Initial Vibrational Level Research Articles

Pulse length control of Na+2 photodissociation by intense femtosecond lasers

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

Dissociative recombination for vibrationally excited ions: the effect of laser photodissociation on the initial vibrational distribution

We report on measurements of dissociative recombination of ions populated in a range of initial vibrational levels with free electrons in the energy range of 0 - 20 eV. A photodis-sociation technique was applied to modify the vibrational distribution, and the influence on the dissociative-recombination (DR) spectrum was studied. To estimate the vibrational distribution in the experiment, the results of a simple model calculation describing a part of the measured spectrum are presented. From the comparison between the experimental data and the model calculation, we conclude that when laser photodissociation is not applied, our result is consistent with a Franck - Condon vibrational distribution from the ion source. When laser photodissociation is applied, a dramatic change of the DR spectrum is observed, which is consistent with the expected effect, based on theoretical photodissociation cross sections.

Dependence of level-resolved energy transfer on initial vibrational level in Li2A1Σu+−Ne collisions

We have investigated collision induced rotational and vibrational energy transfer in the Li2 A1Σu+(vi,ji=30)−Ne system experimentally under single-collision conditions at an effective temperature of 691 K. Over 800 inelastic rate constants have been measured, with the initial vibrational level vi ranging from 2 to 24 and −2≤Δv≤+2. Increasing vi results in a linear increase in the vibrational transition rate constants, which is accompanied by a decrease in the rotationally inelastic transition rate constant. The total inelastic rate constant increases with vi only at the highest values of vi. Net vibrational energy transfer 〈ΔE〉 calculated using rotationally summed rate constants is qualitatively consistent with a simple model. However, explicit inclusion of rotation gives quite different values of 〈ΔE〉. The experimental results are compared with our three-dimensional trajectory calculations on an ab initio potential surface and on a simple repulsive potential surface.

Vibrational predissociation of I2Ne. A quasiclassical dynamical study

A quasiclassical trajectory method is applied to study the vibrational predissociation of the I2(B) … Ne van der Waals system. A potential surface for the complex is proposed and used in the calculations which is fitted with a quantum model in order to reproduce the experimental lifetimes available. The classical results are found to overestimate statistical energy redistribution mechanisms which slow down the predissociation process. The classical calculations, however, predict a behavior of the lifetime versus the initial iodine vibrational level in good agreement with the experimental data.

Classical Trajectory Study of HF Desorption on LiF(001) Surface

AbstractWe report a classical trajectory study of the molecular desorption of a vibrationaly excited HF adsorbed on a corrugated LiF(001) surface. The desorption is induced by either a vibrational deexcitation of the adsorbed molecule (vibrational desorption) or an active motion of the surface ion (thermal desorption). A brief description of the calculation of the gas molecule/surface interaction potential is first presented. By applying this potential, Hamilton's equations of motion can be numerically integrated with the initial conditions selected, thus the dynamics is simulated. The result gives a clear indication about the features of the evolution of the vibrational, rotational and translational energies with increase of time. The angular momentum and translational energy distributions of the desorbed molecule are presented. Desorption probabilities for different initial states of the adsorbed molecule are also predicted, which are influenced by the polar‐angle‐dependent potential and the excitation of the initial vibrational level. The surface phonon mode is found to be strongly coupled to the vibrational, rotational, translational modes of the molecule and surface bond.

A strong collision partner dependence for vibrational energy transfer in benzene and benzene- d6. He versus D 2

State-to-state branching ratios are reported for vibrational energy transfer in benzene and benzene- d 6 due to collisions with D 2 and with He in a supersonic free jet expansion. In both the benzene and benzene- d 6 experiments the initial vibrational level pumped was 6 1. The state-to-state branching ratios are different for D 2 compared with He in C 6H 6 and in C 6D 6. In particular, there is a significant increase in transfer to the 0 0 level for D 2 compared with He. It is postulated that this may be due to the small moment of inertia of D 2, which makes V→R transfer an important mechanism for this collision partner.

Nonlinear resonance and correlated binary collisions in the vibrational predissociation dynamics of I2(B,v)–Ar13 clusters

The vibrational predissociation dynamics of the van der Waals cluster I2(B,v)–Ar13 are studied theoretically from a classical mechanical perspective. The focus is on the ejection of the first Ar atom from the excited cluster. This process is found to occur by two competing mechanisms (1) evaporation of a cluster atom induced by the slow vibrational relaxation of the I2 impurity, and (2) direct ejection of a geometrically favored Ar atom by two or more impulsive collisions with the vibrating I2. In contrast with the picture of independent binary collisions, the multiple argon–iodine interactions leading to successful direct dissociation are correlated in time. The relative propensity of the direct channel is a highly structured function of v, the initial vibrational level of the I2. This behavior results from the energy-dependent frequencies of motion of the diatomic and the localized Ar cluster mode, and is a novel example of a nonlinear resonance between ‘‘system’’ and ‘‘bath’’ degrees of freedom.

