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

Photodissociation of CO2 via the 1Πg state: Wavelength-dependent imaging studies of O(1D2) photoproducts

Photodissociation of CO2 via the 1Πg state is investigated using a time-sliced velocity-mapped ion imaging apparatus combined with a tunable vacuum ultraviolet photolysis source. The main O(1D2) + CO(X1Σ+) channel is directly observed from the measured images of O(1D2) photoproducts at 129.08-134.76 nm. The total kinetic energy release spectra determined based on these images show that the energetic thresholds for the O(1D2) + CO(X1Σ+) photoproducts correspond to the thermochemical thresholds for the photodissociation of CO2(v2 = 0) and CO2(v2 = 1). One significant difference among the CO(X1Σ+, v) vibrational distributions for the predominant CO2(v2 = 0) dissociation is that the population of CO(v = 0) becomes favorable at 130.23-133.45 nm compared to the Boltzmann-like component (v > 0) that always exists at 129.08-134.76 nm. The wavelength dependences of the overall β are found to follow the variation trend of the CO(v = 0) abnormal intensity. The vibrational state-specific β values present a roughly decreasing trend with an increase in v, whereas β(v = 0) appears to be significantly larger than β(v = 1) at 130.23-133.45 nm compared to 134.76 and 129.08 nm. The non-statistical CO(v = 0) with larger β values at 130.23-133.45 nm implies that an additional pathway may open through the conical intersection coupling to the dissociative 21A'  state, except for the ever-existing pathway that yields the Boltzmann-like component. In contrast, at 129.08 nm, the restoration of the statistical equilibrium in the CO(X1Σ+, v) vibrational distribution may be caused by the emergence of novel dissociation pathways arising from the participation of the 31A″ state.

Photodissociation Dynamics of OCS near 150 nm: The S(1SJ=0) and S(3PJ=2,1,0) Product Channels

Vacuum ultraviolet photodissociation dynamics of carbonyl sulfide (OCS) was investigated by using the time-sliced velocity map ion imaging technique. Images of the S(¹S_J=0) and S(³P_J=2,1,0) photofragments formed in the OCS photodissociation were acquired at six photolysis wavelengths from 147.24 to 156.48 nm. Vibrational states of the CO coproducts were partially resolved and identified in the images. Two main dissociation product channels, namely, the spin-allowed S(¹S_J=0) + CO(X¹Σ_g⁺) and spin-forbidden S(³P_J=2,1,0) + CO(X¹Σ_g⁺), were observed. At each photolysis wavelength, the total kinetic energy releases, the relative population of different CO vibrational states, and the anisotropic parameters were derived. Variations of the relative population were noticed between different spin-orbit states of the S(³P_J) channel. It was found that the S(¹S_J=0) + CO(X¹Σ_g⁺) channel is dominated by the ¹Σ⁺ ← ¹Σ⁺ parallel transition of OCS. Interestingly, two types of anisotropic parameters are found at different photolysis wavelengths for the spin-forbidden S(³P_J=2,1,0) + CO(X¹Σ_g⁺) product channel. The anisotropic parameters at 147.24 and 150.70 nm are significantly smaller than at the other four photolysis wavelengths. This phenomenon indicates two different nonadiabatic pathways are responsible for the spin-forbidden channels, which is consistent with the barrier structure in the exit channel of one of the triplet states.

Observation of the Carbon Elimination Channel in Vacuum Ultraviolet Photodissociation of OCS

The textbook mechanism for OCS photodissociation mainly involves the CO + S or CS + O product channel via a single bond fission. However, a third dissociation channel concerning the cleavage of both C-S and C-O bonds yielding SO + C products, though thermodynamically allowed, has never been verified experimentally to date. By using a tunable vacuum ultraviolet laser light and time-sliced velocity map ion imaging technique, we have clearly observed the SO(X³Σ^-) + C(³P_J=0) products as the vacuum ultraviolet laser photon energy gradually exceeds its thermodynamic threshold. The corresponding SO(X³Σ^-) coproducts are highly vibrationally excited and show varying angular distributions from isotropic to anisotropic as the excitation photon energy increases. Theoretical analysis suggests that a fast nonadiabatic pathway plays a dominant role in the formation of the anisotropic SO products. That isotropic products arise as the excitation photon energies approach the thermodynamic threshold can be reasonably explained by the "roaming mechanism".

