Nonadiabatic Dynamics Studied by Liquid-Jet Time-Resolved Photoelectron Spectroscopy

The development of the liquid microjet technique by Faubel and co-workers has enabled the investigation of high vapor pressure liquids and solutions utilizing high-vacuum methods. One such method is photoelectron spectroscopy (PES), which allows one to probe the electronic properties of a sample through ionization in a state-specific manner. Liquid microjets consisting of pure solvents and solute-solvent systems have been studied with great success utilizing PES and, more recently, time-resolved PES (TRPES). Here, we discuss progress made over recent years in understanding the solvation and excited state dynamics of the solvated electron and nucleic acid constituents (NACs) using these methods, as well as the prospect for their future.The solvated electron is of particular interest in liquid microjet experiments as it represents the simplest solute system. Despite this simplicity, there were still many unresolved questions about its binding energy and excited state relaxation dynamics that are ideal problems for liquid microjet PES. In the work discussed in this Account, accurate binding energies were measured for the solvated electron in multiple high vapor pressure solvents. The advantages of liquid jet PES were further highlighted in the femtosecond excited state relaxation studies on the solvated electron in water where a 75 ± 20 fs lifetime attributable to internal conversion from the excited p-state to a hot ground state was measured, supporting a nonadiabatic relaxation mechanism.Nucleic acid constituents represent a class of important solutes with several unresolved questions that the liquid microjet PES method is uniquely suited to address. As TRPES is capable of tracking dynamics with state-specificity, it is ideal for instances where there are multiple excited states potentially involved in the dynamics. Time-resolved studies of NAC relaxation after excitation using ultraviolet light identified relaxation lifetimes from multiple excited states. The state-specific nature of the TRPES method allowed us to identify the lack of any signal attributable to the ¹nπ* state in thymine derived NACs. The femtosecond time resolution of the technique also aided in identifying differences between the excited state lifetimes of thymidine and thymidine monophosphate. These have been interpreted, aided by molecular dynamics simulations, as an influence of conformational differences leading to a longer excited state lifetime in thymidine monophosphate.Finally, we discuss advances in tabletop light sources extending into the extreme ultraviolet and soft X-ray regimes that allow expansion of liquid jet TRPES to full valence band and potentially core level studies of solutes and pure liquids in liquid microjets. As most solutes have ground state binding energies in the range of 10 eV, observation of both excited state decay and ground state recovery using ultraviolet pump-ultraviolet probe TRPES has been intractable. With high-harmonic generation light sources, it will be possible to not only observe complete relaxation pathways for valence level dynamics but to also track dynamics with element specificity by probing core levels of the solute of interest.

Conical intersection and coherent vibrational dynamics in alkyl iodides captured by attosecond transient absorption spectroscopy

The photodissociation dynamics of alkyl iodides along the C–I bond are captured by attosecond extreme-ultraviolet (XUV) transient absorption spectroscopy employing resonant ∼20 fs UV pump pulses. The methodology of previous experiments on CH3I [Chang et al., J. Chem. Phys. 154, 234301 (2021)] is extended to the investigation of a C–I bond-breaking reaction in the dissociative A-band of C2H5I, i-C3H7I, and t-C4H9I. Probing iodine 4 d core-to-valence transitions in the XUV enables one to map wave packet bifurcation at a conical intersection in the A-band as well as coherent vibrations in the ground state of the parent molecules. Analysis of spectroscopic bifurcation signatures yields conical intersection crossing times of 15 ± 4 fs for CH3I, 14 ± 5 fs for C2H5I, and 24 ± 4 fs for i-C3H7I and t-C4H9I, respectively. Observations of coherent vibrations, resulting from a projection of A-band structural dynamics onto the ground state by resonant impulsive stimulated Raman scattering, indirectly reveal multimode C–I stretch and CCI bend vibrations in the A-bands of C2H5I, i-C3H7I, and t-C4H9I.