My research is focussed on studying the spectroscopy and dynamics of high-lying electronic states of molecules using multiphoton excitation with fluorescence or mass-resolved ion detection.

Spectroscopy

Highly-excited molecular states, both Rydberg and ion-pair, usually lying in the vacuum ultraviolet (VUV) absorption region, are excited by absorption of multiple photons from one or more excimer pumped dye lasers. Multiphoton techniques avoid the experimental difficulties involved with using VUV photons. The multiphoton process can be via virtual4 or real intermediate states² or a combination of the two.^1,6 The samples are rotationally cooled by entraining them in a molecular beam. The population of the states is detected by ionisation following further absorption of one or more additional photons. Mass-selection of the ions is achieved through use of a time-of-flight mass spectrometer (TOFMS) and can be used to simplify spectra when more than one isotope occurs in significant abundance.⁶ These experiments are used to obtain molecular constants of the states.^1,2

Dynamics

The same techniques are used to study the interaction between one or more of these states. In particular, the coupling of ion-pair and Rydberg states in small halogen containing molecules has been widely investigated.⁴

The population of excited states is also detected by fluorescence down to low-lying valence states. In these experiments the sample is held at room temperature. Fluorescence techniques are used to study the transfer between ion-pair states of halogens following collisions with various molecules. Recent experiments have shown how two effects, amplified spontaneous emission (ASE) and collisions with residual water had unknowingly interfered with results reported in the literature.^3,5 Fluorescence is also used to characterise valence states that are not accessible by absorption.

SELECTED RECENT PUBLICATIONS

1. An Optical-Optical Double Resonance Study of the Rydberg States of O 2 . II. 'The np and nf ( ungerade ) states excited via Single Rotational Levels of the b 1 S g + Valence State .' A.M. Sjodin, T. Ridley, K.P. Lawley, and R.J. Donovan, J. Chem. Phys ., 2003, 118 , 8791.
2. 'Observation of a new high-energy, shallow-bound Rydberg state in I 2 by optical triple resonance.' A.M. Sjödin, T. Ridley, K.P. Lawley and R.J. Donovan, Chem. Phys . Lett ., 2005, 412 , 110.
3. 'The influence of amplified spontaneous emission in collisional energy transfer studies exemplified by E 0 g + ( 3 P 2 ) to D 0 u + ( 3 P 2 ) transfer in I 2 .' T. Ridley, K.P. Lawley and R.J. Donovan. Chem. Phys ., 2008, 348 , 227.
4. 'Evidence for Rydberg Doorway States in Photoion-pair Formation in Bromomethane (CH 3 Br).' R.J. Donovan, J.T. Hennessy, K.P. Lawley, T. Ridley, S. Wang, P. Brint, and E. Lane, J. Phys. Chem . A, 2008, 112 , 7170.
5. 'Long-range collisional energy transfer between charge-transfer (ion-pair) states of I 2 , induced by H 2 O and I 2 ( X ).' T. Ridley, K.P. Lawley and R.J. Donovan. J. Chem. Phys . 2009, 131 , 234302.
6. 'Characterization of the potential minimum of the F ¢ 0 u + ( 1 D 2 ) ion-pair state of Cl 2 using (1+2 ¢ ) optical-optical double resonance excitation and mass-resolved ion detection.' T. Ridley a , R.J. Donovan and K.P. Lawley, Submitted to J. Chem. Phys . 2011.