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We have analyzed the effects of the spreads in experimental parameters on the reliability of speeds and angular distributions extracted from a generic surface-scattering experiment based on planar laser-induced fluorescence detection. The numerical model assumes a pulsed beam of projectile molecules is directed at a surface. The spatial distribution of the scattered products is detected by imaging the laser-induced fluorescence excited by a thin, pulsed sheet of laser light. Monte Carlo sampling is used to select from realistic distributions of the experimental parameters. The key parameter is found to be the molecular-beam diameter, expressed as a ratio to the measurement distance from the point of impact. Measured angular distributions are negligibly distorted when this ratio is <∼10%. Measured most-probable speeds are more tolerant, being undistorted when it is <∼20%. In contrast, the spread of speeds or of corresponding arrival times in the incident molecular beam has only very minor systematic effects. The thickness of the laser sheet is also unimportant within realistic practical limits. These conclusions are broadly applicable to experiments of this general type. In addition, we have analyzed the specific set of parameters designed to match the experiments on OH scattering from a liquid perfluoropolyether (PFPE) surface in the Paper I [Roman et al., J. Chem. Phys. 158, 244704 (2023)]. This reveals that the detailed form of the molecular-beam profile is important, particularly on apparent angular distributions, for geometric reasons that we explain. Empirical factors have been derived to correct for these effects. [ABSTRACT FROM AUTHOR]

Inelastic collisions of OH with an inert liquid perfluoropolyether (PFPE) surface have been studied experimentally. A pulsed molecular beam of OH with a kinetic energy distribution peaking at 35 kJ mol−1 was directed at a continually refreshed PFPE surface. OH molecules were detected state-selectively with spatial and temporal resolution by pulsed, planar laser-induced fluorescence. The scattered speed distributions were confirmed to be strongly superthermal, regardless of the incidence angle (0° or 45°). Angular scattering distributions were measured for the first time; their reliability was confirmed through extensive Monte Carlo simulations of experimental averaging effects, described in Paper II [A. G. Knight et al., J. Chem. Phys. 158, 244705 (2023)]. The distributions depend markedly on the incidence angle and are correlated with scattered OH speed, consistent with predominantly impulsive scattering. For 45° incidence, the angular distributions are distinctly asymmetric to the specular side but peak at sub-specular angles. This, along with the breadth of the distributions, is incompatible with scattering from a surface that is flat on a molecular scale. New molecular dynamics simulations corroborate the roughness of the PFPE surface. A subtle but unexpected systematic dependence of the angular distribution on the OH rotational state was found, which may be dynamical in origin. The OH angular distributions are similar to those for kinematically similar Ne scattering from PFPE and hence not strongly perturbed by OH being a linear rotor. The results here are broadly compatible with prior predictions from independent quasiclassical trajectory simulations of OH scattering from a model-fluorinated self-assembled monolayer surface. [ABSTRACT FROM AUTHOR]

As part of an ongoing study of the structure and properties of mixtures of ionic liquids in which one component has a hydrocarbon chain and the other a semiperfluorocarbon chain, we now report a study of the mixtures [C8MIM]1−x[C10MIM-F17]x[Tf2N], [C10MIM]1−x[C8MIM-F13]x[Tf2N] and [C10MIM]1−x[C10MIM-F17]x[Tf2N], where [C8MIM][Tf2N] is 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide, [C10MIM][Tf2N] is 1-decyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [C8MIM-F13][Tf2N] is 1-(1H,1H,2H,2H-perfluorooctyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide and [C10MIM-F17][Tf2N] is 1-(1H,1H,2H,2H-perfluorodecyl)-3-methylimidizolium bis(trifluoromethylsulfonyl)imide. The mixtures were investigated using small-angle X-ray (SAXS) and neutron (SANS) scattering complemented by molecular dynamics simulations (with viscosity and surface tension measurements also possible for the mixtures [C10MIM]1−x[C8MIM-F13]x[Tf2N]). Unlike previous studies of [C8MIM]1−x[C8MIM-F13]x[Tf2N], where no strong evidence of alkyl/fluoroalkyl chain segregation or triphilic behaviour was seen (Elstone et al., J. Phys. Chem. B, 2023, 127, 7394–7407), these new mixtures show the formation of small aggregates of varying sizes of each component, even though all were co-miscible across the full range of compositions. Thus, while a clear polar non-polar peak (PNPP) was observed at large or small values of x, at intermediate compositions the small-angle neutron scattering at low q was dominated by scattering from these small aggregates, while at other compositions, there was little or no evidence of the PNPP. The origins of this behaviour are discussed in terms of inter-chain interactions. [ABSTRACT FROM AUTHOR]

The article focuses on the discussion of scattering phenomena involving larger molecules, specifically addressing questions related to the phenylethynyl radical in the interstellar medium. Topics include the challenges in detecting the phenylethynyl radical due to its complex spectrum, potential formation mechanisms in space, and the role of experimental and computational methods in predicting its presence.

