Velocity-map imaging at low extraction fields.

We present a velocity-map imaging (VMI) setup for photoelectron imaging that utilizes low electric extraction fields. This avoids any complications that could arise from electrostatic interactions between the extraction field and the molecular properties that are probed and has a minimal effect on the trajectory of ions in ion beam experiments. By using an attractive potential supplied to the detector, and keeping the electrodes at ground (zero) potential, we show that fringe fields between the VMI arrangement and the vacuum chamber can be eliminated, which is important in experiments on ions.

Ultrafast relaxation dynamics observed through time-resolved photoelectron angular distributions.

Time-resolved photoelectron imaging of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion is presented. Photoelectron angular distributions (PADs) are qualitatively analyzed in terms of the simple s-p model that is based on symmetry arguments. The internal conversion dynamics from the first excited state (1(2)B(3u)) to the ground state ((2)B(2g)) may be observed through temporal changes in the PADs of the spectrally overlapping photoelectron features arising from photodetachment of the ground state and the excited state. A formulism for extracting the population dynamics from the ß(2) anisotropy parameter of overlapping spectroscopic features is presented. This is used to extract the lifetime of the first excited state, which is in good agreement with that observed in the time-resolved photoelectron spectra.

Spectroscopy and dynamics of the 7,7,8,8-tetracyanoquinodimethane radical anion.

The photoelectron spectrum of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion has been measured at 3.1 eV. Additionally, the ultrafast relaxation dynamics of the first excited state (1(2)B(3u)) of TCNQ(-) have been studied using time-resolved photoelectron spectroscopy, which reveals that it undergoes internal conversion back to the ground state ((2)B(2g)) with an associated lifetime of 650 fs and shows evidence of coherent nuclear motion.

Mass spectrometric investigation of anions formed upon free electron attachment to nucleobase molecules and clusters embedded in superfluid helium droplets.

Here we report the first mass spectrometric study of negative ions formed via free electron attachment (EA) to nucleobases (NBs) embedded in helium clusters. Pure and mixed clusters of adenine and thymine have been formed by pickup of isolated NB molecules by cold helium droplets. In contrast to EA of isolated molecules in the gas phase we observe a long-lived parent anion NB- and, in addition, parent cluster ions NB-n up to size n=6. Moreover, we show that a low energy electron penetrating into a doped helium droplet causes efficient damage of the embedded nucleobases via resonant, site selective, dissociative electron attachment.