Highlights from Faraday Discussion 228: time resolved imaging of photo-induced dynamics

Abstract

Summary of recent Faraday Discussions meeting (2021, vol. 228) on time resolved imaging of photo-induced dynamics. The meeting, originally scheduled to take place at IIT Bombay, took place online.

On 1–4 February 2021, 154 delegates from 21 countries gathered online to discuss developments in time resolved imaging of photo-induced dynamics (DOI: 10.1039/D1FD90034J). In this rapidly evolving area, Faraday Discussions provide an ideal forum for open-ended discussion, frank exchange of ideas, and considered critique of cutting-edge work. In the years since the closely related Faraday Discussion on Ultrafast Imaging of Photochemical Dynamics (2016, vol. 194, DOI: 10.1039/C6FD90074G; 10.1039/C6CC90504H), impressive advances have been made in experiments that utilise ultrashort laser, X-ray and electron pulses. The developments in new sources are particularly striking, with several X-ray free-electron lasers (XFELs) coming into operation including the European XFEL (Germany) and the SwissFEL (Switzerland). These offer ever more advanced opportunities to take snapshots of electronic and nuclear motion in matter with unprecedented spatial and temporal resolution.

The meeting took off with a welcome and introduction from the co-chairs of the conference, Dr Adam Kirrander (Edinburgh, UK) and Prof. Gopal Dixit (Mumbai, India), on behalf of the scientific committee: Dr Russell Minns (Southampton, UK), Prof. Taisia Gorkhover (Hamburg, Germany), Prof. Francesca Calegari (Hamburg, Germany), and Prof. Olga Smirnova (Berlin, Germany). The introduction provided an overview of the Faraday Discussion, followed by an expression of the Chairs’ hopes for the discussion and conference as a whole. Originally the meeting was scheduled to take place at IIT Bombay in India, and co-chair Gopal Dixit included a brief presentation about the intended venue and Mumbai, while representatives from the Royal Society of Chemistry explained the practicalities.

The scientific part of the meeting started with the much awaited Spiers memorial introductory lecture delivered by Prof. Toshinori Suzuki (Kyoto, Japan, DOI: 10.1039/D1FD00015B). Prof. Suzuki's lecture covered different experimental aspects of probing and imaging time resolved dynamics in matter as well as important theoretical developments (see Fig. 1). Various experimental methods provide complementary information during photo-induced electronic and structural dynamics, he emphasized. Examples discussed during the talk included strong-field induced charge migration in iodoacetylene, ring-opening of 1,3-cyclohexadiene probed by time resolved X-ray spectroscopy and diffraction methods, time resolved photoelectron spectroscopy of pyrazine, and the role of inelastic scattering during ultrafast electron diffraction from pyridine. Moreover, photo-induced processes in liquid and crystals were also presented, e.g. photo-induced dynamics in photoactive yellow protein probed by X-ray diffraction at XFELs, and transient X-ray and infrared absorption spectroscopy probes of ligand-to-metal charge transfer in the solution phase.

Fig. 1 Schematic of rapid decay via conical intersection from the Spier's memorial lecture. Reproduced from Fig. 1 in DOI: 10.1039/D1FD00015B.

Session 1: time resolved diffraction

Following the introductory lecture, the first session was chaired by Prof. Gopal Dixit (Mumbai, India). Prof. Martin Centurion (Nebraska, USA) gave the first talk where he discussed intense infrared pulse triggered fragmentation of molecules (DOI: 10.1039/D0FD00125B). The strong-field induced ionic products were analysed using time of flight mass spectroscopy coincident in momentum space, which provided the relative yield of various ionic products. Importantly, the structures of different ionic products were obtained by ultrafast electron diffraction. It was emphasized that the independent atom model was not appropriate for simulating ultrafast electron diffraction due to non-availability of diffraction data for ionic species. It was shown that experimental diffraction signals were in excellent agreement with ab initio diffraction simulations. Issues related to strong-field-induced alignment, laser polarization, and differentiating similar species like tropylium and benzylium were also discussed.

