High Vibrational Excitation Research Articles

Overtone Excitation of Nitrogen Molecules via Stimulated Raman Pumping.

Nitrogen bond activation is a pivotal process in chemistry, with bond excitation being fundamental to understanding the underlying mechanisms, making the preparation of molecules in specific quantum states crucial. Here we report the first overtone excitation of the N2 molecule from X1Σg+(v = 0, j = 0, 1, and 2) to X1Σg+(v = 2, j = 0, 1, 2, and 3) using the stimulated Raman pumping (SRP) method in a pulsed molecular beam. N2 was detected using 2+1 resonance-enhanced multiphoton ionization through the a″1Σg+ state. An excitation efficiency of 4% was achieved within the excitation region in which the SRP laser intensity was saturated, indicating the low cross-sectional nature of the process. The SRP detuning spectra for different branches were measured, and the excited N2 [X1Σg+(v = 2)] was further used to access various vibrational states of a″1Σg+, enabling the determination of its vibrational constants. This research opens up new opportunities for studying the specific high vibrational excitation of nitrogen in reactions and scattering experiments and contributes additional precise spectral data for the N2 molecule.

Unimolecular processes in diatomic carbon anions at high rotational excitation

On the millisecond to second time scale, stored beams of diatomic carbon anions C2− from a sputter ion source feature unimolecular decay of yet unexplained origin by electron emission and fragmentation. To account for the magnitude and time dependence of the experimental rates, levels with high rotational and vibrational excitation are modeled for the lowest electronic states of C2−, also including the lowest quartet potential. Energies, spontaneous radiative decay rates (including spin-forbidden quartet-level decay), and tunneling dissociation rates are determined for a large number of highly excited C2− levels and their population in sputter-type ion sources is considered. For the quartet levels, the stability against autodetachment is addressed and recently calculated rates of rotationally assisted autodetachment are applied. Nonadiabatic vibrational autodetachment rates of high vibrational levels in the doublet C2− ground potential are also calculated. The results are combined to model the experimental unimolecular decay signals. Comparison of the modeled to the experimental rates measured at the cryogenic storage ring facility CSR gives strong evidence that C2− ions in quasistable levels of the quartet electronic states are the so far unidentified source of unimolecular decay. Published by the American Physical Society 2024

Frustrated charge transfer in vibrationally inelastic Ar++N2 collisions via hard collision glory scattering

Vibrational energy transfer in collisions between ions and neutrals is a fundamental process in interstellar media, planetary atmospheres, and plasmas. The conventional wisdom is that glancing collisions with large impact parameters are forward-scattered with low vibrational excitation, while hard collisions with small impact parameters are sideway- or backward-scattered with relatively high vibrational excitation. Here, we report experimental observations with a three-dimensional velocity-map imaging crossed-beam apparatus in the inelastic scattering process Ar++N2(v′′ = 0, J′′)→Ar++N2(v′, J′), where all the vibrationally excited N2 products are dominated by forward scattering, contradicting the textbook model. Trajectory surface hopping calculations not only reproduced the experimental observation qualitatively, but also revealed that the vibrational excitation mainly occurs through a transient charge-transfer process. The hard collision glory mechanism, which has so far only been observed in inelastic rotational energy transfer between neutrals, is shown to play a major role for vibrational excitation in the inelastic Ar++N2 collision, via the frustrated charge transfer process.

Expulsion of hydrogen negative ions by the anode layer of negative space charge from Penning discharge with metal hydride cathodes

The method of expulsion of hydrogen negative ions in the axial direction by a negative space charge formed in the plasma of Penning discharge with metal hydride cathodes is discussed. The plasma is produced on hydrogen, which is stored in metal hydride cathodes and injected locally into the discharge cell without any external gas supply due to ion current impact from plasma. The process of nonequilibrium desorption resulted in high vibrational excitation of H2 molecules, which greatly increases the efficiency of H− ion production. The method of efficient expulsion of H− ions depends on the external magnetic field. At a high magnetic field, it is performed by the supply of negative electric bias of the opposite cathode, but at a low one, H− ions are pushed out by the negative space charge of the anode layer, which expands toward the axis. The impact of both methods is considered in the case of DC discharge and in pulsating regime implementation.

High vibrational excitation of the reagent transforms the late-barrier H + HOD reaction into an early-barrier reaction.

