Modern laser sources facilitate a wide range of experimental strategies for interrogating the complex non-adiabatic dynamics operating in the excited states of molecules. Developing detailed insight into such processes is vital in understanding various fundamental processes of biological, environmental, and technological significance. Measurements may be broadly separated into frequency- and time-resolved variants, with a combination of different approaches (with different associated observables) typically being required to reveal a complete mechanistic picture. In the former category, quantum state-resolved information may often be obtained using narrow linewidth lasers. This provides detailed information relating to the starting point on the photochemical reaction coordinate (via the absorption spectrum) and the asymptotic endpoints (i.e. the photoproducts). No direct observation of the intermediate pathways connecting these two limits is generally possible, though, due to the inherently long temporal duration of the laser pulses relative to the typical timescales of non-adiabatic energy redistribution processes. It is therefore desirable to obtain complementary information that monitors real-time evolution along the reaction coordinate as excited state population traverses the potential energy landscape. This may be achieved in time-resolved pump-probe experiments conducted using laser pulses with temporal durations comparable to the ultrafast (i.e. sub-picosecond) timescales of vibrational motion. The use of valence state photoionization for the probe step is a commonly employed methodology and has proved highly instructive in revealing subtle mechanistic details of key energy redistribution pathways operating in a wide range of molecular systems. One common limitation in such measurements, however, is the restricted “view” along the reaction coordinate(s) connecting the initially prepared excited states to various photoproducts. Guided by examples drawn from our own recent work using time-resolved photoelectron imaging, this talk will discuss such issues in detail and highlight some new directions that potentially help overcome them – with particular emphasis placed on the advantages of projecting as deeply as possible into the ionization continuum. The role of complementary measurements using other spectroscopic techniques and the importance of high-level supporting theory to guide data interpretation will also be reinforced.

l0pt Figure In gas-phase experiments using molecular beams, formation of many isomers cannot be prevented and their presence significantly complicates assignment of spectral lines. Current isomer-resolved spectroscopy techniques make use of elaborate double-resonance schemes, requiring at least two fully tuneable laser sources. We present here an alternative approach that utilises electrostatic deflection to spatial separate isomers and create isomer-pure molecular beams. This adds isomer resolution to conventional single-color REMPI spectroscopy, which we demonstrate here for the syn and anti conformers of 3-aminophenol, as shown in Figure 1. This approach furthermore makes the assignment of all transitions to an isomer trivial, without any additional a priori information. This approach can add isomer resolution to any molecular beam based spectroscopy experiment. We show here also the first application of this methodology to study ultrafast dynamics and present the first results of fully isomer-resolved dynamics. In particular, we show that the syn and anti conformers of 2-chlorophenol exhibit very different relaxation dynamics following UV excitation, highlighting the influence of a single hydrogen bond on the underlying ultrafast relaxation processes. Our approach is generally applicable to all isomers that exhibit a difference in dipole moment and will, for example, allow the study of tautomer-resolved dynamics in biomolecules.

Many important physical processes such as non-linear optics and coherent control are highly sensitive to the absolute carrier-envelop-phase (CEP) of driving ultrashort laser pulses. A significant amount of previous theory work has been carried out to study the effect of the absolute CEP on strong-field ionization and related phenomena such as high harmonic generation (HHG) and nonsequential double ionization (NSDI). This makes the measurement of absolute CEP in the photoionization process immensely important in attosecond and strong-field physics. Even though relative CEPs can be measured with a few existing methods, the estimate of the absolute CEP has not been straightforward and has always required theoretical inputs. Recently, we have developed an in-situ method for measuring the absolute CEP of elliptical polarized few-cycle pulse without the assistance of theoretical modelings. Here we will show that the absolute CEP of linear polarized light can also be measured with a similar method. This capability enables the measurement of absolute-phase-resolved strong field ionization for the first time. We are able to compare the experimental results directly with those obtained with numerical solutions of time-dependent Schrodinger equations (TDSE). Preliminary results suggest the TDSE method might have issues in modeling strong field multi-electron dynamics, which have been routinely carried out to help understand the dynamics or calibrate CEP measurement. This failure could be due to the employed single active electron approximation and warrants further investigation. The results of this study will provide theorists with a clear standard for studying strong-field ionization processes in atoms and molecules and will lead to independent experimental measurements of the absolute phase.

