Vibrational Excitation Processes Research Articles

Vibrational wave packet dynamics of H2O+ and H2O by strong-field Fourier transform spectroscopy

The vibrational wave packet dynamics of H2O+ and H2O are investigated by pump–probe measurements using few-cycle intense laser pulses. By a Fourier transform (FT) of the time-dependent yields of the two-body Coulomb explosion pathway, H2O2+ → H+ + OH+, and the parent ion, H2O+, the vibrational excitation processes in the electronic ground states of H2O+ and H2O induced by the intense laser field are retrieved from the phases of the respective vibrational peaks in the FT spectrum. Accurate quantum-dynamics simulations on H2O+ confirm both the peak assignment and the origin of the time-dependent ionization yields of the vibrational wave packet.

In situ quantification of NO synthesis in a warm air glow discharge by WMS-based Mid-IR QCL absorption spectroscopy

Nitric oxide (NO) is one of the most crucial products in the plasma-based nitrogen fixation process. In this work, in situ measurements were performed for quantifying the NO synthesis spatially in a warm air glow discharge, through the method of Mid-infrared quantum cascade laser absorption spectroscopy (QCL-AS). Two ro-vibrational transitions at 1900.076 cm−1 and 1900.517 cm−1 of the ground-state NO(X) were probed sensitively by the help of the wavelength modulation spectroscopy (WMS) approach to increase the signal/noise (S/N) level. The results show a decline trend of NO synthesis rate along the discharge channel from the cathode to the anode. However, from the point of energy efficiency, the cathode region is of significantly low energy efficiency of NO production. Severe disproportionality was found for the high energy consumption but low NO production in the region of cathode area, compared to that in the positive column zone. Further analysis demonstrates the high energy cost of NO production in the cathode region, is ascribed to the extremely high reduced electric field therein not selectively preferable for the processes of vibrational excitation or dissociation of N2 and O2 molecules. This drags down the overall energy efficiency of NO synthesis by this typical warm air glow discharge, particularly for the ones with short electrode gaps. Limitations of further improving the energy cost of NO synthesis by variations of the discharge operation conditions, such as discharge current or airflow rate, imply other effective manners able to tune the energy delivery selectively to the NO formation process, are sorely needed.

Non-equilibrium simulation of energy relaxation for earth reentry utilizing a collisional-radiative model

A high-temperature air collisional-radiative model considering both vibrational and electronic excited states is established to investigate the plasma characteristics along the stagnation-line in the post-shock flow by coupling with the one-dimensional flow model. The results obtained by the model are in reasonable agreement with the experimental data and previous calculation results. The evolutions of different temperatures and energy relaxation processes are analyzed in detail for the height of 76.42 km and Mach number of 40.58. The vibrational and electronic excited modes play a critical role on the energy transfer. The vibrational excitation processes under heavy-particle impact can transfer the translational energy to the vibrational mode, then to the electrons by the vibrational de-excitation processes. The excitation of atoms by heavy-particle impact can gain energy from the translational mode, and the de-excitation processes of high-lying excited states under electron impact result in the energy transfer to electrons. The elastic collisions also play a role on the direct transfer from translational energy to electrons. The calculations are performed for a wide range of flight altitudes and Mach numbers to investigate the thermochemical state and energy relaxation. By comparing the energy transfer processes under different altitudes and Mach numbers, it is found that the energy transfer is dominated by vibrational processes for the low-Height low-Mach condition, and the contribution of electronic excited mode is negligible. The division of thermochemical state and chemical reactions in the Height-Mach number diagram is obtained, which provides the basis for the selection of thermochemical model required for atmospheric reentry calculation.

Efficiency of rovibrational cooling of HeH+ by collisions with He: Cross sections and rate coefficients from quantum dynamics.

