Spin-orbit State Distributions Research Articles

Communication: direct measurements of nascent O((3)P0,1,2) fine-structure distributions and branching ratios of correlated spin-orbit resolved product channels CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2) in VUV photodissociation of CO2.

We present a generally applicable experimental method for the direct measurement of nascent spin-orbit state distributions of atomic photofragments based on the detection of vacuum ultraviolet (VUV)-excited autoionizing-Rydberg (VUV-EAR) states. The incorporation of this VUV-EAR method in the application of the newly established VUV-VUV laser velocity-map-imaging-photoion (VMI-PI) apparatus has made possible the branching ratio measurement for correlated spin-orbit state resolved product channels, CO(ã(3)Π; v) + O((3)P0,1,2) and CO(X̃(1)Σ(+); v) + O((3)P0,1,2), formed by VUV photoexcitation of CO2 to the 4s(10 (1)) Rydberg state at 97,955.7 cm(-1). The total kinetic energy release (TKER) spectra obtained from the O(+) VMI-PI images of O((3)P0,1,2) reveal the formation of correlated CO(ã(3)Π; v = 0-2) with well-resolved v = 0-2 vibrational bands. This observation shows that the dissociation of CO2 to form the spin-allowed CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel has no potential energy barrier. The TKER spectra for the spin-forbidden CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel were found to exhibit broad profiles, indicative of the formation of a broad range of rovibrational states of CO(X̃(1)Σ(+)) with significant vibrational populations for v = 18-26. While the VMI-PI images for the CO(ã(3)Π; v = 0-2) + O((3)P0,1,2) channel are anisotropic, indicating that the predissociation of CO2 4s(10 (1)) occurs via a near linear configuration in a time scale shorter than the rotational period, the angular distributions for the CO(X̃(1)Σ(+); v) + O((3)P0,1,2) channel are close to isotropic, revealing a slower predissociation process, which possibly occurs on a triplet surface via an intersystem crossing mechanism.

Nonadiabatic reactive scattering in atom+triatom systems: Nascent rovibronic distributions in F+H2O→HF+OH

Crossed supersonic jet studies of F + H(2)O --> HF + OH((2)Pi(3/2),(2)Pi(1/2)) have been performed under low density, single collision conditions at E(com) = 6(2) kcal/mol, yielding rotational, vibrational, and spin-orbit state distributions in the nascent OH product by laser induced fluorescence methods. The lowest reaction barriers on the ground and first excited electronic surfaces are DeltaE approximately 4 kcal/mol and DeltaE approximately 25 kcal/mol, correlating with OH((2)Pi(3/2)) and OH((2)Pi(1/2)), respectively. Although only reactions on the ground state potential are Born-Oppenheimer allowed at the experimental collision energies, both ground and excited spin-orbit OH products are observed in a (2)Pi(3/2):(2)Pi(1/2) = 69(1)%:31(1)% branching ratio. This indicates the presence of strong nonadiabatic surface hopping interactions, in agreement with previous results for the F + D(2)O --> DF + OD reaction. Despite clear differences in the rotational distributions between F + H(2)O and F + D(2)O isotopic reactions, the overall electronic branching into spin-orbit manifolds is nearly identical for both OH and OD products. Furthermore, when plotted versus total electronic + rotational energy, the nascent OH and OD populations each lie on single curves, with pronounced kinks in the Boltzmann plots suggestive of microscopic branching in the reaction dynamics. Such an equivalence of electronic and rotational energy release in the OH/OD products is consistent with predominantly nonadiabatic processes taking place in the immediate post-transition state region rather than asymptotically in the exit channel.

OH Fragment from Benzoic Acid Monomer Photolysis: Threshold and Product State Distribution

Photodissociation dynamics of benzoic acid monomer (BAM) at different ultraviolet excitation wavelengths (280-295 nm) has been investigated. The nascent OH product state distributions were measured using the laser-induced fluorescence (LIF) technique. The rotational state distributions, the Lambda-doublet-state ratio, and spin-orbit state distributions of the OH fragment were also measured at 280-294 nm. The OH fragments are vibrationally cold, and their rotational state distributions are peaked at J'' = 3.5 at each photolysis wavelength. No LIF signal of OH fragments was observed at 295 nm. The photodissociation threshold is determined to be 102.5-103.9 kcal/mol for OH channel. The dissociative state and mechanism have been discussed for OH produced from the photodissociation of BAM.