To achieve an efficient 3-D imaging detection of electrons/ions in coincidence, a conventional 2D imaging detector (MCP/phosphor screen) and a fast frame camera are used in the 3D velocity map imaging (VMI) technique[1, 2] . However, it is still difficult to obtain two separate TOF events for two electrons using a conventional MCP detector coupled with a photomultiplier tube (PMT). This is because the phosphor screen is usually made of P47 phosphor which has longer decay time and thus not good to achieve high temporal resolution. Furthermore, due to the very short time separation interval between two electrons, it is imperative to use different phosphor/scintillator for improved 3D electron momentum imaging. Herein, we demonstrate that a scintillator screen coated with poly-para-phenylene laser dye (Exalite 404) can be used to achieve a greatly improved TOF resolution, which is sufficienct for 3D electron imaging.. A silicon photomultiplier tube (si-PMT) is also adopted to suppress the ringing in electric signals, typically associated with MCP pick-off.. The shorter emission lifetime of the poly-paraphenylene dye compared to the conventional P47 phosphor helps achieve an unprecedented dead time ( 0.48 ns). This has greatly enhanced the multi-hit capability of the 3D VMI technique in detecting two or more electrons in coincidence.

Small silver clusters possess remarkable luminescence and photoelectric properties, making them subject of current research.Grandjean, D. et al. Science 2018, 361, 686–690.owever, obtaining vibrations on small, neutral silver clusters remain challenging, due to difficulties in mass-selecting neutral clusters and a lack of easily accessible and widely wavelength-tunable far infrared light sources. Here, we report our study on experimentally probing the vibrational wave packet dynamics on the ground state potential energy surface of the neutral silver tetramer Ag₄, a benchmark system for small neutral metal clusters, and unambiguously assign its structure. We combine femtosecond pump-probe spectroscopy employing the NeNePo (negative-neutral-positive) excitation schemeWolf, S. et al., Phys. Rev.Lett. 74(21), 4177; Hess, H. et al., Eur. Phys. J. D, 16(1), 145-149.ith a cryogenic ion-trap tandem mass spectrometer. A linear polarized ultrafast pump pulse ( ∼ 40 fs, tunable center wavelength from 700 nm - 820 nm) is used to selectively prepare a coherent wave packet by photodetachment from thermalized (20 - 300 K) Ag₄⁻ anions. The wave packet dynamics on the electronic ground state are then probed using a second polarized ultrafast pulse ( ∼ 50 fs, centered at 400 nm), which ionizes Ag₄ in a two-photon process. The mass-selected cation yield as a function of the delay time (0 - 60 ps) between the two laser pulses yields the fs-NeNePo spectrum. Frequency analysis with a resolution down to about 0.5 cm⁻¹ by using Fourier transform of transient traces reveal one prime frequency band (109.5 ± 0.4 cm⁻¹) in all conditions and four bands at 32 cm⁻¹, 78 cm⁻¹, 186 cm⁻¹ and 295 cm⁻¹ dependent on pump wavelengths and temperatures. These frequencies are consists with predicted fundamental vibration frequencies (ν₁, ν₂, ν₅ and ν₆) and one combination (ν₁ + ν₂) for rhombic D_2h geometry of Ag₄. The rephrasing period of the wave packet allows determining vibrational anharmonicities. A strong dependence of the NeNePo cation signal on the polarization of ultrafast pulses is observed, revealing information on the anisotropy of the partial waves involved in the photodetachment process.

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Footnotes:

Grandjean, D. et al. Science 2018, 361, 686–690.H Wolf, S. et al., Phys. Rev.Lett. 74(21), 4177; Hess, H. et al., Eur. Phys. J. D, 16(1), 145-149.w