The article focuses on a general discussion of scattering phenomena at condensed-phase surfaces, particularly in relation to the challenges and techniques involved in measuring angular distributions of evaporating atoms. Topics include the difficulties of measuring angular distributions from cylindrical jets versus flat jets, the prediction of helium atom evaporation patterns, and the observation of surfactant segregation in microjets.

The article focuses on a general discussion about the scattering of larger molecules, particularly examining the differences in potential energy surfaces (PES) between HDO, H2O, and D2O. Topics include the reason for stronger back scattering in HDO compared to H2O and D2O, the role of symmetry and potential terms in scattering phenomena, and specific insights into the n10 term affecting back-scattering.

A crossed beam velocity-map ion-imaging apparatus has been used to determine differential cross sections (DCSs) for the rotationally inelastic scattering of NO(A²Σ+, v = 0, j = 0.5) with CO2, as a function of both NO(A, v = 0, N′) final state and the coincident final rotational energy of the CO2. The DCSs are dominated by forward-peaked scattering for all N′, with significant rotational excitation of CO2, and a small backward scattered peak is also observed for all final N′. However, no rotational rainbow scattering is observed and there is no evidence for significant product rotational angular momentum polarization. New ab initio potential energy surface calculations at the PNO-CCSD(T)-F12b level of theory report strong attractive forces at long ranges with significant anisotropy relative to both NO and CO2. The absence of rotational rainbow scattering is consistent with removal of low-impact-parameter collisions via electronic quenching, in agreement with the literature quenching rates of NO(A) by CO2 and recent electronic structure calculations. We propose that high-impact-parameter collisions, that do not lead to quenching, experience strong anisotropic attractive forces that lead to significant rotational excitation in both NO and CO2, depolarizing product angular momentum while leading to forward and backward glory scattering. [ABSTRACT FROM AUTHOR]

We describe the real-space imaging of the products of molecular scattering, applied to collisions of hydroxyl radicals with low-vapor-pressure-liquid surfaces. A pulsed molecular beam of OD (for technical reasons) with a mean laboratory-frame kinetic energy of 29.5 kJ mol−1 was directed at continually refreshed surfaces of the representative liquids perfluoropolyether, squalane, and squalene. Laser-induced fluorescence (LIF) was excited by pulsed laser light shaped into a planar sheet, tuned to selected rovibronic transitions in the OD A–X band. The LIF emission was imaged and intensified before being captured by an external camera. Sequences of images allowed the evolution of the incident packet and scattered plumes of OD molecules to be observed. The results confirm previous observations of the internal-state distributions of the scattered OD and its differential survival probability on different liquid surfaces. New measurements of the angular distributions found them all to be broad and approximately symmetric, independent of the angle of incidence. This is interpreted as implying a high degree of atomic-scale roughness, rather than a predominant trapping-desorption mechanism, because of the other observed signatures of impulsive scattering; these include the degree of OD rotational excitation, superthermal speeds, and the correlation of speed with scattering angle. This approach has considerable potential to be applied in related gas-surface scattering experiments. It is immune from the difficulties of some other imaging methods that involve charged-particle detection and allows a spatially extended region of the scattering plane perpendicular to the surface to be imaged. [ABSTRACT FROM AUTHOR]

Large-scale molecular dynamics simulations of liquid squalane [C.sub.30][H.sub.62] were performed using an established force field for identifying potentially reactive sites on the surface. An identifiable preference for methyl groups was found at lower temperatures and the width of interface was also found to increase with increasing temperature.

The first measurements of internal energy distributions of the OH produced by a direct mechanism of interfacial reaction between O((super 3)P) atoms and liquid squalane, C(sub 30)H(sub 62), by time-of-flight profile are reported. It was concluded that at least two mechanisms contribute to the production of OH and closer correspondence of translational and rotational energy distributions of higher velocity OH to those from the corresponding gas-phase of O((super 3)P) with smaller OH suggests a very similar, near collinear direct abstraction.

