Carbon cluster cations are generated in the gas phase by laser vaporization of a carbon rod in a pulsed supersonic expansion. C_n⁺ clusters (n = 6,7,10,11,12,15,20) are mass selected using a reflectron time-of-flight mass spectrometer and photodissociated at 355 nm. The main channel for this multiphoton dissociation process is the loss of neutral C₃, resulting in C_n−3⁺ cation fragments. The cationic fragments are reaccelerated into an imaging flight tube with velocity-map imaging grids and detected with an imaging detector. Significant kinetic energy release (KER) is observed for all of these cations, but with much greater KER values detected for the larger species. Specifically, the n = 10,11,12,15 and 20 species known to have monocyclic ring structures produce much greater KER than the n = 6 and 7 species known to have linear structures. Consideration of photon energies for two- or three-photon processes, together with the KER values and estimates for ring strain energies allows investigation of the energetics of the bonding and dissociation in these systems.

Criegee intermediates are zwitterionic carbonyl oxide species that result from alkene ozonolysis in the Earth's troposphere. UV excitation of the simplest Criegee intermediate, CH₂OO, across most of the broad span of the (B ¹A^′) - (X ¹A^′) spectrum results in prompt dissociation to two energetically accessible asymptotes: O (¹D) + H₂CO (X ¹A₁) and O (³P) + H₂CO (a ³A^′′). Dissociation proceeds on multiple singlet potential energy surfaces that are coupled by two regions of conical intersection (CoIn). Velocity map imaging studies reveal a bimodal total kinetic energy (TKER) distribution for the O (¹D) + H₂CO (X ¹A₁) products. The unexpected low TKER component corresponds to highly internally excited H₂CO (X ¹A₁) products. Full dimensional trajectory calculations suggest that the bimodal TKER distribution of the O (¹D) + H₂CO (X ¹A₁) products originates from two different dynamical pathways: a primary pathway evolving through one CoIn region to products and a smaller component sampling both CoIn regions during the dissociation process. Those that access both CoIn regions likely give rise to the more highly internally excited H₂CO (X ¹A₁) products. The remaining trajectories dissociate to O (³P) + H₂CO (a ³A^′′) products after traversing through both CoIn regions. No trajectories follow the more thermodynamically favorable spin-forbidden pathway to O (³P) + H₂CO (X ¹A₁) products. This complementary experimental and theoretical investigation provides insight into the photodissociation of CH₂OO via multiple dissociation pathways through two regions of CoIn that control the branching and energy distributions of products

Ozone photodissociation plays an important role in atmospheric chemistry and has been the focus of many experimental and theoretical studies. In the Hartley band (200-300 nm) there are two spin-allowed dissociation channels, one forming excited state, singlet products (O2(a 1∆g) + O(1D)), and one forming ground state, triplet products (O2(X 3Σg-) + O(3P)). The singlet channel is the primary dissociation channel in the Hartley band, and numerous studies have characterized the dissociation at longer wavelengths within the Hartley band. There has been considerable interest in the triplet channel dissociation at 226 nm following the observation of low velocity O(3P) fragments, but the singlet channel dissociation dynamics at 226 nm has not been previously reported. We report the rotational state distribution and vector correlations of the O2(a 1∆g, v=0) fragments arising from the 226 nm photodissociation of jet-cooled O3. Consistent with previously reported trends, the rotational distribution is shifted to higher rotational states with decreasing wavelength. We observe highly suppressed odd rotational state populations due to a strong Λ-doublet propensity. The measured rotational distribution is in agreement with classical trajectory calculations for the v=0 products, although the distribution is narrower than predicted. The spatial anisotropy follows the previously observed trend of decreasing β with increasing photon energy with β = 0.72 ± 0.14 for v=0, j=38. As expected for a triatomic molecule, the v-j correlation is consistent with v perpendicular to j, but the measured correlation is non-limiting due to rotational and translational depolarization. The j-dependent linewidth of the O2(a 1∆g) REMPI spectrum is also discussed in connection with the lifetime of the resonant O2(d 1Πg) state and predissociation via the II 1Πg valence state.