The photodissociation dynamics of OCS at 248nm: The S(PJ3) atomic angular momentum polarization

The dissociation of OCS has been investigated subsequent to excitation at 248 nm using velocity map ion imaging. Speed distributions, speed dependent translational anisotropy parameters, and the atomic angular momentum orientation and alignment are reported for the channel leading to S((3)P(J)). The speed distributions and beta parameters are in broad agreement with previous work and show behavior that is highly sensitive to the S-atom spin-orbit state. The data are shown to be consistent with the operation of at least two triplet production mechanisms. Interpretation of the angular momentum polarization data in terms of an adiabatic picture has been used to help identify a likely dissociation pathway for the majority of the S((3)P(J)) products, which strongly favors production of J=2 fragment atoms, correlated, it is proposed, with rotationally hot and vibrationally cold CO cofragments. For these fragments, optical excitation to the 2 (1)A(') surface is thought to constitute the first step, as for the singlet dissociation channel. This is followed by crossing, via a conical intersection, to the ground 1 (1)A(') state, from where intersystem crossing occurs, populating the 1 (3)A(')1 (3)A(")((3)Pi) states. The proposed mechanism provides a qualitative rationale for the observed spin-orbit populations, as well as the S((3)P(J)) quantum yield and angular momentum polarization. At least one other production mechanism, leading to a more statistical S-atom spin-orbit state distribution and rotationally cold, vibrationally hot CO cofragments, is thought to involve direct excitation to either the (3)Sigma(-) or (3)Pi states.

Photoinitiated Predissociation of the NO Dimer in the Region of the Second and Third NO Stretch Overtones

Photofragment yield spectra and NO(X(2)Pi(1/2,3/2); v = 1, 2, 3) product vibrational, rotational, and spin-orbit state distributions were measured following NO dimer excitation in the 4000-7400 cm(-1) region in a molecular beam. Photofragment yield spectra were obtained by monitoring NO(X(2)Pi; v = 1, 2, 3) dissociation products via resonance-enhanced multiphoton ionization. New bands that include the symmetric nu(1) and asymmetric nu(5) NO stretch modes were observed and assigned as 3nu(5), 2nu(1) + nu(5), nu(1) + 3nu(5), and 3nu(1) + nu(5). Dissociation occurs primarily via Deltav = -1 processes with vibrational energy confined preferentially to one of the two NO fragments. The vibrationally excited fragments are born with less rotational energy than predicted statistically, and fragments formed via Deltav = -2 processes have a higher rotational temperature than those produced via Deltav = -1 processes. The rotational excitation likely derives from the transformation of low-lying bending and torsional vibrational levels in the dimer into product rotational states. The NO spin-orbit state distribution reveals a slight preference for the ground (2)Pi(1/2) state, and in analogy with previous results, it is suggested that the predominant channel is X(2)Pi(1/2) + X(2)Pi(3/2). It is suggested that the long-range potential in the N-N coordinate is the locus of nonadiabatic transitions to electronic states correlating with excited product spin-orbit states. No evidence of direct excitation to electronic states whose vertical energies lie in the investigated energy region is obtained.

Recoil velocity-dependent spin–orbit state distribution of chlorine photofragments

Abstract We present the results of experimental and theoretical studies of the speed-dependent spin–orbit state distributions of chlorine photofragments produced in the photodissociation of thiophosgene (CSCl 2 ) at 235 nm. Three-dimensional imaging has been employed for observing chlorine photofragments in their ground (Cl) and excited (Cl * ) spin–orbit states. The kinetic energy distributions for Cl and Cl * fragments reflect excitation of several electronic states of the partner fragment CSCl. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] was found to depend on the kinetic recoil energy increasing from 0.1 for low kinetic energy to 0.8 for high kinetic energy. The theoretical interpretation is based on the computation of the CSCl 2 potential energy surfaces (PES) along the C–Cl bond. Two completely different methods of determination of the PES were applied for small and for large values of the C–Cl bond separation R . In case of small and intermediate R values time-dependent density-functional theory has been used. In case of large R values we used an asymptotic method of computation of the PES, which is a generalisation of the Heitler–London approach for many-electron systems. Basis molecular wavefunctions with definite values of the total spin S and the spatial and spin reflection symmetry σ v with respect to reflection of the total electronic wavefunction in the molecular plane were used. The developed theoretical approach was used for the assignment of the molecular states involved in the photodissociation and for the qualitative explanation of the non-statistical population of the spin–orbit states of the chlorine photofragments as function of the kinetic energy. The spin–orbit branching ratio of P (Cl * )/[ P (Cl) + P (Cl * )] predicted by the theory strongly depends on the quantum state of the CSCl fragment. It is large in case of the CSCl(X) + Cl and CSCl(A) + Cl channels and small in case of the CSCl(B) + Cl channel which explains the experimental results.

