Vibrational Relaxation Of CO Research Articles

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.

Vibrational Relaxation of Highly Vibrationally Excited Molecules Scattered from Au(111): Role of the Dissociation Barrier

Energy transfer during molecular collisions at a metal surface represents a sensitive probe of the molecule–surface interaction potential. Here, via molecular dynamics calculations on several first-principles neural network potentials, we find that the vibrational energy transfer dynamics of highly vibrationally excited NO and CO molecules scattered from Au(111) are strongly correlated with their respective potential energy landscapes in the vicinity of the dissociation barrier. Our results not only reproduce the observed significantly less vibrational relaxation of CO (vi = 17) than NO (vi = 16) scattered from Au(111) and attribute it to the different dissociation barriers in the two systems, but also show a dramatic change of dynamics due to a minor adjustment of the energy landscape near the barrier in the CO case. We find that the BEEF-vdW density functional based potential largely overestimates the vibrational relaxation of CO (vi = 17) even in the absence of any nonadiabatic energy loss, despite its good description for CO adsorption on Au(111). We also discuss the possibility of integrating the validated adiabatic potential with first-principles determined diabatic states towards a more complete description of the non-adiabatic energy transfer in these benchmark systems.

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.

Anomalous Humidity Dependence in Photoacoustic Spectroscopy of CO Explained by Kinetic Cooling

Water affects the amplitude of photoacoustic signals from many gas phase molecules. In quartz-enhanced photoacoustic (QEPAS) measurements of CO excited at the fundamental vibrational resonance of CO, the photoacoustic signal decreases with increasing humidity, reaches a pronounced minimum at ~0.19%V, and increases with humidity for higher water contents. This peculiar trend is explained by competing endothermal and exothermal pathways of the vibrational relaxation of CO in N2 and H2O. Near-resonant vibrational–vibrational transfer from CO to N2, whose vibrational frequency is 188 cm−1 higher than in CO, consumes thermal energy, yielding a kinetic cooling effect. In contrast, vibrational relaxation via H2O is fast and exothermal, and hence counteracts kinetic cooling, explaining the observed trend. A detailed kinetic model for collisional relaxation of CO in N2 and H2O is presented. Simulations using rate constants obtained from literature were performed and compared to humidity dependent QEPAS experiments at varying pressure. Agreement between the experiments and simulations confirmed the validity of the model. The kinetic model can be used to identify optimized experimental conditions for sensing CO and can be readily adapted to include further collision partners.

Adiabatic and nonadiabatic energy dissipation during scattering of vibrationally excited CO from Au(111)

A high-dimensional potential energy surface (PES) for CO interaction with the Au(111) surface is developed using a machine-learning algorithm. Including both molecular and surface coordinates, this PES enables the simulation of the recent experiment on scattering of vibrationally excited CO from Au(111). Trapping in a physisorption well is observed to increase with decreasing incidence energy. While energy dissipation of physisorbed CO is slow, due to weak coupling with both the phonons and electron-hole pairs, its access to the chemisorption well facilitates fast vibrational relaxation of CO through nonadiabatic coupling with surface electron-hole pairs.

Electron transfer mediates vibrational relaxation of CO in collisions with Ag(111).

We report experimental results on the state-to-state vibrational relaxation of CO(v = 17) in collisions with Ag(111) at incidence translational energies between 0.27 eV and 0.57 eV. These together with previous results provide a comprehensive set of data on two molecules (CO and NO)-one open and one closed shell-and two metals (Ag and Au). In all four cases, the incidence vibrational energy has been varied over several eV. We find a unifying relation between the probability of vibrational relaxation and the energetics of electron transfer from the metal to the molecule. This argues strongly that electronic friction based theories are not capable of explaining these data.