The collision-induced electronic energy transfer between the CH A2 delta and B(sub 2)Sigma(super --) states with the series of partners He, Ar, H2, N2, CO, and CO2 is investigated. The results do not indicate any enhanced effectiveness in B(sub 2)Sigma(super -) to A(sub 2)Delta coupling for those partners which form strongly bound intermediates, suggesting that this specific electronic channel is controlled by different regions of the potential energy surfaces.

A crossed molecular beam, velocity-map ion imaging apparatus has been used to determine differential cross sections (DCSs) and angle-resolved rotational angular momentum alignment moments for the state-resolved rotationally inelastic scattering of NO(A2Σ+, v = 0, j = 0.5 f1) with Kr at an average collision energy of 785 cm−1. The experimental results are compared to close-coupled quantum scattering (QS) calculations performed on a literature ab initio potential energy surface (J. Kłos et al., J. Chem. Phys., 2008, 129, 244303). DCSs are very strongly forward scattered, with weaker side and backward scattered peaks becoming progressively more important at higher-N′. Good agreement is found between experimental and QS DCSs, indicating that the PES is an accurate reflection of the NO(A)–Kr interaction energies. Partial wave analysis of the QS DCSs isolates multiple scattering mechanisms contributing to the DCSs, including L -type rainbows and Fraunhofer diffraction. Measured alignment moments are not well described by a hard-shell kinematic apse scattering model, showing deviations in the forward scattering hemisphere that are in agreement with QS calculations and arise from attractive regions of the PES. These discrepancies emphasise that established scattering mechanisms for molecules such as NO with lighter noble gases cannot be extrapolated safely to heavier, more polarisable members of the series. [ABSTRACT FROM AUTHOR]

We present a crossed molecular beam velocity-map ion imaging study of state-to-state rotational energy transfer of NO(A²Σ+, v = 0, N = 0, j = 0.5) in collisions with Ne atoms. From these measurements, we report differential cross sections and angle-resolved rotational angular momentum alignment moments for product states N' = 3 and 5-10 for collisions at an average energy of 523 cm-1, and N' = 3 and 5-14 for collisions at an average energy of 1309 cm-1, respectively. The experimental results are compared to the results of close-coupled quantum scattering calculations on two literature ab initio potential energy surfaces (PESs) [Pajón-Suárez et al., Chem. Phys. Lett. 429, 389 (2006) and Cybulski and Fernández, J. Phys. Chem. A 116, 7319 (2012)]. The differential cross sections from both experiment and theory show clear rotational rainbow structures at both collision energies, and comparison of the angles observed for the rainbow peaks leads to the conclusion that Cybulski and Fernández PES better represents the NO(A²Σ+)-Ne interaction at the collision energies used here. Sharp, forward scattered (<10?), peaks are observed in the experimental differential cross sections for a wide range of N' at both collision energies, which are not reproduced by theory on either PES. We identify these as L-type rainbows, characteristic of attractive interactions, and consistent with a shallow well in the collinear Ne-N-O geometry, similar to that calculated for the NO(A²Σ+)-Ar surface [Kłos et al., J. Chem. Phys. 129, 244303 (2008)], but absent from both of the NO(A²Σ+)-Ne surfaces tested here. The angle-resolved alignment moments calculated by quantum scattering theory are generally in good agreement with the experimental results, but both experiment and quantum scattering theories are dramatically different to the predictions of a classical rigid-shell, kinematic-apse conservation model. Strong oscillations are resolved in the experimental alignment moments as a function of scattering angle, confirming and extending the preliminary report of this behavior [Steill et al., J. Phys. Chem. A 117, 8163 (2013)]. These oscillations are correlated with structure in the differential cross section, suggesting an interference effect is responsible for their appearance. [ABSTRACT FROM AUTHOR]

