Spin-orbit Excited State Research Articles

The product rovibrational and spin–orbit state dependent dynamics of the complex reaction H+CO2→OH(2Π;ν,N,Ω, f)+CO: Memories of a lifetime

The product-state-resolved dynamics of the reaction H+CO2→OH(2Π;ν,N,Ω,f)+CO have been explored in the gas phase at 298 K and center-of-mass collision energies of 2.5 and 1.8 eV (respectively, 241 and 174 kJ mol−1), using photon initiation coupled with Doppler-resolved laser-induced fluorescence detection. A broad range of quantum-state-resolved differential cross sections (DCSs) and correlated product kinetic energy distributions have been measured to explore their sensitivity to spin–orbit, Λ-doublet, rotational and vibrational state selection in the scattered OH. The new measurements reveal a rich dynamical picture. The channels leading to OH(Ω,N∼1) are remarkably sensitive to the choice of spin–orbit state: Those accessing the lower state, Ω=3/2, display near-symmetric forward–backward DCSs consistent with the intermediacy of a short-lived, rotating HOCO (X̃ 2A′) collision complex, but those accessing the excited spin–orbit state, Ω=1/2, are strongly focused backwards at the higher collision energy, indicating an alternative, near-direct microscopic pathway proceeding via an excited potential energy surface. The new results offer a new way of reconciling the conflicting results of earlier ultrafast kinetic studies. At the higher collision energy, the state-resolved DCSs for the channels leading to OH(Ω,N∼5–11) shift from forward–backward symmetric toward sideways–forward scattering, a behavior which resembles that found for the analogous reaction of fast H atoms with N2O. The correlated product kinetic energy distributions also bear a similarity to the H/N2O reaction; on average, 40% of the available energy is concentrated in rotation and/or vibration in the scattered CO, somewhat less than predicted by a phase space theory calculation. At the lower collision energy the discrepancy is much greater, and the fraction of internal excitation in the CO falls closer to 30%. All the results are consistent with a dynamical model involving short-lived collision complexes with mean lifetimes comparable with or somewhat shorter than their mean rotational periods. The analysis suggests a potential new stereodynamical strategy, “freeze-frame imaging,” through which the “chemical shape” of the target CO2 molecule might be viewed via the measurement of product DCSs in the low temperature environment of a supersonic molecular beam.

Quantum scattering on coupled ab initio potential energy surfaces for the Cl(2P)+HCl→ClH+Cl(2P) reaction

We present the results of accurate quantum dynamical calculations for the Cl(2P)+HCl→ClH+Cl(2P) reaction using a coupled channel reactive scattering method based on hyperspherical coordinates. The calculations include the three potential energy surfaces (12A′, 22A′ and 12A″) that correlate to the ground states of the reactants and products, as well as electrostatic, Coriolis and spin–orbit couplings. The electrostatic surfaces and couplings are taken from the high quality ab initio computations of A. J. Dobbyn, J. N. L. Connor, N.A. Besley, P. J. Knowles and G. C. Schatz [Phys. Chem. Chem. Phys., 1999, 1, 957], with the barrier heights scaled so that thermal rate coefficients derived from the scattering calculations agree with experimental data. Only the total angular momentum quantum number J = ½ partial wave is calculated; state selected and thermal rate coefficients are obtained using a J-shifting approximation. We study basis set convergence and compare single and multiple surface results, with emphasis on state selected and total cumulative reaction probabilities and thermal rate coefficients, including branching between ground and excited spin–orbit states. The contribution of resonances to the scattering is discussed and analysed. We also compare with scattering results using an earlier, more approximate, set of potential surfaces and couplings due to C. S. Maierle, G. C. Schatz, M. S. Gordon, P. McCabe and J. N. L. Connor [J. Chem. Soc., Faraday Trans., 1997, 93, 709].

