Population Of Vibrational States Research Articles

Photodissociation dynamics of SO2 via the G̃1B1 state: The O(1D2) and O(1S0) product channels.

Produced by both nature and human activities, sulfur dioxide (SO2) is an important species in the earth's atmosphere. SO2 has also been found in the atmospheres of other planets and satellites in the solar system. The photoabsorption cross sections and photodissociation of SO2 have been studied for several decades. In this paper, we reported the experimental results for photodissociation dynamics of SO2 via the G̃1B1 state. By analyzing the images from the time-sliced velocity map ion imaging method, the vibrational state population distributions and anisotropy parameters were obtained for the O(1D2) + SO(X3Σ-, a1Δ, b1Σ+) and O(1S0) + SO(X3Σ-) channels, and the branching ratios for the channels O(1D2) + SO(X3Σ-), O(1D2) + SO(a1Δ), and O(1D2) + SO(b1Σ+) were determined to be ∼0.3, ∼0.6, and ∼0.1, respectively. The SO products were dominant in electronically and rovibrationally excited states, which may have yet unrecognized roles in the upper planetary atmosphere.

Vibration to Vibration: Product Energy Distribution of F + HD Crossed Molecular Beam Experiments.

We investigated the F + HD(v = 1, j = 0) → HF + D reaction using the crossed molecular beam technique combined with the D atom Rydberg tagging time-of-flight spectroscopy. By detecting the products at various scattering angles for different collision energies in the range of 0.8-1.2 kcal/mol, we observed the forward-scattering products of HF(v' = 4) and determined the threshold energy for the opening of this reaction channel. Similar experiments were conducted for the F + HD(v = 0, j = 0) → HF + D reaction within the range of 1.1-1.6 kcal/mol, where forward-scattering products of HF(v' = 3) were observed, and the threshold energy for this reaction channel was determined as well. Furthermore, we measured the differential cross-sections for the F + HD → HF + D reaction in both the vibrational ground state and the excited state of HD and analyzed the vibrational quantum-state distribution of the HF products. It was found that the population of vibrational quantum states of the HF products increases synchronously with the excitation of the reactant HD vibrationally.

Experimentally-measured rate parameters for vibrational quenching of CO by N2

Carbon monoxide is a trace species in Titan's atmosphere that is not consistently found in local thermodynamic equilibrium at all altitudes. Understanding how CO exchanges energy with atmospheric collision partners is therefore crucial for accurate atmospheric modelling. Particularly significant could be vibrational relaxation of CO by nitrogen due to the high collision frequency in Titan's N2-based atmosphere. Some CO(v)-N2 rate coefficients have never been experimentally measured while others were measured decades ago in low-precision experiments. In this work, we present precise experimental measurements of the rate coefficients for the quenching of CO(v = 1,2) by N2. To perform the experiment, an equilibrium mixture of CO, N2, O3, and bath gases was subjected to a temperature-jump, creating a small amount of vibrationally-excited CO. Transient diode laser absorption spectroscopy was then used to observe the evolving vibrational state populations as a function of time. The rate coefficients of interest were found by measuring quenching rate as a function of quencher concentrations. The rate coefficient for quenching of CO(v = 1) by N2 was measured to be k = (1.3 ± 0.2) x 10−14 cm3 molecule−1 s−1 and the rate coefficient for quenching of CO(v = 2) by N2 was measured to be k = (2.1 ± 0.5) x 10−13 cm3 molecule−1 s−1 at 315 K.

A Spectroscopic Thermometer: Individual Vibrational Band Spectroscopy with the Example of OH in the Atmosphere of WASP-33b

