VUV Photodissociation Dynamics of Nitrous Oxide: The O(1SJ=0) and O(3PJ=2,1,0) Product Channels

Vibrational state-specific nonadiabatic photodissociation dynamics of OCS+ via A2Π1/2 (ν1 0 ν3) states

The identification and analysis of quantum state-specific effects can significantly deepen our understanding of detailed photodissociation dynamics. Here, we report an experimental investigation on the vibrational state-mediated photodissociation of the OCS+ cation via the A2Π1/2 (ν1 0 ν3) states by using the velocity map ion imaging technique over the photolysis wavelength range of 263-294 nm. It was found that the electronically excited S+ product channel S+(2Du) + CO (X1Σ+) was significantly enhanced when the ν1 and ν3 vibrational modes were excited. Clear deviations in the branching ratios of the electronically excited S+ channel were observed when the vibrational modes ν1 and ν3 were selectively excited. The results reveal that vibrationally excited states play a vital role in influencing the nonadiabatic couplings in the photodissociation process.

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.

Quantum interference between spin-orbit split partial waves in the F + HD → HF + D reaction

The effect of electron spin-orbit interactions on chemical reaction dynamics has been a topic of much research interest. Here we report a combined experimental and theoretical study on the effect of electron spin and orbital angular momentum in the F + HD → HF + D reaction. Using a high-resolution imaging technique, we observed a peculiar horseshoe-shaped pattern in the product rotational-state-resolved differential cross sections around the forward-scattering direction. The unusual dynamics pattern could only be explained properly by highly accurate quantum dynamics theory when full spin-orbit characteristics were considered. Theoretical analysis revealed that the horseshoe pattern was largely the result of quantum interference between spin-orbit split-partial-wave resonances with positive and negative parities, providing a distinctive example of how spin-orbit interaction can effectively influence reaction dynamics.

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.

Vacuum ultraviolet photodissociation dynamics of CO2 near 133 nm: The spin-forbidden O(3Pj=2,1,0) + CO(X1Σ+) channel

Understanding vacuum ultraviolet (VUV) photodissociation dynamics of CO₂ is of considerable importance in the study of atmospheric chemistry and planetary chemistry. Yet, photodissociation dynamics of the spin-forbidden O(³P_j=2,1,0) + CO(X¹Σ⁺) channel has not been clearly understood so far. Here, we study the O(³P_j) + CO(X¹Σ⁺) dissociation processes in the VUV photodissociation of CO₂ at the photolysis wavelengths between 129.02 and 134.67 nm by using the time-sliced velocity-mapped ion imaging technique. From the vibrational-resolved images of the O(³P_j=2,1,0) photofragment, the total kinetic energy releases, the CO(X¹Σ⁺) cofragment vibrational state distributions, and the product angular distributions have been derived, respectively. The experimental observations show that the total kinetic energy releases for the three ³P_j spin-orbit states (j = 2, 1, 0) exhibit a broad CO(X¹Σ⁺) vibrational energy distribution with significant inverted characteristics, especially at short photoexcitation wavelengths, indicating that the VUV photodissociation could take place in a relatively linear geometry of the triplet state, with one C-O bond extended and the other compressed. Furthermore, a notable photolysis wavelength dependent feature has also been found in the product angular distributions of all three spin-orbit channels (j = 2, 1, 0): Only the vibrational-state specific anisotropy parameter β values at 130.18 nm behave more anisotropic, while all those at other photolysis wavelengths are near the value β = 0.5 for O(³P_j=2,1) channels or β = 0.25 for the O(³P_j=0) channel, with small fluctuations. This anomalous phenomenon suggests that the different nonadiabatic interactions, such as singlet-triplet coupling, may play a key role in the formation of O(³P_j=2,1,0) + CO(X¹Σ⁺) products, with strong photolysis wavelength dependence.

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".