Decomposing the First Absorption Band of OCS Using Photofragment Excitation Spectroscopy

Photofragment excitation spectra of carbonyl sulfide (OCS) have been recorded from 212-260 nm by state-selectively probing either electronically excited S((1)D) or ground state S((3)P) photolysis products via 2 + 1 resonance-enhanced multiphoton ionization. Probing the major S((1)D) product results in a broad, unstructured action spectrum that reproduces the overall shape of the first absorption band. In contrast, spectra obtained probing S((3)P) products display prominent resonances superimposed on a broad continuum; the resonances correspond to the diffuse vibrational structure observed in the conventional absorption spectrum. The vibrational structure is assigned to four progressions, each dominated by the C-S stretch, ν1, following direct excitation to quasi-bound singlet and triplet states. The S((3)PJ) products are formed with a near-statistical population distribution over the J = 2, 1, and 0 spin-orbit levels across the wavelength range investigated. Although a minor contributor to the S atom yield near the peak of the absorption cross section, the relative yield of S((3)P) increases significantly at longer wavelengths. The experimental measurements validate recent theoretical work characterizing the electronic states responsible for the first absorption band by Schmidt and co-workers.

Three-dimensional imaging of carbonyl sulfide and ethyl iodide photodissociation using the pixel imaging mass spectrometry camera

The Pixel Imaging Mass Spectrometry (PImMS) camera is used in proof-of-principle three-dimensional imaging experiments on the photodissociation of carbonyl sulfide and ethyl iodide at wavelengths around 230 nm and 245 nm, respectively. Coupling the PImMS camera with DC-sliced velocity-map imaging allows the complete three-dimensional Newton sphere of photofragment ions to be recorded on each laser pump-probe cycle with a timing precision of 12.5 ns, yielding velocity resolutions along the time-of-flight axis of around 6%-9% in the applications presented.

A fast microchannel plate-scintillator detector for velocity map imaging and imaging mass spectrometry

The time resolution achievable using standard position-sensitive ion detectors, consisting of a chevron pair of microchannel plates coupled to a phosphor screen, is primarily limited by the emission lifetime of the phosphor, around 70 ns for the most commonly used P47 phosphor. We demonstrate that poly-para-phenylene laser dyes may be employed extremely effectively as scintillators, exhibiting higher brightness and much shorter decay lifetimes than P47. We provide an extensive characterisation of the properties of such scintillators, with a particular emphasis on applications in velocity-map imaging and microscope-mode imaging mass spectrometry. The most promising of the new scintillators exhibits an electron-to-photon conversion efficiency double that of P47, with an emission lifetime an order of magnitude shorter. The new scintillator screens are vacuum stable and show no signs of signal degradation even over longer periods of operation.

The vibrationally mediated photodissociation of Cl2

The photodissociation of vibrationally excited Cl(2)(v = 1) has been investigated experimentally using the velocity mapped ion imaging technique. The experimental measurements presented here are compared with the results of time-dependent wavepacket calculations performed on a set of ab initio potential energy curves. The high level calculations allow prediction of all the dynamical information regarding the dissociation, including electronic polarization effects. Using a combination of theory and experiment it was found that there was negligible cooling of the vibrational degree of freedom of the parent molecule in the molecular beam. The results presented are compared with those following the photodissociation of Cl(2)(v = 0). Although the same electronic states are found to be important for Cl(2)(v = 1) as for Cl(2)(v = 0), significant differences were found regarding many of the observables. The overall level of agreement between theory and experiment was found to be reasonable and confirms previous assignments of the photodissociation mechanism.

Electronic polarization effects in the photodissociation of Cl2

Velocity mapped ion imaging and resonantly enhanced multiphoton ionization time-of-flight methods have been used to investigate the photodissociation dynamics of the diatomic molecule Cl(2) following excitation to the first UV absorption band. The experimental results presented here are compared with high level time dependent wavepacket calculations performed on a set of ab initio potential energy curves [D. B. Kokh, A. B. Alekseyev, and R. J. Buenker, J. Chem. Phys. 120, 11549 (2004)]. The theoretical calculations provide the first determination of all dynamical information regarding the dissociation of a system of this complexity, including angular momentum polarization. Both low rank K = 1, 2 and high rank K = 3 electronic polarization are predicted to be important for dissociation into both asymptotic product channels and, in general, good agreement is found between the recent theory and the measurements made here, which include the first experimental determination of high rank K = 3 orientation.

