Rotationally resolved photoionization dynamics of hot CO fragmented from OCS

Two-photon laser-induced fluorescence study of the CO B 1Σ+ (v' = 0) state in a 4850 K plasma plume: Modified molecular constants, evidence of predissociation, and J'-dependent photoionization

We report the two-photon absorption laser-induced fluorescence rotational spectrum of the CO B 1Σ+ ← X 1Σ+ Hopfield-Birge system (v' = 0, v″ = 0) Q-branch in an ∼4850 K, atmospheric pressure plasma torch plume at thermal equilibrium in both the quenching-dominated (low laser intensity) and photoionization-dominated (high laser intensity) regimes. We provide a detailed analysis of the photophysics in these two regimes using a rate equation approach and propose modeling considerations for them as well. In the experimental spectra, distinct rotational transitions up to J″ = 83 are observed, allowing analysis over a very large range of rotational states. Evidence of predissociation is observed for J' ≥ 64 and is likely due to the interaction with the D'1Σ+ electronic state, which has been proposed in the literature but never observed in the v' = 0 state. The line positions of higher rotational states show disagreement with line positions calculated from molecular constants in the available literature, suggesting the need for modifications to the constants, which are reported here. A shift in the B 1Σ+ ← X 1Σ+ absorption spectrum toward higher two-photon energy as a result of the second-order Stark shift was observed in the photoionization-dominated spectrum, and the second-order Stark shift cross section was estimated to be 7 ± 3 × 10-18 cm2. The mean photoionization cross section of the excited upper state was inferred by comparing the line broadening of the two spectra and was estimated to be 11 ± 7 × 10-18 cm2. In addition, weak J'-dependent variations of the photoionization cross section were observed and are reported here.

Resonance-state selective photodissociation dynamics of OCS + hv → CS(X1Σ+) + O(3Pj=2,1,0) via the 21Σ+ state

Understanding vacuum ultraviolet photodissociation dynamics of Carbonyl sulfide (OCS) is of considerable importance in the study of atmospheric chemistry. Yet, photodissociation dynamics of the CS(X1Σ+) + O(3Pj=2,1,0) channels following excitation to the 21Σ+(ν1',1,0) state has not been clearly understood so far. Here, we investigate the O(3Pj=2,1,0) elimination dissociation processes in the resonance-state selective photodissociation of OCS between 147.24 and 156.48 nm by using the time-sliced velocity-mapped ion imaging technique. The total kinetic energy release spectra are found to exhibit highly structured profiles, indicative of the formation of a broad range of vibrational states of CS(1Σ+). The fitted CS(1Σ+) vibrational state distributions differ for the three 3Pj spin-orbit states, but a general trend of the inverted characteristics is observed. Additionally, the wavelength-dependent behaviors are also observed in the vibrational populations for CS(1Σ+, v). The CS(X1Σ+, v = 0) has a significantly strong population at several shorter wavelengths, and the most populated CS(X1Σ+, v) is gradually transferred to a higher vibrational state with the decrease in the photolysis wavelength. The measured overall β-values for the three 3Pj spin-orbit channels slightly increase and then abruptly decrease as the photolysis wavelength increases, while the vibrational dependences of β-values show an irregularly decreasing trend with increasing CS(1Σ+) vibrational excitation at all studied photolysis wavelengths. The comparison of the experimental observations for this titled channel and the S(3Pj) channel reveals that two different intersystem crossing mechanisms may be involved in the formation of the CS(X1Σ+) + O(3Pj=2,1,0) photoproducts via the 21Σ+ state.

