Extreme ultraviolet time-resolved photoelectron spectroscopy of adenine, adenosine and adenosine monophosphate in a liquid flat jet.

Time-resolved photoelectron spectroscopy using an extreme-ultraviolet (XUV) probe pulse was used to investigate the UV photoinduced dynamics of adenine (Ade), adenosine (Ado), and adenosine-5-monophosphate (AMP) in a liquid water jet. In contrast to previous studies using UV probe pulses, the XUV pulse at 21.7 eV can photoionize all excited states of a molecule, allowing for full relaxation pathways to be addressed after excitation at 4.66 eV. This work was carried out using a gas-dynamic flat liquid jet, resulting in considerably enhanced signal compared to a cylindrical jet. All three species decay on multiple time scales that are assigned based on their decay associated spectra; the fastest decay of ∼100 fs is assigned to ππ* decay to the ground state, while a smaller component with a lifetime of ∼500 fs is attributed to the nπ* state. An additional slower channel in Ade is assigned to the 7H Ade conformer, as seen previously. This work demonstrates the capability of XUV-TRPES to disentangle non-adiabatic dynamics in an aqueous solution in a state-specific manner and represents the first identification of the nπ* state in the relaxation dynamics of adenine and its derivatives.

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.

Molecular structure determination: Equilibrium structure of pyrimidine (m-C4H4N2) from rotational spectroscopy (re SE) and high-level ab initio calculation (re) agree within the uncertainty of experimental measurement.

The pure rotational spectrum of pyrimidine (m-C4H4N2), the meta-substituted dinitrogen analog of benzene, has been studied in the millimeter-wave region from 235 GHz to 360 GHz. The rotational spectrum of the ground vibrational state has been assigned and fit to yield accurate rotational and distortion constants. Over 1700 distinct transitions were identified for the normal isotopologue in its ground vibrational state and least-squares fit to a partial sextic S-reduced Hamiltonian. Transitions for all four singly substituted 13C and 15N isotopologues were observed at natural abundance and were likewise fit. Deuterium-enriched samples of pyrimidine were synthesized, giving access to all eleven possible deuterium-substituted isotopologues, ten of which were previously unreported. Experimental values of rotational constants and computed values of vibration-rotation interaction constants and electron-mass corrections were used to determine semi-experimental equilibrium structures (re SE) of pyrimidine. The re SE structure obtained using coupled-cluster with single, double, and perturbative triple excitations [CCSD(T)] corrections shows exceptional agreement with the re structure computed at the CCSD(T)/cc-pCV5Z level (≤0.0002 Å in bond distance and ≤0.03° in bond angle). Of the various computational methods examined, CCSD(T)/cc-pCV5Z is the only method for which the computed value of each geometric parameter lies within the statistical experimental uncertainty (2σ) of the corresponding semi-experimental coordinate. The exceptionally high accuracy and precision of the structure determination is a consequence of the large number of isotopologues measured, the precision and extent of the experimental frequency measurements, and the sophisticated theoretical treatment of the effects of vibration-rotation coupling and electron mass. Taken together, these demanding experimental and computational studies establish the capabilities of modern structural analysis for a prototypical monocyclic aromatic compound.

Relaxation Dynamics of Hydrated Thymine, Thymidine, and Thymidine Monophosphate Probed by Liquid Jet Time-Resolved Photoelectron Spectroscopy.

The relaxation dynamics of thymine and its derivatives thymidine and thymidine monophosphate are studied using time-resolved photoelectron spectroscopy applied to a water microjet. Two absorption bands are studied; the first is a bright ππ* state which is populated using tunable-ultraviolet light in the range 4.74-5.17 eV and probed using a 6.20 eV probe pulse. By reversing the order of these pulses, a band containing multiple ππ* states is populated by the 6.20 eV pulse and the lower energy pulse serves as the probe. The lower lying ππ* state is found to decay in ∼400 fs in both thymine and thymidine independent of pump photon energy, while thymidine monophosphate decays vary from 670 to 840 fs with some pump energy dependence. The application of a computational quantum mechanical/molecular mechanical scheme at the XMS-CASPT2//CASSCF/AMBER level of theory suggests that conformational differences existing between thymidine and thymidine monophosphate in solution account for this difference. The higher lying ππ* band is found to decay in ∼600 fs in all three cases, but it is only able to be characterized when the 5.17 eV probe pulse is used. Notably, no long-lived signal from an nπ* state can be identified in either experiment on any of the three molecules.