Energy Transfer In OH Research Articles

ChemInform Abstract: Quantum Scattering Study of Rotational Energy Transfer in OH(A 2σ +, υ′ = 0) in Collisions with He(1S).

ChemInform Abstract: Quantum Scattering Study of Rotational Energy Transfer in OH(A 2σ +, υ′ = 0) in Collisions with He(1S).

State-to-State Vibrational Energy Transfer in OH A2Σ+ with N2

Vibrational energy transfer from v' = 0 to v' = 1 in the excited OH A2Sigma+ electronic state is investigated in the collisional region of a free-jet expansion. Laser excitation is used to prepare high rotational levels in OH A2Sigma+ (v' = 0, N' = 14-22), which lie above the energetic threshold for OH A2Sigma+ (v' = 1). Subsequent collisions with N2 result in population of a distribution of OH A2Sigma+ (v' = 1) product rotational levels that is characterized through dispersed fluorescence spectra. The majority of products are found in the most near-resonant rotational level of v' = 1, with population falling off exponentially in lower rotational levels. Additionally, the efficiency of vibrational energy transfer is determined by comparing the emission from v' = 1 products with that from the initially prepared v' = 0 level. The fractional transfer decreases by an order of magnitude from the highest to lowest initial rotational levels investigated. This decrease is correlated with an increasingly large change in rotational angular momentum between the initial and final states. The results show that angular momentum constraints are the dominant factor in the efficiency of OH A2Sigma+ v' = 0 to v' = 1 vibrational energy transfer at low collision energies.

Temperature dependence of the collisional energy transfer of OH(v=10) between 220 and 310 K

The temperature dependence of the thermally averaged collisional removal cross section of OH (X 2∏, v=10) by O2, N2O, and CO2 is measured between 220 and 310 K using a two-laser pump–probe technique and a specially designed vacuum-isolated flow cell. OH molecules are generated in v=6–9 by the reaction of hydrogen atoms and ozone. The (10,7) vibrational transition is excited with pulsed near-infrared laser light to create a population of OH (v=10) molecules. The temporal evolution of the v=10 population is monitored as a function of collider gas pressure by a time-delayed ultraviolet laser pulse. The probe step uses laser-induced fluorescence by exciting the B 2∑+–X 2∏ (0,10) transition and detecting the fluorescence from the B 2∑+–A 2∑+ (0,6–8) transitions. From 310 to 223 K, the OH (v=10) removal cross section increases by 35±21, 33±14, and 58±48 percent for the colliders O2, N2O, and CO2, respectively. This inverse temperature dependence is typical of a loss mechanism governed by long-range attractive forces.

State‐resolved collisional energy transfer of OH, NH and H2CO by two‐color resonant four‐wave mixing spectroscopy

AbstractExtra resonances observed in two‐color resonant four‐wave mixing (TC‐RFWM) spectra, which originate from collision‐induced energy transfer processes, were investigated experimentally and theoretically. It is shown that the resonances yield information on state‐to‐state rotational energy transfer (RET) occurring in the ground and excited states of OH and NH. The measurements were performed in situ in an equilibrium medium of high temperature and high density. The results shed more light on the propensity rules for the change in the fine‐structure label and rotational quantum number of NH. A comparison with a recent investigation of NH RET on the ground‐state potential yielded good agreement. A theoretical frequency‐domain picture was developed with the help of exact (off‐diagonal) relaxation matrices and general four‐level schemes. The theoretical results were compared with measured relative TC‐RFWM intensities of the OH radical obtained in a flame. Furthermore, the investigation on diatomic radicals was extended to the polyatomic formaldehyde (H2CO) molecule. Initial cell experiments were performed at low pressure by using CO2 as a quencher. The observation of collision‐induced resonances in the ground state of H2CO indicate the potential of the method for the investigation of RET processes occurring in polyatomic molecules in equilibrium media. Copyright © 2002 John Wiley & Sons, Ltd.