Vacuum ultraviolet photodissociation of OCS via the 21Σ+ state: the S(1D2) elimination channel

The photodissociation of OCS is necessary to model the primary photochemical processes of OCS in the global cycling of sulfur and interstellar photochemistry. Here, by combining the time-sliced velocity-map ion imaging technique with the single vacuum ultraviolet photon ionization method, we have studied the CO(¹Σ⁺, v) + S(¹D₂) photoproduct channel from the OCS photodissociation via the eight different vibrational resonances ( = 1-8) in the 2¹Σ⁺(, 1, 0) ← X¹Σ⁺(0, 0, 0) band. From the measured S(¹D₂) images, the wavelength-dependent CO(¹Σ⁺, v) vibrational state populations have been obtained in the wavelength range of 142.98-154.37 nm. The majority of the CO(¹Σ⁺, v) photoproducts are shown to abruptly populate from low vibrational states to high vibrational states as the photolysis wavelength decreases from 152.38 to 148.92 nm. The anisotropy parameters (β) for the CO(¹Σ⁺, v) + S(¹D₂) channel have also been determined from the images of the S(¹D₂) photoproducts. It is found that the vibrational state-specific β-values present a similar decreasing trend with increasing CO vibrational excitation for all the eight vibrational resonances of OCS*(2¹Σ⁺). These observations indicate that there is a possibility that more than one non-adiabatic dissociation pathways with different dissociation lifetimes are involved in the formation of CO(¹Σ⁺) + S(¹D₂) photoproducts from the initial vibronic levels of the 2¹Σ⁺ state to the final dissociative state.

Time-resolving the UV-initiated photodissociation dynamics of OCS

We present a time-resolved study of the photodissociation dynamics of OCS after UV-photoexcitation at λ = 237 nm. OCS molecules (X1Σ+) were primarily excited to the 11A'' and the 21A' Renner-Teller components of the 1Σ- and 1Δ states. Dissociation into CO and S fragments was observed through time-delayed strong-field ionisation and imaging of the kinetic energy of the resulting CO+ and S+ fragments by intense 790 nm laser pulses. Surprisingly, fast oscillations with a period of ∼100 fs were observed in the S+ channel of the UV dissociation. Based on wavepacket-dynamics simulations coupled with a simple electrostatic-interaction model, these oscillations do not correspond to the known highly-excited rotational motion of the leaving CO(X1Σ+, J ≫ 0) fragments, which has a timescale of ∼140 fs. Instead, we suggest to assign the observed oscillations to the excitation of vibrational wavepackets in the 23A'' or 21A'' states of the molecule that predissociate to form S(3PJ) photoproducts.

Vacuum ultraviolet photodissociation dynamics of OCS + hv → CO(1Σ+) + S(1S0) via the E and F Rydberg states

The state-resolved photodissociation of the CO(1Σ+) + S(1S0) photoproduct channel, formed by vacuum ultraviolet photoexcitation of OCS to a progression of the symmetric stretching vibration (ν1') in the E and F states, has been investigated by using the time-sliced velocity map ion imaging technique. The total kinetic energy release spectra and the vibrational state specific anisotropy parameters (β) were obtained based on the raw images of S(1S0) photoproducts detected in the wavelength ranges of 134.40-140.98 nm, respectively. Except for vibrational band origins, the CO(1Σ+) photoproducts are found to have more significant populations at highly vibrationally excited states as the symmetric stretching vibrational excitation of the E and F states increases. Furthermore, the vibrational-state specific β values for the CO(1Σ+) + S(1S0) channel via the E and F states both show a sudden change from negative to positive in the vicinity of moderately vibronic levels of the E and F states. This anomalous phenomenon suggests that multiple excited states with different symmetries are involved in the photoexcitation process at relatively short photolysis wavelengths due to the strong vibronic couplings existing in the higher vibronic levels of the E and F states, and the formation of CO(1Σ+) + S(1S0) photoproducts may proceed by different nonadiabatic interactions from the prepared excited states to the lower dissociative state 1Σ+, with strong dependence of the initially symmetric stretching excitation in the Rydberg-type transitions.