Photofragment angular momentum distributions in the molecular frame. III. Coherent effects in the photodissociation of polyatomic molecules with circularly polarized light

We extend the a(q) (k)(s) polarization parameter model [T. P. Rakitzis and A. J. Alexander, J. Chem. Phys. 132, 224310 (2010)] to describe the components of the product angular momentum polarization that arise from the one-photon photodissociation of asymmetric top molecules with circularly polarized photolysis light, and provide a general equation for fitting experimental signals. We show that the only polarization parameters that depend on the helicity of the circularly polarized photolysis light are the A(0) (k) and Re[A(1) (k)] (with odd k) and the Im[A(1) (k)] (with even k); in addition, for the unique recoil destination (URD) approximation [for which the photofragment recoil v arises from a unique parent molecule geometry], we show that these parameters arise only as a result the interference between at least two dissociative electronic states. Furthermore, we show that in the breakdown of the URD approximation (for which the photofragment recoil v arises from a distribution of parent molecule geometries), these parameters can also arise for dissociation via a single dissociative electronic state. In both cases, the A(0) (k) and Re[A(1) (k)] parameters (with odd k) are proportional to cosΔφ, and the Im[A(1) (k)] parameters (with even k) are proportional to sinΔφ, where Δφ is the phase shift (or average phase shift) between the interfering paths so that Δφ can be determined directly from the A(q) (k), or from ratios of these A(q) (k) parameters. Therefore, the determination of these A(q) (k) parameters with circularly polarized photolysis light allows the unambiguous measurement of coherent effects in polyatomic-molecule photodissociation.

Photofragment angular momentum distributions in the molecular frame. II. Single state dissociation, multiple state interference, and nonaxial recoil in photodissociation of polyatomic molecules

We present an a(q) (k)(s) polarization-parameter model to describe product angular momentum polarization from the one-photon photodissociation of polyatomic molecules in the molecular frame. We make the approximation that the final photofragment recoil direction is unique and described by the molecular frame polar coordinates (alpha,phi(i)), for which the axial recoil approximation is a special case (e.g., alpha=0). This approximation allows the separation of geometrical and dynamical factors, in particular, the expression of the experimental sensitivities to each of the a(q) (k)(s) in terms of the molecular frame polar angles (chi(i),phi(i)) of the transition dipole moment mu(i). This separation is applied to the linearly polarized photodissociation of polyatomic molecules (asymmetric, symmetric, and spherical top molecules are discussed) and to all dissociation mechanisms that satisfy our recoil approximation, including those with nonaxial recoil and multiple state interference, giving important insight into the geometrical properties of the photodissociation mechanism. For example, we demonstrate that the ratio of polarization parameters A(0) (k)(aniso)/A(0) (k)(iso)=beta (where beta is the spatial anisotropy parameter) is an indication that the dynamics can be explained by a single dissociative state. We also show that for asymmetric top photodissociation, the sensitivity to the a(1) (k)(s) parameters, which can arise either from single-surface or multiple-surface interference mechanisms, is nonzero only for components of the transition dipole moments within the v-d plane of the recoil frame.

Laser detection of spin-polarized hydrogen from HCl and HBr photodissociation: comparison of H- and halogen-atom polarizations

Thermal HCl and HBr molecules were photodissociated using circularly polarized 193 nm light, and the speed-dependent spin polarization of the H-atom photofragments was measured using polarized fluorescence at 121.6 nm. Both polarization components, described by the a(0)(1)(perpendicular) and Re[a(1)(1)(parallel, perpendicular)] parameters which arise from incoherent and coherent dissociation mechanisms, are measured. The values of the a(0)(1)(perpendicular) parameter, for both HCl and HBr photodissociation, are within experimental error of the predictions of both ab initio calculations and of previous measurements of the polarization of the halide cofragments. The experimental and ab initio theoretical values of the Re[a(1)(1)(parallel, perpendicular)] parameter show some disagreement, suggesting that further theoretical investigations are required. Overall, good agreement occurs despite the fact that the current experiments photodissociate molecules at 295 K, whereas previous measurements were conducted at rotational temperatures of about 15 K.

Velocity map imaging in time of flight mass spectrometry

A new variation on time of flight mass spectrometry is presented, which uses a fast framing charge coupled device camera to velocity map image multiple product masses in a single acquisition. The technique is demonstrated on two photofragmentation processes, those of CS(2) and CH(3)S(2)CH(3) (dimethyldisulfide) at a photolysis wavelength of 193 nm. In both cases, several mass fragments are imaged simultaneously, and speed distributions and anisotropy parameters are extracted that are comparable to those obtained by imaging each fragment separately in conventional velocity map imaging studies.