Polanyi's rules predict that a late-barrier reaction yields vibrationally cold products; however, experimental studies showed that the H2 product from the late-barrier H + H2O(|04⟩-) and H + HOD(vOH = 4) reactions is vibrationally hot. Here, we report a potential-averaged five-dimensional state-to-state quantum dynamics study for the H + HOD(vOH = 0-4) → H2 + OD reactions on a highly accurate potential energy surface with the total angular momentum J = 0. It is found that with the HOD vibration excitation increasing from vOH = 1 to 4, the product H2 becomes increasingly vibrationally excited and manifests a typical characteristic of an early barrier reaction for vOH = 3 to 4. Analysis of the scattering wave functions revealed that vibrational excitation in the breaking OH bond moves the location of dynamical saddle point from product side to reactant side, transforming the reaction into an early barrier reaction. Interestingly, pronounced oscillatory structures in the total and product vibrational-state-resolved reaction probabilities were observed for the H + HOD(vOH = 3, 4) reactions, in particular at low collision energies, which originate from the Feshbach resonance states trapped in the bending/torsion excited vibrational adiabatic potential wells in the entrance region due to van der Waals interactions.

A Linearized Semiclassical Dynamics Study of the Multiquantum Vibrational Relaxation of NO Scattering from a Au(111) Surface.

The vibrational relaxation of NO molecules scattering from a Au(111) surface has served as the focus of efforts to understand nonadiabatic energy transfer at metal-molecule interfaces. Experimental measurements and previous theoretical efforts suggest that multiquantal NO vibrational energy relaxation occurs via electron-hole pair excitations in the metal. Here, using a linearized semiclassical approach, we accurately predict the vibrational relaxation of NO from the νi = 3 state for different incident translational energies. We also accurately capture the central role of transient electron transfer from the metal to the molecule in mediating the vibrational relaxation process but fall short of quantitatively predicting the full extent of multiquantum relaxation for high incident vibrational excitations (νi = 16).

Ultrafast electron transfer of different vibronic states in flavodoxin

Understanding the ultrafast electron transfer (ET) processes involving various vibronic excitation in biological systems remains challenging and requires well-defined model proteins for in-depth studies. In this work, we selected three types of mutants with different ET lifetimes in flavodoxin to explore the excitation dependence of the photo-induced ET dynamics. For the ultrafast ET processes within the time window of intramolecular vibrational relaxation, with increasing excitation energies, we observed no change of ET lifetimes but with higher vibrational excitation in products, both of which can be attributed to the unique nature of ultrafast ET. For the slower ET, we did not find any dependence of product vibration excitation relative to the initial different excitation due to the complete vibrational relaxation. These results strongly indicate the vibrational energy can be propagated to the subsequent reactions during ultrafast ET processes.

High energy vibrational excitations of nitromethane in liquid water.

The pathways and timescales of vibrational energy flow in nitromethane are investigated in both gas and condensed phases using classical molecular mechanics, with a particular focus on relaxation in liquid water. We monitor the flow of excess energy deposited in vibrational modes of nitromethane into the surrounding solvent. A marked energy flux anisotropy is found when nitromethane is immersed in liquid water, with a preferential flow to those water molecules in contact to the nitro group. The factors that permit such anisotropic energy relaxation are discussed, along with the potential implications on the molecule's non-equilibrium dynamics. In addition, the energy flux analysis allows us to identify the solvent motions responsible for the uptake of solute energy, confirming the crucial role of water librations. Finally, we also show that no anisotropic vibrational energy relaxation occurs when nitromethane is surrounded by argon gas.

Activation of vibrational-induced CO2 dissociation in cold non-equilibrium plasma

The activation of vibrational-induced dissociation of CO2 in cold non-equilibrium plasma discharges is investigated by means of a 0D self-consistent kinetic model, which, with a state-to-state approach, is able to calculate the CO2 vibrational distribution function (vdf) of the asymmetric mode levels, the electron energy distribution function and the corresponding vibrational-induced and electron impact CO2 dissociation rates. The conditions for the onset of such activation are linked to the achievement of a sufficiently high CO2 vibrational excitation characterized by the presence of a non-equilibrium plateau in the CO2 vdf, resulting from the combined effect of electron–vibrational and vibrational–vibrational collisions, which, by overpopulating the higher vibrational levels, enhances dissociation. Such non-equilibrium conditions are maximized at lower gas temperature, lower pressure and higher power density values. In particular, for the power density, an activation threshold value can be obtained from simulations and its dependence on the gas temperature and pressure can be investigated. The dependence of the maximum vibrational temperature reached at the end of the discharge as a function of the gas temperature and pressure is also analyzed. A satisfactory agreement from our simulation results with the Kotov’s criterion for vibrational activation has been found.