Individual vibrational band spectroscopy presents an opportunity to examine exoplanet atmospheres in detail, by distinguishing where the vibrational state populations of molecules differ from the current assumption of a Boltzmann distribution. Here, retrieving vibrational bands of OH in exoplanet atmospheres is explored using the hot Jupiter WASP-33b as an example. We simulate low-resolution spectroscopic data for observations with the JWST's NIRSpec instrument and use high-resolution observational data obtained from the Subaru InfraRed Doppler instrument (IRD). Vibrational band–specific OH cross-section sets are constructed and used in retrievals on the (simulated) low- and (real) high-resolution data. Low-resolution observations are simulated for two WASP-33b emission scenarios: under the assumption of local thermal equilibrium (LTE) and with a toy non-LTE model for vibrational excitation of selected bands. We show that mixing ratios for individual bands can be retrieved with sufficient precision to allow the vibrational population distributions of the forward models to be reconstructed. A fit for the Boltzmann distribution in the LTE case shows that the vibrational temperature is recoverable in this manner. For high-resolution, cross-correlation applications, we apply the individual vibrational band analysis to an IRD spectrum of WASP-33b, applying an “unpeeling” technique. Individual detection significances for the two strongest bands are shown to be in line with Boltzmann-distributed vibrational state populations, consistent with the effective temperature of the WASP-33b atmosphere reported previously. We show the viability of this approach for analyzing the individual vibrational state populations behind observed and simulated spectra, including reconstructing state population distributions.

Luminescence Measurements of the Hyperthermal Reactions of N/N+ + NH3.

Chemiluminescence from a system of collisions, N+/N/Kr+/Kr/Xe+/Xe + NH3, at collision energies between 10 and 170 eV (center of mass, COM), was measured in the spectral range 300-1000 nm. The energy dependence of the emission excitation cross sections was quantified, and molecular signatures were fit to known spectroscopic constants to determine vibrational-state populations. For both N and N+ collision species, the strongest features were assigned to emissions from NH (A-X) and the atomic hydrogen Balmer series. For each of the spectra resulting from collisions with primary cations, the NH (A-X) emissions had the largest cross sections reaching values of (1.0-1.5) × 10-18 cm2 by 100 eV. Additional features originating from atomic nitrogen and NH (c-a) emissions were also observed. The NH (c-a) emissions accounted for about 8%, 13%, and 15% for total excited populations in collisions with Xe+, N+, and Kr+, respectively. These transitions were consistent with short-range interactions resulting in collision-induced dissociation of the NH3 molecule with apparent energy thresholds between 20 and 30 eV and emission cross sections decreasing with ion mass. Evidence of charge exchange in the N+ + NH3 collisions was observed in the resulting spectra as broad transitions between 420 and 480 nm and were assigned to NH+ emitting from the (B) state. Differences between the spectra were observed as changes in the emission signal with the neutral collisions producing only 30% or 65% of the NH (A-X) emission cross sections compared to the cation results for xenon and krypton, respectively. For N and N+, NH (A) was created in equal amounts at lower collision energies, but the emission for the neutral system increases above that of the cation at collision energies greater than 80 eV COM. In both cases, the threshold energy for appearance was below 10 eV, suggesting an additional pathway for NH (A) formation, namely, hydrogen abstraction or charge exchange and abstraction for the N and N+, respectively. In all cases, the neutral NH (c-a) emission intensity was similar between neutral and cation pairs. The H-α emission line (n = 3-2) decreased to about 10%, 33%, and 50% of the corresponding cation spectra for xenon, krypton, and nitrogen, respectively.

Application of Reflected Shock Wave Configuration to Validate Nonequilibrium Models of Reacting Air

The direct simulation Monte Carlo (DSMC) method is used to model transient thermal and chemical relaxation behind reflected shock waves in oxygen–argon and air mixtures under conditions reproducing earlier shock-tube experiments. Two vibration–translation and three popular DSMC chemical reaction models are tested. Where possible, model parameters are adjusted to match equilibrium and nonequilibrium relaxation times and reaction rates. A number of factors that impact relaxation and reaction model validation are examined, including gas–surface interactions, time-varying freestream properties, location of the observation point, electronic excitation, and nonequilibrium populations of vibrational states probed in the experiments. Comparison of numerical and experimental results has demonstrated that the reflected shock configuration is a platform very convenient for validation and analysis of high-temperature chemical reaction models. Computations have shown that the Bias reaction model is superior to the total collision energy and quantum kinetic models, providing reasonable agreement with measured absorbance time histories and vibrational temperatures in oxygen–argon mixtures and pure . There are some modeling-versus-experiment differences observed for air that may warrant additional studies focused on Zeldovich reaction rates and oxygen–nitrogen vibrational excitation and nonequilibrium dissociation rate.

