Spin-orbit Distributions Research Articles

Infrared Activated Signatures and Jahn-Teller Dynamics of NO-CH4 Collision Complexes.

Bimolecular collision outcomes sensitively depend on the chemical functionality and relative orientations of the colliding partners that define the accessible reactive and nonreactive pathways. Accurate predictions from multidimensional potential energy surfaces demand a full characterization of the available mechanisms. Therefore, there is a need for experimental benchmarks to control and characterize the collision conditions with spectroscopic accuracy to accelerate the predictive modeling of chemical reactivity. To this end, the bimolecular collision outcomes can be investigated systematically by preparing reactants in the entrance channel prior to reaction. Herein, we investigate the vibrational spectroscopy and infrared-driven dynamics of the bimolecular collision complex between nitric oxide and methane (NO-CH4). We recorded the vibrational spectroscopy of NO-CH4 in the CH4 asymmetric stretching region using resonant ion-depletion infrared spectroscopy and infrared action spectroscopy, thus revealing a significantly broad spectrum centered at 3030 cm-1 that extends over 50 cm-1. The asymmetric CH stretch feature of NO-CH4 is explained by CH4 internal rotation and attributed to transitions involving three different nuclear spin isomers of CH4. The vibrational spectra also show extensive homogeneous broadening due to the ultrafast vibrational predissociation of NO-CH4. Additionally, we combine infrared activation of NO-CH4 with velocity map imaging of NO (X2Π, ν″ = 0, J″, Fn, Λ) products to develop a molecular-level understanding of the nonreactive collisions of NO with CH4. The anisotropy of the ion image features is largely determined by the probed rotational quantum number of NO (J″) products. For a subset of NO fragments, the ion images and total kinetic energy release (TKER) distributions show an anisotropic component at low relative translation (∼225 cm-1) indicating a prompt dissociation mechanism. However, for other detected NO products, the ion images and TKER distributions are bimodal, in which the anisotropic component is accompanied by an isotropic feature at high relative translation (∼1400 cm-1) signifying a slow dissociation pathway. In addition to the predissociation dynamics following vibrational excitation, the Jahn-Teller dynamics prior to infrared activation need to be considered to fully describe the product spin-orbit distributions. Therefore, we correlate the Jahn-Teller mechanisms of NO-CH4 to the symmetry-restricted NO (X2Π, ν″ = 0, J″, Fn, Λ) + CH4 (ν″) product outcomes.

Photoionization from the Xe 4d orbitals of XeF2.

We present a comparison of the photoionization dynamics of the 4d shell of XeF2 from threshold to 250eV to those of the prototypical 4d shell of atomic Xe. The new experimental data include spin-orbit and ligand-field-resolved partial cross sections, photoelectron angular distributions, branching fractions, and lifetime widths for the 4d-hole states. The spin-orbit branching fractions and angular distributions are remarkably similar to the corresponding distributions from atomic Xe across a broad energy interval that includes both the intense shape resonance in the f continuum and a Cooper minimum in the same channel. The angular distributions and branching fractions are also in reasonably good agreement with our first-principles theoretical calculations on XeF2. Data are also presented on the lifetime widths of the substate-resolved 4d-hole states of XeF2. While the trends in the widths are similar to those in the earlier experimental and theoretical work, the linewidths are considerably smaller than in the previous measurements, which may require some reinterpretation of the decay mechanism. Finally, we present new data and an analysis of the Auger electron spectra for ionization above the 4d thresholds and resonant Auger spectra for several pre-edge features.

Predissociation dynamics of D2 + hv→ D(1s1/2) + D(2p1/2,3/2, 2s1/2) revealed by the spin-orbit state resolved fragment branching ratios and angular distributions.

