Molecular photoionization cross sections by the Lobatto technique. I. Valence photoionization

Molecular-Frame Photoelectron Angular Distributions of CO in the Vicinity of Feshbach Resonances: An XCHEM Approach

The advent of ultrashort XUV pulses is pushing for the development of accurate theoretical calculations to describe ionization of molecules in regions where electron correlation plays a significant role. Here, we present an extension of the XCHEM methodology to evaluate laboratory- and molecular-frame photoelectron angular distributions in the region where Feshbach resonances are expected to appear. The performance of the method is demonstrated in the CO molecule, for which information on Feshbach resonances is very scarce. We show that photoelectron angular distributions are dramatically affected by the presence of resonances, to the point that they can completely reverse the preferred electron emission direction observed in direct nonresonant photoionization. This is the consequence of significant changes in the electronic structure of the molecule when resonances decay, an effect that is mostly driven by electron correlation in the ionization continuum. The present methodology can thus be helpful for the interpretation of angularly resolved photoionization time delays in this and more complex molecules.

Time-resolved photoelectron imaging of S2 → S1 internal conversion in benzene and toluene

Ultrafast internal conversion of benzene and toluene from the S(2) states was studied by time-resolved photoelectron imaging with a time resolution of 22 fs. Time-energy maps of the photoelectron intensity and the angular anisotropy were generated from a series of photoelectron images. The photoelectron kinetic energy distribution exhibits a rapid energy shift and intensity revival, which indicates nuclear motion on the S(2) adiabatic surface, while the ultrafast evolution of the angular anisotropy revealed a change in the electronic character of the S(2) adiabatic surface. From their decay profiles of the total photoelectron intensity, the time constants of 48 ± 4 and 62 ± 4 fs were determined for the population decay from the S(2) states in benzene and toluene, respectively.

Time-dependent density-functional theory for molecular photoionization with noniterative algorithm and multicenter B-spline basis set: CS2 and C6H6 case studies

In this work a new direct (noniterative) algorithm to solve the time-dependent density-functional theory equations for molecular photoionization has been proposed and implemented, using a multicentric basis set expansion of B-spline functions and complete exploiting of the molecular point-group symmetry. The method has been applied to study the photoionization dynamics of CS2 and C6H6: the results confirmed the expectation of large screening effects in CS2. For C6H6 the screening effects have been found to play a minor role than in CS2, however, also in this case the quality of the final results is definitely improved. The method has proven suitable to study with confidence molecules of medium size, and there is still room for further improvement working on more elaborate treatment of the exchange-correlation functional.

Study of the Electronic Structure of Ni(eta(5)-C(5)H(5))(NO) by Variable-Photon-Energy Photoelectron Spectroscopy and Density Functional Calculations

Photoelectron spectra, with photon energies varying from 18 to 120 eV, have been measured for Ni(eta(5)-C(5)H(5))(NO). Relative partial photoelectron cross sections and branching ratios have been evaluated for the first three valence ionization bands. He I and He II photoelectron spectra have been remeasured for Ni(eta(5)-C(5)H(5))(NO) and Ni(eta(5)-C(5)H(4)CH(3))(NO). In the latter case, the fine structure on the first band differs from that in the previously published spectrum. Density functional calculations have been carried out to determine the ionization potentials of the lowest lying states of Ni(eta(5)-C(5)H(5))(NO) as well as the corresponding photoionization cross sections and the resulting branching ratios using the LCGTO-DF and LDKL-DF methods, respectively. Both experimental and theoretical investigations lead to an ion state ordering (2)E(1) < (2)E(2) approximately (2)A(1)< (2)E(1) and an assignment of (2)E(1) states to the first and third bands with the (2)A(1) and (2)E(2) states comprising the second band. This differs from the original assignment in the literature, where the (2)A(1) ionization was assigned to a high-energy shoulder on the first band. The separation of this shoulder from the main band maximum of 0.23 eV (1850 +/- 81 cm(-)(1)) suggests that it may be caused by excitation of the NO stretching vibration in the ion. The neutral molecule has a NO stretch of 1832 cm(-)(1); the calculated energies for the neutral molecule and the cation are 1845 and 1911 cm(-)(1), respectively. Agreement between calculated and experimental ionization energies and good matching of the theoretical and measured branching ratios support the new assignment of the photoelectron spectrum.