Imaging multiphoton ionization and dissociation of rotationally warm CO via the B+Σ1 and EΠ1 electronic states

Pathways for formation of C⁺ and O⁺ ions when applying (2 + 1) resonance enhanced multiphoton ionization (REMPI) of CO via the B¹Σ⁺ and E¹Π electronic states are characterized with the velocity map imaging technique. By employing an unskimmed pulsed valve, it was possible to obtain sharp images for a wide range of initial CO J-states. Most of the atomic ion production pathways could be assigned as one- or two-photon dissociation of a series of vibrational levels of the CO⁺ X²Σ⁺ and A²Π states. Large enhancements in dissociation of particular CO⁺ vibrational states in these progressions could be accurately assigned to accidental resonances of the REMPI laser with CO⁺ X²Σ⁺-B²Σ⁺ transitions.

Temperature dependence of the photodissociation of CO2 from high vibrational levels: 205-230 nm imaging studies of CO(X1Σ+) and O(3P, 1D) products

The 205-230 nm photodissociation of vibrationally excited CO₂ at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO₂ molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO₂. Photodissociation was observed from vibrationally excited CO₂ with internal energies up to about 20 000 cm^-1, and CO(X¹Σ⁺), O(³P), and O(¹D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO₂ vibrational excitation made this investigation feasible. The internal energies of heated CO₂ molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(X¹Σ⁺) products in v″ = 0. At 230 nm, CO₂ needs to have at least 4000 cm^-1 of rovibrational energy to absorb the UV radiation and produce CO(X¹Σ⁺) + O(³P). CO₂ internal energies in excess of 16 000 cm^-1 were confirmed by observing O(¹D) products. It is likely that initial absorption from levels with high bending excitation accesses both the A¹B₂ and B¹A₂ states, explaining the nearly isotropic angular distributions of the products. CO(X¹Σ⁺) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(X¹Σ⁺), O(³P), and O(¹D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO₂ temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO₂ (and the corresponding available energy) increases. The KER distributions of O(¹D) and O(³P) show broad internal energy distributions in the CO(X¹Σ⁺) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO₂ VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, a³B₂ and b³A₂, and the ground electronic state are involved in the dynamical pathways that lead to product formation.

Photodissociation dynamics of OCS near 214 nm using ion imaging

The OCS photodissociation dynamics of the dominant S((1)D2) channel near 214 nm have been studied using velocity map ion imaging. We report a CO vibrational branching ratio of 0.79:0.21 for v = 0:v = 1, indicating substantially higher vibrational excitation than that observed at slightly longer wavelengths. The CO rotational distribution is bimodal for both v = 0 and v = 1, although the bimodality is less pronounced than at longer wavelengths. Vector correlations, including rotational alignment, indicate that absorption to both the 2(1)A' (A) and 1(1)A″ (B) states is important in the lower-j part of the rotational distribution, while only 2(1)A' state absorption contributes to the upper part; this conclusion is consistent with work at longer wavelengths. Classical trajectory calculations including surface hopping reproduce the measured CO rotational distributions and their dependence on wavelength well, though they underestimate the v = 1 population. The calculations indicate that the higher-j peak in the rotational distribution arises from molecules that begin on the 2(1)A' state but make nonadiabatic transitions to the 1(1)A' (X) state during the dissociation, while the lower-j peak arises from direct photodissociation on either the 2(1)A' or the 1(1)A″ states, as found in previous work.

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.

Slice imaging of the quantum state-to-state cross section for photodissociation of state-selected rovibrational bending states of OCS (v2=0,1,2∣JlM)+hν→CO(J)+S(D21)

Using hexapole quantum state-selection of OCS (v(2)=0,1,2/JlM) and high-resolution slice imaging of quantum state-selected CO(J), the state-to-state cross section OCS (v(2)=0,1,2/JlM)+hnu-->CO(J)+S((1)D(2)) was measured for bending states up to v(2)=2. The population density of the state-selected OCS (v(2)=0,1,2 /JlM) in the molecular beam was obtained by resonantly enhanced multiphoton ionization of OCS and comparison with room temperature bulk gas. A strong increase of the cross section with increasing bending state is observed for CO(J) in the high J region, J=60-67. Integrating over all J states the authors find sigma(v(2)=0):sigma(v(2)=1):sigma(v(2)=2)=1.0:7.0:15.0. A quantitative comparison is made with the dependence of the transition dipole moment function on the bending angle.