Electrically Excited, Supersonic Flow Carbon-Monoxide Laser with Air Species in Laser Mixture

Performance parameters of an electric discharge excited, supersonic flow CO laser operated with significant amounts of air species in the laser mixture is studied experimentally and by kinetic modeling. The results demonstrate that adding nitrogen to the CO–He mixture increases the laser power significantly, by up to a factor of 3. Adding oxygen reduces CO vibrational excitation and results in a steep laser power reduction. Adding air to the laser mixture also produces higher output power, such that the positive effect of nitrogen outweighs the negative effect of oxygen. This result indicates that a chemical CO laser, not dependent on the electric discharge for excitation, may be capable of operating in a mixture of carbon vapor and air. Performance of a supersonic flow chemical laser excited by a reaction of carbon vapor and molecular oxygen is analyzed by kinetic modeling. Peak laser power is predicted at the C vapor mole fraction of 0.2%, when 15% of energy stored in CO vibrational energy mode (4.5% of the reaction enthalpy) is converted to laser power. At higher C vapor mole fractions, flow temperature rise caused by exothermic reactions and by CO vibrational relaxation results in rapid reduction of the predicted laser power.

Electron-Mediated Phonon-Phonon Coupling Drives the Vibrational Relaxation of CO on Cu(100).

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Vibrational Relaxation of Highly Vibrationally Excited CO Scattered from Au(111): Evidence for CO- Formation.

Electronically nonadiabatic dynamics can be important in collisions of molecules at surfaces; for example, when vibrational degrees of freedom of molecules are coupled to electron-hole-pair (EHP) excitation of a metal. Such dynamics have been inferred from a host of observations involving multiquantum relaxation of NO molecules scattered from metal surfaces. Electron transfer forming transient NO- is thought to be essential to the nonadiabatic coupling. The question remains: is this behavior usual? Here, we present final vibrational state distributions resulting from the scattering of CO(vi = 17) from Au(111), which exhibits significantly less vibrational relaxation than NO(vi = 16). We explain this observation in terms of the lower electron affinity of CO compared to NO, a result that is consistent with the formation of a transient CO- ion being important to CO vibrational relaxation.

Communication: Vibrational relaxation of CO(1Σ) in collision with Ar(1S) at temperatures relevant to the hypersonic flight regime.

Vibrational energy relaxation (VER) of diatomics following collisions with the surrounding medium is an important elementary process for modeling high-temperature gas flow. VER is characterized by two parameters: the vibrational relaxation time τvib and the state relaxation rates. Here the vibrational relaxation of CO(ν=0←ν=1) in Ar is considered for validating a computational approach to determine the vibrational relaxation time parameter (pτvib) using an accurate, fully dimensional potential energy surface. For lower temperatures, comparison with experimental data shows very good agreement whereas at higher temperatures (up to 25 000 K), comparisons with an empirically modified model due to Park confirm its validity for CO in Ar. Additionally, the calculations provide insight into the importance of Δν>1 transitions that are ignored in typical applications of the Landau-Teller framework.

Surface vibrational relaxation of N2 studied by CO2 titration with time-resolved quantum cascade laser absorption spectroscopy

A new method for determination of the wall de-excitation probability of vibrationally excited N2 on different surfaces exposed to low-pressure plasmas has been developed. A short dc discharge pulse of only a few milliseconds was applied to a mixture containing 0.05–1% of CO2 in N2 at a pressure of 133 Pa. Due to a nearly resonant fast vibrational transfer between N2(v) and the asymmetric ν3 mode of CO2 the vibrational excitation of these titrating molecules is an image of the degree of vibrational excitation of N2. In the afterglow, the vibrational relaxation of CO2 was monitored in situ using quantum cascade laser absorption spectroscopy. The experimental results were interpreted in terms of a numerical model of non-equilibrium vibrational kinetics in CO2–N2 mixtures. Heterogeneous relaxation was the main quenching process of N2(v) under the conditions of this study, which allowed determination of the value of from the best agreement between the experiment and the model. The new method is suitable for determination in a single plasma pulse with the discharge tube surface pretreated by a low-pressure plasma. The relaxation probability of the first vibrational level of nitrogen γ1 = (1.1 ± 0.15) × 10−3 found for Pyrex and silica is in reasonable agreement with the literature data. Using the new technique the N2(v = 1) quenching probability was measured on TiO2 surface, γ1 = (9 ± 1) × 10−3. A linear enhancement of the N2(v) wall deactivation probability with an increase in the admixture of CO2 was observed for all studied materials. In order to explain this effect, a vibrational energy transfer mechanism between N2(v) and adsorbed CO2 is proposed.