By extending an earlier study [Gianturco et al., J. Chem. Phys. 154, 054311 (2021)] on the purely rotational excitation of HeH+ by He atoms, we report in this paper integral cross sections and rate coefficients for rovibrational excitation and de-excitation processes in HeH+ due to collisions with He. The data were obtained using a new ab initio potential energy surface that includes the vibrational degree of freedom. The results are compared with those computed using the earlier potential energy surface by Panda and Sathyamurthy [J. Phys. Chem. A 107, 7125 (2003)] that additionally accounts for the proton-exchange reaction between HeH+ and He. It is shown that the exchange channel contributes nearly as much as the inelastic channel to the vibrational excitation and de-excitation processes and that the total rate constants pertaining to the purely inelastic processes are largely of the same magnitude as those obtained when both inelastic and reactive channels are included in the dynamics. The inelastic rovibrational rate coefficients involving this astrophysical cation are also found to be much larger than those obtained for anions present in similar interstellar environments.

Electronically induced defect creation at semiconductor/oxide interface revealed by time-dependent density functional theory

Carrier induced defect creation at the semiconductor-oxide interface has been known as the origin of electronic device degradation for a long time, but how exactly the interface lattice can be damaged by carriers (especially low-energy ones) remains unclear. Here we carry out real-time time-dependent density functional theory simulations on concrete $\mathrm{Si}/\mathrm{Si}{\mathrm{O}}_{2}$ interfaces to study the interaction between excited electrons and interface bonds. We show that the normal interface Si-H bonds are generally resistant to electrons due to the delocalized nature and high energy level of the Si-H antibonding states, and due to the high-energy barrier to break the Si-H bond. However, if an additional hydrogen atom exists by attaching to a nearby oxygen atom (forming a ``Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O'' complex), the Si-H bond will be greatly weakened, including the reduction of energy barrier for bond breaking, and the lowering of the antibonding state energy level which favors electron injection. Together with the multiple vibrational excitation process, the corresponding Si-H bond can be broken much more easily. Thus we propose that the Si-H\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}\ifmmode\cdot\else\textperiodcentered\fi{}H-O complex will be the center for defect creation and device degradation. We also explain why such a center might be relatively easy to form during the hydrogen annealing process.

Molecular relaxation effects on vibrational water vapor photoacoustic spectroscopy in air

Photoacoustic spectroscopy is a highly sensitive technique, well suited for and used in applications targeting the accurate measurement of water vapor in a wide range of concentrations. This work demonstrates the nonlinear photoacoustic response obtained for water vapor in air at typical atmospheric concentration levels, which is a result of the resonant vibrational coupling of water and oxygen. Relevant processes in the relaxation path of water in a mixture with air, excited with near-infrared radiation, are identified and a physical model for the acoustic signal measured with a resonant photoacoustic cell is presented. The model is valid for modulation frequencies typical for conventional and quartz-enhanced photoacoustic spectroscopy and provides a simplified means of calibration for photoacoustic water vapor sensors. Estimated values for comprised model coefficients are evaluated from photoacoustic measurements of water vapor in synthetic air. Furthermore, it is shown experimentally that the process of vibrational excitation of nitrogen is of negligible importance in the relaxation path of water vapor and thus insignificant in the photoacoustic heat production in atmospheric measurement environments.

The infrared bands of polycyclic aromatic hydrocarbons in the 1.6–1.7 μm wavelength region

Context. The 3.3 μm aromatic C–H stretching band of polycyclic aromatic hydrocarbon (PAH) molecules seen in a wide variety of astrophysical regions is often accompanied by a series of weak satellite bands at ∼3.4–3.6 μm. One of these sources, IRAS 21282+5050, a planetary nebula, also exhibits a weak band at ∼1.68 μm. While the satellite features at ∼3.4–3.6 μm are often attributed to the anharmonicities of PAHs, it is not clear whether overtones or combination bands dominate the 1.68 μm feature. Aims. In this work, we examine the anharmonic spectra of eight PAH molecules, including anthracene, tetracene, pentacene, phenanthrene, chrysene, benz[a]anthracene, pyrene, and perylene, to explore the origin of the infrared bands in the 1.6–1.7 μm wavelength region. Methods. Density functional theory (DFT) in combination with the vibrational second-order perturbation theory (VPT2) was used to compute the anharmonic spectra of PAHs. To simulate the vibrational excitation process of PAHs, the Wang–Landau random walk technique was employed. Results. All the dominant bands in the 1.6–1.7 μm wavelength range and in the 3.1–3.5 μm C–H stretching region are calculated and tabulated. It is demonstrated that combination bands dominate the 1.6–1.7 μm region, while overtones are rare and weak in this region. We also calculate the intensity ratios of the 3.1–3.5 μm C–H stretching features to the bands in the 1.6–1.7 μm region, I3.1 − 3.5/I1.6 − 1.7, for both ground and vibrationally excited states. On average, we obtain ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈12.6 and ⟨I3.1 − 3.5/I1.6 − 1.7⟩≈17.6 for PAHs at ground states and at vibrationally excited states, respectively.