For molecular photodissociations, the spin-orbit state resolved fragment branching ratios and angular distributions provide deep insight into the dynamics. For the first excited state of the H(2p1/2,3/2) atom, a branching ratio measurement is a challenge because of small energy spacing between them. For the D(2p1/2,3/2) fragments from the predissociation of D2 + 14.76 eV → D(1s) + D(2s, 2p1/2,3/2) in the 2pπC1Πu (υ = 19) state, we made such measurements by pumping the D(2s, 2p1/2,3/2) fragments to high-lying Rydberg states that are subsequently ionized by a delayed-pulse electric field. In the 2pπC1Πu (υ = 19) state, the D2 molecule dissociates via both shape and Feshbach resonances correlating to the channels D(1s) + D(2p3/2) and D(1s) + D(2p1/2), respectively. The measured spin-orbit branching ratios, 2p3/2/(2p1/2 + 2p3/2), correspond to the diabatic limit, 2/3, which indicates strong spin-orbit state mixings near the dissociation limits. The spin-orbit state resolved fragment angular distributions also support the diabatic dissociation mechanism and illustrate simultaneous shape and Feshbach resonances for the R(0) transition.

Quantum-state-resolved studies of aqueous evaporation dynamics: NO ejection from a liquid water microjet.

This work presents the first fully quantum-state-resolved measurements of a solute molecule evaporating from the gas-liquid interface in vacuum. Specifically, laser-induced fluorescence detection of NO(2Π1/2, 3/2, v = 0, J) evaporating from an ∼5 mM NO-water solution provides a detailed characterization of the rotational and spin-orbit distributions emerging from a ⌀4-5 μm liquid microjet into vacuum. The internal-quantum-state populations are found to be well described by Boltzmann distributions, but corresponding to temperatures substantially colder (up to 50 K for rotational and 30 K for spin-orbit) than the water surface. The results therefore raise the intriguing possibility of non-equilibrium dynamics in the evaporation of dissolved gases at the vacuum-liquid-water interface. In order to best interpret these data, we use a model for evaporative cooling of the liquid microjet and develop a model for collisional cooling of the nascent NO evaporant in the expanding water vapor. In particular, the collisional-cooling model illustrates that, despite the 1/r drop-off in density near the microjet greatly reducing the probability of collisions in the expanding water vapor, even small inelastic cross sections (≲ 20 Å2) could account for the experimentally observed temperature differences. The current results do not rule out the possibility of non-equilibrium evaporation dynamics, but certainly suggest that correct interpretation of liquid-microjet studies, even under conditions previously considered as "collision-free," may require more careful consideration of residual collisional dynamics.

MERGERS AND OBLIQUITIES IN STELLAR TRIPLES

Many close stellar binaries are accompanied by a far-away star. The "eccentric Kozai-Lidov" (EKL) mechanism can cause dramatic inclination and eccentricity fluctuations, resulting in tidal tightening of inner binaries of triple stars. We run a large set of Monte-Carlo simulations including the secular evolution of the orbits, general relativistic precession and tides, and we determine the semimajor axis, eccentricity, inclination and spin-orbit angle distributions of the final configurations. We find that the efficiency of forming tight binaries (<~16 d) when taking the EKL mechanism into account is ~ 21%, and about 4% of all simulated systems ended up in a merger event. These merger events can lead to the formation of blue-stragglers. Furthermore, we find that the spin-orbit angle distribution of the inner binaries carries a signature of the initial setup of the system, thus observations can be used to disentangle close binaries' birth configuration. The resulting inner and outer final orbits' period distributions, and their estimated fraction, suggests secular dynamics may be a significant channel for the formation of close binaries in triples and even blue stragglers.