S(D21) atomic orbital polarization in the photodissociation of OCS at 193nm: Construction of the complete density matrix

The absolute velocity-dependent alignment and orientation for S(1D2) atoms from the photodissociation of OCS at 193 nm were measured using the dc slice imaging method. Three main peaks ascribed to specific groups of high rotational levels of CO in the vibrational ground state were found, with rotationally resolved rings in a fourth slow region ascribed to weak signals associated with excited vibrational states of CO. The observed speed-dependent beta and polarization parameters support the interpretation that there are two main dissociation processes: a simultaneous two-surface (A' and A") excitation and the initial single-surface (A') excitation followed by the nonadiabatic crossing to ground state. At 193 nm photodissociation, the nonadiabatic dissociation process is strongly enhanced relative to longer wavelengths. The angle- and speed-dependent S(1D2) density matrix can be constructed including the higher order (K = 3,4) contributions for the circularly polarized dissociation light. This was explicitly done for selected energies and angles. It was found in one case that the density matrix is sensitively affected by the rank 4 terms, suggesting that the higher order contributions should not be overlooked for an accurate picture of the dissociation dynamics in this system.

The roaming atom pathway in formaldehyde decomposition

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.

Correlated vH2 and jCO product states from formaldehyde photodissociation: Dynamics of molecular elimination

A detailed study of the photoinduced molecular elimination pathway of formaldehyde on the ground state surface was carried out using high-resolution dc slice ion imaging. Detailed correlated H(2) rovibrational and CO rotational product quantum state distributions were measured by imaging spectroscopically selected CO velocity distributions following photodissociation at energies from approximately 1800 to approximately 4100 cm(-1) above the barrier to molecular elimination. Excitation to the 2(1)4(1), 2(1)4(3), 2(2)4(1), 2(2)4(3), and 2(3)4(1) bands of H(2)CO are reported here. The dependence of the product rovibrational distributions on excitation energy are discussed in light of a dynamical model which has been formulated to describe the strong product state correlations observed.

Multiphoton processes of CO at 230 nm

High resolution kinetic energy release spectra were obtained for C(+) and O(+) from CO multiphoton ionization followed by dissociation of CO(+). The excitation was through the CO (B (1)Sigma(+)) state via resonant two-photon excitation around 230 nm. A total of 5 and 6 photons are found to contribute to the production of carbon and oxygen cations. DC slice and Megapixel ion imaging techniques were used to acquire high quality images. Major features in both O(+) and C(+) spectra are assigned to the dissociation of some specific vibrational levels of CO(+)(X (2)Sigma(+)). The angular distributions of C(+) and O(+) are very distinct and those of various features of C(+) are also different. A dramatic change of the angular distribution of C(+) from dissociation of CO(+)(X (2)Sigma(+), nu(+) = 1) is attributed to an accidental one-photon resonance between CO(+)(X (2)Sigma(+), nu(+) = 1) and CO(+)(B (2)Sigma(+), nu(+) = 0) and explained well by a theoretical model. Both kinetic energy release and angular distributions were used to reveal the underlying dynamics.