Coherent control of the vibrational dynamics of aligned heteronuclear diatomic molecules

We present an analytical pulse design protocol for controlling the vibrational dynamics of polar diatomic molecules within a given electronic state. Altering the potential energy function via the position-dependent electric permanent dipole moment, the vibrational state population dynamics is directly controlled using appropriately shaped laser pulses in the midinfrared regime. The optimal pulse shapes---that are expected to drive the molecule along user-defined quantum pathways---are obtained by reverse engineering, that is, solving the Schr\"odinger equation of the nuclei inversely in a relevant subspace. The proposed control scheme is validated by accurately solving the full time-dependent Schr\"odinger equation of the ${\mathrm{HeH}}^{+}$ molecular ion with two completely different methods: (1) propagating the complex population amplitudes of many field-free eigenstates or (2) propagating directly the nuclear wave packet on a grid. We find that besides smooth transitions, arbitrary Rabi oscillations as well as vibrational ladder climbing can be efficiently controlled with the present scheme. As a result, the molecule is successively excited beyond the potential barrier, leading to enhanced dissociation in the ground electronic state. Rotational effects and possible extensions of the presented control are also briefly discussed.

ExoMol photodissociation cross-sections – I. HCl and HF

ABSTRACT Photon initiated chemistry, i.e. the interaction of light with chemical species, is a key factor in the evolution of the atmosphere of exoplanets. For planets orbiting stars in UV-rich environments, photodissociation induced by high-energy photons dominates the atmosphere composition and dynamics. The rate of photodissociation can be highly dependent on atmospheric temperature, as increased temperature leads to increased population of vibrational excited states and the consequent lowering of the photodissociation threshold. This paper inaugurates a new series of papers presenting computed temperature-dependent photodissociation cross-sections with rates generated for different stellar fields. Cross-sections calculations are performed by solving the time-independent Schrödinger equation for each electronic state involved in the process. Here, photodissociation cross-sections for hydrogen chloride and hydrogen fluoride are computed for a grid of 34 temperatures between 0 and 10 000 K. Use of different radiation fields shows that for the Sun and cooler stars the photodissociation rate can increase exponentially for molecular temperatures above 1000 K; conversely the photodissociation rates in UV rich fields instead are almost insensitive to the temperature of the molecule. Furthermore, these rates show extreme sensitivity to the radiation model used for cool stars, suggesting that further work on these may be required. The provision of an ExoMol data base of cross-sections is discussed.

Infrared frequency comb spectroscopy of CH2I2: Influence of hot bands and pressure broadening on the ν1 and ν6 fundamental transitions.

Direct frequency comb spectroscopy was utilized to measure the vibrational absorption spectrum of diiodomethane, CH2I2, from 2960 to 3125cm-1. The data were obtained using a CH2I2 concentration of (6.8 ± 1.3) × 1015moleculecm-3 and a total pressure of 10-300mbar with either nitrogen or argon as the bath gas. The rovibrational spectra of two fundamental transitions, ν6 and ν1, were recorded and analyzed. We suggest that a significant contribution to the observed congested spectra is due to the population in excited vibrational states of the low energy ν4 I-C-I bend, resulting in transitions 61 04n n and 11 04n n, where the integer n is the initial vibrational level v = 1-5. PGOPHER was used to fit the experimental spectrum, allowing for rotational constants and other spectral information to be reported. In addition, it was found that the peak widths for the observed transitions were limited by pressure broadening, resulting in a pressure broadening parameter of (0.143 ± 0.006) cm-1 atm-1 by N2 and (0.116 ± 0.006) cm-1 atm-1 by Ar. Further implications for other dihaloalkane infrared spectra are discussed.