Doppler Tomographic Observations of Kepler-13b

AbstractWe present Doppler tomographic observations of the transiting planet Kepler-13b (aka KOI-13b), a highly inflated hot Jupiter orbiting the Teff=8500 K primary of a hierarchical triple stellar system. As the planet transits the rapidly rotating host star, it successively blocks regions of the stellar disk with different radial velocities, causing a “bump” in the stellar spectral line shape, which we resolve spectroscopically. The manner in which this perturbation moves across the stellar line during the transit gives information on the relative alignment between the stellar spin and planetary orbital angular momentum vectors. This is a powerful statistical probe of planetary migration processes, as the expected spin-orbit misalignment distributions from dynamically cold migration (disk interactions) and dynamically hot migration (planet-planet scattering, Kozai cycles) are significantly different: the former will lead to primarily aligned orbits; the latter to a wide distribution. Doppler tomography also promises to be a powerful technique for confirming transiting planet candidates around rapidly rotating stars like Kepler-13A, which are not amenable to radial velocity follow-up and thus are currently a poorly sampled region of parameter space.

ON THE FORMATION OF HOT JUPITERS IN STELLAR BINARIES

We study the production of Hot Jupiters (HJs) in stellar binaries. We show that the "eccentric Kozai-Lidov" (EKL) mechanism can play a key role in the dynamical evolution of a star-planet-star triple system. We run a large set of Monte Carlo simulations including the secular evolution of the orbits, general relativistic precession, and tides, and we determine the semi-major axis, eccentricity, inclination and spin-orbit angle distributions of the HJs that are produced. We explore the effect of different tidal friction parameters on the results. We find that the efficiency of forming HJs when taking the EKL mechanism into account is higher then previously estimated. Accounting for the frequency of stellar binaries, we find that this production mechanism can account for about 30% of the observed HJ population. Current observations of spin-orbit angles are consistent with this mechanism producing \sim 30% of all HJs, and up to 100% of the misaligned systems. Based on the properties of binaries without a HJ in our simulations, we predict the existence of many Jupiter-like planets with moderately eccentric and inclined orbits and semi-major axes of several AU.

Global observations of thermospheric temperature and nitric oxide from MIPAS spectra at 5.3μm

[1] We present vertically resolved thermospheric temperatures and NO abundances in terms of volume mixing ratio retrieved simultaneously from spectrally resolved 5.3 μm emissions recorded by the Michelson Interferometer for Passive Atmospheric Spectroscopy (MIPAS) in its upper atmospheric observation mode during 2005–2009. These measurements are unique since they represent the first global observations of temperature and NO for both day and night conditions taken from space. A retrieval scheme has been developed which accounts for vibrational, rotational and spin-orbit non-LTE distributions of NO. Retrieved polar temperature and NO profiles have a vertical resolution of 5–10 km for high Ap values, and degrade to 10–20 km for low Ap conditions. Though retrieved NO abundances depend strongly on the atomic oxygen profile used in the non-LTE modeling, observations can be compared to model results in a consistent manner by applying a simple correction. Apart from this, total retrieval errors are dominated by instrumental noise. The typical single measurement precision of temperature and NO abundances are 5–40 K and 10–30%, respectively, for high Ap values, increasing to 30–70 K for Tk and 20–50% for NO VMR for low Ap conditions. Temperature and NO profiles observed under auroral conditions are rather insensitive to smoothing errors related to the mapping of a priori profile shapes. However, for extra-polar and low Ap conditions, a potential systematic bias in the retrieved nighttime temperature and NO profiles related to smoothing errors has been identified from a comparison to Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) simulations. We have constructed a solar minimum monthly climatology of thermospheric temperature and NO from MIPAS observations taken during 2008–2009. MIPAS temperature distributions agree well, on average, with the Mass Spectrometer and Incoherent Scatter radar model (NRLMSISE-00), but some systematic differences exist. MIPAS temperatures are generally colder than NRLMSISE-00 in the polar middle thermosphere (mainly in the summer polar region) by up to 40 K; and are warmer than NRLMSISE-00 in the lower thermosphere around 120–125 km by 10–40 K. Thermospheric NO daytime distributions agree well with the Nitric Oxide Empirical Model (NOEM), based on Student Nitric Oxide Explorer (SNOE) observations. A comparison of MIPAS NO number density with the previous climatology for the declining phases of the solar cycle based on HALOE and SME data shows that MIPAS is generally larger with values ranging from 10 to 40%, except in the auroral region and at the equatorial latitudes above 130 km where the MIPAS/HALOE+SME ratio varies from 1.6 to 2. Day-night differences in MIPAS NO show daytime enhancements of up to 140% in the tropical and midlatitudes middle thermosphere. In the lower thermosphere, the diurnal amplitude is smaller and NO concentrations are generally higher during night by about 10–30%, particularly in the auroral regions.