Slice imaging of quantum state-to-state photodissociation dynamics of OCS

Slice imaging experiments are reported for the quantum state-to-state photodissociation dynamics of OCS. Both one-laser and two-laser experiments are presented detecting CO(J) or S((1)D(2)) photofragments from the dissociation of hexapole state-selected OCS(v(2) = 0,1,2 / J = 1,2) molecules. We present data using our recently developed large frame CCD centroiding detector and have implemented a new high speed MCP high voltage pulser with an effective slice width of only 6 ns. Slice images are presented for quantum state-to-state photolysis, near 230 nm, of vibrationally excited OCS(v(2) = 0,1,2). Two-laser pump-probe experiments detecting CO(J = 63 or 64) show a dramatic change in the beta parameter for the same final state of CO(J) when the photolysis energy is reduced by about 1000 cm(-1). We attribute the observed change from large positive to large negative beta to a large increase of the molecular frame deflection angle at very slow recoil velocity, due to a breakdown of the axial recoil. Two-laser experiments on the S((1)D(2)) fragment reveal single well-separated rings in the slice images correlating with individual CO(J) states. Strong polarization effects of the probe laser are observed, both in the angular distribution of the intensity of single S((1)D(2)) rings and in the resolution of the radial velocity distribution. It is shown how the broadening of the velocity distribution can be reduced by a directed ejection of the electron in the ionization process perpendicular to the slice imaging plane. The dissociation energy of OCS(v(2) = 0, J = 0) --> CO(J = 0) + S((1)D(2)) is determined with high accuracy D(0) = (34 608 +/- 24) cm(-1).

State-to-state photodissociation of carbonyl sulfide (ν2=0,1∣JlM). II. The effect of initial bending on coherence of S(D21) polarization

Photodissociation studies using ion imaging are reported, measuring the coherence of the polarization of the S((1)D(2)) fragment from the photolysis of single-quantum state-selected carbonyl sulfide (OCS) at 223 and 230 nm. A hexapole state-selector focuses a molecular beam of OCS parent molecules in the ground state (nu2=0mid R:JM=10) or in the first excited bending state (nu2=1mid R:JlM=111). At 230 nm photolysis the Im[a1 (1)(parallel, perpendicular)] moment for the fast S(1D2) channel increases by about 50% when the initial OCS parent state changes from the vibrationless ground state to the first excited bending state. No dependence on the initial bending state is found for photolysis at 223 nm. We observe separate rings in the slow channel of the velocity distribution of S(1D2) correlating to single CO(J) rotational states. The additional available energy for photolysis at 223 nm is found to be channeled mostly into the CO(J) rotational motion. An improved value for the OC-S bond energy D0=4.292 eV is reported.

Photoionization dynamics in CS fragmented from CS2 studied by high-resolution photoelectron spectroscopy

The photoionization dynamics of CS have been studied using high-resolution laser photoelectron spectroscopy. The photodissociation of CS2 at ~308 nm results in highly rotationally excited CS in its X1Σ+ singlet ground state, as well as in rotationally cold CS in the excited a3Π triplet state. The ground-state CS fragments are formed together with sulfur in its 3P, 1D, and 1S electronic states; triplet CS is produced in coincidence with ground-state sulfur (3P). In both channels the photoelectron spectra are dominated by Δv = 0 propensity, but transitions involving Δv = 1 and 2 are also observed. Key words: photoelectron spectroscopy, photoionization, photodissociation, excited states, reactive intermediates.

Probing of the hot-band excitations in the photodissociation of OCS at 288 nm by DC slice imaging

The photodissociation dynamics of OCS at 288 nm has been investigated using the DC (direct current) slice imaging technique, which is a recently developed high-resolution "slicing" approach that directly measures the central slice of the photofragment distribution in imaging experiments. By analyzing a DC sliced image of S(1D2) photofragments we observe dissociation originating from OCS molecules excited up to v2 = 4 in the molecular beam. The measured translational energy distribution was used to determine the branching ratio for the contribution from each initial bending state (0 v2 0) of OCS and relative photodissociation cross section ratios compared to v2 = 1. Large negative anisotropy parameters determined as a function of the S(1D2) fragment recoil speed indicate that the photodissociation of OCS at 288 nm occurs exclusively from the 11A′′(1Σ–) bending excited potential surface that can be accessed through a perpendicular transition.Key words: DC slicing imaging, OCS, photodissociation, hot-band excitation.