Vibrationally resolved photoelectron spectroscopy of oligothiophene radical anions.

Vibrationally resolved photoelectron spectroscopy of terthiophene, quaterthiophene, and quinquethiophene radical anions is presented. The increased spectral resolution afforded by the combination of slow photoelectron velocity-map imaging and ion cooling in a cryogenic ion trap allows the characterization of vibronic structures within the S0 and T1 states. Analysis of the spectra, aided by electronic structure calculations and Franck-Condon simulations, revealed evidence for significant contributions from kinetically trapped higher energy conformers in the anion-to-triplet transitions. Unlike the lowest energy structures, where all the thiophene linkers are in the trans configuration, these higher energy conformers contain at least one cis linker. We also found that the adiabatic Franck-Condon simulations drastically underestimated the intensities of some vibronic features in the singlet ground state spectra due to large geometry changes upon photodetachment and anharmonic couplings in the singlet state.

Photoelectron spectroscopy of anthracene and fluoranthene radical anions.

We report the slow electron velocity map imaging spectroscopy of cryogenically cooled anthracene and fluoranthene radical anions, two similarly sized polycyclic aromatic hydrocarbon molecules. The results allow us to examine the lowest energy singlet and triplet states in the neutral molecules on equal footing from the anionic ground state. The analysis of the experimental spectra is aided by harmonic calculations and Franck-Condon simulations, which generally show good agreement with experimental values and spectra. The electron affinity of fluoranthene is measured to be 0.757(2) eV, which is larger than that of anthracene at 0.532(3) eV. The lowest energy triplet state in anthracene is observed at 1.872(3) eV above the singlet ground state, while that of fluoranthene is observed at 2.321(2) eV above its singlet ground state. Comparisons of experimental and calculated spectra show that in addition to the Franck-Condon active modes, there is a clear presence of vibrational modes that gain intensity via vibronic coupling in both the singlet and triplet states in both molecules. In addition, the triplet state generally exhibits increased vibronic coupling compared to the singlet state, with the fluoranthene triplet state exhibiting evidence of distortion from C2v symmetry.

A multi-plate velocity-map imaging design for high-resolution photoelectron spectroscopy.

A velocity map imaging (VMI) setup consisting of multiple electrodes with three adjustable voltage parameters, designed for slow electron velocity map imaging applications, is presented. The motivations for this design are discussed in terms of parameters that influence the VMI resolution and functionality. Particularly, this VMI has two tunable potentials used to adjust for optimal focus, yielding good VMI focus across a relatively large energy range. It also allows for larger interaction volumes without significant sacrifice to the resolution via a smaller electric gradient at the interaction region. All the electrodes in this VMI have the same dimensions for practicality and flexibility, allowing for relatively easy modifications to suit different experimental needs. We have coupled this VMI to a cryogenic ion trap mass spectrometer that has a flexible source design. The performance is demonstrated with the photoelectron spectra of S- and CS2-. The latter has a long vibrational progression in the ground state, and the temperature dependence of the vibronic features is probed by changing the temperature of the ion trap.

The effects of added hydrogen on a helium atmospheric-pressure plasma jet ambient desorption/ionization source.

We present mass spectrometric data demonstrating the effect that hydrogen has on a helium-based dielectric-barrier discharge (DBD) atmospheric-pressure plasma jet used as an ambient desorption/ionization (ADI) source. The addition of 0.9 % hydrogen to the helium support gas in a 35-W plasma jet increased signals for a range of test analytes, with enhancement factors of up to 68, without proportional increases in background levels. The changes in signal levels result from a combination of changes in the desorption kinetics from the surface and increased ion production in the gas phase. The enhancement in ADI-MS performance despite the quenching of key plasma species reported in earlier studies suggests that ionization with a H2/He plasma jet is the result of an alternate mechanism involving the direct generation of ionized hydrogen.