Photodissociation of 1-bromo-2-chloroethane at 266 nm

The photodissociation of CH 2BrCH 2Cl at 266 nm has been investigated on the universal crossed molecular beam machine. The primary dissociation step leads exclusively to the formation of CH 2CH 2Cl radicals and Br atoms in the electronic ground state as well as in the spin–orbit excited state, with a branching ratio 2±1:8±1. Photofragment total c.m. translational energy distribution P( E t ) has been obtained and about 64% of the available energy is partitioned into translational energy for Br channel and about 28.5% of the available energy is partitioned into translational energy for Br ∗ channel. The anisotropy parameters are determined to be β(Br ∗)=0.8±0.2 and β(Br)=−0.6±0.2, respectively. Some CH 2CH 2Cl radicals with large internal excitation (corresponding to formation of ground state Br channel) may undergo secondary dissociation to form CH 2CH 2+Cl. The experimental results are discussed in terms of a model that involves the initial excitation of two repulsive electronic states: one from an parallel transition to the 3 Q 0 state, and the other from a perpendicular transition to the 3 Q 1 , 1 Q states.

Photodissociation dynamics of carbonyl chloride fluoride and its implications for phosgene three body decay

The photodissociation dynamics of COFCl has been studied by monitoring Cl fragments by resonance enhanced multi-photon ionisation and time-of-flight techniques at dissociation wavelengths near 235 nm. The COFCl heat of formation and the dissociation energy for C–Cl bond fission were for the first time experimentally determined: ΔfH00 (COFCl)=-397±15 kJ mol-1 and D0(COF–Cl)=364±8 kJ mol-1. 35% of the available energy is channelled into FCO rotation and bending vibration. The remaining energy is released as product translation. Other than in phosgene, no spin–orbit state selective behaviour is observed. The observations agree with a decay mechanism within the COFCl molecular plane as well as with a fragmentation via out-of-plane movement of the departing Cl atom. The structural and electronic similarity of COFCl and COCl2 allows conclusions to be drawn on the COCl2 dissociation. The findings are evidence for a previously proposed decay mechanism for the asynchronous concerted three body decay of phosgene into CO+2Cl. The first bond cleavage produces ground and excited spin–orbit state Cl and Cl*, respectively, with large kinetic energy release, while breaking the second C–Cl bond exclusively generates slow ground state Cl atoms.

State-to-state inelastic scattering from vibrationally activated OH–H2 complexes

State-selective infrared excitation of o-H2–OH via the pure OH overtone transition has been used to induce a half-collision inelastic scattering event between the OH radical and ortho-H2 under restricted initial orientation conditions. The time evolution and final state distribution of the OH products from vibrational predissociation have been evaluated by ultraviolet probe laser-induced fluorescence measurements. The half-collision scattering takes place with ∼3350 cm−1 of energy available to the OH (v=1)+o-H2 products, an energy that exceeds the classical barrier to reaction. The OH (v=1) products are preferentially populated in high rotational levels with a distribution that is consistent with an energy gap law. A significant fraction of the OH fragments are promoted to the excited spin–orbit state in the predissociation process. A strong lambda-doublet propensity is also found, indicating that the OH unpaired pπ orbital is preferentially aligned perpendicular to the rotational plane of the OH products. Finally, the OH rotational and fine structure distributions are compared with those obtained in previous full collision inelastic scattering studies at energies below the threshold for reaction.

Rate constants for the reaction of Ar+ with O2 and CO as a function of temperature from 300 to 1400 K: Derivation of rotational and vibrational energy effects

Rate constants for the charge-transfer reactions of Ar+ with O2 and CO have been measured in a high temperature flowing afterglow from 300 to 1400 K. Comparisons between our results and the previous flow drift tube studies of Dotan and Lindinger at 300 K illustrate the effects of internal excitation on the reactivity. The rate constants measured for both systems agree favorably with the drift tube results from 300 to 900 K. Rotational and translational energy decreases charge transfer equally, consistent with previous experiments, indicating a long-lived collision complex forms during the reaction. The flowing afterglow rate data deviate from the drift tube results above 900 K as a result of populating vibrationally excited states of the neutral reagents. Charge transfer from the thermally populated spin-orbit excited state of Ar+ with O2 and CO only slightly enhances the rate constants at 1400 K. Populating the v″>0 levels reduces the threshold for accessing excited state products, and the rate constants for vibrationally excited states are much larger than for v″=0. The data suggest most of the enhancement comes from v″>2.