Then another talk was given by Prof. Peter Weber (Brown, USA) where he presented a general method to extract transient molecular structures in excited molecules with sub-Ångstrom spatial resolution from ultrafast X-ray scattering data (DOI: 10.1039/D0FD00118J). Experimental results for the molecules N-methylmorpholine measured at the Linac Coherent Light Source (LCLS) were discussed. It was demonstrated that the method is robust as long as the database of structures used in the analysis is sufficiently large and dense in the vicinity of the sought structure so that good matches to the experimentally observed scattering patterns can be obtained. It was also discussed that the vibrational state distributions play no role within the present detection limits. It was anticipated that the method should also be applicable to ultrafast electron diffraction data. The talk stimulated intense discussion of the possibility of employing machine learning methods such as deep neural networks and artificial intelligence-based methods to automatize the determination of structures from measured diffraction data.

When a molecule is excited by a pump pulse, it is essential to understand what processes take place in the different excited states. Dr Adi Natan (SLAC, USA) presented experimental results on a detailed understanding of multiphoton processes in iodine using time resolved X-ray scattering to address such issues (DOI: 10.1039/D0FD00126K). His analysis disentangled different-order multiphoton processes via the anisotropy of the time resolved X-ray scattering. Legendre decomposition method and Fourier analysis were employed to analyse anisotropic components up to 8th-order of the diffraction data, allowing different dissociation channels and vibrational modes to be resolved. Moreover, velocities corresponding to different dissociation pathways were assigned. The feasibility to image the build-up of different orders of anisotropies during pump pulse were discussed.

Not only experimentalist, but also theoreticians presented exciting work. Prof. Kenneth Lopata (Louisiana, USA) presented the possibility to capture static holes in core electronic states and ring currents using time resolved X-ray scattering (DOI: 10.1039/D0FD00124D). How the scattering signal is sensitive to the core hole in a molecule and the position of the core hole at different atoms within the molecule was discussed, with the oxazole molecule used to demonstrate the idea of mapping the core hole. The scattering signals corresponding to core holes at different carbon atoms within oxazole were distinct. Moreover, the contributions of elastic and inelastic signals to the total signals were also shown. It was anticipated that time-dependent ring currents in an excited molecule would be essential to capture the electron dynamics well before ring-opening reactions, where electronic ring currents have a potential role in bond softening.

Along the same direction, Prof. Jean Christophe Tremblay (Metz, France) presented the possibility of imaging light-induced charge migration and subsequently ring currents in magnesium porphyrin (DOI: 10.1039/D0FD00116C). The role of electron–electron correlation and non-local pure dephasing on the light-induced charge migration was presented. It was pointed out that correlation-mediated changes in the charge migration lead to significant changes in ring current and consequently on the induced magnetic field. The time-dependent Liouville-von Neumann approach was employed to simulate the electron dynamics, and a Kossakowski-type model was used to capture the effect of vibrationally mediated dephasing of the electronic motion. The transient electronic flux density was imaged by time resolved X-ray scattering. Even after the pump pulse, presence of coherences scattering signal was interesting despite the dephasing due to the dissipative environment. It was anticipated that magnesium porphyrin has the potential for light-induced molecular magnets. Participants have discussed the feasibility of performing circular dichroism measurement to probe ring current in molecules as time-evolving charge distribution is accompanied by the electronic current obeying the continuity equation.

X-ray pulses from XFELs are ultrashort and ultra-intense, which causes severe damage to the molecule under experiment via processes like photoionization, Auger decay, and other secondary processes. Prof. Linda Young (Argonne, USA) presented work that explored the relative contributions of coherent and incoherent scattering signals at different energies and pulse durations (DOI: 10.1039/D0FD00106F). The molecule 1,3-cyclohexadiene was used to explore the sensitivity of molecular bonding and electron correlation to the simulated scattering signal using the independent atom model, Hartree–Fock, and density functional theory to simulate signals (see Fig. 2). As a consequence of multiphoton absorption during scattering processes, the possibility of imaging intramolecular charge transfer and dissociation within CH₃I molecules was also explored. Ultrashort X-ray pulses carry unavoidable bandwidth due to the energy-time uncertainty. How to extract useful signals from strong background signals was debated. Overall Session 1 had lively discussions (DOI: 10.1039/D1FD90023D) that were recorded by Royal Society of Chemistry staff.

