Development and Construction of a new Photoelectron Imaging Spectrometer for Studying the Spectroscopy and Ultrafast Dynamics of Molecular Anions

Abstract

We present a detailed account of the development, construction, and commissioning of a new experiment for studying the spectroscopy and ultrafast dynamics of molecular anions in the gas phase. The new instrument incorporates: an electrospray ionisation source, which is capable of generating a vast class of molecular anions; a Wiley-McLaren time-of-flight mass spectrometer; and a compact photoelectron imaging arrangement for anions, which negates the use of pulsed high voltages. We use this
instrument in conjunction with a femtosecond laser system to perform the first ultrafast time-resolved photoelectron imaging experiments on molecular anions generated through electrospray ionisation.
A method for reconstructing three dimensional charged particle distributions from their associated two dimensional projections on an imaging detector plane is described. This new method utilises: (1) onion-peeling in polar co-ordinates (POP) to perform the reconstruction; and (2) basis set concepts to significantly enhance the algorithms computational speed. We compare this new POP algorithm with other reconstruction algorithms, which shows that the method is as good as the benchmark pBASEX method
in terms of accuracy. Importantly, we show that it is also computationally fast, allowing images to be reconstructed as they are acquired in a typical imaging experiment.
Original work is presented which investigates the spectroscopy and ultrafast excited dynamics of the 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anion. The photoelectron spectrum of TCNQ– is measured at 3.1 eV, which is used to gain insight into the electronic structure and geometries of both the anion and neutral states. Time-resolved photoelectron imaging experiments explore the relaxation dynamics of its first excited 1 2B3u state, which we show undergoes internal conversion back to the 2B2g
ground state on a timescale of 650 fs. Results also provide evidence of a wave packet motion on the excited state, which exhibits a characteristic frequency of 30 cm–1.
Finally, we describe, for the first time, a formulism which allows ultrafast relaxation timescales to be extracted from the photoelectron angular distributions of
isoenergetic photoelectron features. As an example, we use the time-resolved photoelectron angular distributions of a nearly isoenergetic feature in the photoelectron images of TCNQ–. From this model we extract a relaxation time for the 1 2B3u state, which quantitatively agrees with those extracted from fits to the features in the photoelectron spectra derived from the images.