Session J05: Time-resolved Molecular Dynamics and Femtochemistry

A Generalized Interpretation of Time Resolved Photoelectron Spectroscopy Experiments

We present a generalized interpretation of one photon pump - one photon probe, angle and time resolved photoelectron spectroscopy experiments in molecules. Using a previously derived mathematical formalism based on the density matrix, we are able to clarify the different aspects of molecular wavepacket dynamics that are observed in each beta parameter. As a general result, we find that rotational coherences are separated out from vibronic population dynamics by the highest order beta parameter. Data from linear, symmetric and asymmetric tops molecules is presented to demonstrate the validity of this interpretation.   

Femtosecond Time-Resolved Coulomb Explosion Imaging of UV-Induced Photodissociation of Iodomethane

The UV-induced photodissociation of iodomethane (CH3I) and the ensuing molecular dynamics is investigated by time-resolved Coulomb explosion imaging. We utilize a UV-IR pump-probe setup with a coincident 3D ion momentum imaging apparatus to measure yields and kinetic energies of all ionic fragments as a function of the time-delay between the pump and probe pulses. Excitation at a wavelength of 258 nm initiates a resonant one photon dissociation into neutral fragments, which results in C-I bond cleavage. The dissociation products are then strong-field ionized, using the IR probe pulse. Analysis of the delay-dependent kinetic energy release, for each fragmentation channel, allows the time evolution of the internuclear distance to be extracted. The results highlight the sensitivity of Coulomb explosion imaging as probe of structural dynamics on ultrafast timescales.   

Comparison of UV and Strong-field Driven Dynamics of Bromoform (CHBr$_{\mathrm{3}})$ probed by Femtosecond XUV Transient Absorption Spectroscopy

UV excitation and strong-field ionization induced dynamics of Bromoform (CHBr$_{\mathrm{3}})$ have been studied using table-top XUV transient absorption spectroscopy. Element-specific core-to-valence transitions provide an atomic scale perspective, sensitive to changes in the local valence electronic structure, with ultrafast time resolution. The formation of both neutral and ionized fragments is probed with the same measurement. Strong-field ionization (10$^{\mathrm{14}}$~W/cm$^{\mathrm{2}})$ of Bromoform yields predominantly neutral Br atoms. No evidence of transient species or molecular products was observed. While depletion of the parent molecule signal is rapid (\textless 30~fs), product formation occurs on a slower, 70~fs timescale for spin-orbit excited Br* atoms. Ground state Br atoms form even more slowly and only after a pronounced delay, strongly indicating a multi-step relaxation/dissociation mechanism. In contrast, UV dissociation of Bromoform leads to parent signal depletion and Br/Br* product formation with comparable (\textless 100~fs) time constants. The branching between Br {\&} Br* is comparable to that observed in the strong-field experiment.   

A Comparison of Strong and Weak Field Ionization as a Probe of Excited State Molecular Dynamics

Ionization can serve as a universal probe of excited state molecular dynamics, such as internal conversion, dissociation, and isomerization. We conduct time-resolved photoelectron and photoion spectroscopy measurements of excited state state dynamics in both weak field (UV-pump/VUV-probe) and strong field (UV-pump/IR-probe) ionization (WFI and SFI). We investigate the relative merits of WFI versus SFI as probes in two different classes of molecules - halogenated methanes (CH2I2) and cyclic organic molecules (uracil). Experimentally, both WFI and SFI approaches show similar dynamics - CH2I2 undergoes rapid internal conversion followed by dissociation and uracil has substantial population trapping in the excited state in addition to rapid internal conversion back to the ground state. Theoretically, we compare the experimental results with electronic structure and dynamics calculations. We find that while SFI and WFI provide qualitatively similar information about the excited state dynamics, only WFI results can be compared quantitatively with calculations.   

Vibrational Coherences and Proton-Transfer Dynamics of Ionized Phenoxide in Aqueous Solution Observed by Few-Femtosecond Transient Absorption Spectroscopy.

Optical transient absorption spectroscopy with few-cycle (6-fs) pulses elucidates ionization-induced vibrational coherences and proton-transfer dynamics of small molecules in aqueous solution. Strong-field ionization of sodium phenoxide (C$_{\mathrm{6}}$H$_{\mathrm{5}}$O$^{\mathrm{-\thinspace }}$Na$^{\mathrm{+}})$ yields the phenoxyl radical (C$_{\mathrm{6}}$H$_{\mathrm{5}}$O$^{\mathrm{^{\circ}-}})$ and the hydrated electron. The former appears as an absorption band at 400 nm atop the broad absorption feature of the hydrated electron. The phenoxyl radical absorption band exhibits pronounced amplitude and energy modulations in the time domain, which upon Fourier transformation, yields its vibrational frequencies. These frequencies are assigned with the aid of density-functional theory calculations and are comparable to those obtained from gas-phase and argon matrix measurements. Time-domain analysis of the vibrational coherences furnishes Huang-Rhys factors and dephasing times for the vibrational modes that are coupled to the ionization transition. Finally, the transient absorption spectrum reveals a feature at 428 nm that is assigned to the phenol radical cation (C$_{\mathrm{6}}$H$_{\mathrm{5}}$OH$^{\mathrm{^{\circ}+}})$. The formation of the radical cation species within the instrument response of our experiment suggests that ionization-induced proton transfer (IIPT) from the solvent (water) to the phenoxyl radical occurs on the sub-10-fs timescale. Our results shed light on the elementary ultrafast dynamics that accompany the interaction of ionizing radiation with molecules of biological relevance.   