Vacuum ultraviolet photodissociation dynamics of N2O via the C1Π state: The N(2Dj=5/2, 3/2) + NO(X2Π) product channels

We study the vacuum ultraviolet photodissociation dynamics of N₂O via the C¹Π state by using the time-sliced velocity map ion imaging technique. Images of N(²D_{j=5/2, 3/2}) products from the N atom elimination channels were acquired at a set of photolysis wavelengths from 142.55 to 148.19 nm. Vibrational states of the NO(X²Π) co-fragments were partially resolved in experimental images. From these images, the product total kinetic energy release distributions (TKERs), branching ratios of the vibrational states of NO(X²Π) co-fragments, and the vibrational state specific angular anisotropy parameters (β) have been determined. Notable features were found in the experimental results: the TKERs show that the NO(X²Π) co-fragments are highly vibrationally excited. For the highly vibrationally excited state of NO(X²Π), a bimodal rotational structure is found at all the studied photolysis wavelengths. Furthermore, the vibrational state specific β values of both spin-orbit channels (j = 3/2, 5/2) clearly show a monotonic decrease as the vibrational quantum number of NO(X²Π) increases. These observations suggest that multiple dissociation pathways play a role in the formation of the N(²D_{j=5/2, 3/2}) + NO(X²Π) products: one corresponds to a fast dissociation pathway through the linear state (the C¹Π state) following the initial excitation to a slightly bent geometry in the vicinity of the linear C¹Π configuration, leading to the low rotationally excited components with relatively large β values; the other corresponds to a relatively slow dissociation pathway through the bent C(3¹A') or C(3¹A″) state, leading to moderately rotationally excited NO(X²Π) products with smaller β values.

Photodissociation Dynamics of Nitrous Oxide near 145 nm: The O(1S0) and O(3P J=2,1,0) Product Channels

We report the study of photodissociation dynamics of nitrous oxide in the vacuum ultraviolet region, using the time-sliced velocity map ion imaging technique. Ion images of the O(¹S₀) and O(³P _J=2,1,0) products were measured at nine photolysis wavelengths from 142.55 to 148.79 nm. The product channels O(¹S₀) + N₂(X¹Σ_g⁺) and O(³P _J=2,1,0) + N₂(A³Σ_u⁺) have been observed. For these dissociation channels, the total kinetic energy releases of the dissociated products were acquired. With vibrational structures of the N₂ coproducts partially resolved in the experimental images, the branching ratios of different vibrational states of the N₂ coproducts were determined, and the vibrational state specific anisotropy parameters (β values) were derived. Analysis shows that the O(¹S₀) + N₂(X¹Σ_g⁺) channel is primarily formed via nonadiabatic couplings between the C (¹Π) state and the higher-lying D (¹Σ⁺) state of the N₂O. A moderate rotational excitation and high vibrational excitation of N₂(X¹Σ_g⁺) products have been observed through this pathway. On the other hand, for the O(³P _J=2,1,0) + N₂(A³Σ_u⁺) channels, where a slightly higher rotational excitation of N₂ coproducts have been observed, the possible pathway would be via nonadiabatic couplings from the C (¹Π) state to the lower-lying A(¹Σ^-)state.

VUV Photodissociation Dynamics of Nitrous Oxide: The N((2)DJ=3/2,5/2) and N((2)PJ=1/2,3/2) Product Channels

We report on an experimental study of the vacuum ultraviolet photodissociation dynamics of nitrous oxide as a function of photolysis wavelength. In this study, both the N((2)DJ) + NO(X(2)Π) and N((2)PJ) + NO(X(2)Π) product channels were investigated using the time-sliced velocity ion imaging technique. Images of the N((2)DJ=5/2,3/2) and N((2)PJ=3/2,1/2) products were measured at seven and ten, respectively, photolysis wavelengths between 124.44 and 133.20 nm. The vibrational states of the NO products were partially resolved in the acquired raw ion images. The total kinetic energy release and the branching ratios of different vibrational states of NO products were determined. The vibrational state distributions of NO were found to be inverted for the N((2)DJ=5/2,3/2) and N((2)PJ=3/2,1/2) product channels. This phenomenon indicates that the N-O bond is highly vibrational excited during the breaking of the N-N bond. Vibrational state resolved anisotropic parameters β in both the N((2)DJ) and the N((2)PJ) channels were acquired. The small β values (around 0.5) in the dissociation process suggest that transition states in a bent configuration play an important role in the formation of N + NO products.