Relaxation of Vibrationally Excited OH Radical by SO.

Molecular energy transfer in gas-phase collisions is of large interest in kinetics models. Vibrational excitation and deactivation processes can be concurrent with chemical reaction and/or sources of nascent excited species. In the present work, a full-dimension quasi-classical trajectory study of the vibrational deactivation of OH(v' = 1,5) in collisions with SO is presented. A global potential energy surface for the ground electronic state of HSO2, previously reported, is used to represent the interatomic interactions. The specific initial-state OH deactivation cross section and the corresponding SO activation cross sections are reported. Models for the maximum impact parameter and excitation function are discussed, while some details on the energy transfer mechanisms are also given. Specific initial-state deactivation thermal rate coefficients and vibrational average deactivation thermal rate coefficients are also presented and compared to previous results in the literature.

Orientation-specific switching of inelastic electron tunneling in an oxygen-pyridine complex adsorbed onto an Ag(110) surface.

Here, we report the development of a molecular rotary switch (a "stator-rotor" consisting of a single oxygen molecule as a stator and a single pyridine molecule as a rotor) on a silver surface. The pyridine molecule was bonded to the oxygen molecule and was found to rotate to enable "ON" or "OFF" vibrational conductance through the oxygen molecule. Four stable sites around the oxygen molecule were observed, and vibration conductance turned on and off depending on the site at which the pyridine molecule bonded. The spatially resolved mapping of the vibrational change revealed two locations of maximal vibration intensity, separated by ∼3 Å. These positions acted as two conducting channels. The two distinct vibrational energy levels were associated with the switching process. Adsorption-induced electron transfer between the silver layers and the molecules enhanced the local interactions between the molecules. The two vibration modes were excited by resonant tunneling despite substantial interactions between the molecules, which resulted in a decrease in tunneling conductance. An independent pathway exists for the vibrational excitation process by tunneling electrons and intermolecular interactions.

Validity of the Born-Oppenheimer approximation in the indirect-dissociative-recombination process

An alternative method is introduced to solve a simple two-dimensional models describing vibrational excitation and dissociation processes during the electron-molecule collisions. The model works with one electronic and one nuclear degree of freedom. The two-dimensional $R$-matrix can be constructed simultaneously on the electronic and nuclear surfaces using all three forms developed previously for electron-atom and electron-molecule collisions. These are the eigenchannel $R$-matrix form, inversion technique of Nesbet and Robicheaux, and the Wigner-Eisenbud-type form using expansion over the poles of the symmetrized Hamiltonian. The 2D $R$-matrix method is employed to solve a simple model tailored to describe the dissociative recombination and the vibrational excitation of H$_2^+$ cation in the singlet ungerade symmetry $^1\Sigma_u$. These results then serve as a (near-exact) benchmark for the following calculation in which the $R$-matrix states are replaced by their Born-Oppenheimer approximations. The accuracy of this approach and its correction with the first-order nonadiabatic couplings are discussed.

$$\hbox {H}^{+} + \hbox {O}_2$$ H + + O 2 system revisited: four-state quasidiabatic potential energy surfaces and coupling potentials

The global adiabatic and quasidiabatic potential energy surfaces for the ground and first three excited ( $$1-4\, ^3\text {A}^{\prime \prime }$$ ) electronic states of $$\hbox {H}^{+} +\hbox {O}_2$$ system are reported on a finer grid points in the Jacobi coordinates using Dunning’s cc-pVTZ basis set and internally contracted multi-reference (single and double) configuration interaction method. Ab initio procedures have been used to compute the corresponding quasidiabatic surfaces and radial coupling potentials which are relevant for the dynamical studies of inelastic vibrational excitation and charge transfer processes. Nonadiabatic couplings arising out of relative motion of proton and the vibrational motions of $$\hbox {O}_2$$ between the adiabatic electronic states have also been analyzed. Adiabatic and quasidiabatic potential energy surfaces have been constructed using ab initio procedure for the lowest four 1-4 $$^{3}A''$$ electronic states of $$\hbox {H}^{+}$$ + $$\hbox {O}_{2}$$ collision system. The computed quasidiabatic surfaces and coupling potentials would be helpful in understanding the collision dynamics of inelastic and charge transfer processes in the system.