The HeCl2 potential: A combined scattering-spectroscopic study

Total differential scattering angular distribution measurements for He scattering from Cl2 are reported. The scattering data are combined with previous excitation spectroscopy and vibrational predissociation product state distributions for He–Cl2 to determine the potential-energy surfaces for the ground X and excited B electronic states. The potentials are somewhat deeper, De=−38.2 cm−1 for the X state, than previously estimated. The X state potential minimum is for the ‘‘T’’ shaped configuration with a Rmin value of 3.55 Å. The potentials are quite successful at describing the scattering data, the rotationally resolved spectroscopy of the complex, and the product state distributions for vibrational predissociation. The B state potential underestimates the dependence of the rate of vibrational predissociation on the initial vibrational level.

The structure and dissociation dynamics of the Ne2Cl2 van der Waals complex

The structure and dynamics of Ne2Cl2 and Ne3Cl2 are studied by laser pump–probe spectroscopy. Analysis of a rotationally resolved B←X excitation band shows that Ne2Cl2 has a distorted tetrahedral structure with a Ne–Ne bond length of 3.23 Å and Ne2 center of mass to Cl2 center of mass distance of 3.12 Å. This structure is very close to that predicted by summing the atom–atom interactions. Excitation spectral shifts suggest a Ne3Cl2 structure with the neon atoms encircling the Cl2 bond axis. The total van der Waals binding energy of Ne2Cl2 is found to be between 145.6 and 148.6 cm−1, which is 20 cm−1 greater than 2*D0(Ne–Cl2)+D0(Ne2). For Cl2 stretching levels below υ′=10, transfer of one Cl2 vibrational quantum to the van der Waals vibrational modes is sufficient to dissociate both neon atoms from the complex. This indicates that the two neon atoms need not dissociate via independent, impulsive ‘‘half-collisions’’ which would require two Cl2 vibrational quanta. Observation of a NeCl2 dissociation fragment, however, indicates that such a sequential mechanism competes with the direct dissociation. Cl2 fragment rotational state population distributions for different initial vibrational levels are characterized using a simple rotational surprisal analysis. Comparison of these surprisal plots to those of the NeCl2 dissociation shows that as the size of the complex increases, so does the degree of statistical redistribution during the reaction. Even for Ne2Cl2, however, the extent of product rotational excitation is only weakly dependent upon the amount of energy available to the products and is always less than predicted by a statistical distribution between the translational and rotational product degrees of freedom.

ChemInform Abstract: Rovibrational Energy Transfer from the (00, K′= 0) and (72, K′= 0) Levels of S1 Glyoxal in Crossed Beam Collisions with H2.

Crossed molecular beams have been used to study rotationally and vibrationally inelastic scattering from the (72, K’=0) and the (00, K’=0) levels of 1Au (S1) trans‐glyoxal in collision with H2. The center‐of‐mass collision energy was approximately 80 meV (650 cm−1). Laser excitation was used to select each of the initial vibrational levels with rotational states limited to J’=0–10, K’=0. Dispersed fluorescence was used to monitor quantitatively the final rotational and vibrational levels populated in single collision scattering. Vibrationally inelastic scattering from each initial level has an absolute cross section large enough to allow the process to be observed easily with resolution of accompanying ΔK rotational state changes. Although many final vibrational levels are energetically accessible and in principle observable, only those reached by ±1 quantum changes in the lowest frequency mode, the OHC–CHO torsion ν7, are seen. From the initial level 72, the cross sections for Δυ7=+1 and Δυ7=−1 are nearl...

Evidence for final state interactions in the vibrational predissociation of ICl–Ne complexes

The nascent ICl rotational state distribution is examined following vibrational predissociation of ICl–Ne A state complexes from a range of initial vibrational levels, vA. The energy released to fragments after the loss of one quantum of ICl vibrational excitation varies dramatically over the initial vA levels investigated. The ICl rotational product distributions are shown to be strongly dependent on the energy available to fragments. The efficiency of energy transfer into ICl rotations is similar for vibrational predissociation of ICl–Ne complexes prepared in vA=11, 14, and 19. The energy dependence of the peaks in the ICl rotational product distributions is consistent with an impulsive final interaction between ICl and Ne, analogous to a rotationally inelastic half-collision.

