Vibrational Population Distribution 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.

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.

Signatures of Reaction Mechanisms Encoded in the Vibrational Population Distribution of Small-Molecule Products: Photodissociation of Symmetric-Triazine Using 266 nm Radiation

We utilize rotationally resolved Chirped-Pulse Fourier Transform millimeter-wave spectroscopy to study photodissociation dynamics of 1,3,5-Triazine (symmetric-Triazine) to form 3 HCN molecules. The state-specific vibrational population distribution (VPD) of the photofragments contains mechanistic details of the reaction. This photodissociation is performed using 266 nm radiation transverse to a seeded supersonic jet. The vibrational cooling inefficiency in the jet preserves the VPD of the photofragments, while rotational cooling enhances the signal of low-J pure-rotational transitions. The multiplexed nature of the spectrometer enables simultaneous sampling of several "vibrational satellites" of the J = 1 ← 0 transition of HCN. Excited state populations along the HCN bend (v2) and CN stretch (v3) modes are observed, which show ≥3.2% vibrational excitation of the photofragments. Observation of an at least bimodal VPD, along the even-v states of v2, implies an asymmetric partitioning of vibrational energy among the HCN photofragments. This suggests a sequential dissociation mechanism of symmetric-Triazine initiated by 266 nm radiation.

Air thermochemistry in the converging section of de Laval nozzles on hypersonic wind tunnels

State-to-state simulations of nonequilibrium flow in nozzles are made for a range of reservoir conditions and geometries. The geometry of the converging section and throat has little influence on the thermochemistry of the flow. Higher reservoir pressure and temperature both drive the thermochemistry toward equilibrium. For reservoir temperatures of 1500, 4000, and 7000 K, the flow property that has the largest departure from equilibrium is the N2 vibrational temperature, the O mass fraction, and the N mass fraction, respectively. Even at the lowest reservoir pressure, these departures from equilibrium are only 14%, 8%, and 2% for the 1500, 4000, and 7000 K reservoirs, respectively. The differences in these flow properties at the throat between the nonequilibrium and equilibrium simulations are maintained throughout in the nonequilibrium simulations of the diverging section. Applying the simplification of equilibrium flow in the converging section and around the throat yields almost no observable errors in the vibrational population distributions in the diverging section. The simplification is recommended for most practical intents and purposes, and the current work provides important quantitative information to make informed judgments when applying it.

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.

Selective excitation of vibrational states in three-state Na2 molecule by a chirped laser pulse

We have theoretically investigated the population transfer and the selective excitation of vibrational states of the target state in a three-state Na2 molecule using the chirped adiabatic passage. The population transfer among electronic states is studied. Compared with the single negative chirped pulse, complete population transfer can be achieved by using a positive chirped pulse. The effects of laser parameters on vibrational population distribution are discussed, and the selective excitation of vibrational states can be achieved by adjusting the central frequency. Finally, we give the method of selective excitation of specific vibrational states.

Roaming Dynamics and Conformational Memory in Photolysis of Formic Acid at 193\u2009nm Using Time-resolved Fourier-transform Infrared Emission Spectroscopy

In photodissociation of trans-formic acid (HCOOH) at 193 nm, we have observed two molecular channels of CO + H2O and CO2 + H2 by using 1 μs-resolved Fourier-transform infrared emission spectroscopy. With the aid of spectral simulation, the CO spectra are rotationally resolved for each vibrational state (v = 1–8). Each of the resulting vibrational and rotational population distributions is characteristic of two Boltzmann profiles with different temperatures, originating from either transition state pathway or OH-roaming to form the same CO + H2O products. The H2O roaming co-product is also spectrally simulated to understand the interplay with the CO product in the internal energy partitioning. Accordingly, this work has evaluated the internal energy disposal for the CO and H2O roaming products; especially the vibrational-state dependence of the roaming signature is reported for the first time. Further, given a 1 μs resolution, the temporal dependence of the CO/CO2 product ratio at v ≥ 1 rises from 3 to 10 of study, thereby characterizing the effect of conformational memory and well reconciling with the disputed results reported previously between absorption and emission methods.

Adaptive strong-field control of vibrational population in NO2.

An adaptive closed-loop system employing coincidence time-of-flight feedback is used to determine the optimal pulse shapes for manipulating the branching ratio of NO dications following double ionization by an intense laser pulse. Selection between the long-lived NO2+ and the dissociative N+ + O+ final states requires control of the vibrational population distribution in the transient NO2+. The ability to both suppress and enhance NO2+ relative to N+ + O+ is observed, with the effectiveness of shaped pulses surpassing near Fourier transform-limited pulses by about an order of magnitude in each direction, depending on the pulse energy. The control is subsequently investigated using velocity map imaging, identifying plausible dissociation pathways leading to N+ + O+. Combining the information about the N+ + O+ dissociation with a well-defined control objective supports the conclusion that the primary control mechanism involves selectively populating long-lived NO2+ vibrational states.

