Vibrational Relaxation Of OH Research Articles

Detection of vibrationally excited hyroxyl radicals by absorption of the DFB diode laser radiation at the transition (ν = 3) ← (ν = 1)

Using a DFB laser diode at 1509.3 nm the OH(v = 1) formed in the reaction O(1D) + H2O was detected by absorption at the transition (v = 3)←(v = 1). The advantage of the method is the ability to obtain reliable data on absolute concentrations of vibrationally excited radicals. The spectrum of transient absorption near Q(3/2) line of 2Π3/2 term was recorded. At a pressure of 8 Torr He, the rotation distribution was found to be in thermal equilibrium. The yields of OH(v = 1) and OH(v = 2) in the chemical reaction were determined. The rate constants of vibrational relaxation of OH(v = 1) on H2O and SF6 were measured.

Unified picture of vibrational relaxation of OH stretch at the air/water interface

The elucidation of the energy dissipation process is crucial for understanding various phenomena occurring in nature. Yet, the vibrational relaxation and its timescale at the water interface, where the hydrogen-bonding network is truncated, are not well understood and are still under debate. In the present study, we focus on the OH stretch of interfacial water at the air/water interface and investigate its vibrational relaxation by femtosecond time-resolved, heterodyne-detected vibrational sum-frequency generation (TR-HD-VSFG) spectroscopy. The temporal change of the vibrationally excited hydrogen-bonded (HB) OH stretch band (ν=1→2 transition) is measured, enabling us to determine reliable vibrational relaxation (T1) time. The T1 times obtained with direct excitations of HB OH stretch are 0.2-0.4 ps, which are similar to the T1 time in bulk water and do not noticeably change with the excitation frequency. It suggests that vibrational relaxation of the interfacial HB OH proceeds predominantly with the intramolecular relaxation mechanism as in the case of bulk water. The delayed rise and following decay of the excited-state HB OH band are observed with excitation of free OH stretch, indicating conversion from excited free OH to excited HB OH (~0.9 ps) followed by relaxation to low-frequency vibrations (~0.3 ps). This study provides a complete set of the T1 time of the interfacial OH stretch and presents a unified picture of its vibrational relaxation at the air/water interface.

Chemical reaction versus vibrational quenching in low energy collisions of vibrationally excited OH with O

Quantum scattering calculations are reported for state-to-state vibrational relaxation and reactive scattering in O + OH(v = 2 - 3, j = 0) collisions on the electronically adiabatic ground state (2)A'' potential energy surface of the HO2 molecule. The time-independent Schrödinger equation in hyperspherical coordinates is solved to determine energy dependent probabilities and cross sections over collision energies ranging from ultracold to 0.35 eV and for total angular momentum quantum number J = 0. A J-shifting approximation is then used to compute initial state selected reactive rate coefficients in the temperature range T = 1 - 400 K. Results are found to be in reasonable agreement with available quasiclassical trajectory calculations. Results indicate that rate coefficients for O2 formation increase with increasing the OH vibrational level except at low and ultralow temperatures where OH(v = 0) exhibits a slightly different trend. It is found that vibrational relaxation of OH in v = 2 and v = 3 vibrational levels is dominated by a multi-quantum process.

Rate Coefficients for Vibrational Relaxation of OH(X2Π, v = 1–4) by He

The vibrational relaxation of OH(X(2)Π) by collisions with rare gases is very slow due to small molecular interactions. No measurement of the rate coefficients has been made for relaxation of relatively low vibrational levels v ≤ 4 of OH by He, and there is only one report of the upper limit for v = 2, <1 × 10(-14) cm(3) molecule(-1) s(-1). In this article, we have studied vibrational relaxation of the levels v = 1-4 of OH(X(2)Π) by collisions with He. A gaseous mixture of O3 and H2 in a carrier gas at 70-130 Torr of He was irradiated at 266 nm, and OH(X(2)Π, v ≤ 4) was generated in the reaction O((1)D) + H2. A single vibrational level of OH was detected with laser-induced fluorescence (LIF) via the A(2)Σ(+)-X(2)Π transition. Time-resolved LIF intensities of OH(v) were recorded, and kinetic analysis was made by an originally developed integrated profiles method (IPM). On the basis of the evaluation of the pressure-dependent rate coefficients of diffusion loss and the effect of impurities on the kinetics, the rate coefficients of vibrational relaxation for OH(X(2)Π, v = 1-4) by He have been determined to be (2.9 ± 1.5) × 10(-17), (1.4 ± 0.4) × 10(-16), (5.2 ± 0.5) × 10(-16), and (1.6 ± 0.2) × 10(-15) cm(3) molecule(-1) s(-1) for v = 1, 2, 3, and 4, respectively (the confidence limits are 2σ). The rate coefficients are larger at higher vibrational levels and smoothly correlate to those reported previously for v = 10-12.