We present a crossed molecular beam scattering study, using velocity-map ion-imaging detection, of state-to-state rotational energy transfer for NO(A²Σ+) in collisions with the kinematically identical colliders He and D2. We report differential cross sections and angle-resolved rotational angular momentum polarization moments for transfer of NO(A, v = 0, N = 0, j = 0.5) to NO(A, v = 0, N' = 3, 5-12) in collisions with He and D2 at respective average collision energies of 670 cm-1 and 663 cm-1. Quantum scattering calculations on a literature ab initio potential energy surface for NO(A)-He [J. Kłos et al., J. Chem. Phys. 129, 244303 (2008)] yield near-quantitative agreement with the experimental differential scattering cross sections and good agreement with the rotational polarization moments. This confirms that the Kłos et al. potential is accurate within the experimental collisional energy range. Comparison of the experimental results for NO(A) + D2 and He collisions provides information on the hitherto unknown NO(A)-D2 potential energy surface. The similarities in the measured scattering dynamics of NO(A) imply that the general form of the NO(A)-D2 potential must be similar to that calculated for NO(A)-He. A consistent trend for the rotational rainbow maximum in the differential cross sections for NO(A) + D2 to peak at more forward angles than those for NO(A) + He is consistent with the NO(A)-D2 potential being more anisotropic with respect to NO(A) orientation. No evidence is found in the experimental measurements for coincident rotational excitation of the D2, consistent with the potential having low anisotropy with respect to D2. The NO(A) + He polarization moments deviate systematically from the predictions of a hard-shell, kinematic-apse scattering model, with larger deviations as N' increases, which we attribute to the shallow gradient of the anisotropic repulsive NO(A)-He potential energy surface. [ABSTRACT FROM AUTHOR]

Research was conducted to examine the rotational state-specific dynamics of SiF C2-Delta-B2-Sigma+ collision-induced transfer. Discrete rotational levels of the SiiF C2-delta, v = 0 state were prepared using laser excitation on the C2-Delta-X2T-Pi transition which allowed their state-specific collisional behavior to be investigated. Results indicate that the rotational angular momentum of SiF C2-Delta has little effect on the rate of total quenching by simple partners.

We present the implementation of a new crossed-molecular beam, velocity-map ion-imaging apparatus, optimized for collisions of electronically excited molecules. We have applied this apparatus to rotational energy transfer in NO(A²Σ+, v = 0, N = 0, j = 0.5) + Ar collisions, at an average energy of 525 cm-1. We report differential cross sections for scattering into NO(A²Σ+, v = 0, N' = 3, 5, 6, 7, 8, and 9), together with quantum scattering calculations of the differential cross sections and angle dependent rotational alignment. The differential cross sections show dramatic forward scattered peaks, together with oscillatory behavior at larger scattering angles, while the rotational alignment moments are also found to oscillate as a function of scattering angle. In general, the quantum scattering calculations are found to agree well with experiment, reproducing the forward scattering and oscillatory behavior at larger scattering angles. Analysis of the quantum scattering calculations as a function of total rotational angular momentum indicates that the forward scattering peak originates from the attractive minimum in the potential energy surface at the N-end of the NO. Deviations in the quantum scattering predictions from the experimental results, for scattering at angles greater than 10°, are observed to be more significant for scattering to odd final N'. We suggest that this represents inaccuracies in the potential energy surface, and in particular in its representation of the difference between the N- and O-ends of the molecule, as given by the odd-order Legendre moments of the surface. [ABSTRACT FROM AUTHOR]

The depolarization of OH(X 2Π3/2,v=0,J=1.5–6.5,e) rotational angular momentum (RAM) in collisions with He and Ar under thermal conditions (298 K) has been studied using two-color polarization spectroscopy (PS). Orientation or alignment of the OH RAM was achieved using circularly or linearly polarized pulsed excitation, respectively, on the off-diagonal OH A 2Σ+-X 2Π(1,0) band. The evolution of the ground-state OH(X) RAM polarization, exclusively, was probed using an independent, linearly polarized pulse tuned to the diagonal OH A 2Σ+-X 2Π(0,0) band. The PS signal decay rate constant kPS decreases with increasing rotational quantum number for OH(X)+Ar but does not vary monotonically for OH(X)+He. The measured kPS equals the sum kRET+kΛ+kdep, where kRET, kΛ, and kdep are the rate constants for rotational energy transfer, Λ-doublet changing collisions, and rotationally elastic depolarization (of orientation or alignment of the OH(X) angular momentum, as specified), respectively. Values of kdep can be extracted from the measured kPS with prior knowledge of kRET and kΛ. Because kRET and kΛ were not previously available for collisions of Ar with OH(X, v=0), we performed exact, fully quantum-mechanical scattering calculations on a new potential energy surface (PES) presented here for the first time. The raw experimental results show that kdep is systematically markedly higher for alignment than for orientation for OH(X)+Ar but much more weakly so for OH(X)+He. Calculated kRET and kΛ values at 298.15 K are consistent with a substantial contribution from kdep for OH(X)+Ar but not for OH(X)+He. This may point to the role of attractive forces in elastic depolarization. The experimental results provide a very sensitive test of the ability of the most recent ab initio OH(X)–He PES of Lee et al. [J. Chem. Phys. 113, 5736 (2000)] to reproduce kRET+kΛ accurately. [ABSTRACT FROM AUTHOR]