Singlet–triplet excitation spectrum of the CO–He complex. II. Photodissociation and bound-free CO(a 3Π←X 1Σ+) transitions

The dissociating states of the triplet–excited CO–He complex are studied by means of scattering calculations on ab initio diabatic potential energy surfaces produced in the preceding paper (Paper I). With the aid of an effective transition dipole function and the bound states of the CO–He complex in the ground singlet state we obtain the photoabsorption cross section as a function of the excitation energy and generate the full UV spectrum of the singlet–triplet transition. It was found that the dominant contributions to the spectrum, in the energy range from −5 to +10 cm−1 relative to the band origin at 48 473.201 cm−1, originate from resonances that correspond to higher spin–orbit levels of the excited CO(a 3Π)–He complex with approximate quantum number |Ω|=1. Rapid predissociation, with the triplet CO fragment decaying into its lower spin–orbit levels with Ω=0, limits the lifetime of these excited levels to, typically, 10–700 ps. We also predict the rotational and spin–orbit state distribution of the triplet CO fragment and the maximum deflection angle of the photodissociation products in a molecular beam experiment.

The 212.8-nm photodissociation of formic acid: Degenerate four-wave mixing spectroscopy of the nascent OH(X 2Πi) radicals

The 212.8-nm photodissociation dynamics of formic acid was investigated utilizing degenerate four-wave mixing spectroscopy. The background-free rotational spectrum of the nascent OH radicals was obtained, and a cold rotational energy distribution peaking at N″=3 was extracted from the DFWM spectrum. The distribution was well approximated by a Boltzmann distribution with a rotational temperature of Trot∼716 K, which corresponds to an average rotational energy of ∼498 cm−1. The observation of a nonstatistical spin–orbit state distribution, with a preference for the low-energy F1 manifold, implies the absence of any interactions with nearby triplet states during dissociation. Preferential population of the Λ-doublet was observed, indicating that the ν7 H–O–C bending vibration in HCOOH(Ã) and the recoil impulse are the principal sources of the OH rotation.

Photodissociation of hydrogen sulfide at 157.6 nm: Observation of SH bimodal rotational distribution

Photodissociation of the H2S molecule at 157.6 nm was studied experimentally using the Rydberg tagging technique. Translational energy distributions of the H-atom product from the H2S photodissociation were measured, and the SH(X 2Π)+H(2S) channel was found to be the dominant dissociation process. Spin-orbit and rovibrational state distributions were also obtained for the SH product, which was found to be both vibrationally and rotationally excited. An intriguing bimodal rotational distribution in the lowest two vibrational states, v=0 and 1, has been clearly observed for the SH product, indicating that there are two distinctive dissociation mechanisms involved in the photodissociation of H2S at 157 nm excitation.

Photodissociation of HCl at 193.3 nm: Spin–orbit branching ratio

HCl was photodissociated by ultraviolet (uv) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the chlorine fragment, [Cl(2P1/2)]/[Cl(2P3/2)]=0.69±0.02, in excellent agreement with recent theoretical studies. The H atom angular distribution studied by changing the uv photolysis laser polarization confirmed a dominant A 1Π←X 1Σ+ electronic transition in the photoexcitation process (β=−1.01±0.04 and β*=−0.94±0.07).