Vibrational energy transfer in OH A 2Σ+ between 195 and 295 K

Vibrational energy transfer (VET) v′=1→0 and electronic quenching of v′=1 and 0 in the A 2Σ+ electronically excited state of the OH radical has been studied over the temperature range 195 to 295 K. The colliders investigated were N2, O2, and CO2. Laser-induced fluorescence experiments were conducted in a flow cell with photolytic production of OH; both fluorescence intensity and time decay measurements were made. The VET cross sections are found to increase with decreasing temperature, suggestive of attractive force interactions in the VET process.

Rate-equation modeling of single- and multiple-quantum vibrational energy transfer of OH (A 2 Σ + , v ′ =0 to 3)

A computer code based on a kinetic rate-equation model for describing the collisional dynamics of OH (A 2Σ+) in laser-induced fluorescence experiments was developed. In this work, the capabilities of the simulation code are extended to include the vibrational states up to the OH (A 2Σ+, v′=3) level. The calculation of quenching, rotational and vibrational relaxation rate coefficients for different collider species is discussed. Problems that arise for the description of vibrational relaxation include the branching ratio between single- and multiple-quantum steps and the form of the nascent rotational distribution after a vibrational relaxation step. Experimental spectra recorded under a variety of conditions are simulated using a consistent set of model assumptions. The calculations must include vibrational relaxation steps up to Δv=3 to account for the experimental intensity distributions. Effects due to polarized laser excitation become more important for vibrational states with v′>1. Areas for future work are identified, including determination of experimental rate coefficients for state-changing and depolarizing collisions in the upper vibrational levels.

Picosecond laser-induced fluorescence measurement of rotational energy transfer of OH A^2Σ^+ (v′ = 2) in atmospheric pressure flames

Rate constants for total and state-specific rotational energy transfer (RET) of OH A(2) ?(+) (v? = 2) have been measured directly in atmospheric methane-air and methane-oxygen flames for the first time to our knowledge. We used a picosecond Raman-excimer laser (tau(l) = 300 ps, lambda = 268 nm) to excite the P(11) (12.5) and Q(11) (16.5) A -X transitions in the (2, 0) band of OH molecules. We analyzed the resultant fluorescence with a high-resolution spectrometer in combination with a fast-gated, intensified CCD camera (tau(g) = 400 ps). We recorded the temporal evolution of the emission spectrum by shifting the detection time with respect to the laser pulse. Measured emission spectra were inverted to yield the time-dependent population of rotational levels in the excited state. We calculated rate constants for RET from the results of the fit. The total RET in v? = 2 is similar to v? = 0, 1. The state-specific rates are represented well by a simple energy-gap law.

Measurement of vibrational energy transfer of OH (A 2 Σ + ,v ′ =1→0) in low-pressure flames

Σ+) was measured in a low-pressure H2/O2 flame for three rotational levels of OH (v′=1). Rate coefficients for collisions with H2O and N2 were determined. At 1600 K, kVET (N2) is (in 10-11 cm3s-1) 10.1±2, 6.1±1.8, and 3.8±1.3 for N′=0, 5, and 13, respectively. The kVET (H2O) is <1.1±1.8. The kQ (N2) is <2.4±8 for both vibrational levels. The kQ (H2O) in v′=1 is 59.1±6.5, 54.7±6.4, and 54.9±6.6 for N′=0, 5, and 13, respectively, and, determined indirectly, 74.6±10.4, 70.6±10.3, and 63.4±7.3 for N′=0, 5, and 13 in v′=0. A multi-level model of OH population dynamics, which is being developed for the quantitative simulation of experimental LIF spectra, was extended to include VET. It was attempted to simulate state-to-state-specific VET coefficients for N2 collisions. From these simulations it appears that angular momentum conservation does not determine the N dependence of the vibrational relaxation step.

A scaling formalism for the representation of rotational energy transfer in OH (A 2Σ+) in combustion experiments

In order to predict spectra and temporal decays of the OH radical measured by Laser-Induced Fluorescence (LIF), the collisional energy transfer between different quantum states must be taken into account. At the elevated temperatures relevant for combustion studies, the large number of interacting quantum states and corresponding state-to-state energy transfer coefficients precludes the experimental measurement of all necessary information for the appropriate mixtures of colliders. Therefore, a scaling procedure has been devised which allows the representation of the matrix of Rotational Energy Transfer (RET) coefficients on the basis of measured data with four scaling coefficients. The scaling coefficients have been determined by comparison of the calculated RET rates with available measured data. The mathematical formalism for the scaling law - the ECS-EP law - is based on the Energy Corrected Sudden (ECS) law and includes an Exponential Power law (EP) for the representation of the basis coefficients.