We show that weak residual magnetic fields can significantly affect the preparation and measurement of molecular rotational angular momentum alignment in a typical gas-phase stereodynamics apparatus. Specifically, polarization spectroscopy, a third-order nonlinear spectroscopic technique, is used to prepare and probe the collisional and noncollisional losses of rotational angular momentum alignment of OH X 2Π. Residual magnetic fields of the order of the geomagnetic field are shown to have a significant effect on the prepared polarization on a submicrosecond timescale. This can be expected to be a significant effect for many gas-phase free radicals, such as those of interest in combustion, atmospheric chemistry, and the burgeoning field of cold molecules. We demonstrate a simple experimental remedy for this problem. [ABSTRACT FROM AUTHOR]

We present the first results from a novel experimental approach to the measurement of state-to-state differential scattering cross-sections for inelastic scattering of electronically excited CN A2Π with Ar. Photodissociation of ICN with linearly polarized 266 nm radiation generates CN X2Σ+ (υ″=0,J″) with a near mono-energetic speed distribution and large anisotropy. Saturated optical pumping of the nascent CN X2Σ+ transfers this speed distribution without distortion to selected rotational quantum states of the A2Π (υ′=4) level. The products of rotational energy transfer within the A2Π (υ′=4) level into the J′=0.5, F2, f, state are probed using frequency modulated stimulated emission spectroscopy on the A–X (4,2) band with a single frequency external cavity tunable diode laser. Doppler profiles of transitions from individual rotational, spin-orbit and lambda doublet specific levels are acquired for different geometrical arrangements of photolysis polarization and probe propagation directions. The resulting Doppler profiles, which for this J′=0.5 state cannot display a rotational angular momentum alignment, are combined to yield composite Doppler profiles depending on speed and translational anisotropy, which are analyzed to determine fully state-to-state resolved differential scattering cross-sections. [ABSTRACT FROM AUTHOR]

A theoretical analysis of two-color polarization spectroscopy (TCPS) is presented as an extension of a previous analysis of one-color PS. Three commonly used schemes in which pump and probe transitions share a common level are considered. Diagrammatic techniques are used to isolate the photon interaction sequences that can contribute to the signal. A perturbation-theory analysis expressing the signal in terms of spherical tensor moments is applied. The analysis emphasises the significance of orientation and alignment tensor moments of rotational angular momentum and their collisional evolution. The assumed context is transitions between single rotational states of gas-phase molecules that subsequently suffer discrete collisions. The time scale of the measurements is assumed to be long relative to the periods of molecular motion, as would typically be the case for signals excited by nanosecond-pulsed lasers from samples at moderate pressures. The Doppler motion of the probed species is included, as is an analytical solution to the integration over the Maxwell–Boltzmann distribution of velocities. The effects of nuclear hyperfine depolarization and velocity-changing collisions are discussed. It is shown that when pump- and probe-laser pulses are separated in time, TCPS creates and probes either orientation or alignment of rotational angular momentum in the common level shared by pump and probe transitions. Example simulations of one- and two-color polarization spectroscopies are included to demonstrate the resulting simplification of the measured signal using TCPS. TCPS is therefore a viable spectroscopic technique for the determination of rotational angular momentum orientation and alignment relaxation rates in molecular gases, of interest because they are sensitive probes of inelastic collisions. [ABSTRACT FROM AUTHOR]