A new allylic 1,6 H-atom transfer mechanism is established through infrared (IR) excitation of the 2-butenal-oxide Criegee intermediate [CH₃CH=CHCHOO]. Rapid 1,6 H-atom transfer is facilitated for certain conformers of 2-butenal oxide by extended conjugation across the vinyl and carbonyl oxide groups. A low-energy conformer (tZZ) of 2-butenal oxide is identified by IR action spectroscopy in the fundamental CH region with ultraviolet (UV) detection of OH products by laser-induced fluorescence (LIF). The strongest observed IR transition at 2996 cm⁻¹is consistent with the anharmonic frequency computed for the tZZ conformer. A low energy reaction pathway involving isomerization of 2-butenal oxide from a lower energy conformer (tZZ) to a higher energy conformer (cZZ), followed by 1,6 H-atom transfer via a 7-membered ring transition state with relatively low ring strain, is theoretically predicted and shown experimentally to yield the OH products. The rapid appearance of OH products (ca. 2.3 ± 1.0 × 10⁸ s⁻¹) agrees with a statistical RRKM calculation for an effective reaction rate (k_eff(E) on the order of 10⁸ s⁻¹ at ca. 3000 cm⁻¹) including tunneling. Unimolecular decay involves a combination of conformational isomerization and unimolecular dissociation via 1,6 H-atom transfer. The excellent agreement between experiment and theory confirms the allylic 1,6 H-atom transfer mechanism in 2-butenal-oxide Criegee intermediate and provides a novel pathway for non-photolytic OH generation upon alkene ozonolysis in the troposphere.

The laser-induced fluorescence (LIF) and wavelength-resolved emission spectroscopic techniques have been used to study the rotational levels, the vibrational predissociation (VP) products and the product-state branching ratios of the à state of the C₃Ar van der Waals complex. The excited states were prepared by exciting the complex bands associated with 0 8⁻ 0-000, 0 4⁺ 0-000 and 0 0 2-000 bands of the à ¹Π_u - ~X ¹Σ_g system of C₃. The superscripts "-" and "+" denote the lower and upper Renner components, respectively. The type A and C bands of the complex bands are in pairs and they are separated by 2ν_b (b, van der Waals bending vibration).A.J. Merer, Y.-C. Hsu, Y.-R. Chen, and Y.-J. Wang, J. Chem. Phys. 143, 194304(2015).f 11 bands, only two, associated with the 002-000 band of C₃, are rotationally resolved with a laser of 0.04 cm⁻¹resolution. The lifetimes of these complex bands are in the order of a few to a few tens of picoseconds estimated from the rotational linewidths of the LIF spectra. The VP processes are quite complex; more than one vibrational state of the C₃ fragments was identified from each upper complex level. The fragment states were the pure bending levels (0 v₂ 0) and the combination levels (1 v₂ 0) of the à state. A.J. Merer, Y.-C. Hsu, Y.-R. Chen, and Y.-J. Wang, J. Chem. Phys. 143, 194304(2015).O

