Ultrafast Dynamics of Gas Phase Anions

Abstract

The ultrafast dynamics of a number of molecules have been studied in the gas phase through the use of frequency- angle- and time-resolved photoelecron imaging. A particular emphasis has been applied to the behaviour of biologically relevant molecules following photoexcitation. The gaseous ions were produced from an electrospray ionisation source and interrogated by a purpose-built velocity-map imaging photoelectron spectrometer with a minimum temporal resolution of ~50 fs.
Firstly, the details of a global kinetic fitting routine for a time-resolved photoelectron spectrum are presented. Through fitting the constituent photoelectron images to a kinetic fit, the photoelectron angular anisotropy of the constituent features of the time-resolved spectrum is preserved. Secondly, the dynamics of the green fluorescent protein model chromophore following UV excitation were explored, identifying internal conversion of the initially produced excited state population to a lower lying excited state before photodetachment. Thirdly, frequency- and angle-resolved photoelectron imaging is employed to investigate the dynamics of anionic resonances of para-benzoquinone, that have been implicated in facilitating electron attachment to this moiety. Fourthly, the photoelectron spectra of a series of carboxylic acids are presented in order to assess the feasibility of producing anions by attaching carboxylic acids to neutral chromophores. Fifthly, a time-resolved photoelectron spectrum of the biological chromophore and carboxylic acid, all-trans¬-retinoic acid is presented. Finally, highly anisotropic photoelectron spectra of the dianion, antimony tartrate are presented. In order to explain the anisotropy, classical trajectories of electrons on the molecular electrostatic potential energy surface are calculated. From this, the observed anisotropy can be assigned the the shape of the molecular repulsive Coulomb barrier.