Optically pumped and matrix‐assisted anti‐Stokes Raman spectroscopy

AbstractAnti‐Stokes Raman spectroscopy is an excellent method to suppress optical backgrounds from Raman scatters and surrounding materials, even in electronic resonance conditions. The upshift of scattered light can avoid the fluorescence or phosphorescence that is usually red‐shifted with respect to the excitation wavelength. However, this method's competence is circumscribed by its low scattering efficiency, which results from an insufficient population of upper vibrational states that obey temperature‐dependent Boltzmann statistics. To overcome this limitation, we propose that the mode‐selective absorption of overtone or combination vibrations leads to enhanced anti‐Stokes Raman scattering under simultaneous optical pumping and Raman excitation. In this work, we applied near‐infrared (NIR) pumping resonant with vibrational overtones of nonfluorescent systems. Vibrationally resonant pumping is efficient for population transfer, and the excess energy is thermodynamically distributed over all fundamental vibrations. Furthermore, we present that NIR absorbing molecules can be employed as matrices to amplify anti‐Stokes Raman signals of weak scattering targets.

Boundary conditions for fluid-dynamic parameters of a single-component gas flow with vibrational deactivation on a solid wall

Boundary conditions for fluid-dynamic parameters of a strongly non-equilibrium singlecomponent rarefied gas flow in the slip regime are obtained using kinetic-theory methods. The gas flow is described in the frame of the state-to-state approach assuming vibrational energy exchange as the slow relaxation process. The set of governing equations including conservation equations coupled with additional relaxation equations for vibrational state populations is presented. The gas-solid surface interaction is considered on the basis of the specular-diffusive model, and possible vibrational deactivation/excitation processes on the wall are taken into account. The obtained boundary conditions depend on the accommodation and deactivation coefficients along with the transport coefficients such as the multi-component vibrational energy diffusion and thermal diffusion coefficients; the thermal conductivity; the bulk and shear viscosity coefficients and the relaxation pressure. The dependence of boundary conditions on the normal mean stress has been obtained for the first time. In the particular case of the gas without internal degrees of freedom, the slip velocity and the temperature jump can be reduced to the well-known in the literature expressions. Implementation of the state-specific boundary conditions should not cause additional computational costs in numerical simulations of viscous flows in the state-to-state approach, since the slip/jump equations depend on the transport coefficients which have to be evaluated regardless of the boundary conditions used in the code.

Higher criteria for the regularity of a one-dimensional local field

A coupled problem of gasdynamics, vibrational relaxation, and dissociation in the flow of oxygen behind reflected shock waves is studied. The detailed state-to-state kinetic approach is used, which is based on a coupled solution of the momentum and energy conservation equations with the balance equations for molecular vibrational state populations and concentrations of oxygen atoms. Initial conditions corresponding to recent experiments in shock tubes are considered. For different models of physicochemical processes, a comparison is made with experimental data; varying the model parameters yields satisfactory agreement of all gas-dynamic parameters with the measured ones. The key feature of the proposed approach is the allowance for partial vibrational-chemical relaxation in the time interval between the incident and reflected shock waves. When relaxation between the shocks is not frozen, the reflected shock wave propagates through a vibrationally nonequilibrium gas, which significantly affects kinetics and gas dynamics. Accounting for partial relaxation ensures good agreement between the pressure calculated behind the front of the reflected shock wave and the pressure measured in the experiment. On the other hand, comparison with the vibrational temperature calculated indirectly from spectroscopic experimental data under the assumption of frozen relaxation shows noticeable differences near the shock wave front. We conclude that the technique for extracting gas-dynamic parameters fromspectroscopic data has to be improved by taking into account vibrational excitation before the reflected shock wave.

Tackling voltage losses

Voltage losses limit the performance of organic solar cells, yet their origins are not fully understood. Now, a theoretical model encompassing electronic state hybridization and thermal population of vibrational states explains the reduced non-radiative voltage losses in efficient non-fullerene acceptor systems.

State-to-state modeling of oxygen relaxation taking into account electron kinetics

Coupled vibrational, chemical and electronic kinetics in oxygen is studied using the detailed state-to-state model. Simulation of zero-dimensional oxygen relaxation in the presence of free electrons in the temperature range 300 – 1000 K is carried out; the master equations for the vibrational state populations and densities of excited species are solved together with the Boltzmann equation for the electron density distribution. Different models for heavy-particle interactions are assessed. It is shown that the forced harmonic oscillator model is suitable in the entire temperature range. The effect of the initial conditions and the presence of free electrons is discussed; it is shown that including electrons modifies the main relaxation mechanisms. Recommendations are given for extending the kinetic scheme to high-temperature regimes.