Reactive collision of electrons with CO+ in cometary coma

Context. In order to improve our understanding of the kinetics of the cometary coma, theoretical studies of the major reactive collisions in these environments are needed. Deep in the collisional coma, inelastic collisions between thermal electrons and molecular ions result in recombination and vibrational excitation, the rates of these processes being particularly elevated due to the high charged particle densities in the inner region. Aims. This work addresses the dissociative recombination, vibrational excitation, and vibrational de-excitation of electrons with CO+ molecular cations. The aim of this study is to understand the importance of these reactive collisions in producing carbon and oxygen atoms in cometary activity. Methods. The cross-section calculations were based on multichannel quantum defect theory. The molecular data sets, used here to take into account the nuclear dynamics, were based on ab initio R-matrix approach. Results. The cross-sections for the dissociative recombination, vibrational excitation, and vibrational de-excitation processes, for the six lowest vibrational levels of CO+ – relevant for the electronic temperatures observed in comets – are computed, as well as their corresponding Maxwell rate coefficients. Moreover, final state distributions for different dissociation pathways are presented. Conclusions. Among all reactive collisions taking place between low-energy electrons and CO+, the dissociative recombination is the most important process at electronic temperatures characterizing the comets. We have shown that this process can be a major source of O(3P), O(1D), O(1S), C(3P) and C(1D) produced in the cometary coma at small cometocentric distances.

Low-energy collisions between electrons and BeD+

Multichannel quantum defect theory is applied in the treatment of the dissociative recombination and vibrational excitation processes for the BeD+ ion in the 24 vibrational levels of its ground electronic state (). Three electronic symmetries of BeD** states (, , and ) are considered in the calculation of cross sections and the corresponding rate coefficients. The incident electron energy range is 10−5–2.7 eV and the electron temperature range is 100–5000 K. The vibrational dependence of these collisional processes is highlighted. The resulting data are useful in magnetic confinement fusion edge plasma modeling and spectroscopy, in devices with beryllium based main chamber materials, such as ITER and JET, and operating with the deuterium–tritium fuel mix. An extensive rate coefficients database is presented in graphical form and also by analytic fit functions whose parameters are tabulated in the supplementary material.

Quantum dynamics calculations for e+ + LiH → Li+ + [H−; e+] dissociative positron attachment using a pseudopotential model

We theoretically discuss the dynamics of the chemical reaction process induced by positron attachment, e+ + LiH → Li+ + [H−; e+]. Simple model pseudopotentials were developed on the basis of the electrostatic interaction between e+ and LiH with a large dipole moment, where some numerical parameters were chosen to qualitatively reproduce the previous-reported ab initio positron-bound adiabatic potential energy curve of [LiH; e+]. Using the developed pseudopotential models, we have performed time-independent quantum scattering calculations using the hyperspherical coordinate. Although the main scattering processes of the e+ + LiH collision are rotational and vibrational excitation processes, it is found that the calculated probabilities for the dissociative positron attachment (DPA) process are not very small within the present pseudopotential framework. This suggest that the DPA process to LiH can be observed in the future experiments.

State-specific simulation of oxygen vibrational excitation and dissociation behind a normal shock

A complete vibration–vibration–translation (V–V–T) bound–bound transitions rates database for O2–O2 collisions is generated using the forced harmonic oscillator (FHO) model with temperature range between 1000 and 20,000K. The postshock process of oxygen vibrational excitation and dissociation is simulated by solving the system of master equations coupled with the one-dimensional compressible flow equations. The results agree well with existing shock tube data. The state-specific calculations show that inclusion of pure vibration–vibration (V–V) processes could reduce the discrepancy with experimental data and the quasi-steady-state (QSS) approximation is valid when dissociation starts to dominate.