A scaling formalism for the representation of rotational energy transfer in OH ( A 2 Σ + ) in combustion experiments

A scaling formalism for the representation of rotational energy transfer in OH ( A 2 Σ + ) in combustion experiments

Quantum scattering studies of the Λ doublet resolved rotational energy transfer of OH(X 2Π) in collisions with He and Ar

Three dimensional potential energy surfaces for the collision systems OH(X 2Π)+He and OH(X 2Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate 2Πx and 2Πy states split into two states of 2A′ and 2A″ symmetry, respectively, when the C∞v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close-coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet resolution show, within the same spin–orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F1(2Π3/2) state and an e/f conserving propensity in the F2(2Π1/2) state, while transitions between the two spin–orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen.

State-to-state rotational energy transfer in OH (A 2?+, ??=1)

State-to-state rotational energy transfer (RET) coefficients for thermal collisions of OH (A2Σ+, υ′=1) with He, Ar, N2, CO2, and H2O at 300K were determined from time-resolved laser-induced fluorescence (LIF) measurements. The RET coefficients are very similar in both qualitative behaviour and absolute magnitude to those measured previously for OH (A2Σ+, υ′=0).

State-specific rotational energy transfer in OH (A 2?+, v?=0) by some combustion-relevant collision partners

State-to-state rotational energy transfer (RET) co-efficients were determined for inelastic collisions of OH (A 2Σ+, v′=0) with N2, CO2, and H2O at 300 K. The experimental procedure described previously allows the direct evaluation of state-specific RET coefficients from time-resolved laser-induced fluorescence (LIF) measurements without any assumptions on the RET. The results show strikingly different RET behaviour for the three collision partners. The data can serve as a basis for a comparison with dynamic collision models.

Parity propensities in rotational energy transfer of OH X 2Πi with helium

Rotational energy transfer in OH (X 2Πi, v=2) induced by collisions with He, has been studied using a two-laser technique. Strong propensities are found for conservation of total parity in collisions which change the rotational level and spin orbit component. These results have implications concerning the shape of the OH–He potential.

Rotational energy transfer in OH (A 2Σ+, v′=0): A method for the direct determination of state-to-state transfer coefficients

We have determined state-to-state rate coefficients for rotational and fine structure transitions of OH (A 2Σ+, v′=0) in thermal collisions with He and Ar at 300 K. The temporal evolution of single fluorescence lines within the A–X, 0–0 band of OH were measured, exciting either the F2(4) or F2(5) state by a nanosecond laser pulse. The OH radical was produced in a discharge flow cell, containing predominantly He or Ar, at various pressures between 1 and 6 mbar. The time resolution in the experimental setup was sufficient to evaluate the rotational energy transfer coefficients directly from the time dependence of two fluorescence lines. The observed average rate coefficients for collisions of OH (A, v′=0) with Ar are approximately 3 times larger than those with He. The two rare gases show different qualitative behavior. Whereas the almost isoenergetic transitions with ΔJ=1 and ΔN=0 are favored in collisions with Ar, those with ΔJ=ΔN=−2 are favored in collisions with He. In addition, a strong preference for transitions conserving the parity of the OH, a propensity rule, previously reported for rotational relaxation in the A state of OH, was found for collisions with He but not for collisions with Ar. Our experimental results for He and for Ar are in good agreement with recent quantum mechanical calculations of the energy transfer coefficients.

Vibrational energy transfer in OH X 2Πi, v=2 and 1

Using an infrared pump/ultraviolet probe method in a flow discharge cell, vibrational energy transfer in OH X 2Πi has been studied. OH is prepared in v=2 by overtone excitation, and the time evolution of population in v=2 and 1 monitored by laser-induced fluorescence. Rate constants for vibrational relaxation by the colliders H2O, NH3, CO2, and CH4 were measured. Ratios of rate constants for removal from the two states, k2/k1, range from two to five.