Fig. 2 Schematic of intense X-ray scattering. Reproduced from Fig. 1 in DOI: 10.1039/D0FD00106F.

Session 2: ultrafast X-ray science

The first session on the second day of the conference was focussed on ultrafast X-ray science and was chaired by Dr Adam Kirrander (Edinburgh, UK). Prof. Jonathan Marangos (Imperial, UK) presented the first talk on ghost imaging in the spectral domain (DOI: 10.1039/D0FD00122H). This linked nicely to the presentations the previous day. Within the time resolved pump–probe configuration, spectral absorption of X-ray photons was observed. It is known that X-ray pulses from XFELs are stochastic in nature and carry shot-to-shot noise as they are generated via the SASE process. Spectral correlation in the fluctuation between shot-to-shot is a key element in ghost imaging. A soft X-ray pulse was used for sudden ionization in isopropanol, and the photoelectron kinetic energy was analysed as a function of pump–probe delay. Moreover, the capability to disentangle the overlapping signal stemming from pump–probe pulses was discussed, which was helpful to access the strength of transitions such as 1s → π* in the oxygen atom of isopropanol in a time resolved fashion.

The second talk of this session was focussed on extracting meaningful signals from fluctuating pulses of XFELs and was presented by Prof. Jan M. Rost from Dresden, Germany (DOI: 10.1039/D0FD00117A). A deep neural network was employed to purify the fluctuating spectra corresponding to the non-linear two-photon process, which is a process sensitive to the character of an ionizing pulse. A sufficient training set of photoelectron spectra stemming from fluctuating pulses was obtained by solving a one-dimensional time-dependent Schrödinger equation and training the model with a combination of random Hamilton matrices. The effects of chirp and partial coherence in fluctuating pulses were included. Emphasis on the form of synthetic Hamilton matrices was discussed.

Prof. Thomas Pfeifer from Heidelberg Germany presented the experimental realization of non-linear two-photon spectroscopy at a free-electron laser (DOI: 10.1039/D0FD00107D). Transient absorption spectroscopy was used to probe non-resonant ionization dynamics in neon. Dynamics stemming from strong coupling among resonant bound states in doubly charged neon were presented. Moreover, XUV-initiated site-specific dissociation dynamics in oxygen and diiodomethane were also probed using transient absorption spectroscopy with high spectral and temporal resolutions. It is anticipated that the presented experimental method will be useful for other non-linear coherent spectroscopy schemes in the XUV and X-ray spectral range.

The second session on ultrafast X-ray science was held the next day and was chaired by Prof. Linda Young (Argonne, USA). The first presentation, by Prof. Daniel Neumark (Berkeley, USA), demonstrated transient wave-mixing spectroscopy to probe decay dynamics of highly excited 3s Rydberg states using an attosecond XUV pulse (DOI: 10.1039/D0FD00113A). An XUV pulse was used to create doubly charged O₂, and two IR pulses were used for wave-mixing (see Fig. 3). Analysis of experimental results and input from quantum dynamical simulation concluded that the lifetime of vibrational states varied greatly across the manifold of excited electronic states. The lifetimes are dictated by electronic autoionization, and the rate of autoionization critically relates to the internuclear distance between two ionic decay channels. In addition, the rates are sensitive to the location on the potential with respect to decay pathways. The talk concluded that an intricate interplay of vibrational dynamics, autoionization, and tunnelling affects decay dynamics.

Fig. 3 Schematic of transient wave-mixing in O₂. Reproduced from the graphical abstract image in DOI: 10.1039/D0FD00113A.

Dr Thomas Wolf (SLAC, USA) presented the next talk in this session (DOI: 10.1039/D0FD00112K). Resonant Auger spectroscopy was used to probe excited-state dynamics in thymine, where an ultraviolet pulse was used to excite ππ* transition and probed by X-ray pulse. Internal conversion leads to ππ* excitation to n–π* excitation, which was in resonance with the oxygen K-edge. Analysis of experimental results leads to a distinction between spectator and contributor decay pathways. Also, absorption spectra lead to the presence of intersystem crossing on the picosecond timescale. This talk also leads to an intriguing historical prospect of Auger spectroscopy. Participants mentioned that Lise Meitner was the first to discuss the same decay concept, which was reported by Pierre Auger later, and therefore it should be known as Auger–Meitner spectroscopy.