State-to-state dynamics at the gas-liquid metal interface: Rotationally and electronically inelastic scattering of NO[2Π1/2(0.5)] from molten gallium

Jet cooled NO molecules are scattered at 45° with respect to the surface normal from a liquid gallium surface at E(inc) from 1.0(3) to 20(6) kcal/mol to probe rotationally and electronically inelastic scattering from a gas-molten metal interface (numbers in parenthesis represent 1σ uncertainty in the corresponding final digits). Scattered populations are detected at 45° by confocal laser induced fluorescence (LIF) on the γ(0-0) and γ(1-1) A(2)Σ ← X(2)Π(Ω) bands, yielding rotational, spin-orbit, and λ-doublet population distributions. Scattering of low speed NO molecules results in Boltzmann distributions with effective temperatures considerably lower than that of the surface, in respectable agreement with the Bowman-Gossage rotational cooling model [J. M. Bowman and J. L. Gossage, Chem. Phys. Lett. 96, 481 (1983)] for desorption from a restricted surface rotor state. Increasing collision energy results in a stronger increase in scattered NO rotational energy than spin-orbit excitation, with an opposite trend noted for changes in surface temperature. The difference between electronic and rotational dynamics is discussed in terms of the possible influence of electron hole pair excitations in the conducting metal. While such electronically non-adiabatic processes can also influence vibrational dynamics, the γ(1-1) band indicates <2.6 × 10(-4) probability for collisional formation of NO(v = 1) at surface temperatures up to 580 K. Average translational to rotational energy transfer is compared from a hard cube model perspective with previous studies of NO scattering from single crystal solid surfaces. Despite a lighter atomic mass (70 amu), the liquid Ga surface is found to promote translational to rotational excitation more efficiently than Ag(111) (108 amu) and nearly as effectively as Au(111) (197 amu). The enhanced propensity for Ga(l) to transform incident translational energy into rotation is discussed in terms of temperature-dependent capillary wave excitation of the gas-liquid metal interface.

Nonstatistical spin dynamics in photodissociation of H2O at 157nm

Photodissociation of H2O via the A band at 157nm has been reinvestigated using the high resolution H atom Rydberg tagging technique. The spin-orbit population distributions were found to be highly nonstatistical for the OH(v=0,1,2) product channels, while nearly statistical for the OH(v=4) channel. These results suggest that the dissociation dynamics of H2O at 157nm is remarkably different for different vibrationally excited OH channels. The result presented here is not entirely consistent with the direct dynamical picture of the A band dissociation of H2O.

Photodissociation channels for N2O near 130 nm studied by product imaging

The photodissociation of N(2)O at wavelengths near 130 nm has been investigated by velocity-mapped product imaging. In all, five dissociation channels have been detected, leading to the following products: O((1)S)+N(2)(X (1)Sigma), N((2)D)+NO(X (2)Pi), N((2)P)+NO(X (2)Pi), O((3)P) + N(2)(A (3)Sigma(+) (u)), and O((3)P) + N(2)(B (3)Pi(g)). The most significant channel is to the products O((1)S) + N(2)(X(1)Sigma), with strong vibrational excitation in the N(2). The O((3)P) + N(2)(A,B):N((2)D,(2)P) + NO branching ratio is measured to be 1.4 +/- 0.5, while the N(2)(A) + O((3)P(J)):N(2)(B) + O((3)P(J)) branching ratio is determined to be 0.84+/-0.09. The spin-orbit distributions for the O((3)P(J)), N((2)P(J)), and N((2)D(J)) products were also determined. The angular distributions of the products are in qualitative agreement with excitation to the N(2)O(D (1)Sigma(+)) state, with participation as well by the (3)Pi(v) state.