Discharge Regimes Transition and Characteristics Evolution of Nanosecond Pulsed Dielectric Barrier Discharge.

Discharge regime transition in a single pulse can present the breakdown mechanism of nanosecond pulsed dielectric barrier discharge. In this paper, regime transitions between streamer, diffuse, and surface discharges in nanosecond pulsed dielectric barrier discharge are studied experimentally using high resolution temporal–spatial spectra and instantaneous exposure images. After the triggering time of 2–10 ns, discharge was initiated with a stable initial streamer channel propagation. Then, transition of streamer-diffuse modes could be presented at the time of 10–34 ns, and a surface discharge can be formed sequentially on the dielectric plate. In order to analyze the possible reason for the varying discharge regimes in a single discharge pulse, the temporal–spatial distribution of vibrational population of molecular nitrogen N2 (C3Πu, v = 0,1,2) and reduced electric field were calculated by the temporal–spatial emission spectra. It is found that at the initial time, a distorted high reduced electric field was formed near the needle electrode, which excited the initial streamer. With the initial streamer propagating to the dielectric plate, the electric field was rebuilt, which drives the transition from streamer to diffuse, and also the propagation of surface discharge.

Multiquantum Transitions in Oxygen and Nitrogen Molecules in Hypersonic Nonequilibrium Flows

A computational fluid dynamics analysis was performed with state-to-state kinetics of oxygen and nitrogen for assessing the influence of vibrational excitation and dissociation on hypersonic flowfields in the range of Mach 7–20. Flows around an axisymmetric forebody geometry were considered by using multiquantum vibrational–translation and dissociation rate coefficients from an ab initio database. The nonequilibrium vibrational energy distributions were modeled by the master kinetic equation. Output quantities of interest included shock standoff distance, translational and vibrational temperatures, and population fractions in the quantum energy states. Both a two-temperature model and a state-to-state kinetics model were used to compare with existing experimental data. The objective of the study was to understand the energy exchange process of multiquantum transitions in the shock layer of a blunt body flow and to assess the maximum number of multiquantum jumps required in describing the nonequilibrium behavior. Multiquantum transitions caused an increase in population in upper energy states due to more efficient energy exchanges. The greater dissociation for the higher multiquantum jumps resulted in reduced shock standoff distance. At the higher Mach numbers, population depletion from the upper energy states demonstrated preferential dissociation. At high temperatures, results indicated that multiquantum transitions had a large influence on the macroscopic parameters of the gas flow behind the shock, especially on the temperature profiles as well as on vibrational population distributions in the upper energy states.

A global model study of the population dynamics of molecular hydrogen and the generation of negative hydrogen ions in low-pressure ICP discharge with an expansion region: effects of EEPF

A global model is developed to study the effects of electron energy probability function (EEPF) on the population dynamics of molecular hydrogen and the generation of ions in low-pressure inductively coupled plasma (ICP) discharge with an expansion region. The EEPFs are measured using a commercial Langmuir probe and are found to change from bi-Maxwellian to Maxwellian type with increasing pressure. In the simulations, the measured EEPFs are employed to consider a non-Maxwellian EEPF in the low-pressure regime. The chemical reactions involving the H2(v) vibrational states are extended based on our previous study where some important reactions affecting the hydrogen vibrational population and thus the production were omitted. Compared with assuming a Maxwellian EEPF, the measured EEPF with a bi-Maxwellian distribution in the low-pressure regime produces a higher density of ions, due to a lower effective electron temperature and a higher electron density. As the pressure increases, the densities of ions under these two EEPF cases tend to be equal. The hydrogen vibrational population, together with the creation and loss mechanisms of different vibrational levels, is further investigated at different pressures under two EEPF cases, due to its key role played in the generation of ions. The dependence of the evaluated reactions contributing to the production of vibrational states on the EEPF is weakened with increasing vibrational level and pressure, whereas the evaluated reactions contributing to the destruction of vibrational states are almost independent of the EEPF. It is found that not only the indirect electron-vibration excitation (EV) and the resonant electron-vibration excitation (eV) but the vibrational-translational relaxation in collisions with molecular hydrogen (VT) are responsible for a shrinking plateau of vibrational population distribution with increasing pressure.

