I. V. Hertel

Efficient deactivation of a model base pair via excited-state hydrogen transfer

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1ππ* state and the electronic ground state with a 1ππ* charge-transfer state that is strongly stabilized by the transfer of a proton.

Ultrafast dynamics in isolated molecules and molecular clusters

During the past decade the understanding of photo-induced ultrafast dynamics in molecular systems has improved at an unforeseen speed and a wealth of detailed insight into the fundamental processes has been obtained.This review summarizes our present knowledge on ultrafast dynamics in isolated molecules and molecular clusters evolving after excitation with femtosecond pulses as studied by pump–probe analysis in real time. Experimental tools and methods as well as theoretical models are described which have been developed to glean information on primary, ultrafast processes in photophysics, photochemistry and photobiology. The relevant processes are explained by way of example—from wave packet dynamics in systems with a few atoms all the way to internal conversion via conical intersections in bio-chromophores. A systematic overview on characteristic systems follows, starting with diatomic and including larger organic molecules as well as various types of molecular clusters, such as micro-solvated chromophore molecules. For conciseness the focus is on molecular systems which remain unperturbed by the laser pulses—apart from the excitation and detection processes as such. Thus, only some aspects of controlling and manipulating molecular reactions by shaped and/or very intense laser pulses are discussed briefly for particularly instructive examples, illustrating the perspectives of this prospering field.The material presented in this review comprises some prototypical examples from earlier pioneering work but emphasizes studies from recent years and covers the most important and latest developments until January 2006.

Laser ablation of dielectrics with temporally shaped femtosecond pulses

A significant improvement in the quality of ultrafast laser microstructuring of dielectrics is demonstrated by using temporally shaped pulse trains with subpicosecond separation. The sequential energy delivery induces a material softening during the initial steps of excitation changing the energy coupling for the subsequent steps. This leads to lower stress, cleaner structures, and provides a material-dependent optimization process.

Single-photon ionization of C60- and C70-fullerene with synchrotron radiation: determination of the ionization potential of C60

Abstract The photoionization threshold region of C60 has been studied using single-photon ionization with synchrotron radiation. Mass selection is realized with a time-of-flight mass spectrometer. The ionization potential of C60 has been determined to 7.58+0.04−0.02 eV.Significant structure in the photoionization yield is seen in the energy range up to 1 eV above threshold.

Interaction between liquid water and hydroxide revealed by core-hole de-excitation

The hydroxide ion plays an important role in many chemical and biochemical processes in aqueous solution. But our molecular-level understanding of its unusual and fast transport in water, and of the solvation patterns that allow fast transport, is far from complete. One proposal seeks to explain the properties and behaviour of the hydroxide ion by essentially regarding it as a water molecule that is missing a proton, and by inferring transport mechanisms and hydration structures from those of the excess proton. A competing proposal invokes instead unique and interchanging hydroxide hydration complexes, particularly the hypercoordinated OH-(H2O)4 species and tri-coordinated OH-(H2O)3 that can form a transient hydrogen bond between the H atom of the OH- and a neighbouring water molecule. Here we report measurements of core-level photoelectron emission and intermolecular Coulombic decay for an aqueous hydroxide solution, which show that the hydrated hydroxide ion is capable of transiently donating a hydrogen bond to surrounding water molecules. In agreement with recent experimental studies of hydroxide solutions, our finding thus supports the notion that the hydration structure of the hydroxide ion cannot be inferred from that of the hydrated excess proton.

Collision Experiments with Laser Excited Atoms in Crossed Beams

Publisher Summary This chapter discusses collision experiments using laser excited atoms in crossed beams. With the advent of tunable narrow-band lasers, especially cw dye lasers, the situation has changed and it has become clear that it should be possible to excite atoms optically within the scattering region of an otherwise conventional crossed-beam experiment. In this way a steady-state upper-state population could be reached that may be comparable to the ground-state population. When an atomic beam is excited, it is free of internal collisions and, for right angle intersection with the laser, free of Doppler broadening. Then the laser properties allow selecting the state into which the atom is excited. Specific fine- and hyperfine-structure states may be chosen as well as a particular combination of sub-states. The novel techniques allow preparing states with an angular momentum different from zero and to vary systematically the alignment and orientation of the resulting non-spherical interaction potentials. Frequency doubling of dye lasers can also widen the scale of possible applications. Very high powers are needed and one probably would have to use a pulsed laser. An alternative to frequency doubling is the direct two-photon excitation of atoms.

Collisions of excited Na atoms with H2 molecules. I. Ab initio potential energy surfaces and qualitative discussion of the quenching process

Potential energy surfaces have been calculated for the four lowest electronic states of Na (3 2S, 3 2P)+H2(1Σ+g) by means of the RHF–SCF and PNO–CEPA methods. For the so‐called quenching process of Na (3 2P) by H2 at low initial translational energies (E–VRT energy transfer) the energetically most favorable path occurs in C2v symmetry, since—at intermediate Na–H2 separation—the ? 2B2 potential energy surface is attractive. From the CEPA calculations, the crossing point of minimal energy between the ? 2A1 and ? 2B2 surfaces is obtained at Rc = 3.57 a.u. and rc = 2.17 a.u. with an energy difference to the asymptotic limit (R = ∞, r = re) of −0.06 eV. It is thus classically accessible without any initial translational energy, but at low initial translational energies (∼0.1 eV) quenching will be efficient only for arrangements of collision partners close to C2v symmetry. There is little indication of an avoiding crossing with an ionic intermediate correlating asymptotically with Na+ and H2− as was assumed in ...

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

Internal conversion in highly excited benzene and benzene dimer: femtosecond time-resolved photoelectron spectroscopy

Abstract Photoelectron spectra of benzene molecules and dimers excited to the S2 electronic state by 140 fs laser pulses at a wavelength of 200 nm have been measured by combining the pump–probe technique with the coincidence detection of ions and electrons. Their time dependence allows one to directly follow the evolution of internal conversion processes in different electronic states on a fs time-scale. Analysis of the corresponding electron energy distribution reveals large variations in the geometry of the different electronic states in both benzene and in its dimer.

Angular momentum transfer and charge cloud alignment in atomic collisions: intuitive concepts, experimental observations and semiclassical models

The authors discuss intuitive concepts to describe alignment and orientation effects in collision processes with, or leading to, an atomic np state. For direct excitation one can understand the atomic angular momentum transferred in terms of a rolling ball, and for excitation (de-excitation) in a molecular picture one can visualise the alignment angle of the atomic p charge cloud in terms of a transition from a body-fixed molecular picture (small internuclear distances R) to a space-fixed picture (large R). These concepts are illustrated by experimental results for e+Na* and Na++Na* collisions. Semiclassical theory is discussed for both the direct and the molecular inelastic processes, giving a theoretical foundation for these models. Detailed results are reported for the time development of the charge cloud in Na++Na* collisions as a model case, illustrating the concept of body-fixed versus space-fixed electron motion and its limitations. Further examples are the molecular process N2+Na* and the atomic process Xe+Ba* at thermal energies. In all cases long-range rotational ( Sigma - Pi ) coupling determines the charge cloud motion.