Non-equilibrium plasma kinetics of CO2 in glow discharges: a comparison with existing modeling and experimental results

We report results obtained by our 0D, time-dependent self-consistent model for the description of the CO2 plasma kinetics in glow discharge conditions, comparing our results with the simulation and experimental results reported by Grofulovic et al (2018 Plasma Sources Sci. Technol. 27 115009; 2019 PhD Thesis) and Klarenaar et al (2017 Plasma Sources Sci. Technol. 26 115008). Our model is based on the simultaneous solution of the kinetic equations describing the vibrational, the electronic excited states and the plasma chemistry and of the electron Boltzmann equation for the calculation of the electron energy distribution function (eedf). The results for the vibrational level densities of CO2 show a satisfactory agreement with the Grofulovic’s model results, despite the differences in the vibrational energy level scheme and in the kinetic processes included with the correspondent rate coefficients, with a good match also with the corresponding experimental results. Moreover, conditions characterized by higher power density (5–50 W cm−3) have been investigated to understand the behavior of the CO2 plasma discharge when a higher vibrational excitation is present. Large deviations of the vibrational distributions of CO2 and CO from equilibrium ones are predicted both in discharge and post discharge conditions. In particular, the CO2 vibrational distribution presents a behavior similar to a Treanor distribution for v < 15 while a deactivation of the plateau in the vibrational distribution function after v > 15 appears as a consequence of the dissociation induced by vibrational excitation mechanism, i.e. pure vibrational mechanism, becoming important at higher power densities. Finally, the results dependence on the selection of the CO2 electron molecule dissociation cross section, i.e. Phelps (1973 J. Appl. Phys. 44 4464 or Cosby (1993 Report No. AD-A266 464 WL-TR-93-2004 (Dayton, OH: Wright-Patterson Airforce Base)), has been investigated, showing that its more opportune choice is still a problem to be discussed for the description of conditions in which the electron impact dissociation dominates the kinetics, while once vibrational excitation is activated, CO2 dissociation is essentially driven by vibrational-induced dissociation, depending to a minor extent from that choice.

The systematic study of quenching rate coefficients of NO(X2Π) colliding by NO(A2Σ+) and N2(A3Σ+u, B3Πg)

MRCI approach was used to investigate the PECs for NO and N2. These PECs are in good agreement with the RKR potential, and by using RKR potential we obtain accurate Frank-Condon factors. Based on Rosen-Zener approximation and Frank-Condon factors, we calculate the quenching rate coefficients of NO(X2Π,v*=0–3) colliding by NO(A2Σ+,v = 0–20), N2(A3Σ+u,v = 0–23) and N2(B3Πg,v = 0–28). The higher vibrational excitation v* is more conducive to the occurrence of quenching. Moreover, rate coefficients of NO(X2Π,v*) and N2(X1Σ+g,v*) colliding by NO(A2Σ+,v = 0–20) are calculated at high temperature. Our results provide more accurate numerical basis for the entry of aircraft into the atmosphere at high temperature.

Absorption Spectra of Electrified Hydrogen Molecules

Molecular hydrogen normally has only weak, quadrupole transitions between its rovibrational states, but in a static electric field it acquires a dipole moment and a set of allowed transitions. Here we use published ab initio calculations of the static electrical response tensors of the H2 molecule to construct the perturbed rovibrational eigensystem and its ground state absorptions. We restrict attention to two simple field configurations that are relevant to condensed hydrogen molecules in the interstellar medium (ISM): a uniform electric field and the field of a pointlike charge. The energy eigenstates are mixtures of vibrational and angular momentum eigenstates so there are many transitions that satisfy the dipole selection rules. We find that mixing is strongest among the states with high vibrational excitation, leading to hundreds of absorption lines across the optical and near-infrared. These spectra are very different from that of the field-free molecule, so if they appeared in astronomical data they would be difficult to assign. Furthermore, in a condensed environment the excited states likely have short lifetimes to internal conversion, giving the absorption lines a diffuse appearance. We therefore suggest electrified H2 as a possible carrier of the diffuse interstellar bands (DIBs). We further argue that in principle it may be possible to account for all of the DIBs with this one carrier. However, despite electrification, the transitions are not very strong and a large column of condensed H2 would be required, making it difficult to reconcile this possibility with our current understanding of the ISM.