Vibrational CARS measurements in a near-atmospheric pressure plasma jet in nitrogen: II. Analysis

The understanding of the ro-vibrational dynamics in molecular (near)-atmospheric pressure plasmas is essential to investigate the influence of vibrational excited molecules on the discharge properties. In a companion paper Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), results of ro-vibrational coherent anti-Stokes Raman scattering (CARS) measurements for a nanosecond pulsed plasma jet consisting of two conducting molybdenum electrodes with a gap of 1 mm in nitrogen at 200 mbar are presented. Here, those results are discussed and compared to theoretical predictions based on rate coefficients for the relevant processes found in the literature. It is found, that during the discharge the measured vibrational excitation agrees well with predictions obtained from the rates for resonant electron collisions calculated by Laporta et al (2014 Plasma Sources Sci. Technol. 23 065002). The predictions are based on the electric field during the discharge, measured by electric field induced second harmonic generation Kuhfeld et al (2021 J. Phys. D: Appl. Phys. 54 305204), Lepikhin et al (2020 J. Phys. D: Appl. Phys. 54 055201) and the electron density, which is deduced from the field and mobility data calculated with Bolsig+ Hagelaar and Pitchford (2005 Plasma Sources Sci. Technol. 14 722–33). In the afterglow a simple kinetic simulation for the vibrational subsystem of nitrogen is performed and it is found, that the populations of vibrational excited states develop according to vibrational-vibrational transfer on timescales of a few microseconds, while the development on timescales of some hundred microseconds is determined by the losses at the walls. No significant influence of electronically excited states on the populations of the vibrational states visible in the CARS measurements () was observed.

Vibrational self-quenching rates of CO(v=1,2) measured via transient diode laser absorption spectroscopy

New laboratory measurements of the rate coefficients for the vibrational relaxation of CO(v=1) and CO(v=2) through collisions with CO have been performed. The experimental approach involved creating a temperature-jump in an equilibrium mixture of CO and bath gases by photolyzing a trace amount of O3 in the mixture with a pulsed UV laser. This shifted a small fraction of the CO population to excited vibrational states. Transient diode laser absorption spectroscopy was then used to observe the evolving vibrational state populations as a function of time. Rate coefficients were deduced from the change in collisional relaxation rate with quencher concentration. This work was motivated by the need for a robust non-LTE model to describe Titan's upper atmosphere. Because the gas collision frequencies are not sufficient to reach thermal equilibrium in a non-LTE environment, the distribution of states is not easily predicted. To properly model such a region, all contributing vibrational energy transfer mechanisms must be thoroughly characterized. Despite their importance, the uncertainties associated with several CO quenching rate parameters in the literature are large. In this work, the rate coefficient for CO(v=2) self-quenching was measured to be (2.7 ± 0.6) x 10−12 cm3 molecule−1 s−1and the overall rate coefficient for CO(v=1) self-quenching was measured to be (1.8 ± 0.3) x 10−12 cm3 molecule−1 s−1. Each represents a significant decrease in uncertainty compared with values being used in current Titan non-LTE models.

Simultaneous Retrievals of Nighttime O(3P) and Total OH Densities From Satellite Observations of Meinel Band Emissions

AbstractRetrieving the total number density [OH], which is used in the chemical balance equations and is the sum of both ground and excited vibrational state populations, is a challenging problem to such a degree there exist no such estimates from recent space observations. We present a novel retrieval approach to simultaneously and self‐consistently derive both [O(3P)] and total [OH] in the nighttime mesosphere and lower thermosphere which operates with the Meinel band volume emission rates and their ratios. Its application to the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) and Scanning Imaging Absorption spectroMeter for Atmospheric CHartographY (SCIAMACHY) observations shows a good agreement of the retrieved [O(3P)] taking into account the measurement uncertainties and variation of model inputs used. However, retrieved [OH] show a large discrepancy, mainly above 90 km. SCIAMACHY [OH] is generally in good agreement with WACCM [OH] after accounting for uncertainties, while SABER OH is larger than WACCM [OH] by up to a factor of 2.