OH/OD product state distributions arising from the reaction of gas-phase O(3P) atoms at the surface of the liquid hydrocarbon squalane C30H62/C30D62 have been measured. The O(3P) atoms were generated by 355 nm laser photolysis of NO2 at a low pressure above the continually refreshed liquid. It has been shown unambiguously that the hydroxyl radicals detected by laser-induced fluorescence originate from the squalane surface. The gas-phase OH/OD rotational populations are found to be partially sensitive to the liquid temperature, but do not adapt to it completely. In addition, rotational temperatures for OH/OD(v′=1) are consistently colder (by 34±5 K) than those for OH/OD(v′=0). This is reminiscent of, but less pronounced than, a similar effect in the well-studied homogeneous gas-phase reaction of O(3P) with smaller hydrocarbons. We conclude that the rotational distributions are composed of two different components. One originates from a direct abstraction mechanism with product characteristics similar to those in the gas phase. The other is a trapping-desorption process yielding a thermal, Boltzmann-like distribution close to the surface temperature. This conclusion is consistent with that reached previously from independent measurements of OH product velocity distributions in complementary molecular-beam scattering experiments. It is further supported by the temporal profiles of OH/OD laser-induced fluorescence signals as a function of distance from the surface observed in the current experiments. The vibrational branching ratios for (v′=1)/(v′=0) for OH and OD have been found to be (0.07±0.02) and (0.30±0.10), respectively. The detection of vibrationally excited hydroxyl radicals suggests that secondary and/or tertiary hydrogen atoms may be accessible to the attacking oxygen atoms.© 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A diagrammatic perturbation theory description of one-color polarization spectroscopy (PS) is developed which emphasizes the significance of orientation and alignment tensor moments of the rotational angular momentum, and their collisional evolution. The influences of Doppler motion, velocity-changing collisions, decay of population, orientation and alignment, and nuclear hyperfine depolarization on the calculated PS signal are discussed. Illustrative simulations are presented of the evolution of the PS signal as a function of pump–probe laser delay. These are generated by a Monte Carlo integration of the derived equations for the signal electric field over typical experimental pump and probe laser temporal profiles and velocity distributions for a commonly studied system, the OH A 2Σ+–X 2Π (0,0) band. These predictions are compared with a preliminary set of results obtained in an experimental apparatus designed for one-color polarization spectroscopy using independent pump and probe lasers. Measurements are presented using linearly polarized pump light on the Q1(2.5) transition of the OH A 2Σ+–X 2Π (0,0) band with He as the collision partner. The decay of the experimental PS pump–probe signal is discussed with reference to inelastic collisional population transfer rates in the literature. It is concluded that the collisional depolarization of rotational alignment is rapid, with a rate approximately twice that of population transfer. This is consistent with previous measurements in atmospheric pressure flames. PS is shown to be a viable novel spectroscopic method for determining rotational angular momentum orientation and alignment relaxation rates, which are valuable quantities because they are sensitive probes of the forces involved in inelastic collisions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We describe an experimental approach to the determination of the nascent internal state distribution of gas-phase products of a gas–liquid interfacial reaction. The system chosen for study is O([sup 3]P) atoms with the surface of liquid deuterated squalane, a partially branched long-chain saturated hydrocarbon, C[sub 30]D[sub 62]. The nascent OD products are detected by laser-induced fluorescence. Both OD (v[sup ′]=0) and (v[sup ′]=1) were observed in significant yield. The rotational distributions in both vibrational levels are essentially the same, and are characteristic of a Boltzmann distribution at a temperature close to that of the liquid surface. This contrasts with the distributions in the corresponding homogeneous gas-phase reactions. We propose a preliminary interpretation in terms of a dominant trapping-desorption mechanism, in which the OD molecules are retained at the surface sufficiently long to cause rotational equilibration but not complete vibrational relaxation. The significant yield of vibrationally excited OD also suggests that the surface is not composed entirely of –CD[sub 3] endgroups, but that secondary and/or tertiary units along the backbone are exposed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effect of collisions on the magnitude of polarization spectroscopy (PS) signals from the OH radical on the A [sup 2]Σ[sup +]–X [sup 2]Π (0,0) band has been studied. OH was produced by the 266-nm photolysis of H[sub 2]O[sub 2] and detected by one-color degenerate PS, using ≈5-ns pulses from a Nd:YAG pumped dye laser. Spectra with both circular and linear pump polarizations are reported, together with signal dependence on OH number density and pump pulse fluence. The relative line intensities in the spectra and measured square dependence on OH number density are consistent with a description of PS as a variant of four-wave mixing spectroscopy. The pump pulse fluence dependence is fitted well by literature saturation curves. The collisional dependence of the PS signal was investigated by adding increasing pressures of He, Ar, or N[sub 2] collider gases for fixed overlapping pump and probe pulses. The principal finding is the very rapid loss of the PS signal with increasing collider pressure. The resulting phenomenological rate constants are in the range 5–9×10[sup -9] cm[sup 3] s[sup -1]. We discuss these rate constants with reference to the literature rotational energy transfer and dephasing rate constants. We propose that the very large observed values may be explained by the effect of elastic velocity changing collisions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]