Time-Resolved Kinetic Chirped-Pulse Rotational Spectroscopy in a Room-Temperature Flow Reactor

Chirped-pulse Fourier transform millimeter-wave spectroscopy is a potentially powerful tool for studying chemical reaction dynamics and kinetics. Branching ratios of multiple reaction products and intermediates can be measured with unprecedented chemical specificity; molecular isomers, conformers, and vibrational states have distinct rotational spectra. Here we demonstrate chirped-pulse spectroscopy of vinyl cyanide photoproducts in a flow tube reactor at ambient temperature of 295 K and pressures of 1-10 μbar. This in situ and time-resolved experiment illustrates the utility of this novel approach to investigating chemical reaction dynamics and kinetics. Following 193 nm photodissociation of CH2CHCN, we observe rotational relaxation of energized HCN, HNC, and HCCCN photoproducts with 10 μs time resolution and sample the vibrational population distribution of HCCCN. The experimental branching ratio HCN/HCCCN is compared with a model based on RRKM theory using high-level ab initio calculations, which were in turn validated by comparisons to Active Thermochemical Tables enthalpies.

Controlling vibrational population distribution of the Na2 molecule by ultrashort pulses in the three-state system

The population transfer and vibrational population distribution of the target state in the three-state Na2 molecule are theoretically investigated by using the time-dependent wave packet method. The population can be efficiently transferred to the target state by the process of adiabatic passage by light induced potentials (APLIP). The vibrational population distribution of the target state can be controlled by adjusting the laser parameters, such as the pulse width, time delay, intensity and envelope. The calculations show the vibrational population distribution variations under different laser parameters where the preparation of the specific state for molecules can be rationalised. In addition, the influence of the ionization state on the population transfer is studied.

Effect of thermal nonequilibrium on ignition in scramjet combustors

The effect of thermal nonequilibrium on flame ignition and stabilization in supersonic scramjet combustors is studied using reaction-layer resolved high resolution numerical simulations. The flow configuration is based on the HyShot-II experiment, with a sonic H2 jet injected into a supersonic crossflow. A detailed H2/air chemical kinetics model is used to simulate the combustion process. Nonequilibrium is described using a multi-temperature model, where the vibrational population distribution of each species is treated using a species-specific temperature. Ab initio derived chemical rates are used for key initiation reactions, while a vibrationally-coupled model is used to describe other reactions under nonequilibrium conditions. The simulations reveal an interesting phenomenon, namely, temperature inversion driven flame stabilization. While conventional wisdom states that ignition should be delayed behind a compression shock due to underpopulation of vibrational levels, the results show that ignition occurs much earlier than in equilibrium flows. The main reason is the generation of a temperature inversion region due to cross-stream fuel injection. Here, the slower vibrational relaxation produces regions of higher vibrational temperatures with combustible mixtures, while the nonequilibrium-driven translational temperature is higher than the equilibrium temperature. The ab initio rates show that certain reactions are relatively insensitive to vibrational energy distribution. Hence, for the same total energy, the reaction rates are increased when the translational energy is higher, leading to stable combustion with enhanced dissociation very close to the nozzle exit. Further analysis shows much of the combustion heat release occurs in this zone where O2 vibrational temperatures are higher than translational temperatures. Even though the presence of H2O quenches nonequilibrium relatively quickly, the initial differences in temperatures created by the nonequilibrium flow is sufficient to substantially alter ignition location.

Highly vibrationally excited CO generated in a low-temperature chemical reaction between carbon vapor and molecular oxygen

A chemical flow reactor is used to study the vibrational population distribution of CO produced by a reaction between carbon vapor generated in an arc discharge and molecular oxygen. The results demonstrate formation of highly vibrationally excited CO, up to vibrational level v=14, at low temperatures, T=400–450K, with population inversion at v=4–7, in a collision-dominated environment, 15–20Torr. The average vibrational energy per CO molecule formed by the reaction is 0.6–1.2eV/molecule, which corresponds to 10–20% of reaction enthalpy. The results show feasibility of development of a new CO chemical laser using carbon vapor and oxygen as reactants.

Activated Dissociation of HCl on Au(111).

We report zero-coverage reaction probabilities (S0) for HCl dissociative adsorption on Au(111) obtained by the seeded molecular beam hot-nozzle method. For measurements at normal incidence with mean translational energies ranging from 0.94 to 2.56 eV (nozzle temperatures 296 to 1060 K), S0 increased from 6 × 10(-6) to 2 × 10(-2). S0also increased with increasing nozzle temperature for fixed incidence energy associated with the motion normal to the surface. Accounting for the influence of the vibrational state population and translational energy distributions in the incident beam, we are able to compare the experimental results to recent theoretical predictions. These calculations, performed employing 6-D quantum dynamics on an electronically adiabatic potential energy surface obtained using density functional theory at the level of the generalized gradient approximation and the static surface approximation, severely overestimate the reaction probabilities when compared with our experimental results. We discuss some possible reasons for this large disagreement.