Molecular vibrational dynamics in polyvinyl alcohol studied by femtosecond coherent anti-stokes Raman spectroscopy

We have performed femtosecond time-resolved coherent anti-stokes Raman spectroscopy (CARS) to study the vibrational dynamics in polyvinyl alcohol (PVA) film. We observed femtosecond coherent vibrational relaxation and CARS signal beats in PVA at room temperature. We found that the coherent vibrational relaxation of anti-symmetric CH2 stretching modes in PVA is faster than that of symmetric modes, probably due to faster vibrational energy transfer. The coherent vibrational relaxation of OH stretching modes was observed to be slower than that of CH2 modes, because OH stretching modes have less resonant energy transfer rate compared to CH2 modes.

Ultracold collisions and reactions of vibrationally excited OH radicals with oxygen atoms

We report a quantum dynamics study of O + OH (v = 1, j = 0) collisions on its ground electronic state, employing two different potential energy surfaces: the DIMKP surface by Kendrick and Pack, and the XXZLG surface by Xu et al. A time-independent quantum mechanical method based on hyperspherical coordinates has been adopted for the dynamics calculations. Energy-dependent probabilities and rate coefficients are computed for the elastic, inelastic, and reactive channels over the collision energy range E(coll) = 10(-10)-0.35 eV, for J = 0 total angular momentum. Initial state-selected reaction rate coefficients are also calculated from the J = 0 reaction probabilities by applying a J-shifting approximation, for temperatures in the range T = 10(-6)-700 K. Our results show that the dynamics of the collisional process and its outcome are strongly influenced by long-range forces, and chemical reactivity is found to be sensitive to the choice of the potential energy surface. For O + OH (v = 1, j = 0) collisions at low temperatures, vibrational relaxation of OH competes with reactive scattering. Since long-range interactions can facilitate vibrational relaxation processes, we find that the DIMKP potential (which explicitly includes van der Waals dispersion terms) favours vibrational relaxation over chemical reaction at low temperatures. On the DIMKP potential in the ultracold regime, the reaction rate coefficient for O + OH (v = 1, j = 0) is found to be a factor of thirteen lower than that for O + OH (v = 0, j = 0). This significantly high reactivity of OH (v = 0, j = 0), compared to that of OH (v = 1, j = 0), is attributed to enhancement caused by the presence of a HO(2) quasibound state (scattering resonance) with energy near the O + OH (v = 0, j = 0) dissociation threshold. In contrast, the XXZLG potential does not contain explicit van der Waals terms, being just an extrapolation by a nearly constant function at large O-OH distances. Therefore, long-range potential couplings are absent in calculations using the XXZLG surface, which does not induce vibrational relaxation as efficiently as the DIMKP potential. The XXZLG potential leads to a slightly higher reactivity (a factor of 1.4 higher) for O + OH (v = 1, j = 0) compared to that for O + OH (v = 0, j = 0) at ultracold temperatures. Overall, both potential surfaces yield comparable values of reaction rate coefficients at low temperatures for the O + OH (v = 1, j = 0) reaction.

