275 to 350 km. Since the initial vibrational distribution of the nascent O2+ was the same for the two releases, this result implies an altitude dependence in the quenching of O2+ vibrational states. Building on previous evidence that O2+ is vibrationally excited in the nighttime thermosphere, we further conclude that this vibrational distribution is altitude dependent. In terms of 6300 Å airglow production, the effect is manifested in an altitude dependence of f(1D). Additionally, quenching by O(3P) was found to contribute very little to the depopulation of the nascent O(1D), with QO = 0 giving the best fit to the RED AIR observations.