Vibrational relaxation of CO2(ν2) by O(3P) in the 142–490 K temperature range

Laboratory measurements of the quenching of CO2(ν2) by O atoms are presented over the 142–490 K temperature range relevant to the 75–120 km altitude region of the terrestrial atmosphere. The primary cooling mechanism in this region occurs when CO2 is efficiently excited through collisions with ambient O atoms, populating the bending vibrational (ν2) modes. A significant fraction of the vibrationally excited CO2 relaxes through spontaneous 15‐μm emission that escapes into space, thereby removing kinetic energy from this region of the atmosphere and generating a local cooling effect. The rate coefficient for the vibrational relaxation of CO2(ν2) by O atoms, kO(ν2), is measured using transient diode laser absorption spectroscopy. A slight negative temperature dependence is observed for kO(ν2), with values ranging from 2.7 (±0.4) × 10−12 cm3 s−1 at 142 K to 1.3 (±0.2) × 10−12 cm3 s−1 at 490 K.

Force Field Optimization using Dynamics and Ensemble Averaged Data: Vibrational Spectra and Relaxation in Bound MbCO

Force field parameters are ingredients for realistic atomistic simulations of gas- and condensed-phase systems. Here we discuss the effect of including averaged data from explicit MD simulations in optimizing potential energy functions. It is shown that vibrational frequencies (FeC and CO stretch and FeCO bend) and CO vibrational relaxation times ((v = 1) --> (v = 0) (T(10)) and (v = 2) --> (v = 1) (T(21))) in the active site of CO-bound myoglobin (MbCO) can be well represented with a single set of force field parameters. It is further demonstrated that parameters fitted in a subsystem of MbCO comprising the CO ligand, heme group and proximal histidine, are transferable to investigating the full protein and to providing quantitatively correct results. In particular, it is possible to calculate the CO and FeC stretch and the FeCO bending frequency to within approximately 5%; the relaxation time of the first vibrationally excited state including quantum corrections of T(10) approximately 25 ps is calculated close to the experimental value (17 ps), and the ratio T(10)/T(21) approximately 2 agrees favorably with experimental estimates. In contrast, following the more traditional approach of fitting frequencies from analyzing the Hessian matrix leads to a force field that captures frequencies correctly but not relaxation of vibrations.

Anharmonic Coupling in Molecular Dynamics Simulations of Ligand Vibrational Relaxation in Bound Carbonmonoxy Myoglobin

Vibrational relaxation of CO bound to myoglobin (MbCO) following photoexcitation is investigated using nonequilibrium molecular dynamics (MD) simulations. It is found that harmonic potential energy functions for bond vibrations are not suited to simultaneously and accurately describe vibrational de-excitation and the vibrational spectroscopy of the bound ligand. Only when anharmonic (e.g. Morse) potentials are introduced for both the C-O and the adjacent Fe-C(CO) bonds to allow anharmonic coupling, rapid (tens of ps) relaxation of the vibrationally excited CO is possible. To capture both relaxation and vibrational spectroscopy, the parameters of the potential energy functions are fitted by an interactive, nonlinear least-squares procedure using averages over multiple MD trajectories. The sensitivity of cooling rate to the difference in vibrational frequency between coupled modes is demonstrated. Potential cooling mechanisms are suggested, based on the sensitivity of the CO relaxation rate to changes in the force field parameters of local degrees of freedom. Accounting for quantum correction leads to relaxation rates around 20 ps, in good agreement with experiment. Finally, the importance of electronic effects is explored by fitting a 2D potential energy surface to ab initio data to describe the strengthening and weakening of the CO bond as a function of Fe-C(CO) bond length, and vice versa.