C–Cl bond fission dynamics and angular momentum recoupling in the 235 nm photodissociation of allyl chloride

The photodissociation dynamics of allyl chloride at 235 nm producing atomic Cl((2)P(J);J=1/2,3/2) fragments is investigated using a two-dimensional photofragment velocity ion imaging technique. Detection of the Cl((2)P(1/2)) and Cl((2)P(3/2)) products by [2+1] resonance enhanced multiphoton ionization shows that primary C-Cl bond fission of allyl chloride generates 66.8% Cl((2)P(3/2)) and 33.2% Cl((2)P(1/2)). The Cl((2)P(3/2)) fragments evidenced a bimodal translational energy distribution with a relative weight of low kinetic energy Cl((2)P(3/2))/high kinetic energy Cl((2)P(3/2)) of 0.097/0.903. The minor dissociation channel for C-Cl bond fission, producing low kinetic energy chlorine atoms, formed only chlorine atoms in the Cl((2)P(3/2)) spin-orbit state. The dominant C-Cl bond fission channel, attributed to an electronic predissociation that results in high kinetic energy Cl atoms, produced both Cl((2)P(1/2)) and Cl((2)P(3/2)) atomic fragments. The relative branching for this dissociation channel is Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))]=35.5%. The average fraction of available energy imparted into product recoil for the high kinetic energy products was found to be 59%, in qualitative agreement with that predicted by a rigid radical impulsive model. Both the spin-orbit ground and excited chlorine atom angular distributions were close to isotropic. We compare the observed Cl((2)P(1/2))/[Cl((2)P(1/2))+Cl((2)P(3/2))] ratio produced in the electronic predissociation channel of allyl chloride with a prior study of the chlorine atom spin-orbit states produced from HCl photodissociation, concluding that angular momentum recoupling in the exit channel at long interatomic distance determines the chlorine atom spin-orbit branching.

Theoretical studies of intersystem crossing effects in the O(3P, 1D)+H2 reaction

We have studied the influence of intersystem crossing on the reaction dynamics of the O+H2 reaction by performing trajectory surface hopping (TSH) calculations with accurate potential-energy surfaces and global spin–orbit coupling surfaces that we have generated using a four state model proposed by Hoffmann and Schatz. In the TSH calculations, we develop a new mixed representation that treats the reactant and product asymptotes in the adiabatic representation, and the singlet–triplet crossing region in the diabatic representation. This representation thus correctly describes O and OH fine structure-resolved cross sections, and it also treats intersystem crossing effects arising from the singlet–triplet crossing. Our calculations are based on the 1 3A′ and 1 3A″ states of Walch and Kuppermann, and the 1 1A′ state of Dobbyn and Knowles. The globally determined spin–orbit coupling matrix is derived from complete active space self-consistent field calculations using the two-electron Breit–Pauli Hamiltonian. Our dynamics calculations show that the triplet O+H2 cross section is modestly increased (up to 20% at collision energies &amp;gt;10 kcal/mol above the reactive threshold) by intersystem crossing, and product rotational excitation is also increased. In addition, we find that the OH spin–orbit distributions favor the Π3/22 state by a 2:1 ratio over Π1/22. This result is consistent with observations for O atom reactions with alkanes.