A time-dependent study of the dynamics of IBr photodissociation

Time-dependent calculations of the photodissociation of IBr are presented. We have calculated the dissociative absorption spectrum in the range of frequencies from v = 14705 cm −1, the v = 0 threshold, to v = 22321cm −1, establishing the yields of Br( 2P 3 2 ) in the ground state and Br ∗( 2P 1 2 ) in the excited spin-orbit state. The effect of the vibrational excitation of the IBr molecule on the photodissociation yields was also examined. Two independent photodissociation paths arising from vibrational levels of the ground X( 1∑ o +) state and of the excited A( 3Π 1) state which are only a few cm −1 apart are considered. We show that the percentage yields of Br ∗ and Br are strongly dependent on the photodissociation path.

Temperature-dependent study of the kinetics of Ta(a4F3/2) with O2, N2O, CO2 and NO

M. L. Campbell and K. L. Hooper, J. Chem. Soc., Faraday Trans., 1997, 93, 2139 DOI: 10.1039/A700915A

Spin–orbit relaxation kinetics of Br(4 2P1/2)

Pulsed and steady-state photolysis techniques have been employed to measure the rate coefficients for collisional deactivation of the spin–orbit excited state of atomic bromine, Br(4 2P1/2). Pulsed lifetime studies for quenching by Br2 and CO2 yielded absolute rate coefficients at room temperature of kBr2=1.2±0.1×10−12 and kCO2=1.5±0.3×10−11 cm3/molecule s. The rate coefficients for quenching by rare gases, N2, O2, NO, NO2, N2O, CO, CO2, COS, SO2, SF6, CF4, CH4, H2S, H2, D2, HBr, HCl, and HI, relative to that for Br2 were determined in a steady-state photolysis experiment. Correlation of the deactivation probabilities with energy defect for the case of electronic-to-vibrational energy transfer is demonstrated.

Photodissociation of CF3Br at 193 nm: evidence for a distorted dissociation pathway

The photodissociation of CF3Br at 193 nm has been studied by photofragment translational spectroscopy. The primary dissociation step leads to the formation of CF3 radicals and Br atoms in the electronic ground state and in the spin-orbit excited state, with relative quantum yields φ(Br) = 0.47 ± 0.05 and φ(Br∗) = 0.53 ∓ 0.05, respectively. At higher laser fluences the slowest and internally hottest CF3 radicals undergo photoinduced secondary dissociation to CF2 + F. The anisotropy parameters derived from measurements with a polarized photolysis laser are β(Br) = 1.8 ± 0.2 and β(Br∗) = 0.7 ± 0.3. The experimental results are discussed in terms of a model that involves the initial excitation of two repulsive electronic states 3Q0 and 1Q1 via transitions polarized parallel and perpendicular to the CBr bond, respectively. From the observed β parameters it is concluded that in roughly two thirds of the molecules dissociation proceeds via a distorted geometry in which the molecular symmetry C3v is reduced to Cs through the effect of e-type bending vibrations.

Photofragment translational spectroscopy of IBr at 304 nm: Polarization dependence and dissociation dynamics