Vibrational relaxation of CO 2 (1 2 0 1) by argon

We present experimental measurements of the vibrational relaxation of CO 2 (1 2 0 1) by argon, at ambient temperature (295 ± 2 K). The CO 2 molecules were directly excited to the (1 2 0 1, J = 14) ro-vibrational state by a tunable laser radiation at ∼2 μm. Time-resolved infrared fluorescence technique was used to study the collisional relaxation process. The bimolecular deactivation rate constant of CO 2 (1 2 0 1) by argon was found to be (825 ± 43 Torr −1 s −1) while the self-deactivation by CO 2 (0 0 0 0) was determined to be (3357 ± 135 Torr −1 s −1). The radiative life-time of the vibrational combination band (1 2 0 1), τ[CO 2 (1 2 0 1)], was found to be (5.55 ± 0.27) μs. Modern angular momentum theory was used to explain values of the deactivation rate measured. It is concluded that the presence of the (0 8 0 0) state acts like an angular momentum sink leading to a fast deactivation rate of the CO 2 (1 2 0 1) by argon.

Density-functional theory study of vibrational relaxation of CO stretching excitation on Si(100)

A first-principles theory is presented for calculating the lifetime of adsorbate vibrations on semiconductor or insulator surfaces, where dissipation of the vibrational energy to substrate phonons is the dominant relaxation mechanism. As an example, we study the stretching vibration of CO/Si(100), where a lifetime of 2.3 ns has been measured recently [K. Lass, X. Han, and E. Hasselbrink, J. Chem. Phys. 123, 051102 (2005)]. Density-functional theory (DFT) calculations for the local modes of the adsorbate, including their anharmonic coupling, are combined with force field calculations for the substrate phonons. Using the DFT-Perdew-Burke-Ernzerhof functional, we have determined the most stable adsorption site for CO on top of the lower Si atom of the Si surface dimer, the local normal modes of CO, and the multidimensional potential energy surface for the CO vibrations. The anharmonic stretching frequency of adsorbed CO obtained in DFT-PBE is 5% lower than the experimental value, while the B3LYP functional reproduces the CO stretching frequency with only 1.4% error. The coupling between the anharmonic vibrational modes and the phonon continuum is evaluated within first-order perturbation theory, and transition rates for the CO vibrational relaxation are calculated using Fermi's golden rule. The lifetime of 0.5 ns obtained with DFT-PBE is in qualitative agreement with experiment, while using vibrational frequencies from the B3LYP functional gives a much too long lifetime as compared to experiment. We find that the numerical value of the lifetime is very sensitive to the harmonic frequencies used as input to the calculation of the transition rate. An empirical adjustment of these frequencies yields excellent agreement between our theory and experiment. From these calculations we conclude that the most probable microscopic decay channel of the CO stretching mode is into four lateral shift/bending quanta and one phonon.

Vibrational relaxation of CO2(ν2) by atomic oxygen

In the Earth's upper atmosphere, collisions with ambient O atoms efficiently excite the CO2 [0000] vibrational ground‐state population to the first excited, [0110] or ν2, vibrational bend state. Subsequent relaxation of the ν2 population occurs through spontaneous emission of 15‐μm radiation. Much of this radiation escapes into space, thereby removing ambient kinetic energy from the atmosphere. This cooling mechanism is especially important at altitudes between the mesopause and the lower thermosphere, approximately 80–120 km, where the O‐atom density is relatively high and the kinetic temperature is rising. Laboratory measurements have been performed to better characterize the CO2(ν2)‐O vibrational relaxation rate coefficient kO(ν2). A 266‐nm laser pulse photolyzed trace amounts of O3 in a CO2‐O3‐rare gas mixture, simultaneously creating O atoms and raising the gas temperature to create a nonequilibrium CO2 vibrational distribution. Transient diode laser absorption spectroscopy was used to monitor CO2 vibrational level population reequilibration. A global nonlinear least squares fitting technique was used to interpret the kinetic data, yielding kO(ν2) = (1.8 ± 0.3) × 10−12 cm3s−1. The result is in good agreement with previous laboratory measurements, with published kO(ν2) values in the (1.2–1.5) × 10−12 cm3s−1 range and at the low end of the (2–6) × 10−12 cm3s−1 range estimated from the analysis of upper atmospheric data.