Vibrational relaxation of trapped molecules in solid matrices: OH(A Σ2+;v=1)/Ar

The vibrational relaxation of OH(A (2)Sigma(+);v=1) embedded in solid Ar has been studied over 4-80 K. The interaction model is based on OH undergoing local motions in a cage formed by a face-centered cubic stacking where the first shell atoms surround the guest and connect it to the heat bath through 12 ten-atom chains. The motions confined to the cage are the local translation and libration-rotation of OH and internal vibrations in OH...Ar, their energies being close to or a few times the energies of nearby first shell and chain atoms. The cage dynamics are studied by solving the equations of motion for the interaction between OH and first shell atoms, while energy propagation to the bulk phase through lattice chains is treated in the Langevin dynamics. Calculated energy transfer data are used in semiclassical procedure to obtain rate constants. In the early stage of interaction, OH transfers its energy to libration-rotation intramolecularily and then to the vibrations of the first shell and chain atoms on the time scale of several picoseconds. Libration-to-rotational transitions dispense the vibrational energy in small packages comparable to the lattice frequencies for ready flow. Energy propagation from the chains to the heat bath takes place on a long time scale of 10 ns or longer. Over the solid argon temperature range, the rate constant is on the order of 10(6) s(-1) and varies weakly with temperature.

ChemInform Abstract: Vibrational Relaxation of OH and CH Fundamentals of Polar and Nonpolar Molecules in the Condensed Phase

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Vibrational Relaxation of OH and CH Fundamentals of Polar and Nonpolar Molecules in the Condensed Phase

Studies of vibrational energy flow in various polar and nonpolar molecules that follows the ultrafast excitation of the CH and OH stretch fundamentals, modeled using semiclassical methods, are reviewed. Relaxation rates are calculated using Landau-Teller theory and a time-dependent method, both of which consider a quantum mechanical solute molecule coupled to a classical bath of solvent molecules. A wide range of decay rates are observed, ranging from 1 ps for neat methanol to 50 ps for neat bromoform. In order to understand the flow rates, it is argued that an understanding of the subtle mixing between the solute eigenstates is needed and that solute anharmonicities are critical to facilitating condensed phase vibrational relaxation. The solvent-assisted shifts of the solute vibrational energy levels are seen to play a critical role of enhancing or decreasing lifetimes.

Nascent vibrational distributions and relaxation rates of diatomic products of the reactions of O( 1D) with CH 4, C 2H 6, CH 3F, CH 2F 2 and CHF 3 studied by time resolved Fourier transform infrared emission

Time resolved Fourier transform infrared (TRFTIR) emission has been used to study the reactions of CH 4, C 2H 6, CH 3F, CH 2F 2 and CHF 3 with O( 1D). One hundred and ninety-three nanomters photolysis of N 2O was used to prepare O( 1D), and emission analysed from OH( ν = 1–4) for the two hydrocarbons and HF( ν = 1–6) from CH 3F, CH 2F 2 and CHF 3. For the O( 1D) + CH 4 reaction, the nascent OH vibrational distribution showed a population inversion between ν = 1 and 2, and was in excellent agreement with previous laser induced fluorescence and TRFTIR data, as well as with quasi-classical trajectory calculations. Time resolved populations were analysed to yield rate constants for vibrational relaxation of OH( ν) with CH 4, and found to be consistent with stepwise deexcitation rather than chemical removal being dominant. Reaction with C 2H 6 produced a monotonically decreasing population in ν = 1–4 and more rapid relaxation rates than those with methane. For the fluorinated methanes, nascent vibrational populations in HF( ν = 1–6) were measured and shown to be very similar, all monotonically decreasing with ν, and fitting the same vibrational surprisal plot, showing a larger than statistical partitioning of the available energy in vibration. Relaxation rate constants of HF( ν) with the parent fluorinated methane showed values, which increased with increasing H atom content.

Vibrational relaxation of OH by oxygen atoms

The collisional removal of OH( v = 1) by O( 3P) atoms is investigated. OH is generated by 193 nm photolysis of H 2O 2, and O( 3P) atoms are generated by a microwave discharge in O 2 diluted in Ar. OH( v = 0 and 1) concentrations are monitored by laser-induced fluorescence vs. the time after the photolysis laser pulse. From comparison of these concentrations with kinetic simulations, the room-temperature total removal rate constant for OH( v = 1) in collisions with O( 3P) is determined to be (3.9 ± 0.6) × 10 −11 cm 3 molecule −1 s −1. This value is slightly larger than the OH( v = 0)–O( 3P) reaction rate constant, but the difference is within the experimental uncertainty.

