After sudden ionization of a large molecule, the positive charge can migrate throughout the system on a sub-femtosecond time scale, purely guided by electronic coherences. The possibility to actively explore the role of the electron dynamics in the photo-chemistry of bio-relevant molecules is of fundamental interest for understanding, and perhaps ultimately controlling, the processes leading to damage, mutation and, more generally, to the alteration of the biological functions of the macromolecule. Attosecond laser sources can provide the extreme time resolution required to follow this ultrafast charge flow. In this review we will present recent advances in attosecond molecular science: after a brief description of the results obtained for small molecules, recent experimental and theoretical findings on charge migration in bio-relevant molecules will be discussed.

No abstract

Nitromidazoles are relevant compounds of multidisciplinary interest, and knowledge of their physical-chemical parameters as well as their decomposition under photon irradiation is needed. Here we report an experimental and theoretical study of the mechanisms of VUV photofragmentation of 2- and 4(5)-nitromidazoles, compounds used as radiosensitizers in conjunction with radiotherapy as well as high-energy density materials. Photoelectron-photoion coincidence experiments, measurements of the appearance energies of the most important ionic fragments, density functional theory, and single-point coupled cluster calculations have been used to provide an overall insight into the energetics and structure of the different ionic/neutral products of the fragmentation processes. The results show that these compounds can be an efficient source of relevant CO, HCN, NO, and NO molecules and produce ions of particular astrophysical interest, like the isomers of azirinyl cation ( m/ z 40), predicted to exist in the interstellar medium, and protonated hydrogen cyanide ( m/ z 28).

Sudden ionisation of a relatively large molecule can initiate a correlation-driven process dubbed charge migration, where the electron density distribution is expected to rapidly move along the molecular backbone. Capturing this few-femtosecond or attosecond charge redistribution would represent the real-time observation of electron correlation in a molecule with the enticing prospect of following the energy flow from a single excited electron to the other coupled electrons in the system. Here, we report a time-resolved study of the correlation-driven charge migration process occurring in the nucleic-acid base adenine after ionisation with a 15–35 eV attosecond pulse. We find that the production of intact doubly charged adenine – via a shortly-delayed laser-induced second ionisation event – represents the signature of a charge inflation mechanism resulting from many-body excitation. This conclusion is supported by first-principles time-dependent simulations. These findings may contribute to the control of molecular reactivity at the electronic, few-femtosecond time scale.

The valence and core-level photoelectron spectra of gaseous indole, 2,3-dihydro-7-azaindole, and 3-formylindole have been investigated using VUV and soft X-ray radiation supported by both an ab initio electron propagator and density functional theory calculations. Three methods were used to calculate the outer valence band photoemission spectra: outer valence Green function, partial third order, and renormalized partial third order. While all gave an acceptable description of the valence spectra, the last method yielded very accurate agreement, especially for indole and 3-formylindole. The carbon, nitrogen, and oxygen 1s core-level spectra of these heterocycles were measured and assigned. The double ionization appearance potential for indole has been determined to be 21.8 ± 0.2 eV by C 1s and N 1s Auger photoelectron spectroscopy. Theoretical analysis identifies the doubly ionized states as a band consisting of two overlapping singlet states and one triplet state with dominant configurations corresponding to holes in the two uppermost molecular orbitals. One of the singlet states and the triplet state can be described as consisting largely of a single configuration, but other doubly ionized states are heavily mixed by configuration interactions. This work provides full assignment of the relative binding energies of the core level features and an analysis of the electronic structure of substituted indoles in comparison with the parent indole.

In the present work, we studied the photoinduced ion chemistry of the halogenated pyrimidines, a class of prototype radiosensitizing molecules, in the energy region 9-15 eV. The work was stimulated by previous studies on inner shell site-selective fragmentation of the pyrimidine molecule, which have shown that the fragmentation is governed by the population/formation of specific ionic states with a hole in valence orbitals, which in turn correlate to accessible dissociation limits. The combined experimental and theoretical study of the appearance energies of the main fragments provides information on the geometric structure of the products and on the role played by the specific halogen atom and the site of halogenation in the dissociation process. This information can be used to gain new insights on the elementary mechanisms that could possibly explain the enhanced radiation damage to the DNA bases or to the medium in which the bases are embedded, thereby contributing to their radiosensitizing effect.

No abstract

The fragmentation of uracil molecules and pure and nano-hydrated uracil clusters by (12)C(4+) ion impact is investigated. This work focuses on the fragmentation behavior of complex systems and the effect of the environment. On the one hand, it is found that the environment in the form of surrounding uracil or water molecules has a significant influence on the fragmentation dynamics, providing an overall 'protective' effect, while on the other hand we observe the opening of specific fragmentation channels. In particular, we report on the first observation of a series of hydrated fragments. This indicates a strong interaction between uracil and water molecules, holding the water clusters bound to the observed molecular fragments.