Vibrational effects in the quantum dynamics of the H + D2 + charge transfer reaction

The H + D(v = 0,1 and 2) charge transfer reaction is studied using an accurate wave packet method, using recently proposed coupled diabatic potential energy surfaces. The state-to-state cross section is obtained for three different channels: non-reactive charge transfer, reactive charge transfer, and exchange reaction. The three processes proceed via the electronic transition from the first excited to the ground electronic state. The cross section for the three processes increases with the initial vibrational excitation. The non-reactive charge transfer process is the dominant channel, whose branching ratio increases with collision energy, and it compares well with experimental measurements at collision energies around 0.5 eV. For lower energies the experimental cross section is considerably higher, suggesting that it corresponds to higher vibrational excitation of D(v) reactants. Further experimental studies of this reaction and isotopic variants are needed, where conditions are controlled to obtain a better analysis of the vibrational effects of the D reagents.

Modelling the non-local thermodynamic equilibrium spectra of silylene (SiH2).

This paper sets out a robust methodology for modelling spectra of polyatomic molecules produced in reactive or dissociative environments, with vibrational populations outside local thermal equilibrium (LTE). The methodology is based on accurate, extensive ro-vibrational line lists containing transitions with high vibrational excitations and relies on the detailed ro-vibrational assignments. The developed methodology is applied to model non-LTE IR and visible spectra of silylene (SiH2) produced in a decomposition of disilane (Si2H6), a reaction of technological importance. Two approaches for non-LTE vibrational populations of the product SiH2 are introduced: a simplistic 1D approach based on the Harmonic approximation and a full 3D model incorporating accurate vibrational wavefunctions of SiH2 computed variationally with the TROVE (Theoretical ROVibrational Energy) program. We show how their non-LTE spectral signatures can be used to trace different reaction channels of molecular dissociations.

Determining the Effect of Hot Electron Dissipation on Molecular Scattering Experiments at Metal Surfaces.

Nonadiabatic effects that arise from the concerted motion of electrons and atoms at comparable energy and time scales are omnipresent in thermal and light-driven chemistry at metal surfaces. Excited (hot) electrons can measurably affect molecule–metal reactions by contributing to state-dependent reaction probabilities. Vibrational state-to-state scattering of NO on Au(111) has been one of the most studied examples in this regard, providing a testing ground for developing various nonadiabatic theories. This system is often cited as the prime example for the failure of electronic friction theory, a very efficient model accounting for dissipative forces on metal-adsorbed molecules due to the creation of hot electrons in the metal. However, the exact failings compared to experiment and their origin from theory are not established for any system because dynamic properties are affected by many compounding simulation errors of which the quality of nonadiabatic treatment is just one. We use a high-dimensional machine learning representation of electronic structure theory to minimize errors that arise from quantum chemistry. This allows us to perform a comprehensive quantitative analysis of the performance of nonadiabatic molecular dynamics in describing vibrational state-to-state scattering of NO on Au(111) and compare directly to adiabatic results. We find that electronic friction theory accurately predicts elastic and single-quantum energy loss but underestimates multiquantum energy loss and overestimates molecular trapping at high vibrational excitation. Our analysis reveals that multiquantum energy loss can potentially be remedied within friction theory whereas the overestimation of trapping constitutes a genuine breakdown of electronic friction theory. Addressing this overestimation for dynamic processes in catalysis and surface chemistry will likely require more sophisticated theories

Clustering and multiphoton effects in velocity map imaging of methyl chloride

We have investigated different effects caused by clustering and multiphoton processes in the photodissociation of CHCl using velocity map imaging (VMI). Single-photon dissociation of CHCl at 193.3 nm yields CH and Cl fragments probed subsequently by resonance enhanced multiphoton ionisation (REMPI) at 333.52 nm and 235.31 nm, respectively. In the clusters, the CH fragments are strongly ‘caged’ losing most of their kinetic energy. Consequently, the VMI is dominated by a sharp central peak. On the other hand, the Cl fragments lose a broad range of energies in the cluster, yielding a blurred isotropic ‘central blob’ in VMI. At an increased 193 nm photon flux, another central feature appears in the CH images for isolated CHCl molecules. This ‘hour-glass’ shaped image corresponds to the CHCl multiphoton ionisation at 193 nm and subsequent CHCl ion photodissociation at 333 nm. The ion photodissociation is a parallel process with the anisotropy β-parameter values between +1.3 and +2.0. The total kinetic energy release (TKER) distribution suggests a high CH vibrational excitation. The CHCl ion photodissociation is discussed and compared to the photodissociation of similar CHBr ion.