Nascent OH[formula omitted]) product state distributions from the reaction of O(1D) with ethylene.: A laser-induced fluorescence study

The full characterization of the OH(X2Π, v″=0–3, N″, J″, Λ″) product state distributions for the O(1D)+C2H4→OH+C2H3 reaction was experimentally performed using the laser-induced fluorescence (LIF) technique. Statistical spin–orbit distributions were obtained, while some preference for the formation of the Π(A′) Λ-doublet level was observed. The rovibrational populations obtained suggest that the reaction preferentially evolves via insertion, yielding rovibrationally cold OH through slow decomposition of an alcohol-type collision complex and rovibrationally excited OH by fast decomposition. Moreover, some evidences were found about the implication of an abstraction mechanism, which would produce rotationally cold and highly vibrationally excited OH.

Bond-breaking in quantum state selected clusters: Inelastic and nonadiabatic intracluster collision dynamics in Ar–H2O→Ar+H(2S)+OH(2Π1/2,3/2±;N)

High-resolution vibrationally mediated IR+UV photodissociation methods are used to investigate the dynamics of H–OH bond breaking in quantum state selected H2O and Ar–H2O van der Waal complexes prepared in a slit supersonic jet expansion. This capability is based on the following strategy: (i) Specific rovibrational quantum states of the Ar–H2O dimer and H2O monomer are optically selected in the second overtone (vOH=3←0) region with an injection seeded, Fourier transform limited (Δν≈160 MHz) optical parametric oscillator. (ii) Selective H–OH bond cleavage of the vibrationally excited H2O subunit in the cluster is achieved by 248 nm or 222 nm UV photolysis. (iii) Multibody collision dynamics between the H, OH, and Ar photofragments are probed via laser induced fluorescence (LIF) on the asymptotic OH rotational, lambda-doublet and spin–orbit distributions. Comparison between cluster (Ar–H2O) and monomer (H2O) data explicitly samples the influence of the Ar “solvent” on the UV photolysis dynamics and in particular highlights the dominant role of intracluster collisions as the fragments recoil. Most importantly, the OH fine structure distributions are found to be dramatically different for Ar–H2O vs H2O photolysis, indicating the major contribution of nonadiabatic events in the photofragmentation dynamics.

Vibrationally mediated photolysis dynamics of H2O in the vOH=3 manifold: Far off resonance photodissociation cross sections and OH product state distributions

Vibrationally mediated photodissociation dynamics of water on the first excited electronic state surface (Ã) has been studied with slit jet-cooled H2O prepared in the complete polyad of vOH=3 overtone stretch levels (|03+〉, |12+〉, |12−〉, and |03−〉). (Notationally, |n1n2±〉 refers to symmetric/antisymmetric combinations of local mode OH stretch excitation, roughly corresponding to n1 and n2 quanta in the spectator and photolyzed OH bond, respectively.) At 248 nm photolysis wavelength the Condon point for bond cleavage occurs in the classically forbidden region, primarily sampling highly asymmetric H+OH exit valley geometries of the potential energy surface. Rotational, vibrational, spin orbit, and lambda doublet distributions resulting from this “far off resonance” photodissociation process are probed via laser induced fluorescence, exploiting the high efficiency laser excitation and light collection properties of the slit jet expansion geometry. Only vibrationally unexcited OH products are observed for both |12±〉 and |03±〉 initial excitation of H2O, despite different levels of vibration in the spectator OH bond. This is in contrast with “near-resonance” vibrationally mediated photolysis studies by Crim and co-workers in the |04−〉 and |13−〉 manifold, but entirely consistent with theoretical predictions from a simple two-dimensional quantum model. Photolysis out of the rotational ground H2O state (i.e., JKaKc=000) yields OH product state distributions that demonstrate remarkable insensitivity to the initial choice of H2O vibrational stretch state, in good agreement with rotational Franck–Condon models. However, this simple trend is not followed uniformly for rotationally excited H2O precursors, which indicates that these Franck–Condon models are insufficient and suggests that exit channel interactions do play a significant role in photodissociation dynamics of H2O at the fully state-to-state level.