Dr Ruaridh Forbes (SLAC, USA) gave the last talk of this session, and he presented results from an experiment performed at SACLA XFEL in Japan (DOI: 10.1039/D0FD00115E). An intense IR pulse was used for strong-field ionizing of methyl iodide, and an XUV pulse was used to probe the charge transfer process following multiple ionization. Various fragment ions were collected in velocity map imaging at different delay times between IR and XUV pulses. Covariance mapping in the recoil frame established a correlation between the momenta of various fragment ions in a time resolved manner. Understanding about charge transfer, dissociation, and ionization processes, was obtained by analysing kinetic energy distributions of different iodine ions at various pump–probe delay times. The classical over-the-barrier model was used to get deep insight into fragmentation dynamics. The highly interesting discussion of all the papers in the ultrafast X-ray science session was recorded (DOI: 10.1039/D1FD90026A).

Session 3: time resolved ultrafast spectroscopy

Dr Russell Minns (Southampton, UK) chaired the first part of the session. The first talk in this session was unique as it was jointly presented by Prof. Albert Stolow and Prof. Varun Makhija (Ottawa, Canada). This talk consisted of a conversation between the two scientists discussing making a molecular movie of dissociation dynamics in NO₂ (DOI: 10.1039/D0FD00128G). Their paper used time resolved photoelectron spectra, with a VUV pulse launching a nonadiabatically coupled vibronic wavepacket that are probed by UV pulses. Nonadiabatic coupling between Rydberg-valence states and between Rydberg–Rydberg states leads to predissociation and the formation of fragments. Time-windowed Fourier transforms were employed to extract the coherences between the coupled states. An anharmonic coupling between vibrational modes leads to intramolecular vibrational energy redistribution, and a “standard” normal mode picture of vibrational coupling was not adequate to describe the experiment.

Next, Prof. Oleg Kornilov (Berlin, Germany) presented interesting results on time resolved photoelectron spectroscopy of methyl orange and metanil yellow in the solution phase (DOI: 10.1039/D0FD00111B). Relaxation dynamics via dark states in both the molecules were followed when the molecule was photoexcited by visible light, and an XUV pulse was used for ionization from the photoexcited molecule. With the help of theoretical simulations, using time-dependent density functional theory and the surface hopping method, the role of intramolecular charge transfer state was established. By comparing the experimental binding energies of excited states with the simulated ones, the suitability of different kinds of exchange–correlation functional within time-dependent density functional theory was discussed.

Prof. Siva Rama Krishnan (Chennai, India) gave the third talk on probing state-selective photoelectron dynamics (DOI: 10.1039/D0FD00120A). An XUV pulse was used to trigger the coupled electron-nuclear dynamics in acetylene. The photo-induced acetylene–vinylidene system has been proven as a benchmark system to learn about proton migration and coupled electron-nuclear dynamics around the conical intersection. Dissociation and isomerization pathways were identified by analysing energy spectra and angular distribution in coincidence photoelectron–photoion spectroscopy using velocity map imaging. It was emphasized that the employed experimental setup probes strong and relatively weak ionization pathways and state-specific photo-induced dynamics. Analysis of photoelectron asymmetry parameters helped to identify electronic states in the cation, which was crucial to understanding the dynamics.

After a short break, Prof. Spiridoula Matsika (Pennsylvania, USA) discussed theoretical findings on nonadiabatic excited-state dynamics in uracil (DOI: 10.1039/D0FD00110D). The role of dynamic electron correlation in the dynamics, using different electronic structure methods, was explored in detail. Potential energy surfaces for excited-state dynamics were simulated using different methods such as TDDFT, CASCF, MRCIS, and XMS-CASPT2. Surface hopping was used for nuclear dynamics. It was found that the dynamics were very sensitive to the choice of electronic structure method. A correlated method like MRCIS precited the important role of the S₂ state, whereas other “lower-level” methods provided the importance to the S₁ state. Future experiments were anticipated to settle the dust about employing adequate theoretical methods to discuss nonadiabatic dynamics in uracil.

