Decay of high Rydberg states: A paradigm for intramolecular dynamics in a congested bound level structure coupled to a continuum

Probing Ultrafast Purely Electronic Charge Migration in Small Peptides

A pump–probe experiment that can examine a pure charge migration on a time scale short compared to the onset of nuclear motion is discussed. The mass spectrometric studies of Schlag et al. suggest that short peptide terminated by an aromatic amino acid are particularly suitable test compounds. The pump pulse needs to ionize the molecule on a time scale short compared to the period of the electronic motion, typically sub-fs. However, ionization occurs preferentially when the electrical field of the light is maximal so that the duration of the pulse envelope can be somewhat longer. Detection by photoelectron spectrometry of the peptide cation, to produce a dication, is shown to be able to probe the electronic rearrangement.

Molecular Theory of ZEKE Spectroscopy

In this paper, we derive the double-resonance and single-resonance two photon absorption cross sections for zero electron kinetic energy (ZEKE) spectroscopy using the inverse Born–Oppenheimer approximation (IBOA) as a basis set. Quantum chemistry calculations can then be used to evaluate the ZEKE spectra. To show how to analyze the vibronic spectra of ZEKE spectroscopy, a couple of examples have been analyzed. Quantum chemistry calculations can be employed to obtain the potential surfaces of molecules and their ions, which can be used to calculate the Franck–Condon factors involved in the vibronic transitions. We also show that using the IBOA as a basis set, the autoionization of the ZEKE states can be calculated.

The elimination of redundant constraints in surprisal analysis of unimolecular dissociation and other endothermic processes

It is well understood that energy rich polyatomic molecules do not dissociate promptly because the number, P, of their energy states far exceeds the number, N, of the decay channels. In the simplest RRK theory, the fraction N/P is the probability of dissociation. We discuss the distribution of the decay rates of maximal entropy and conclude that it is governed by at most N linearly independent constraints, N < P, or, more typically, N << P. This mathematical requirement already severely restricts the number of constraints. Beyond it however, on physical grounds, one constraint, or at most a few, may be dominant. We discuss why just one constraint, or a few but less than N, can be sufficient to describe the product state distribution.

Pulsed-field ionization spectroscopy of high Rydberg states (n=50–200) of bis(η6-benzene)chromium

The ionization behavior of the high Rydberg states of bis(eta(6)-benzene)chromium in the presence of ac and/or dc fields has been explored. The application of an ac scrambling field at the time of laser excitation lengthens the lifetime of the Rydberg state by almost two orders of magnitude. The lifetime enhancement by the scrambling field is much more effective for n<100 than it is for n>100 Rydberg states. The pulsed-field ionization of Rydberg states of n<100 shows the typical diabatic ionization behavior for low n. The two distinct ionization behaviors observed for the relatively low (n=50-100) and high (n=100-200) Rydberg states suggest that the former originate from the optically accessed nf Rydberg series, whereas the latter are due to np Rydberg series. Based on the understanding of the ionization behavior of bis(eta(6)-benzene)chromium, the accurate ionization potential is deduced to give IP=5.4665+/-0.0003 eV. Optimization of the various electric field conditions greatly enhances the spectral sensitivity of the mass-analyzed threshold ionization (MATI) spectroscopy. The high-resolution MATI spectrum of the title molecule obtained here provides precise cationic vibrational frequencies for many skeletal and benzene ring modes. A number of vibrational modes are newly identified, and the ambiguity regarding to some mode assignments is now clearly resolved through the Frank-Condon analysis based on ab initio calculations.

MOLECULES IN HIGH RYDBERG STATES

The unusual properties of high molecular Rydberg states with principal quantum number n>>100 have enabled the development of new research tools for the study of molecules and ions in the gas phase. These tools range from spectroscopic techniques such as zero kinetic energy (ZEKE) photoelectron spectroscopy and mass-analyzed threshold ionization (MATI) spectroscopy to techniques that are suited to the investigation of photodissociation processes, bimolecular reactions, and state-selected ion-molecule reactions. This review summarizes recent progress in the understanding of the properties of high molecular Rydberg states and gives an overview of recent chemical applications of these states.