Fast vibrational relaxation of OH( v=9) by ammonia and ozone

Vibrational relaxation of OH(X 2 Π) in the v=9 state by O 3 and NH 3 has been studied via flash-photolysis using high-resolution direct infrared absorption detection of OH in the v=11←9 band. OH( v=9) removal by O 3 occurs with a nearly gas-kinetic rate (k O 3 =1.37(15)×10 −10 cm 3/ s) . Collisional removal by NH 3 (k NH 3 =7.60(50)×10 −10 cm 3/ s) is two to three times higher than the hard-sphere collision limit, suggesting a resonant V–V energy transfer induced by long-range dipole–dipole interactions. An upper limit of 5×10 −14 cm 3/ s is placed on OH( v=9) vibrational relaxation by argon.

Vibrational Relaxation of OH and OD Stretching Vibrations of Phenol and Its Clusters Studied by IR−UV Pump−Probe Spectroscopy

The vibrational relaxation of the OH/OD stretching mode of phenol and its cluster with Ar or H2O has been observed directly by nanosecond IR−UV pump−probe spectroscopy. The OH/OD vibrational level was prepared by a tunable nanosecond IR light pulse, and the decay of the pumped level as well as the rise of the relaxed level were observed by two-photon ionization with a tunable UV laser. For the OH stretch level of phenol-d0, two components were observed: a fast component with a decay time of a few nanoseconds and a slow component with a decay time longer than 1 μs. It was also found that the decay rate of the fast component coincides with that of the rise of relaxed levels. The results indicate that the intramolecular vibrational redistribution (IVR) of the OH stretching vibration of bare phenol-d0 is classified as an intermediate-strong-coupling case. For phenol-d1, only the slow component was identified for the decay of the OD stretching vibration, whereas the relaxed level showed a rise with a few nano...

Product branching fractions in the reactions of NH(a 1Δ) and NH(X 3Σ−) with NO

Reactions of the imidogen radical NH with NO have been investigated in its electronically excited a 1Δ and ground X 3Σ− states, where UV- and VUV-LIF were used to monitor radical and atomic species in the 193 nm photolysis of HNCO/NO mixtures at 295±3 K in 20 Torr He. The rate constants for the reactions of NH(a 1Δ; v=0,1)+NO→products (1) and NH(X 3Σ−; v=0,1)+NO→products (2) were determined. No apparent effect on the rate constants of the vibrational excitation of NH was observed in either reaction. H and OH were observed as products in both reactions, whereas physical quenching of NH(a 1Δ) to NH(X 3Σ−) is dominant in Reaction (1). In addition, O(3P) was found to be produced in Reaction (1) for the first time. Product branching fractions in these reactions were determined using proper reference reactions, along with addition of Xe to control the relative contributions of Reactions (1) and (2). The branching fractions in Reaction (1) were: 0.57 for quenching (1a), 0.20 for H+N2O (1b), 0.14 for OH+N2 (1c), 0.08 for the sum of O+N2H (1d), and O+N2+H (1e), and in Reaction (2): 0.65 for H+N2O (2a), 0.30 for OH+N2 (2b). These results were found to be consistent with former theoretical predictions that Reactions (1) and (2) produced on the same doublet surfaces. It was elucidated that the observed delay of OH(v=0) production was due to the vibrational relaxation of OH(v≥1) produced in the reactions.

OH vibrational distribution in the reaction O( 1D)+CH 4

Vibrationally excited OH radicals were generated by 266 nm laser photolysis of ozone in the presence of methane. Temporal profiles of OH (ν=0, 1, 2) concentrations were directly monitored by standard LIF techniques at various CH 4 pressures. The total vibrational distribution of OH product in the reaction O ( 1D) + CH 4 was found to be much hotter than the statistical distribution P(ν=0): P(ν=1): P(ν=2): P(ν=3): P(ν=4) = 0.18: 0.29: 0.37: 0.15: 0.01. The rate constants for vibrational relaxation of OH(ν=1, 2, 3) in collisions with CH 4 were measured to be (5.1±1.0) × 10 −13, (1.5±0.4) × 10 −12 and (6.8±2.0) × 10 −12 cm 3/s, respectively.

