Session Z04: Focus Session: Ultrafast Non-adiabatic Dynamics in Molecules

New perspectives on electrocyclic photochemistry using ultrafast diffraction probes

Electrocyclic chemistry has been fascinating chemists for many decades due to the concerted formation and cleavage of multiple bonds and the resulting stereospecificity of electrocyclic reactions. They can proceed both via thermal and photochemical pathways with opposite stereospecificity as predicted by the famous Woodward-Hoffmann rules. The existence of photochemical pathways and their femtosecond timescales makes these reactions available for detailed studies using time-resolved methods. The availability of novel ultrafast experimental methods with direct structural sensitivity such as ultrafast electron diffraction and X-ray diffraction provides revolutionary insights into such reaction dynamics. Such methods allow for following the involved structural dynamics in real space and time on the level of individual atomic distance changes during the evolution from the reactant to the photoproduct.

We have investigated electrocyclic ring-opening reactions in a series of molecules sharing the structural backbone of 1,3-cyclohexadiene. In 1,3-cyclohexadiene, we follow the ring-opening reaction directly in space and time in the evolution of atomic pair distribution functions. Using the 1,3-cyclohexadiene derivative α-phellandrene, we confirm the Woodward-Hoffmann rules in real time by obtaining unambiguous signatures of a specific conformer of α-phellandrene evolving into the photoproduct isomer predicted by the Woodward-Hoffmann rules. In a further derivative,  α-terpinene, we observe the structural relaxation into the pericyclic minimum of the excited state, the photochemical “transition state” of the reaction in the vicinity of the conical intersection with the electronic ground state. The observed structural rearrangements suggest a new perspective on the origins of the stereoselectivity of electrocyclic reactions.   

Which form of the molecular Hamiltonian is the most suitable for simulating the nonadiabatic quantum dynamics at a conical intersection? [1]

Choosing an appropriate representation of the molecular Hamiltonian is one of the challenges faced by simulations of the nonadiabatic quantum dynamics around a conical intersection. The adiabatic, exact quasidiabatic, and strictly diabatic representations are exact and unitary transforms of each other, whereas the approximate quasidiabatic Hamiltonian ignores the residual nonadiabatic couplings in the exact quasidiabatic Hamiltonian. A rigorous numerical comparison of the four different representations is difficult because of the exceptional nature of systems where the four representations can be defined exactly and the necessity of an exceedingly accurate numerical algorithm that avoids mixing numerical errors with errors due to the different forms of the Hamiltonian. Using the quadratic Jahn-Teller model and high-order geometric integrators, we are able to perform this comparison and find that only the rarely employed exact quasidiabatic Hamiltonian yields nearly identical results to the benchmark results of the strictly diabatic Hamiltonian, which is not available in general. In this Jahn-Teller model and with the same Fourier grid, the commonly employed approximate quasidiabatic Hamiltonian led to inaccurate wavepacket dynamics, while the Hamiltonian in the adiabatic basis was the least accurate, due to the singular nonadiabatic couplings at the conical intersection. Finally, I will describe a rigorous procedure [2] for assessing the importance or residual nonadiabatic couplings neglected in approximate diabatization schemes.   

UV-induced Dissociation Dynamics of Bromoform Probed by Ultrafast Electron Diffraction

The UV photochemistry of bromoform significantly contributes to stratospheric ozone depletion. It is a major source of bromine radicals, which have an ozone destruction potential 100 times higher than that of chlorine atoms. After 267 nm excitation, multiple reaction pathways are energetically accessible. The dominant single-photon dissociation channel is C-Br bond fission, but questions remain regarding the transient intermediates participating in the process as well as potential alternative reaction channels. For example, HBr and Br₂ elimination have been reported, too, but largely attributed to multiphoton excitation. A recent femtosecond XUV transient absorption study found no evidence for a Br₂ species and, supported by theoretical calculations, it was concluded that the dominant reaction mechanism is direct dissociation of atomic Br.¹ However, an alternative roaming-like mechanism was previously reported in which a transitory iso-CHBr₃ is formed.² Here the UV-induced photodissociation of bromoform is investigated by ultrafast electron diffraction (UED). With an excitation ratio of only 0.3%, multiphoton processes are vanishingly small, and all results can be confidently assigned to single-photon channels. Dissociation trajectories based on structures retrieved from UED patterns spaced by as little as 50 fs time delays will be presented. Parallels and distinguishing features between experimental results and theoretically predicted trajectories will be discussed.

¹B. W. Toulson et al., Struct. Dyn. 6, 054304 (2019)

²A. S. Mereshchenko et al., Nat. Chem. 7, 562–568 (2015)   

Site Selective Coulomb Explosion Imaging of Excited State Structural Rearrangements in CS2

Structural imaging of transient excited-state species remains a key goal of molecular physics, promising to unveil rich information about the dynamics underpinning photochemical transformations. Here, we present such a study, employing site-selective ionization and time-resolved Coulomb explosion imaging to interrogate structural dynamics of ultraviolet (UV) excited carbon disulfide (CS₂). This prototypical system exhibits many of the fundamental motifs of ultrafast photochemistry, such as strong non-adiabtic coupling of several potential energy surfaces and motion along multiple nuclear coordinates. Substantial changes in the recorded three-dimensional ion momenta in the first several hundred femtoseconds following photoexcitation arise from Coulomb explosion of highly bent and stretched nuclear geometries transiently populated prior to photodissociation. Through examination of correlated ion momentum distributions and comparison to a simple classical model, a comprehensive picture of the photoinduced nuclear dynamics can be extracted, involving initial bending and stretching motion ultimately leading into photodissociation into S and CS photoproducts.   