Dr Marco Ruberti (Imperial, UK) discussed a state-of-the-art ab initio theoretical approach to describe many-body quantum ionic coherences extracted from transient absorption spectroscopy on the attosecond timescale (DOI: 10.1039/D0FD00104J). Interaction of XUV as a pump pulse and X-ray as a probe with pyrazine was simulated. A time-dependent B-spline restricted correlation space-algebraic diagrammatic construction method was employed for molecular photoionization. The reduced ionic density matrix of the photoion–photoelectron system was calculated to simulate attosecond charge dynamics. Identifications of crucial coherent channels during complex many-electron dynamics induced by the pump pulse were performed. Also, the widely used sudden ionization concept in photoionization in low photon energy was found to be inappropriate.

The last talk of this session was presented by Prof. Majed Chergui (Lausanne, Switzerland) where he discussed the dissociation and recombination of ligands in nitrosylmyoglobin using femtosecond X-ray emission spectroscopy (DOI: 10.1039/D0FD00131G). It was shown that more than one species is formed in photo-induced nitrosylmyoglobin, as evident from the analysis of femtosecond X-ray absorption spectra. It was conveyed that these findings settle the longstanding issue of thermal versus electronic effects in the formation of transient structures where haem porphyrin transits to high-spin configuration and pentacoordinate iron moves out-of-plane. It is expected that these findings will be crucial for understanding spin cross-over dynamics in haem proteins. The animated and probing discussion of all the papers in this session can be found in (DOI: 10.1039/D1FD90024B).

Session 4: strong-field physics

Prof. Francesca Calegari (Hamburg, Germany) chaired the strong-field physics session on the third day. Dr Alicia Palacios (Madrid, Spain) gave the first talk where she presented a theoretical XUV pump-XUV probe study on glycine, with an interest in the role of nuclear motion on electronic coherences (DOI: 10.1039/D0FD00121J). In the simulation, an XUV pulse launches an electronic wavepacket by sudden ionization in glycine. The triggered electronic wavepacket evolves under the influence of nuclear motion, which is probed by another XUV pulse. It was shown that electronic wavepacket could be tailored by tuning the central frequency of the ionizing XUV pulse, which leads to the desired fragmentation of glycine. Moreover, few fragmentation channels encode the information about the early dynamics of the wavepacket. The cation dynamics were probed as a function of pump–probe delay time. Special emphasis has been made on the careful and balanced treatment of electronic wavefunction in the continuum and the role of non-adiabatic couplings among various electronic states of the cation. It was envisioned that the simultaneous detection of photoelectron and photo-ions in a time resolved manner would be the appropriate experimental approach to probe the simulated dynamics.

Prof. Fernando Martín (Madrid, Spain) gave the next talk, which focussed on controlling angular distributions of photoelectrons in H₂ (DOI: 10.1039/D0FD00114G). This theory talk demonstrated how the parameters of the laser pulses could be tuned to achieve control over the emission direction of the photoelectrons. An XUV pulse was used as a pump pulse to trigger ionization and an IR pulse was used to probe photoelectrons. The onset of the nuclear motion with probe IR pulse can break the inversion symmetry in H₂, which leads to the asymmetric angular distribution of photoelectrons in the molecular frame, something that led to animated discussions among the theoreticians in the audience.

Of course, not only XUV pulses can ionize atoms or molecules, but IR pulses can also trigger ionization via strong-field ionization. So while Dr Alicia Palacios and Prof. Fernando Martin discussed the results of XUV-induced ionization, Dr Andrew Maxwell (UCL, UK) presented a theoretical analysis of strong IR pulse-induced ionization and the effect of the polarisation of the IR pulse on the ionization process (DOI: 10.1039/D0FD00105H). He discussed the conditions for interferences to yield electron vortices with nonzero orbital angular momentum. An alternative explanation was provided for the situation when an atom is ionized by a pair of intense two counter-rotating circularly polarized light fields. Results from the strong-field approximation, time-dependent density functional theory, and the R-matrix method were found to be in good agreement. Several possibilities to measure the orbital angular momentum of electrons directly or indirectly were discussed in detail.