Manipulating photodissociation dynamics by frequency chirped laser pulses

The photodissociation dynamics of the D2+ molecular ion is investigated theoretically in the presence of linearly as well as so-called arbitrarily varying frequency laser pulses. After a sudden ionization of the neutral system, the impact of several chirped probe pulses is explored in terms of total dissociation probabilities, kinetic energy release and angular distribution of the photofragments. All the calculated quantities are presented as a function of the delay time of the probe pulse and a comparison between positive negative as well as zero chirp situations is discussed. Furthermore, by tracing the maxima of the vibrating nuclear density, special kind of frequency chirps are constructed with the aim of maximizing the dissociation probability of the system. Our treatment of the light-matter interaction incorporating strong nonadiabaticity, is carried out in the light-induced conical intersection (LICI) framework. The phase modulation of the pulses makes possible the modification of several properties of the created LICI, leading to interesting observations in the studied quantities.   

Light-induced and Natural Nonadiabatic Phenomena in Diatomics

Nonadiabatic effects play a very important role in controlling chemical dynamical processes. They are strongly related to avoided crossings (AC) or conical intersections (CIs) which can either be present naturally or induced by classical or quantized laser light ("light-induced avoided crossings" (LIACs) and "light-induced conical intersections" (LICIs)). In the latter situation the radiation field mixes the nuclear and electronic degrees of freedom in an optical cavity. Here we show to what extent the classical or cavity's quantized field description of the electric field are equivalent. Solving the time-dependent nuclear Schr\"{o}dinger equation we can simulate either LIAC or LICI situations in the NaI molecule, which is a strongly coupled diatomic in field free case. Obtained results undoubtedly demonstrate a significant difference between the impact of the LIAC and that of the LICI on the dynamics of the molecule, as well as the collective effect of light-induced and natural nonadiabatic phenomena.   

Spatio-temporal imaging of multimode vibrational wave packets in strong-field ionization and excitation of iodomethane

We report on the experimental characterization of a vibrational wave packet created in iodomethane molecule (CH$_{\mathrm{3}}$I) irradiated by an intense laser field. Using a combination of a pump-probe setup employing two 25 fs, 800 nm pulses and an ion momentum imaging apparatus, we identify the signatures of vibrational motion in different electronic states by channel-selective Fourier spectroscopy. The delay-dependent yields of parent ions and iodine fragments from dissociative ionization are dominated by the vibrations in the ground state of the neutral molecule and manifest the opposite phase behavior, which is consistent with the bond softening mechanism of vibrational excitation [1]. For the doubly charged parent ions vibrational motion in the ground cationic state plays a more important role. Finally, in the Coulomb explosion channels structures reflecting vibrations in the first excited state of the cation can be clearly identified if the pump pulse intensity is kept sufficiently low to avoid the dissociation of this state within the pump. [1] Z. Wei et al., Nature Communications 8, 735 (2017)   

Non-Born-Oppenheimer electronic wavepacket in molecular nitrogen at 14 eV probed by time-resolved photoelectron spectroscopy

Attosecond pulses provide a means to coherently excite a set of electronic states in atoms and molecules. Here we use a train of attosecond pulses to excite a pair of electronic states in molecular nitrogen. The excitation frequency is centered at 14 eV, and populates two coupled electronic states -- the valence state $ b^{\prime}$ $^1\Sigma_u^+ ~v~=~13$ and the Rydberg state $c_4^{\prime}$ $^1\Sigma_u^+ ~v~=~4$. The resulting electronic wavepacket is probed by a time-delayed 800~nm pulse which, through two-photon absorption, ionizes the molecule. The pump-probe final state-resolved photoelectron spectrum exhibits a quantum beat with a period of about 240 fs, corresponding to a frequency of 4.1 $\pm$ 0.2 THz. The phase of the quantum beat depends on the vibrational quantum number of the final state in the cation. The zeroth order Born-Oppenheimer representations of the electronic states strongly perturb each other since they have the same symmetry. Optical excitation of the intermediate levels causes major zeroth order population oscillations at the observed frequency that are then mapped onto the photoelectron spectrum, qualitatively reproducing the experimental observations.