Probing Ultrafast Dynamics with Time-resolved Multi-dimensional Coincidence Imaging: Butadiene

Time-resolved coincidence imaging of photoelectrons and photoions represents the most complete experimental measurement of ultrafast excited state dynamics, a multi-dimensional measurement for a multi-dimensional problem. Here we present the experimental data from recent coincidence imaging experiments, undertaken with the aim of gaining insight into the complex ultrafast excited-state dynamics of 1,3-butadiene initiated by absorption of 200 nm light. We discuss photoion and photoelectron mappings of increasing dimensionality, and focus particularly on the time-resolved photoelectron angular distributions (TRPADs), expected to be a sensitive probe of the electronic evolution of the excited state and to provide significant information beyond the time-resolved photoelectron spectrum (TRPES). Complex temporal behaviour is observed in the TRPADs, revealing their sensitivity to the dynamics while also emphasising the difficulty of interpretation of these complex observables. From the experimental data some details of the wavepacket dynamics are discerned relatively directly, and we make some tentative comparisons with existing ab initio calculations in order to gain deeper insight into the experimental measurements; finally, we sketch out some considerations for taking this comparison further in order to bridge the gap between experiment and theory.Comment: 18 pages, 10 figures. Pre-print of JMO submissio

Switched wave packets: A route to nonperturbative quantum control

The dynamic Stark effect due to a strong nonresonant but nonionizing laser field provides a route to quantum control via the creation of novel superposition states. We consider the creation of a field-free "switched" wave packet through adiabatic turn-on and sudden turn-off of a strong dynamic Stark interaction. There are two limiting cases for such wave packets. The first is a Raman-type coupling, illustrated by the creation of field-free molecular axis alignment. An experimental demonstration is given. The second case is that of dipole-type coupling, illustrated by the creation of charge localization in an array of quantum wells

Accessing the molecular frame through strong-field alignment of distributions of gas phase molecules

A rationale for creating highly aligned distributions of molecules is that it enables vector properties referenced to molecule-fixed axes (the molecular frame) to be determined. In the present work, the degree of alignment that is necessary in order for this to be achieved in practice is explored. Alignment is commonly parametrised in experiments by a single parameter, , which is insufficient to enable predictive calculations to be performed. Here it is shown that, if the full distribution of molecular axes takes a Gaussian form, this single parameter can be used to determine the complete set of alignment moments needed to characterise the distribution. In order to demonstrate the degree of alignment that is required in order to approach the molecular frame, the set of alignment moments corresponding to a few chosen values of are used to project a model molecular frame photoelectron angular distribution into the laboratory frame. These calculations show that needs to approach 0.9 in order to avoid significant blurring to be caused by averaging

Sequential and direct ionic excitation in the strong-field ionization of 1-butene molecules

We study the Strong-Field Ionization (SFI) of the hydrocarbon 1-butene as a function of wavelength using photoion-photoelectron covariance and coincidence spectroscopy. We observe a striking transition in the fragment-associated photoelectron spectra: from a single Above Threshold Ionization (ATI) progression for photon energies less than the cation D0–D1 gap to two ATI progressions for a photon energy greater than this gap. For the first case, electronically excited cations are created by SFI populating the ground cationic state D0, followed by sequential post-ionization excitation. For the second case, direct sub-cycle SFI to the D1 excited cation state contributes significantly. Our experiments access ionization dynamics in a regime where strong-field and resonance-enhanced processes can interplay

Following the excited state relaxation dynamics of indole and 5-hydroxyindole using time-resolved photoelectron spectroscopy

Time-resolved photoelectron spectroscopy was used to obtain new information about the dynamics of electronic relaxation in gas-phase indole and 5-hydroxyindole following UV excitation with femtosecond laser pulses centred at 249 nm and 273 nm. Our analysis of the data was supported by ab initio calculations at the coupled cluster and complete-active-space self-consistent-field levels. The optically bright 1La and 1Lb electronic states of 1\u3c0\u3c0* character and spectroscopically dark and dissociative 1\u3c0\u3c3* states were all found to play a role in the overall relaxation process. In both molecules we conclude that the initially excited 1La state decays non-adiabatically on a sub 100 fs timescale via two competing pathways, populating either the subsequently long-lived 1Lb state or the 1\u3c0\u3c3* state localised along the N-H coordinate, which exhibits a lifetime on the order of 1 ps. In the case of 5-hydroxyindole, we conclude that the 1\u3c0\u3c3* state localised along the O-H coordinate plays little or no role in the relaxation dynamics at the two excitation wavelengths studied.Peer reviewed: YesNRC publication: Ye