After a break, the second part of this session resumed with three experimental talks. Ultraviolet light-induced dissociation in the molecule OCS studied using time resolved ion-momentum spectroscopy was presented by Prof. Jochen Küpper (Hamburg, Germany, DOI: 10.1039/D0FD00119H). The induced dissociation was probed by the strong-field ionization, which yields CO and S fragmentation. Quantum simulations based on wavepacket dynamics were found to support the experiment findings. It was anticipated that further experimental and theoretical work is needed to have an adequate understanding of observed fast oscillations during the dissociation process.

Strong-field mediated hydrogen migration and bond formation in CH₃OH using velocity map imaging were presented by Dr Rituparna Das (Ahmadabad, India, DOI: 10.1039/D0FD00129E). By tuning the wavelength, pulse duration, intensity, and polarisation of the laser, she was able to control strong-field mediated processes. Several dissociation pathways were observed, which were in turn supported by quantum chemistry calculations. Possible mechanisms for the formation of H₂⁺ and H₃⁺ ions from strong-field ionized CH₃OH was also discussed.

In the final talk of this session, Prof. Nina Rohringer (Hamburg, Germany) discussed the results of X-ray optical wave mixing to image valence charge density (DOI: 10.1039/D0FD00130A). This related to an extent with discussions about time resolved X-ray diffraction on the first day, since it is challenging to extract information about the valence electron density from diffraction. Prof. Rohringer presented a theoretical description of X-ray domain wave mixing based on a quantum electrodynamics framework. She has also related her findings to a controversy regarding previous results on X-ray parametric down-conversion. During the discussion, recorded in full for all papers in this session in (DOI: 10.1039/D1FD90025K), Gopal Dixit highlighted the possibility to image valence charge density using X-ray diffraction with a low-pass Fourier method.

Poster sessions

Two poster sessions were conducted during the conference to accommodate the large number of registered posters. Both poster sessions were very engaging and the discussions around them were lively. The best student poster was awarded a prize, as judged by a panel led by Gopal Dixit (Mumbai, India), ably assisted by Russel Minns (Southampton, UK), Jochen Küpper (Hamburg, Germany), Kenneth Lopata (Louisiana USA), Francesca Calegari (Hamburg, Germany), Jean Christophe Tremblay (Metz, France), Adi Natan (SALC, USA), Jon Marangos (Imperial, UK) and Adam Kirrander (Edinburg, UK). The panel awarded a joint best-poster award to Nicola Mayer (Berlin, Germany) and Maria Elena Castellani (Durham, UK) for their posters on probing Rydberg states in non-collinear bicircular high-harmonic generation via the spin–orbit coupling, and probing the ultrafast dynamics of the isolated adenosine-5-triphosphate dianion through time resolved photoelectron imaging, respectively.

Concluding remarks

Following the rich tradition of Faraday Discussions, the meeting wrapped up with a final session focussed on the concluding remarks on the fourth day. Prof. Misha Ivanov (Berlin, Germany) had kindly taken on the challenging task to summarize the entire meeting succinctly and provided an interesting outlook towards making a molecular movie in the realm of quantum mechanics (DOI: 10.1039/D1FD90033A). Prof. Taisia Gorkhover (SLAC, USA) chaired this last session. During the concluding remark, Prof. Ivanov successfully managed to bring together the many themes running throughout the conference, and also pointed to other potential players in the area of molecular movie making, such as laser-induced electron diffraction and high harmonic generation spectroscopy. He noted that by analysing highly nonlinear processes within a single laser cycle, strong-field techniques can overcome the limitations imposed by the pulse duration in making the movie (see Fig. 4). How lovely it would be to have a molecular movie in a true quantum sense by visualizing how entanglement between different coupled electronic-nuclear degrees of freedom develops during photo-induced dynamics, he concluded.

Fig. 4 Chiral current generated by coherent electronic excitation of a chiral molecule. Image courtesy of A. Ordonez, D. Ayuso and O. Smirnova.

Acknowledgements

G. D. acknowledges support from the Ramanujan fellowship (SB/S2/RJN-152/2015). A. K. acknowledges support from the Leverhulme Trust (RPG-2020-208) and the EPSRC (EP/V006819/1 and EP/V049240/1).

---