X-ray induced Coulomb explosion images of toluene and its isomers

We present the progress of an experiment on the soft x-ray induced Coulomb explosion of toluene and other C7H8 isomers. Coulomb explosion imaging is a powerful technique that can intuitively link fragment ion momenta to the full three-dimensional molecular geometry. This technique is particularly suited to high-repetition-rate XFEL facilities and has proven effective when probing polyatomic molecules with planar symmetry and ”marker atoms” with high x-ray absorption cross sections that help define a fragmentation within the molecular plane [1]. We show that Coulomb explosion imaging can also be extended to the aforementioned pure hydrocarbons, some of which are not planar, and note that the kinetic energies and angular distributions of the ensuing fragments vary depending on the cyclic or acyclic geometries of the molecule. We additionally show promising results employing time-resolved Coulomb explosion imaging to observe ultrafast ring reconfiguration of toluene upon strong-field ionization.

[1] R. Boll., et al. X-ray multiphoton-induced Coulomb explosion images complex single molecules. Nat. Phys. 18, 423–428 (2022).

Investigating the UV-induced isomerization of Norbornadiene to Quadricyclane using ultrafast X-ray scattering

The UV-induced isomerization of norbornadiene (NB) to quadricyclane (QC) is investigated using femtosecond gas-phase X-ray (16 keV) scattering. The 200-nm UV excites NB to the 3p Rydberg state, from where it isomerizes to QD via a conical intersection within sub-100-fs. Our preliminary results indicate that subsequent to the isomerization, which can be resolved due to the short pulse durations of both UV and X-ray pulses, the molecule is likely to dissociate into smaller products. Time-resolved identification of the isomerization and dissociation products and involved transient intermediates will elucidate this photochemical reaction. These findings are supported by ab initio calculations.   

XUV-IR pump-probe dissociative ionization of CO2

We theoretically investigated strong-field XUV pump- IR probe dissociative ionization in full dimensionality by propagating the coupled nuclear motion on the A²Π_u, B²Σ_u⁺, C²Σ_g⁺, a⁴Σ_g^-, and b⁴Π_u of CO₂⁺ in full (3D) dimensionality. Our ab initio calculations are based on adiabatic Born-Oppenheimer (BO) potential-energy surfaces and non-adiabatic, laser-dipole, and fine-structure couplings between BO states we obtained by applying the multi-configurational self-consistent-field quantum-chemistry code GAMESS. This allowed us to provide kinetic energy release (KER) spectra for dissociation into the in the O(³P_g) + CO⁺( X²Σ⁺) and O⁺(⁴S_u) + CO(X¹Σ⁺) channels, the relative importance of the mentioned couplings, and branching ratios for O⁺ versus CO⁺ yields for different laser parameters and initial molecular orientations to the linearly polarized laser electric field. Our KER spectra reveal the ro-vibrational excitations of CO⁺ fragments along a dominant 3ω dissociation paths. Addressing the nuclear dynamics near the conically intersecting A²Π_u and B²Σ_u⁺ states, we reproduce the core-hole oscillation period 115 fs in good agreement with the experiment of Timmers et al., Phys. Rev. Lett. 113, 113003 (2014). In addition, we predict and interpret as due to quantum beats between specific vibration and electronic states slower 62 fs oscillation.   

Excited State Nonadiabatic Dynamics Of o-Nitrophenol: Theoretical Insight into Pump-probe Experiments and Dissociation Mechanisms

Photoexcitation of o-nitrophenol is known to lead to many subsequent nonadiabatic events, such as internal conversion, intersystem crossing, excited state intramolecular proton transfer and dissociation to form HONO, an important atmospheric species. In this work, time-resolved photoionization and time-resolved photoelectron spectra were taken by pumping o-nitrophenol at 255 nm (4.9 eV) and probing at 153 nm (8.1 eV). The measurements revealed a time dependent ion fragment signal of mass 109 amu. Based on our computations this signal is attributed to the formation of the 2-hydroxyphenoxy ion and NO. Most previous experimental time resolved studies of o-nitrophenol excited the molecule to the first excited singlet state (S₁). In this work, however, at 4.9 eV, o-nitrophenol is pumped to the fourth excited singlet state (S₄) which allows the observation of the dissociation channel leading to 2-hydroxyphenoxy and NO as the dominant one. Electronic structure calculations were used to interpret the experiments and provide insight into the dynamics. Calculations based on multireference perturbation theory (CASPT2) of the potential energy surfaces created by linear interpolation of internal coordinates along the reaction coordinate of NO dissociation through a bicyclic, spiro like, intermediate showed that the photodissociation is most likely to occur on the first excited triplet state surface (T₁). Therefore, the findings provide strong evidence for internal conversion, intersystem crossing and NO formation following photoexcitation to S₄. The combination of photoelectron and photoion spectroscopy, together with computational results, provides robust evidence of intersystem crossing that is difficult to establish with only a single technique.