Rovibrational energy transfer from the (0, K′=0) and (72,K′=0) levels of S1 glyoxal in crossed beam collisions with H2

Crossed molecular beams have been used to study rotationally and vibrationally inelastic scattering from the (72, K′=0) and the (00, K′=0) levels of 1Au (S1) trans-glyoxal in collision with H2. The center-of-mass collision energy was approximately 80 meV (650 cm−1). Laser excitation was used to select each of the initial vibrational levels with rotational states limited to J′=0–10, K′=0. Dispersed fluorescence was used to monitor quantitatively the final rotational and vibrational levels populated in single collision scattering. Vibrationally inelastic scattering from each initial level has an absolute cross section large enough to allow the process to be observed easily with resolution of accompanying ΔK rotational state changes. Although many final vibrational levels are energetically accessible and in principle observable, only those reached by ±1 quantum changes in the lowest frequency mode, the OHC–CHO torsion ν7, are seen. From the initial level 72, the cross sections for Δυ7=+1 and Δυ7=−1 are nearly identical, mirroring the symmetry of UP and DOWN vibrational transitions recently observed in I*2 inelastic scattering from H2, D2, and He. The total cross section for pure rotationally inelastic scattering from 00 is about nine times larger than that for rovibrationally inelastic scattering, whereas the two cross sections are nearly equivalent for 72. With resolution of ΔK rotational state changes, it is seen that the cross sections for these processes scale approximately exponentially with the amount of energy ΔE transferred between translational motion and the internal degrees of freedom of the glyoxal molecule. For the initial level (00, K′=0), not only do these rotational and rovibrational cross sections obey the same energy scaling law, but where similar ΔE occurs, they are the same size. In contrast, the ΔK rotationally state-resolved cross sections for transitions from (72, K′=0) fall off much more rapidly in the vibrationally elastic (Δυ7=0) channel than in the vibrationally inelastic (Δυ7=±1) channels. Consequently, some rovibrational transitions have cross sections comparable to the pure rotational transitions with the same value of ΔK. These relationships contradict the conventional rule of thumb prescribing large rotational and small vibrational cross sections.

Resonances in molecular photodissociation

Calculations of the photodissociation of a collinear model of CO 2 are reported which address the characterization of resonance structures predicted in the absorption line shape. Results from excited initial vibrational levels indicate the existence of hyperspherical resonant states predicted by Schinke and Engel. Resonance structures are also found in reactive scattering probabilities on the final electronic state surface.

Electron-impact dissociation of HCl.

A recently developed impact-parameter method is applied to electron-impact dissociation processes involving the A /sup 1/Pi state of HCl. Calculations were performed for initial vibrational levels of the X /sup 1/..sigma../sup +/ state up to v/sub i/ = 2 and for rotational temperatures to 1000 K. The cross sections increase by approximately 50% from v/sub i/ = 0 to v/sub i/ = 2. The dependence on rotational temperature is small except for an anomalous enhancement at 0 K that the theory predicts for Pi states. Since the predicted cross sections are an order of magnitude larger than those for photodissociation, electron-impact dissociation processes will play an important role in lasers involving HCl.

Chemical timing 3. The picosecond dynamics of intramolecular vibrational redistribution from 11 levels in S1 p-difluorobenzene vapor

Collisional quenching of S1–S0 fluorescence (chemical timing) has been used to study the dynamics of intramolecular vibrational redistribution (IVR) from 11 initial vibrational levels in S1 p-difluorobenzene (pDFB). The levels range in energy from 1615 to 3310 cm−1 with total state densities ranging from about 10 to 2000 per cm−1. The IVR follows intermediate case kinetics of radiationless transition theory for levels below about 2500 cm−1 and statistical case kinetics for higher levels. The IVR involves coupled states spanning an energy width of 0.02–0.3 cm−1. The IVR lifetimes vary from about 15 to 300 ps with much variability among initial levels. A low frequency out-of-plane mode appears to be an accelerating mode for IVR. Average matrix elements for the level coupling associated with the IVR are found by analysis of the kinetic data to be in the range 0.01–0.1 cm−1. Vibration-rotation coupling appears to be an important contributor to IVR, at least for the lower levels.