The vibrational predissociation (VP) products of eighteen rovibrational levels of the à state of the C₃Ar complex have been studied.Y.-J. Wang and Y.-C. Hsu, J. Chem. Phys. 153, 124303(2020).^,S.−C. Hsiao and Y.−C. Hsu, to be published.hese complex levels are associated with the 0 2^- 0, 0 2^+ 0, 0 4^- 0, 0 8^- 0, 0 4^+ 0, and 0 0 2 vibrational levels of C_3(Ã). The distributions of the fragment branching ratios versus the square root of the excess energies (the sum of the translational and rotational energy of the VP product) obtained from these complex levels do not necessarily follow the momentum gap law G.E. Ewing, J. Chem. Phys. 71, 3143(1979).r energy gap law. J.A. Beswick and J. Jortner, J. Chem. Phys. 69, 512(1979).ffects such as spectroscopic perturbation,E.J. Bohac, M.D. Marshall, and R.E. Miller, J. Chem. Phys. 98,6642(1993) and references therein.nergy gap,^c,d angular momentum,^a threshold predissociation,^a and intramolecular vibrational redistributionA. Buchachenko, N. Halberstadt, B. Lepetit, and O. Roncero, Int. Rev. Phys. Chem. 22, 153(2003).n the VP processes have been previously reported. In this work, these effects will be examined and the VP mechanism of the à state of the C_3 S.-C. Hsiao and Y.-C. Hsu, to be published.TG.E. Ewing, J. Chem. Phys. 71, 3143(1979).oJ.A. Beswick and J. Jortner, J. Chem. Phys. 69, 512(1979).EE.J. Bohac, M.D. Marshall, and R.E. Miller, J. Chem. Phys. 98,6642(1993) and references therein.eA. Buchachenko, N. Halberstadt, B. Lepetit, and O. Roncero, Int. Rev. Phys. Chem. 22, 153(2003).o

In 2017, we presented at ISMS a new Time-Resolved Kinetic Chirped-Pulse (TReK-CP) spectrometer.Zaleski, D. P.; Prozument, K.; ISMS 2017, WH02.y coupling a UV photolysis source to a chirped pulse millimeter-wave (mm-wave) spectrometer, we demonstrated the ability to measure kinetic and thermodynamic properties of the photolysis of acrylonitrile (CH₂CHCN), including product branching ratios and rotational and vibrational thermalization rates at reasonable time resolution (ca. 10 μs). However, sensitivity to vibrationally excited states and pre-collisional dynamics was limited, so the observation of truly nascent molecules was still out of reach. Here, we present improvements to the TReK-CP design that enables detection of nascent product molecules from the photolysis of acrylonitrile, with particular focus on the formation of cyanoacetylene (HC₃N). Moving to the 260-295 GHz mm-wave band significantly improves sensitivity to small polyatomics, enabling detection of HC₃N within 1 μs after photolysis in a variety of vibrational states. We have also devised a new detection scheme that enables a time resolution of 1 μs, amongst other improvements. Revisiting the photolysis of acrylonitrile with these improvements has led to surprising observations. We will present evidence that HC₃N has different dynamics than the primary photoproduct, HCN, which clearly forms rotationally hot and is undetectable until the first collisional event takes place. Meanwhile, cyanoacetylene forms slower, exhibiting low temperature state distributions, a large kinetic isotopic effect, and strong kinetic dependence on the initial temperature of the precursor. This is consistent with the theoretical prediction that the final step, CH₂CCN → HC₃N + H, occurs with little excess energy.Zaleski, D. P.; Harding, L. B.; Klippenstein, S. J.; Ruscic, B.; Prozument, K. J. Phys. Chem. Lett. 2017, 8, 6180.e will also show that we are, in fact, detecting truly nascent cyanoacetylene, in that the kinetics show a distinct change from first to second order on the collisional timescale of the reactor.

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Zaleski, D. P.; Prozument, K.; ISMS 2017, WH02.B Zaleski, D. P.; Harding, L. B.; Klippenstein, S. J.; Ruscic, B.; Prozument, K. J. Phys. Chem. Lett. 2017, 8, 6180.W