Water Photolysis and Its Contributions to the Hydroxyl Dayglow Emissions in the Atmospheres of Earth and Mars.

Airglow is a well-known phenomenon in the Earth's upper atmosphere, which arises from the emissions of energetic atoms and molecules. The Meinel band emission from high vibrationally excited OH(X) radicals is one of the more important contributors to the airglow from the mesosphere/lower thermosphere. The H + O3 reaction has long been regarded as the dominant source of these OH(X, high v) radicals. Here we demonstrate that vacuum ultraviolet (VUV) photolysis of water vapor at λ ∼ 112.8 nm represents another source of exceptionally highly vibrationally excited OH(X) radicals, with a nascent vibrational state population distribution that maximizes at v = 9 and extends to at least the v = 15 level. Atmospheric chemistry modeling indicates that OH(X, high v) radicals from H2O photolysis might be detectable in the OH Meinel band dayglow in the upper atmosphere of Earth and should dominate the corresponding emission from the Martian atmosphere. VUV photolysis of H2O also produces electronically excited OH(A) radicals, and simultaneous detection of emissions from OH(X, high v) and OH(A) is shown to offer a route to identifying high-oxygen exoplanetary atmospheres.

Photodissociation Dynamics of OCS near 150 nm: The S(1SJ=0) and S(3PJ=2,1,0) Product Channels.

Vacuum ultraviolet photodissociation dynamics of carbonyl sulfide (OCS) was investigated by using the time-sliced velocity map ion imaging technique. Images of the S(1SJ=0) and S(3PJ=2,1,0) photofragments formed in the OCS photodissociation were acquired at six photolysis wavelengths from 147.24 to 156.48 nm. Vibrational states of the CO coproducts were partially resolved and identified in the images. Two main dissociation product channels, namely, the spin-allowed S(1SJ=0) + CO(X1Σg+) and spin-forbidden S(3PJ=2,1,0) + CO(X1Σg+), were observed. At each photolysis wavelength, the total kinetic energy releases, the relative population of different CO vibrational states, and the anisotropic parameters were derived. Variations of the relative population were noticed between different spin-orbit states of the S(3PJ) channel. It was found that the S(1SJ=0) + CO(X1Σg+) channel is dominated by the 1Σ+ ← 1Σ+ parallel transition of OCS. Interestingly, two types of anisotropic parameters are found at different photolysis wavelengths for the spin-forbidden S(3PJ=2,1,0) + CO(X1Σg+) product channel. The anisotropic parameters at 147.24 and 150.70 nm are significantly smaller than at the other four photolysis wavelengths. This phenomenon indicates two different nonadiabatic pathways are responsible for the spin-forbidden channels, which is consistent with the barrier structure in the exit channel of one of the triplet states.

Non-Boltzmann vibrational energy distributions and coupling to dissociation rate.

In this article, we propose a generalized model for nonequilibrium vibrational energy distribution functions. The model can be used, in place of equilibrium (Boltzmann) distribution functions, when deriving reaction rate constants for high-temperature nonequilibrium flows. The distribution model is derived based on the recent ab initio calculations, carried out using potential energy surfaces developed using accurate computational quantum chemistry techniques for the purpose of studying air chemistry at high temperatures. Immediately behind a strong shock wave, the vibrational energy distribution is non-Boltzmann. Specifically, as the gas internal energy rapidly excites to a high temperature, overpopulation of the high-energy tail (relative to a corresponding Boltzmann distribution) is observed in ab initio simulations. As the gas excites further and begins to dissociate, a depletion of the high-energy tail is observed, during a time-invariant quasi-steady state. Since the probability of dissociation is exponentially related to the vibrational energy of the dissociating molecule, the overall dissociation rate is sensitive to the populations of these high vibrational energy states. The non-Boltzmann effects captured by the new model either enhance or reduce the dissociation rate relative to that obtained assuming a Boltzmann distribution. This article proposes a simple model that is demonstrated to reproduce these non-Boltzmann effects quantitatively when compared to ab initio simulations.