The photodissociation dynamics of IBr has been studied at 304 nm by state-selective photofragment translational spectroscopy. Velocity distributions, anisotropy parameters, and relative quantum yields are obtained for the ground I(2P3/2) and spin–orbit excited state I*(2P1/2) iodine atoms, which are produced from photodissociation of IBr at this wavelength. Two sharp velocity distributions observed for the I channel suggest the two dissociation pathways that correlate with ground-state iodine formation. Based on the expected translational energy release and the energy separation between the peaks, the two distributions have been assigned to dissociation of IBr to form I(2P3/2)+Br(2P3/2) and I(2P3/2)+Br*(2P1/2) with the former channel appearing at higher translational energy. The I* distribution shows one strong peak indicating that there is one dominant channel for formation of I* atoms at this wavelength which has been assigned to dissociation of IBr to form I*(2P1/2)+Br(2P3/2) with a quantum yield of 0.1. The I* signal formed from the I*(2P1/2)+Br*(2P1/2) dissociation channel is observed very weakly. The observed anisotropy parameter indicates that the I+Br* product (β=−0.7) is formed mainly from the perpendicular 1Π1(2341)←X transition while the I*+Br channel (β=1.8) is formed predominantly from the parallel 3Π0+(2341)←X transition followed by curve crossing to the 3Σ0+−(2422) state. The recoil energy dependence of the anisotropy parameter in the I atom produced in the I+Br channel shows a positive β value above maximum of the peak recoil energy and a negative value below the peak maximum of the recoil energy distribution. These results are interpreted in terms of the presence of more than one path for the formation of I+Br photoproduct with opposite polarization for their absorbing transitions, most likely the 3Π0+(2341)←X and the 3Π1(2341)←X transitions. The possible excited state dynamics which give the observed results are discussed in terms of the previously proposed potential energy diagrams for IBr and ICl.

Fullyab initioinvestigation of bound and predissociating states of the NeOH(X) complex

New ab initio potential energy surfaces (PESs) are reported for the interaction of Ne(1S0) with the OH radical in its ground (X 2Π) electronic state. These are then used in the variational calculation of the bound vibrational states of the NeOH(X) complex. The calculated dissociation energy (D0) is 26.2 cm−1, which lies within the experimental estimate (23–30 cm−1). The ab initio PESs are also used to determine the positions and widths of the metastable levels of the complex which correlate with the first excited rotational state (j=5/2, ω=3/2) and the first excited spin–orbit state (j=1/2, ω=1/2) of OH(X 2Π). The predissociation rates are strongly dependent on the bending motion, the intermolecular stretching vibrational quantum number and the parity. The predissociation lifetimes are in good agreement with estimates from stimulated emission pumping experiments of [Chuang, Andrews, and Lester, J. Chem. Phys. 103, 3418 (1995)]. A quantum flux method is used to study the redistribution of the predissociation flux as a function of the fragment separation.

Intermolecular vibrations and spin–orbit predissociation dynamics of NeOH (X 2Π)

Stimulated emission pumping of NeOH is used to access the bound intermolecular vibrational levels supported by the OH X 2Π3/2+Ne potential energy surface as well as predissociative levels correlating with the spin–orbit excited state of OH X 2Π1/2+Ne which lie more than 100 cm−1 above the dissociation limit. Intermolecular stretching intervals and rotor constants yield the radial dependence of the average interaction between Ne and OH X 2Π3/2,1/2, while the spacings between angular levels provide information on the anisotropy of the potentials. The lifetime of spin–orbit predissociative levels is found to increase by a factor of 3 upon intermolecular stretching excitation, from 8.5 ps to more than 27 ps. This increase in lifetime is attributed to the fall-off in the coupling between the two spin–orbit states with increasing intermolecular separation distance. The dominant coupling for spin–orbit predissociation is provided by the difference potential, the change in the intermolecular potential when the unpaired electron of OH lies in or out of the NeOH plane. The energies, rotor constants, and lifetimes of the bound and predissociative NeOH levels observed experimentally are compared with theoretical calculations of these quantities based on ab initio potentials for Ne+OH X 2Π by Yang and Alexander (accompanying paper). Remarkably good agreement is found between experiment and theory, given the weakness of the Ne+OH X 2Π interaction.