Rotational and spin-orbit effects in the dynamics of O(3Pj)+hydrocarbon reactions. II. Models for spin-orbit propensities

The spin-orbit states of OH have been found in part I to be unequally populated in reactions of O(3Pj) with saturated hydrocarbons. We clarify the correlations in these systems within an adiabatic, Ω-conserving approximation, and show that the observed OH spin-orbit distributions are not well reproduced in this adiabatic limit. However, the inclusion of strong nonadiabatic coupling between surfaces of the same Ω and parity results in a much improved prediction. The necessary mixing is demonstrated to be compatible with a previous independent characterization of the entrance channel surfaces in O(3Pj) + CH4 collisions, and with the known propensities for inelastic, j-changing collisions of O(3Pj) with closed shell partners. The nonadiabatic model which agrees best with the observed OH spin-orbit ratios suggests that the O(3Pj) reactivity should vary only moderately with j.

Selective production of photofragments by monitoring the shape of asymmetric resonances in OH photodissociation: Dependence on initial vibrational states

Quantum mechanical analysis is presented on the vibrational state dependence of the total dissociation cross sections and the branching ratios of O(3Pj, j=0,1,2) in the predissociation of OH. Two transformation matrices, each of which describes the relation between an atomic term limit and the correlating molecular states, are constructed and incorporated in the close coupling calculations. The branching ratios of O(3Pj, j=0,1,2) depend very sensitively on the vibrational levels (v=0–4) of the initial X 2Π state. The variations of the spin–orbit distributions as a function of the excitation energy near the asymmetric resonances change markedly depending on the vibrational levels. The variations are either redshifted or blueshifted from the resonance position, depending on the degree of asymmetry of the resonances. The widths of the variations tend to increase with increasing vibrational quantum number of the initial state, suggesting the possibility of choosing the proper linewidths in the experiments to selectively produce the photofragments in one-photon process. Discussion is presented on the applicability of the theoretical scheme to analyze the recent measurements by Neumark and co-workers [J. Chem. Phys. 103, 2495 (1995)] on the product fine structure distributions in the predissociation of O2.

Predissociation dynamics of the O2 B 3Σ−u state: Vibrational state dependence of the product fine-structure distribution

The predissociation of the O2B 3Σ−u state (υ=0–11) is investigated using fast beam photofragment translational spectroscopy. The energy resolution of the experiment, 7–10 meV, is sufficient to yield the correlated fine structure distribution P(j1,j2) for the two O(3Pj) fragments. These spin–orbit branching ratios depend markedly on the vibrational quantum number, providing detailed insight into a relatively unexplored facet of molecular dissociation dynamics. No less than four repulsive states are expected to mediate the predissociation of the B 3Σ−u state, primarily via spin–orbit coupling, and the couplings among these states at long range (R∼5–7 Å) determine the final spin–orbit distributions P(j1,j2). We have attempted to model these distributions in both the adiabatic and diabatic limits, with neither limit proving very successful. A more phenomenological approach to fitting our data suggests that products with j1=j2=2 result from single transitions between adiabatic potentials at long range, whereas the populations in the other product states are determined by multiple transitions among the repulsive states.

Inelastic scattering of [formula omitted] with atomic and molecular colliders. Rotational and fine-structure excitations

The inelastic collisions of NO ( T rot ⩽ 5 K) with Ar, Xe, CO, N 2, O 2, N 2O and CO 2 were studied in molecular beams at center-of-mass collision energies 750–2500 cm −1. Rotational, Λ-doublet and spin-orbit distributions of scattered NO( 2 Π 1 2 3 2 ) were determined. In all the scattering experiments (with the possible exception of Ar) no preferences wer observed in the Λ-doublet populations. The rotational distributions all appear Boltzmann-like, with somewhat different rotational temperatures. The populations of the 2 Π 1 2 and 2 Π 3 2 spin-orbit states depend on the nature of the collider, but do not show any clear relationship with the extent of rotational excitation. No evidence of long-range attractive interactions is revealed in any of the systems studied.