Dissociation of organic halides has been use for studying ultrafast processes over the last three decades given their relative simplicity and the significance in atmospheric chemistry. Specifically, photofragmentation of alkyl bromides with UV light has attracted substantial attention because of the ozone depletion potential of Br atoms. This presentation summarizes our recent results on the ultrafast photodissociation mechanisms of n-butyl bromide resolved using femtosecond time-resolved mass spectrometry. Multiple dissociative pathways occur upon photo excitation of n-butyl bromide include C-Br scission, C-C dissociation, and hydrogen elimination leading to unsaturated carbon bonds. The dissociative A state is accessed via two UV photon adsorption of two UV pump photons. This state undergoes direct dissociation of the C-Br bond within  160 fs. Three photon excitation reaches the n-5p Rydberg state, where several competing fragmentation pathways are monitored. The fastest relaxations occur in states which are highly excited and have C-H dissociation leading to double and triple C-C bond formation with lifetimes of  500 fs. Dissociation on the ion-pair state occurs within 10 ps to produce the butyl radical. Additionally, β elimination of HBr from the parent molecule occurs within 4 ps. The depopulation of the 5p Rydberg state through internal conversion activates vibrations along the carbon backbone and produces an the intermediate (bromopropyl radical) within 600 fs. The bromopropyl radical undergoes a concerted ring-closure and Br elimination into highly stable cyclopropane within 7.5 ps. The reaction pathways and potential energy curves were identified with the aid of density functional theory calculations. These results elucidate the elementary steps and mechanism which are fundamental in atmospheric chemistry and provide insight into how electronic photoexcitation is dissipated into the vibrational motions of the carbon backbone of simple hydrocarbons.

Transition state spectroscopy experiments, based on negative-ion photodechament, allow for the direct probing of the vibrational structure and metastable resonances that are characteristic of the neutral reactive surface. Here, we study the four-atom F + NH₃ → HF + NH₂ reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled NH₃F⁻ anions. The resulting spectra reveal features associated with a manifold of vibrational Feshbach resonances in the post-transition state product well of this reactive surface. Beyond this, the spectra contain structure reporting on reactive resonances in the pre-transition state reaction complex well. Quantum dynamical calculations performed on a full-dimensional potential surface show excellent agreement with the experimental results, allowing for the assignment of spectral structure and demonstrating that key dynamics of this bimolecular reaction are well described by this theoretical framework.

r0pt Figure The dynamics of aniline in water, after excitation along its lowest energy absorption (267 nm), has been investigated, from the femto (fs) to the nanoseconds (ns) scale, by pump-probe broadband transient absorption (TA) methods. The complex prompt TA spectrum, which evolves over the fs to ns scales, is analyzed by using a pump-repump-probe scheme that permits to interrogate the nature of the contributing species. The results permit us to identify, in addition to the long-living ππ^* state responsible of the fluorescence, the formation of a charge transfer to solvent state (CTTS) that will autoionize to form the fully solvated cation and electron. The nature of this CTTS state is discussed in terms of the 3s/πσ^* state characterized in the gas phaseJ. O. F. Thompson et al. J. Chem. Phys. 142, 114309 (2015); doi:10.1063/1.4914330nd the specific water-solute interactions established.

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Footnotes:

J. O. F. Thompson et al. J. Chem. Phys. 142, 114309 (2015); doi:10.1063/1.4914330a

We simulate laser-induced dynamics in acetylene (C₂H₂) using fully-experimental structural parameters. The rotation-vibration energy structure, including anharmonicities, is defined by the global spectroscopic Hamiltonian for the ground electronic state of C₂H₂ built from the extensive high resolution spectroscopy studies on the molecule, transition dipole moments from intensities, and effects of the (inelastic) collisions are parametrized from line broadenings using the relaxation matrix [J. Chem. Phys. 154, 144308 (2021)]. The approach, based on an effective Hamiltonian outperforms today's ab initio computations both in terms of accuracy and computational cost, however, is limited to a few small molecules. With such accuracy, the Hamiltonian permits to study the inside machinery of theoretical pulse shaping [J. Chem. Phys. 156, 084302 (2022)] for laser quantum control. With an adequate pulse shaping technique (in mid-IR) based on "super-Gaussian" pulses, we show a realistic and performant path to the population of a "dark" ro-vibrational state in C₂H₂.

Intramolecular vibrational redistributionis often assumed in Rice–Ramsperger–Kassel–Marcus and other rate calculations. In contrast, experimental spectroscopy, computational results, and models based on Anderson localization have shown that ergodicity is achieved rather slowly during molecular energy flow and the statistical assumption might easily fail due to quantum localization. Here, we develop a simple model for the interplay of IVR and energy transfer and simulate the model with near-exact quantum dynamics for 10-degree of freedom system. We find that there is a rather sharp “phase transition” as a function of molecular anharmonicity “a” between a region of facile energy transfer and a region limited by IVR with incomplete accessibility of the state space. The very narrow transition range of the order parameter ä" happens to lie right in the middle of the range expected for molecular vibrations, thus demonstrating that reactive energy transfer dynamics occurs not far from the localization boundary,with implications for controllability of reactions. This work is publised on JCP: doi: 10.1063/5.0043665 I1pt Figure