Spin–orbit predissociation dynamics of NeOH (X2Π)

A joint theoretical–experimental study has been carried out to examine the unimolecular dissociation dynamics of NeOH (X 2Π) complexes containing spin–orbit excited OH radicals. The spin–orbit predissociation process is a direct manifestation of the difference in the two potential-energy surfaces (PESs) required to describe the interaction of Ne (1S0) with the OH radical in its ground (X 2Π) electronic state. The theoretical investigation involved the determination of the ab initio PESs as well as the energies and widths of the NeOH resonances correlating with the spin–orbit excited state of OH (X 2Π1/2). In addition, the redistribution of predissociation flux is mapped out as a function of the fragment separation using the quantum flux method. Experimentally, the NeOH resonances derived from the spin–orbit excited state of OH have been observed by stimulated emission pumping (SEP). The energies and linewidths of the predissociative resonances are extracted from the SEP spectra and compared with theoretical predictions. A dramatic increase in the spin–orbit predissociation lifetime is observed upon intermolecular stretching excitation which principally reflects the reduced splitting between the two PESs with increasing intermolecular separation. Comparison of the experimentally measured and theoretically calculated observables provides a stringent test of the ab initio PESs and insight into the photodissociation mechanism.

Excess Energy and Structural Dependence of the Rate of Energy Redistribution during the Photodissociation of Iodotoluenes

Abstract : The ortho (2-), meta (3-1), and para (4-) iodotolunes (C7H7I) photodissociated at 266 nm and 30.4 nm and studied with state-selective one- dimensional photo-fragment translational spectroscopy with state-selective velocity, and translational energy distributions are obtained for the ground I(2P3/2) and spin-orbit excited state I*(2P1/2) iodine atoms produced photodissociation. As has been observed in iodobenzene, the ground state I channel observed in each isomer of iodotoluene exhibits both a prompt alkyl iodide-like dissociation channel following a parallel excitation to the alkyl iodide 30(zero)(n, sigma*) repulsive state and curve crossing to the 1Q1(n, sigma*) state, and a slower, indirect dissociation channel following a competitive excitation to aromatic charge transfer (pi, pi*) predissociate excited states at both 266 nm and 304 nm. The I* channel observed at both 266 nm and 304 nm for each isomer results from prompt dissociation resulting from parallel absorption to an alkyl iodide type 3Q(zero)(n, sigma*) state. The rapid I and I* dissociative channels observed for each isomer are found to exhibit strong dependence on the excess excitation. This is discussed in terms of the increase in the density of the vibrational states of the toluene ring. Dissociation times and rates of internal energy redistribution (IER) from the slow dissociative channel (Beta) are estimated for each isomer. For 2- iodotoluene, the rate of IER for the slow I channel increases from 20 kcal/mol- ps at 304 nm to 30 kcal/mol-ps at 266 nm, while the rates of IER for 3- and 4- iodotoluene (16 kcal/mol-ps and 15 kcal/mol-ps, respectively) remain unchanged as the photon energy is increased

Infrared fluorescence observations of electronic to vibrational energy transfer from Br* (4 2P [formula omitted]) to SO 2

Time-resolved infrared fluorescence observations of the Br* (4 2P 1 2 ) spin—orbit excited state and the SO 2 symmetric (ν 1) and antisymmetric (ν 3) stretching modes have been obtained following 532 nm laser photolysis of Br 2/SO 2 and IBr/SO 2 mixtures. The total Br* quenching rate coefficient by SO 2 was determined from direct Br* observations to be (1.1 ± 0.2) × 10 −13 cm 3 molecule −1 s −1 at 298 K. Kinetic evidence from SO 2 fluorescence and the nearly complete reduction of emission through a cold gas filter suggests that the ν 3 fundamental level is excited preferentially despite the existence of nearly resonant three-quantum stretch levels.

Simulation of photodissociation dynamics of HBr adsorbed on a LiF (001) surface

Computer simulations are performed to study the dynamics of the photodissociation of a single HBr adsorbed on a LiF (001) surface. A cluster model, including 50 moving atoms in two layers surrounded by five static walls consisting of 274 fixed atoms, is used to represent the surface. Since only one HBr molecule is allowed to sit on the surface, interactions between coadsorbates and possible collisions between adsorbed species are not considered in this model. The equilibrium properties of the HBr molecule on the surface prior to the photon absorption are obtained through Monte Carlo simulations, while the subsequent dissociation dynamics is described by molecular dynamics. Two dissociation channels corresponding to the ground and excited spin–orbit states of bromine are explicitly considered and transitions from one excited-state surface to another are treated with the Landau–Zener model. The kinetic energy and angular distributions of the dissociated H atoms as well as the Br/Br* ratio are determined from 600 trajectories. At 100 K and 193 nm, the calculated kinetic-energy distribution of the H fragment agrees with experimental data very well, as does the Br/Br* ratio. Neither the H kinetic energy nor the Br/Br* ratio is significantly affected by the presence of the surface. The energy exchange between the H atom and the surface is minimal, but increases when H is replaced by deuterium. The calculated H fragment angular distribution deviates from experimental data by approximately 15°. The deviation can be attributed to the scattering of the dissociating H fragment by a coadsorbate. We have also investigated the influence of the bulk temperature and the incident photon wavelength on the dynamics.