Assessment of vibration–dissociation coupling models for hypersonic nonequilibrium simulations

The fidelity of three widely-used two-temperature vibration–dissociation coupling models, including the Park model (1988), the Macheret–Fridman model (1994), and the coupled vibration–dissociation–vibration (CVDV) model (1963), is numerically investigated via a comparison with state-specific results and existing shock tube data for oxygen flows. Under the hypothetical condition where a Boltzmann distribution corresponding to a vibrational temperature is assumed, it is found that the CVDV model with a proper parameter is capable of providing the most accurate results, whereas the Park and Macheret–Fridman models present similar prediction accuracy for which the nonequilibrium dissociation rate coefficients could be as much as three orders of magnitude lower than state-specific values. However, for actual postshock flows with intensive vibrational excitation and dissociation processes, the CVDV model still shows significant discrepancies relative to state-specific results and experimental data, presenting a much lower vibrational temperature and a higher degree of dissociation. The essential cause for the disagreement lies in the existence of non-Boltzmann distributions observed when the incubation period is over and dissociation starts to dominate. Recommendations on further modification of two-temperature vibration–dissociation coupling models are presented.

Theoretical resonant electron-impact vibrational excitation, dissociative recombination and dissociative excitation cross sections of ro-vibrationally excited BeH+ ion

A theoretical study of resonant vibrational excitation, dissociative recombination and dissociative excitation processes of the beryllium monohydride cation, BeH+, induced by electron impact, is reported. Full sets of ro-vibrationally-resolved cross sections and of the corresponding Maxwellian rate coefficients are presented for the three processes. Particular emphasis is given to the high-energy behaviour. Potential curves of , and symmetries and the corresponding resonance widths, obtained from R-matrix calculations, provide the input for calculations which use a local complex-potential model for resonant collisions in each of the three symmetries. Rotational motion of nuclei and isotopic effects are also discussed. The relevant results are compared with those obtained using a multichannel quantum defect theory method. Full results are available from the Phys4Entry database.

Electron impact on vibrationally cold

The dissociative recombination and vibrational excitation processes induced by electron impact on vibrationally cold are investigated in the framework of the multichannel quantum defect theory for electron energies below 1 eV. The thermal rate coefficients for the electron temperature range from 10 to 5000 K are reported.

Investigating the role of vibrational excitation in simulating charged-particle tracks in liquid pyrimidine

We report on our results of a study into the sensitivity of charged-particle (electron) track simulations in liquid pyrimidine, to the vibrational cross sections and vibrational energy loss distribution function employed in those simulations. We achieve this by repeating the earlier investigation of Fuss et al. [J. Appl. Phys. 117 , 214701 (2015)], but now incorporating more accurate data for the vibrational integral cross sections and the energy loss distribution function that have recently become available. We find that while changes in absorbed dose or particle range are quite minor, due to the energy transferred via vibrational excitations being low in comparison to that for other processes such as ionisation, at the very end of the tracks, where non-ionizing interactions dominate, the significantly large numbers of vibrational excitation processes increases the electrons’ ability to induce other effects (e.g. sample heating, bond breaking and radical formation) that might cause damage.

The role of electron-impact vibrational excitation in electron transport through gaseous tetrahydrofuran.

In this paper, we report newly derived integral cross sections (ICSs) for electron impact vibrational excitation of tetrahydrofuran (THF) at intermediate impact energies. These cross sections extend the currently available data from 20 to 50 eV. Further, they indicate that the previously recommended THF ICS set [Garland et al., Phys. Rev. A 88, 062712 (2013)] underestimated the strength of the electron-impact vibrational excitation processes. Thus, that recommended vibrational cross section set is revised to address those deficiencies. Electron swarm transport properties were calculated with the amended vibrational cross section set, to quantify the role of electron-driven vibrational excitation in describing the macroscopic swarm phenomena. Here, significant differences of up to 17% in the transport coefficients were observed between the calculations performed using the original and revised cross section sets for vibrational excitation.