An experimental study of HF photodissociation: Spin–orbit branching ratio and infrared alignment

Single rotational levels of HF (v=3) were prepared by using overtone excitation and these molecules were then photodissociated by ultraviolet (UV) radiation at 193.3 nm. Time-of-flight spectra of the hydrogen atom fragment provided the spin–orbit state distribution of the fluorine fragment. Changing the UV photolysis laser polarization confirmed an A 1Π←X 1Σ+ electronic transition in the photodissociation step. Photodissociation of HF at 121.6 nm is also reported. Infrared (IR) induced alignment of the diatom was studied by monitoring the IR laser polarization dependence of the H-atom product angular distribution. Depolarization due to hyperfine interaction was studied by using the R(0) transition. Agreement with theory is excellent.

Photodissociation of HOCl via the 1 1A″ state. Model calculations of the spin-orbit and Λ-doublet product state distributions

A model is proposed for predicting the OH( 2Π) spin-orbit and Λ-doublet and Cl( 2P) spin-orbit product state distributions resulting from the photodissociation of HOCl via its 1 1A″ excited state. The model predicts that the OH(Π(A″)) Λ-doublet states will be preferentially populated but that there will be no strong preference for either OH spin-orbit state. Some correlation between the OH and Cl spin-orbit states is also predicted. Comparisons are made with predicted quantum state distributions resulting from the photodissociation of HOCl via its 2 1A′ state.

Laser initiated half reaction study of H+O2→OH+O

The H+O2 reaction system was studied under geometry limited half reaction conditions. The weakly bonded complex O2–H2S was formed by supersonic expansion, and reaction was initiated by 193 nm photoirradiation of the complex. Rotational, spin-orbit, and lambda doublet state distributions of product OH were determined by a laser-induced fluorescence (LIF) technique. The populations of the two spin-orbit states were observed to be statistical. The population of the Π(A′) level was almost twice that of the Π(A″) level, and the planar geometry was suggested for reaction path. These populations of the fine structures of OH were similar to those of OH formed under bimolecular reaction conditions. On the other hand, the rotational state distribution of OH from the half reaction has two components and the dominant one shows a very cold rotational distribution, in sharp contrast with that of the bimolecular reaction where rotation is highly excited. This cold rotational distribution could be partially explained by the absorption of a part of available energy by the internal motion of SH. However, the distribution with a peak at the lowest rotational level could not be explained by this effect, but ascribed to the exit interaction between SH and OH and/or the entrance channel specificity, i.e., the reaction occurs in limited impact parameters.

Reactant rotational energy effect on energy distribution in products. The role of the electronic angular momentum

In reactions of O(1D) with CH4 clusters, CD4 and propane, monomers and clusters, preference for the 2Π32 spin-orbit state of the OH product is observed. New results on the effect of the initial rotational temperature of propane on the spin-orbit state distribution are presented. The preference for the 2Π32 state and the rotational energy effect are attributed to conservation of electronic angular momentum during curve crossing in the entrance channel and to conservation of the projection of the electronic angular momentum on the internuclear axis through the reaction. A quantitative model rationalizes all the experimental observations.

Quantum resolved studies of electron-stimulated reactions on adsorbate covered Pt(111) surfaces

Using laser resonance-enhanced ionisation spectroscopy, we have studied electron (6–350 eV) stimulated dissociation of NO 2 coadsorbed with up to 0.75 monolayer of atomic O on Pt(111). Several dramatic effects on NO 2 dissociation occur due to the presence of O. There is a large ( × 26) enhancement in the specific dissociation yield, a narrowing of the NO translational energy distributions, and a distinct propensity ( > 4:1 at low J) for populating the upper Ω = 3 2 NO spin-orbit level over the Ω = 1 2 level. The spin-orbit state distribution of the O( 3 P J) dissociation fragment is (5.0): (2.5): (1.0) for J = 2, 1 and 0, which is within experimental error of the statistical ( T → ∞) 2 J + 1 limit. The enhanced yield probably results from an increased excited state lifetime due to a reduction in substrate charge-transfer screening. We have also detected O( 3 P J = 2,1,0) and NO X 2 Π 3 2 , 1 2 (v = 5) above an electron (6–350 eV) beam irradiated Pt(111) surface containing coadsorbed O 2 and NO at 90 K. We conclude that both O( 3 P J) and NO(v = 5) are laser-induced photodissociation fragments of NO 2 desorbates. The NO 2 is probably the reaction product of a collision between an O atom (created by electron-stimulated dissociation of adsorbed O 2) and an NO(a). We correlate the 10 eV NO 2 production threshold with the dissociative ionization of the 3σ g molecular bonding orbital of O 2(a).