Stimulated emission pumping of intermolecular vibrations in OH–Ar(X 2Π)

Stimulated emission spectroscopy has been used to access the intermolecular bending and stretching vibrations supported by the OH(X 2Π)+Ar(1S0) potential-energy surfaces. Manifolds of OH–Ar bending levels, correlating with the j= (3)/(2) , (5)/(2) , and (7)/(2) rotational levels of OH(2Π3/2), have been observed with zero to three quanta of intermolecular stretch. OH–Ar complexes have also been prepared in intermolecular vibrational levels of the spin–orbit excited state correlating with OH(2Π1/2). The first dissociation limit, producing OH(2Π3/2) v=0, j= (3)/(2) +Ar(1S0) fragments, has been determined to lie between 93 and 103 cm−1 above the zero-point level. Complexes prepared in metastable levels, detected up to 200 cm−1 beyond the first dissociation limit, undergo predissociation by using OH rotational or spin–orbit excitation to break the OH–Ar intermolecular bond.

The infrared photofragmentation of Ar+2. Evidence of excited state population from dimer and cluster ionization

Argon dimer ions have been generated via three different techniques: (1) autoionization; (2) vertical ionization of neutral Ar2; (3) ionization and subsequent fragmentation of argon cluster ions. In experiments (2) and (3) the dimers and clusters are formed via the adiabatic expansion of argon in a supersonic beam. In each case Ar+2 ions have been mass selected and subjected to single-photon infrared excitation (912–1094 cm−1) using a line-tunable carbon dioxide laser in a crossed-beam arrangement. Only those Ar+2 ions with internal energies within 1000 cm−1 of a dissociation limit yield Ar+ photofragments, the kinetic energy spread of which has been measured using an electrostatic analyzer. The photofragment kinetic energy spectra of dimer ions formed by autoionization do not exhibit any dependence on the angle of laser polarization; it is proposed that such behavior is due to the presence of a high thermal rotational temperature (500 K). In contrast, the corresponding spectra of Ar+2 formed via vertical ionization, exhibit two quite distinct features, one of which shows a strong dependence on laser polarization angle. Calculations show that the latter behavior is most probably due to photodissociation out of an excited spin–orbit state of Ar+2. A very pronounced increase in Ar+2 infrared photodissociation signal is observed as a function of increasing nozzle stagnation pressure. To account for such behavior it is proposed that, following ionization, argon cluster ions fragment to give dimer ions in excited vibrational/rotational levels both in the electronic ground and an excited spin–orbit state.

Spin-orbit relaxation rates of bismuth (6p3,2D3/2) following photolysis of trimethylbismuth at .lambda. = 193 nm

Rate coefficients for the collisional relaxation of the first excited spin-orbit state of Bi (6p{sup 3}, {sup 2}D{sub 3/2}) have been measured at 295 K for Ar, CO{sub 2}, SF{sub 6}, H{sub 2}, D{sub 2}, HF, and DF. The excited Bi atoms were prepared by excimer laser photolysis of trimethylbismuth (TMB) at 193 nm and monitored directly in emission. The rate coefficient for quenching by the precursor TMB and a lower limit for removal by CH{sub 3} photofragments have also been established. Where applicable, our results are compared with the earlier work of Bevan and Husain and of Trainor. The suitability of long-range interaction models is discussed for those cases where isotopic substitution leads to markedly different quenching rates.