Vibrational relaxation of OH(v= 1) and OD(v= 1) by HNO3, DNO3, H2O, NO and NO2

A pulsed photolysis, laser-induced fluorescence method has been used to study the kinetics of OH(v= 1) and OD(v= 1). The following rate constants (cm3 molecule–1 s–1) for relaxation of these vibrationally excited radicals have been measured at 298 ± 4 K: (a) for OH(v= 1) with HNO3, H2O, NO and NO2: (2.54 ± 0.11)× 10–11, (1.36 ± 0.1)× 10–11, (3.8 ± 0.6)× 10–11 and (4.8 ± 0.8)× 10–11; (b) for OD(v= 1) with DNO3, NO and NO2: (1.53± 0.2)× 10–11, (2.7 ± 0.3)× 10–11 and (4.3 ± 0.3)× 10–11. Rate constants are also reported for recombination of OH(v= 0) and, for the first time, OD(v= 0) with NO and NO2 in the presence of 18 Torr Ar. The ratios of rate constants (kOH/kOD) under these conditions are (1.0 ± 0.16) for association with NO and (0.99 ± 0.17) for combination with NO2. The rate data for all processes involving radical–radical collisions are discussed in terms of models developed by Quack and Troe for treating processes proceeding via strongly bound collision complexes.

Time‐resolved emission studies of vibrationally excited hydroxyl radicals: OH(X2π, v″=9)

Vibrationally excited hydroxyl radicals OH(v″=9) are produced following the laser photolysis of 03/H2/He mixtures at 248 nm. The temporal profile of OH(v″=9) density was monitored by observation of the Δv=6 emission near 625 nm. Variation of the ozone pressure yields an estimate of the rate of vibrational relaxation of OH(v″=9) by 03, k ≈ 2 × 10−10 cm3 molecule−1 s−1. Comparison of the observed overtone emission intensity with that arising from the 0(21D2) precursor at 630 nm yields a factor‐of‐two estimate of the Einstein coefficient for spontaneous emission, A9→3 ≈ 0.006 s−1. This is in agreement with a previous experimental study, but does not support a theoretical value that has been extensively utilized in modeling of atmospheric Meinel emission intensity.

Energy Transfer Processes in the Stratosphere

During the past decade or two considerable fundamental information has been acquired on the mechanisms and rates of molecular energy transfer among vibrational, rotational, and electronic states. It is a well-known fact that the rate and even the products of a chemical reaction can be markedly influenced by the degree of internal state excitation of the reactants. Also, it is clear that many of the chemical processes occurring in the upper atmosphere produce atoms or molecules in excited states. It will be the purpose of this paper to review the state of knowledge of energy transfer mechanisms and kinetic rates as it pertains to atmospheric constituents and to attempt to indicate possible important roles of energy transfer in the physics and chemistry of the stratosphere. Some specific examples will include vibrational relaxation of OH and O3, the possible role of atomic species as quenchants, and electronic/vibrational energy transfer processes.

Formation of Vibrationally Excited OH by the Reaction H + O_3

An earlier study [Chem. Phys. Lett. 1, 619 (1968)] concluded that the reaction H + O(3) ? OH + O(2) forms OH predominantly in the highest accessible vibrational levels, upsilon = 8 and 9. We have extended this earlier work (1) by using fourier transform spectroscopy which is capable of giving more precise values for the relative vibrational populations at low intensities, (2) by recording emission down to lower background pressures (1 x 10(-4) Torr), and (3) by treating the vessel walls so as to remove OHdagger (vibrationally excited OH in it ground (2)II electronic state) more effectively. This involved using a room temperature vessel coated with silica gel. Under these conditions (provided that the values available for the radiational lifetime of OHdagger are correct) vibrational relaxation of OHdagger should have been largely arrested. We conclude that the relative rate constants for formation of OHdagger in levels upsilon are k(upsilon = 6) < 0.4, k(upsilon = 7) asymptotically equal to 0.4, k(upsilon = 8) asymptotically equal to 0.8, and k(upsilon = 9) = 1.00.