Spin-orbit state distribution of atomic oxygen ( 3P J) produced by photodissociation of nitrogen dioxide

The spin-orbit state distribution of O( 3P J ) produced by photodissociation of NO 2 was determined by application of a tunable VUV laser. The distributions in the dissociation by 355, 337 and 266 nm excitation were all similar, i.e. 3P 2: 3P 1: 3P 0≈1:0.19:0.03, while the photolysis at 212 nm yielded 1:0.35:0.08.

Translational and internal state distributions of NO produced in the 193 nm explosive vaporization of cryogenic NO films: Rotationally cold, translationally fast NO molecules

We report the translational, rotational, and spin-orbit state distributions of fast NO molecules which are generated by the 193 nm laser vaporization of 30 K multilayer NO films. Rotational distributions in v=0 are obtained by laser multiphoton ionization for five different velocities ranging from 900 to 2200 m s−1, corresponding to translational energies ET=0.14 to 0.71 eV. In every case, the average molecular rotational energy is more than 10 times smaller than the component of translational energy normal to the surface. Average rotational energies 〈ER〉 range from 0.009±0.002 to 0.024±0.006 eV (with corresponding best fit temperatures, TRot =105 to 220 K). For the molecules with ET=0.14 and 0.22 eV, the spin–orbit population ratios are typically comparable with TRot. For higher translational energies, the typical spin–orbit ratios are larger than expected from TRot and increase to a value F2/F1 of 1.1±0.50.4 (statistical ratio) for NO molecules with ET =0.71 eV. In some cases, the rotational distributions have a non-Boltzmann, high J component. Preliminary investigations for molecules with ET=0.56 eV indicate that the vibrational distribution v=1/v=0 is 3±1% (T≊785 K). The rotations in v=1 are also cold (〈ER〉≊0.01 eV, TRot ≊130 K). The mechanism that causes the ejection of translationally fast, rotationally cold NO molecules is considered in terms of either a collisional cooling process following desorption or rotationally constrained desorption dynamics.

The electronic energy transfer from Kr( 3P 2) to 12CO and 13CO at thermal energy

The title reactions have been studied by observing CO(d 3Δ i-a 3Π r, a′ 3Σ +-a 3Π r) emissions in flowing-afterglow and beam apparatus. The d-a/a′-a ratio for 12CO was smaller than that for 13CO by a factor of 2. The rovibrational and spin-orbit state distribution in d 3Δ i of 12CO and 13CO were determined from a spectral simulation. The rotational distribution was reproduced by Boltzmann temperatures of 600–750 K for the endoergic 12CO (d: ν′ = 20, 21) and 13CO (d: ν′ = 21) levels and 1400 K for the exoergic 13CO (d: ν′ = 20) level, without showing any spin-orbit propensity for the 3Δ 1,2,3 sub-levels.

Investigation of no scattering from organic monolayers: Spin-orbit state and vibrational state population distributions

The final internal energy distribution of NO molecules scattered from two different organic monolayers is investigated. Inelastic scattering of NO( 2Π 1 2 ) to give NO( 2Π 3 2 ) is found to be particularly efficient, the population in the upper NO( 2Π 3 2 ) state increasing with collision energy. A simple theoretical model is proposed to interpret these results and to explain their similarity to those observed in NO scattering from metallic surfaces. No vibrational excitation of the NO scattered from either of the two organic surfaces is observed, even though rotational excitation to channels well above the vibrational excitation threshold is seen.

Non-Boltzmann rotational and inverted spin–orbit state distributions for laser-induced desorption of NO from Pt(111)

The internal state distributions of NO desorbed from a Pt(111) surface by visible and near-visible laser radiation (355, 532, and 1064 nm) were measured by laser-induced fluorescence. Non-Boltzmann rotational state distributions and inverted spin–orbit populations were observed and both were found to be relatively insensitive to the desorption-laser wavelength. It is suggested that the internal state distributions arise from the charge exchange processes occuring during desorption via a short-lived negative-ion resonance intermediate.