High-pressure dc glow discharges in hollow diamond cathodes

We report the generation and characterization of dc helium microdischarges at several times atmospheric pressure in monolithic diamond hollow-cathode devices having cavity diameters on the order of 100 μm. I–V characteristics indicated operation in the glow discharge regime even at nearly 10 atm, while spectroscopic measurements of the N2 C3Πu → B3Πg emission returned rotational temperatures always around 420 K, with a pressure-dependent vibrational population distribution. The variation of breakdown voltage with pressure closely followed Paschen’s law, but with offsets in both axes that we tentatively ascribe to strong diffusive loss and a partial thermalization of electron energies under the high pressures considered here.

Formation of deeply bound ultracold LiRb molecules via photoassociation near the Li 2S1/2+Rb 5P3/2 asymptote

We present spectra of ultracold $^{7}\mathrm{Li}^{85}\mathrm{Rb}$ molecules in their electronic ground state formed by spontaneous decay of weakly bound photoassociated molecules. Beginning with atoms in a dual-species magneto-optical trap, weakly bound molecules are formed in the 4(1) electronic state, which corresponds to the ${B}^{1}\ensuremath{\Pi}$ state at short range. These molecules spontaneously decay to the electronic ground state and we use resonantly enhanced multiphoton ionization to determine the vibrational population distribution in the electronic ground states after spontaneous emission. Many of the observed lines from the spectra are consistent with transitions from the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ ground electronic state to either the ${B}^{1}\ensuremath{\Pi}$ or the ${D}^{1}\ensuremath{\Pi}$ electronic state that has been previously observed, with levels possibly as low as ${X}^{1}{\ensuremath{\Sigma}}^{+}({v}^{\ensuremath{'}\ensuremath{'}}=2)$ being populated. We do not observe decay to weakly bound vibrational levels of the ${X}^{1}{\ensuremath{\Sigma}}^{+}$ or ${a}^{3}{\ensuremath{\Sigma}}^{+}$ electronic state in the spectra. We also deduce a lower bound of $3900 \mathrm{cm}{}^{\ensuremath{-}1}$ for the dissociation energy of the $\mathrm{LiRb}{}^{+}$ molecular ion.

New insight into dissociative photoionization of N2O at ∼20 eV using threshold photoelectron–photoion coincidence velocity imaging

Dissociative photoionization (DPI) of N2O at ∼20eV has been reinvestigated with threshold photoelectron–photoion coincidence (TPEPICO) velocity imaging. In threshold photoelectron spectrum, a shoulder peak at 20.045eV is observed close to the ground vibrational level of C2Σ+ state at 20.100eV. Through comparing the coincident mass spectra recorded at 20.045 and 20.100eV, the assignment of the shoulder band is corrected to a vibrational excited D2Π ionic state from the previous conclusions of the vibrationless level of b4Π or hot band of C2Σ+ state. For the dominant photofragment of NO+ at 20.045eV, TPEPICO time-sliced velocity image is measured to obtain the corresponding total kinetic energy and angular distributions. Interestingly, both the bimodal vibrational population and angular distribution of NO+ fragment from dissociation of N2O+(D2Π) are very similar to those of N2O+(C2Σ+) ions. With the aid of potential energy curves, the DPI mechanisms of N2O via D2Π ionic state at 20.045eV along the NO+(X1Σ+)+N(2D) and NO+(X1Σ+)+N(2P) dissociation channels are clarified, in which the internal conversion from D2Π to B2Π state is the rate-determined step.

Molecular halogen elimination from halogen-containing compounds in the atmosphere

Atmospheric halogen chemistry has drawn much attention, because the halogen atom (X) playing a catalytic role may cause severe stratospheric ozone depletion. Atomic X elimination from X-containing hydrocarbons is recognized as the major primary dissociation process upon UV-light irradiation, whereas direct elimination of the X2 product has been seldom discussed or remained a controversial issue. This account is intended to review the detection of X2 primary products using cavity ring-down absorption spectroscopy in the photolysis at 248 nm of a variety of X-containing compounds, focusing on bromomethanes (CH2Br2, CF2Br2, CHBr2Cl, and CHBr3), dibromoethanes (1,1-C2H4Br2 and 1,2-C2H4Br2) and dibromoethylenes (1,1-C2H2Br2 and 1,2-C2H2Br2), diiodomethane (CH2I2), thionyl chloride (SOCl2), and sulfuryl chloride (SO2Cl2), along with a brief discussion on acyl bromides (BrCOCOBr and CH2BrCOBr). The optical spectra, quantum yields, and vibrational population distributions of the X2 fragments have been characterized, especially for Br2 and I2. With the aid of ab initio calculations of potential energies and rate constants, the detailed photodissociation mechanisms may be comprehended. Such studies are fundamentally important to gain insight into the dissociation dynamics and may also practically help to assess the halogen-related environmental variation.