A hierarchical family of three-dimensional potential energy surfaces for He-CO

A hierarchical family of five three-dimensional potential energy surfaces has been developed for the benchmark He-CO system. Four surfaces were obtained at the coupled cluster singles and doubles level of theory with a perturbational estimate of triple excitations, CCSD(T), and range in quality from the doubly augmented double-zeta basis set to the complete basis set (CBS) limit. The fifth corresponds to an approximate CCSDT/CBS surface (CCSD with iterative triples/CBS, denoted CBS+corr). The CBS limit results were obtained by pointwise basis set extrapolations of the individual counterpoise-corrected interaction energies. For each surface, over 1000 interaction energies were accurately interpolated using a reproducing kernel Hilbert space approach with an R-6+R-7 asymptotic form. In each case, both three-dimensional and effective two-dimensional surfaces were developed. In standard Jacobi coordinates, the final CBS+corr surface has a global minimum at rCO=2.1322a0,R=6.418a0, and gamma=70.84 degrees with a well depth of -22.34 cm-1. The other four surfaces have well depths ranging from -14.83 cm-1 [CCSD(T)/d-aug-cc-pVDZ] to -22.02 cm-1 [CCSD(T)/CBS]. For each of these surfaces the infrared spectrum has been accurately calculated and compared to experiment, as well as to previous theoretical and empirical surfaces. The final CBS+corr surface exhibits root-mean-square and maximum errors compared to experiment (4He) of just 0.03 and 0.04 cm-1, respectively, for all 42 transitions and is the most accurate ab initio surface to date for this system. Other quantities investigated include the interaction second virial coefficient, the integral cross sections, and thermal rate coefficients for rotational relaxation of CO by He, and rate coefficients for CO vibrational relaxation by He. All the observable quantities showed a smooth convergence with respect to the quality of the underlying interaction surface.

Vibrational relaxation of vibrationally and rotationally excited CO molecules by He atoms

This work presents a detailed quantum mechanical study of rovibrationally inelastic He+CO collisions in a wide range of translational and internal energies of the collision partners. Fully converged coupled states calculations of rate constants for vibrational relaxation of CO(v=1) by He are found to be in excellent agreement with experimental measurements at temperatures between 35 and 1500 K. The role of rotational energy for vibrational relaxation of CO is investigated and it is illustrated that the CO molecules in the first excited vibrational state can exhibit near-resonant vibrational relaxation when they are initially in high rotational excitation and the collision energy is small. A reduced channel coupled states approach neglecting low vibrational states in the basis set is implemented for calculations of rate constants for vibrational and rotational energy transfer in collisions of vibrationally excited CO molecules with He atoms. It is shown that initial vibrational excitation significantly increases rate constants for vibrationally inelastic collisions but does not affect purely rotational energy transfer.

Vibrational relaxation of CO in ultracold He3 collisions

We present results of quantum mechanical scattering calculations on the vibrational relaxation of CO induced by collisions with He3 at ultracold temperatures and compare them with the corresponding results for the He–CO4 system. The low energy cross sections are controlled by shape resonances supported by the van der Waals well. For a thermal rotational population the cross sections for the quenching of the v=1 vibrational level are larger for He3 than for He4 collisions. Because of the absence of a Feshbach resonance, the cross sections at low energies for the quenching of the j=1 rotational level are much smaller than those for He–CO3. Very good agreement is found for the rate coefficients for the vibrational relaxation of the v=1 level with the measurement data that are available at temperatures above 35 K. There are small discrepancies between the calculated rate coefficients with earlier calculations for the relaxation of the v=1 vibrational level, similar to those found for He–CO4 collisions, but the discrepancies tend to be larger with decrease of temperature.