Quantum dynamics studies of isotope effects in the Mg+(3p) + HD \u2192 MgH+/MgD+ + D/H insertion reaction

The time-dependent wave packet quantum dynamics studies for the Mg+(3p) + HD → MgH+/MgD+ + D/H diabatic reaction are carried out for the first time on recently developed diabatic YHWCH potential energy surfaces [Phys. Chem. Chem. Phys., 2018, 20, 6638–6647]. The results of reaction probabilities and total integral cross sections show a dramatic preference to the formation of MgD+ over MgH+ owing to the insertion reaction mechanism in the title reaction. The MgD+/MgH+ branching ratio witnesses a monotonic decrease from 10.58 to 3.88 at collision energy range of 0.01 to 0.20 eV, and at the collision energy of 0.114 eV, it is close to the experimental value of 5. The rovibrational state-resolved ICSs of the two channels show the products MgD+ have higher vibrational excitation and hotter rotational state distributions. The opacity function P(J) suggests that the MgH+ + D channel and MgD+ + H channel are dominated by high-b and low-b collisions, respectively. Both forward and backward scattering peaks are found in the differential cross section curves, whereas the angle distributions of products are not strictly forward-backward symmetric because of the short lifetime of the complex in the reaction.

HCS(A2A″)-based insights into the effect of vibrational excitation on the reactions C+SH (v = 0–20, j = 0) → S+CH, H+CS

The effect of vibrational excitation on reaction C+SH (v = 0–20, j = 0) → S+CH, H+CS is investigated on the excited potential energy surface of HCS(A2A″) by the quasi-classical trajectory method. The obtained reaction probability, total integral cross section (ICS), and the impact parameter show that the influence of vibration excitation presents different characteristics on different reaction channels. The vibrational state-resolved ICSs, differential cross sections as well as two-angle distribution functions P(θr), P(ϕr) of products for different vibrational quantum numbers of reactant are investigated. These results show that (i) the products have obvious forward–backward scattering feature; (ii) for different reactions, the distribution P(θr) varies with vibrational quantum number of reactant; (iii) at high vibrational excitations of the reactant, the insertion mechanism becomes apparent in this reaction, so the product molecules are more positively oriented along the positive direction of the scattering plane.

Photodissociation Dynamics of Nitrous Oxide near 145 nm: The O(1S0) and O(3P J=2,1,0) Product Channels.

We report the study of photodissociation dynamics of nitrous oxide in the vacuum ultraviolet region, using the time-sliced velocity map ion imaging technique. Ion images of the O(1S0) and O(3P J=2,1,0) products were measured at nine photolysis wavelengths from 142.55 to 148.79 nm. The product channels O(1S0) + N2(X1Σg+) and O(3P J=2,1,0) + N2(A3Σu+) have been observed. For these dissociation channels, the total kinetic energy releases of the dissociated products were acquired. With vibrational structures of the N2 coproducts partially resolved in the experimental images, the branching ratios of different vibrational states of the N2 coproducts were determined, and the vibrational state specific anisotropy parameters (β values) were derived. Analysis shows that the O(1S0) + N2(X1Σg+) channel is primarily formed via nonadiabatic couplings between the C (1Π) state and the higher-lying D (1Σ+) state of the N2O. A moderate rotational excitation and high vibrational excitation of N2(X1Σg+) products have been observed through this pathway. On the other hand, for the O(3P J=2,1,0) + N2(A3Σu+) channels, where a slightly higher rotational excitation of N2 coproducts have been observed, the possible pathway would be via nonadiabatic couplings from the C (1Π) state to the lower-lying A(1Σ-)state.

Non-equilibrium Microwave Plasma for Efficient High Temperature Chemistry.

A flowing microwave plasma based methodology for converting electric energy into internal and/or translational modes of stable molecules with the purpose of efficiently driving non-equilibrium chemistry is discussed. The advantage of a flowing plasma reactor is that continuous chemical processes can be driven with the flexibility of startup times in the seconds timescale. The plasma approach is generically suitable for conversion/activation of stable molecules such as CO2, N2 and CH4. Here the reduction of CO2 to CO is used as a model system: the complementary diagnostics illustrate how a baseline thermodynamic equilibrium conversion can be exceeded by the intrinsic non-equilibrium from high vibrational excitation. Laser (Rayleigh) scattering is used to measure the reactor temperature and Fourier Transform Infrared Spectroscopy (FTIR) to characterize in situ internal (vibrational) excitation as well as the effluent composition to monitor conversion and selectivity.