Vibrational energy transfer from highly excited anharmonic oscillators: Quasiclassical Monte Carlo trajectory study of Br2–Ar and Br2–Br system

Thermally averaged rate coefficients for vibrational energy transfer of Br2 by Ar and Br atoms are calculated by a quasiclassical Monte Carlo trajectory method. In the Br2–Br system, both the nonreactive and the reactive energy transfer rates are calculated using the potential energy surface derived from a VB-DIM method. The quantum number dependence of the transition rates is investigated over a wide range of initial vibrational levels at T=2000–3500 K. It is found that the vibrational scaling law is very much different from the predictions based on the simple first-order theories of the vibrational energy transfer. A surprisal analysis of the results shows that the surprisals depend linearly on the absolute values of the quantum number difference. The characteristics of the energy transfer in the highly excited vibrational states are discussed.

Theoretical study of the reaction H + ClCH3 → HCl + CH3

Abstract The reaction H + ClCH 3 has theoretically studied in a LEPS potential energy surface with a single-particle approximation for the methyl group. The LEPS adjustable parameters were selected to reach a good agreement with experimental values of activation energy and exothermicity. A wide set of quasi-classical trajectories for that system has been calculated within a energy range covering the significative values of relative velocities at temperatures between 300 and 1000 K. Calculated reactive cross sections increase with translational energy and with the initial vibrational level, but they are not influenced by rotational excitation of the reactants. Microscopic and total reaction rate constants have been obtained within the temperature range and agree quite well with available experimental results. Final energy distribution shows that most of the exoergicity is consumed in increasing the relative velocity of the products, while HCl molecules remain in their vibrational ground state.

Nascent NO vibrational distribution from 2485 Å NO2 photodissociation

The initial NO vibrational level distribution has been determined for NO2 photodissociation at 2485 Å. Excitation spectra of the NO vibrational levels were measured by using both the NO A 2Σ+←X 2Π and B 2Π←X 2Π transitions, the latter being somewhat stronger due to saturation effects. It was determined that the NO population was strongly inverted, with most of the nascent NO being in v=6–8; the thermodynamic limit is v=8. Injection locking of the KrF laser output permitted study of the 2491 Å NO2 band, and it was evident that the increased absorption in this region gave greatly enhanced signal levels in the excitation spectra, at those wavelengths where NO2 and NO absorption lines coincide. It was demonstrated that in the 2640–2850 Å wavelength region, NO2 can be detected by use of a single dye laser, simultaneously dissociating NO2 and electronically exciting the resultant vibrationally hot NO. Deactivation of NO(v=8) by NO2 was found to proceed with a rate coefficient of 1.1×10−11 cm3 molecule−1 s−1, whereas the coefficient for quenching by N2 and He was ≤2×10−15 cm3 molecule−1 s−1. The peculiar NO rotational distributions noted by Zacharias et al. in their study of NO2 dissociation at 3371 Å were also observed in the present work.

Model for fast, nonadiabatic collisions between alkali atoms and diatomic molecules

Equations for collisions involving two potential surfaces are presented in the impact parameter approximation. In this approximation, a rectilinear classical trajectory is assumed for the translational motion, leading to a time-dependent Schroedinger’s equation for the remaining degrees of freedom. Model potentials are considered for collisions of alkali atoms with diatomic molecules that lead to a particularly simple form of the final equations. Using the Magnus approximation, these equations are solved for parameters chosen to model the process Cs+O2→Cs++O2−, and total cross sections for ion-pair formation are obtained as a function of energy. The results exhibit oscillations that correspond qualitatively to those seen in recent measurements. In addition, the model predicts that the oscillations will become less pronounced as the initial vibrational level of O2 is increased.

On the mechanism for vibrational autoionization in H2

The cross sections for the reactions of H2+ with He and Ne to form HeH+ and NeH+ are known to depend strongly on the H2+ vibrational level. When the H2+ reactant ions are produced by autoionization of Rydberg states, the measurement of these cross sections provides a means for studying the final vibrational state distribution of H2+ ions resulting from the autoionization process. Relative cross sections for hydride ion formation were determined for approximately 100 autoionizing Rydberg states in para-H2 between the H2+2𝒥g+,v=0 and v=6 convergence limits. The relative cross sections for formation of HeH+ and NeH+ (and thus the final vibrational state distributions of H2+) are nearly the same for all the autoionizing Rydberg states between any two H2+ vibrational convergence limits v and v+1 regardless of the initial vibrational level of the autoionizing state. Combining this observation with elementary considerations, we show that vibrational autoionization proceeds predominantly (≳ 95%) via a Δv=1 mechanism when this channel is energetically allowed. When autoionization via Δv=1 is forbidden, autoionization proceeds predominantly (≳ 75%) via the minimum change in Δv. In cases when autoionization proceeds via Δv≳1, the fraction of ions left in the highest available vibrational level decreases slightly with increasing initial vibrational level.