The destruction of hot molecules by photodissociation influences the composition and dynamics of exoplanets, particularly in the presence of a UV-rich stellar environments. We compute temperature-dependent photodissociation cross sections and rates for molecules found in these atmospheres, for building a more realistic model of the planetary chemistry. The cross sections are calculated by solving the nuclear-motion Schrödinger equation as part of the ExoMol project using codes Duo, DVR3D and ExocrossYurchenko et al Comput Phys Commun 2016 202 262–275; Tennyson et al Comput Phys Commun 2004 163 85-116; Yurchenko et al A&A 2018 614 A131 using the methodology previously describedPezzella et al Phys. Chem. Chem. Phys. 2021 23 16390–16400 Photodissociation rates are computed integrating the cross section with different stellar field models representing different star types.
New tools and results for HF, HCl and HCN. Cross sections and rates for the diatomics are compared with previously available dataHeays et al A&A 2017 602 A105 finding a good agreement for the interstellar medium for low temperatures. Both cross sections and rates have a dramatic temperature dependence for temperatures above 1000 K. Our results for HCN are compared with the results obtained by previous works employing the time dependent Schrödinger equationChenel et al J. Chem. Phys. 2016 144 144306; Aguado et al Astrophys. J. 2017 838 33

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Footnotes:

Yurchenko et al Comput Phys Commun 2016 202 262–275; Tennyson et al Comput Phys Commun 2004 163 85-116; Yurchenko et al A&A 2018 614 A131 , Pezzella et al Phys. Chem. Chem. Phys. 2021 23 16390–16400. Heays et al A&A 2017 602 A105, Chenel et al J. Chem. Phys. 2016 144 144306; Aguado et al Astrophys. J. 2017 838 33 .

l0pt Figure Acetone is one of the most abundant ketones in the atmosphere, with 73 million tonnes emitted or formed in the atmosphere annually. Additionally, as the simplest ketone, understanding the photodynamics of acetone can improve our understanding of the photolysis pathways of other ketones. CO formation from acetone photolysis was studied over the whole S₁ ← S₀ absorption spectrum,Jacob, L.S.D.; Lee, K.L.K.; Schmidt, T.W.; Nauta, K.; Kable, S.H. J. Chem. Phys. 2022, 156, 094303owever, this talk focuses on the longer wavelengths (305-320 nm). Resonance enhanced photoionisation (REMPI) and photofragment excitation (PHOFEX) of the CO photofragment at photolysis wavelengths longer than 300 nm, combined with laser-induced fluorescence (LIF) of acetone, found CO was forming from a unimolecular pathway, attributed to roaming. Although roaming is often associated with 'cold' rotational distributions, this does not seem to be the case for CO formed from roaming in acetone. The CO products had significant rotational excitation up to J  ∼ 80. Fourier transform infrared spectroscopy was used to obtain quantum yields of the photolysis products of acetone from 285 – 325 nm at various pressures of synthetic air and nitrogen bath gas (3-760 Torr total pressure). Carbon monoxide was found to have a quantum yield of up to 10% in non-oxidative conditions at 3 Torr and 760 Torr. In an atmosphere of synthetic air at actinic wavelengths, this pathway was reduced to a maximum of 3%.

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Jacob, L.S.D.; Lee, K.L.K.; Schmidt, T.W.; Nauta, K.; Kable, S.H. J. Chem. Phys. 2022, 156, 094303h

The spectroscopy and predissociation dynamics of several vibronic levels (v^′ = 0-6) of the SH A²Σ⁺ state have been studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. By measuring the product translational energy distributions as a function of excitation wavelength, the H + S(³P_J) photofragment yield (PFY) spectra are obtained across the various A²Σ⁺ ← X²Π bands. The PFY spectra of the A²Σ⁺ v^′ = 3-6 states exhibit broad linewidths ( > 4 cm⁻¹), indicating that these levels undergo rapid predissociation with lifetimes on the order of picosecond. The measured spin-orbit branching fractions of the H + S(³P_J=2,1,0) product channels provide insights to the predissociation dynamics of the A²Σ⁺ state, which change dramatically as the vibrational level v^′ increases. The A²Σ⁺ v^′ = 0 state of SH predissociates mainly through adiabatic coupling to the ⁴Σ⁻ repulsive state, while all three repulsive states (⁴Σ⁻, ²Σ⁻, and ⁴Π) are involved in the dissociation pathways for higher vibrational levels.