SPS Seminar - Action spectroscopy of gas-phase ions

When:  Thursday 15 December at 14.00

Where:  Microsoft Teams – Robert Hooke/Online

Speaker:  James N Bull (University of East Anglia)

Hosted by: Andrew James

Abstract:

Action spectroscopy, usually performed in the gas phase, encompasses a range of spectroscopic techniques that monitor the ‘actions’ of molecules resulting from the absorption of light. Potential actions include ejection of an electron (e.g. photoelectron and photodetachment spectroscopy), fragmentation of the molecule (photodissociation spectroscopy), and isomerisation or changes in molecular shape (photoisomerisation spectroscopy). Our laboratory seeks to develop new action spectroscopy instrumentation and techniques, based on measuring several actions, and apply these methods to investigate fundamental processes and properties, including: (1) understanding the structure and function of photoactive biological chromophores, (2) high-precision molecular structure and property determination, and (3) probing the photochemistry of atmospherically and astrochemically relevant molecules.

The first part of this talk will introduce the technique of photoisomerisation action spectroscopy, which employs elements of ion mobility spectrometry and electronic spectroscopy to prepare and study isomer-selected ions (such reactant selectivity is rarely achievable in solution-based studies) and to probe isomerisations directly. When coupled with other action spectroscopy techniques, including photoelectron, photodetachment, and their femtosecond time-resolved variants, we can obtain unparalleled insight into the detailed excited state dynamics, for example measuring isomeric product distributions, internal conversion routes, and competitive dynamical timescales, and fingerprinting the presence of barriers in excited-state reactions. The second part of the talk will focus on astrochemistry-related studies performed in a cryogenic cooling (T ≈ 13 K) environment under ultrahigh vacuum (P ≈10^-14 mbar), providing ‘molecular cloud in a box’ conditions. Experiments on target species, including carbon cluster anions and radical cations of naphthalene, azulene, and cyanonaphthalene, provide characterisation of heat- and light-induced cooling dynamics over the ultraslow timescale (millisecond to seconds). The measured dynamics include monitoring the competitions between statistical dissociation, recurrent fluorescence, and infrared (vibrational) cooling. Understanding the dynamical interplay of these cooling processes is central to predicting molecular stability and, in turn, abundance of these molecules in space.