Chemical Physics zyxwvutsrqpo ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Chemical Physics 184 ( 1994) 357-363 Resonant two-photon ionization detection of atomic iodine resulting from photodissociation of ally1 iodide under vibrational (C-H overtone) excitation A.V. Baklanov, V.P. Maltsev Institute of Chemical Kinetics and Combustion, Institutskaya 3, Novosibirsk 63(X790, Russian Federation L. Karlsson, B. Lindgren, U. Sassenberg Stockholm University, Department of Physics, Box 6730. S-113 85 Stockholm Sweden Received 25 November 1993; in final form 7 February 1994 Abstract The resonant two-photon ionization (R2PI or ( 1 + 1) REMPI) method was used to achieve highly sensitive detection of atomic iodine. The atoms were excited and ionized by tuned VW and visible radiation, subsequently. The formation of iodine atoms in the fine structurestates 2P3,2 and 2P,,2 was observed after the photolysis of ally1 iodide (C,H,I) by UV radiation (355 nm). The atomic iodine absorption at 151.469 and 159.358 nm corresponding to the transitions 2Ps,s-(3P2)5d[3]s,2 and ‘PllT ( 3P2) 5d[ 21s/2, respechvely, were obtained under both collisional and collisionless conditions. It was shown, that the distribution of I atoms on hyperhne structurelevels, produced from the photodissociation of CsHsI did not differ from the equilibrium one. The time-profile of atomic iodine appearancewas measured when C3HSI was excited at the C-H vibrational overtone frequency (v=5, h=703.5 nm). zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction The pumping of C-H, O-H or N-H vibrational over- tones are examples of the universal method of the nar- row bandwidth excitation of molecules to study their dynamics on the ground state electronic surface [ 11. The unimolecular dissociation reactions of several mol- ecules excited through vibrational overtones were investigated by time-resolved techniques [2-51. In these papers, the kinetics or the time-profile of the dis- sociation product appearance was measured as a func- tion of the initial molecular excitation energy. The unique property of the tetramethyldioxetane TMDO molecule was used by Crim et al. [ 21. They detected the fluorescence of the electronically excited acetone molecule produced by the TMDO dissociation. The laser induced fluorescence (LIF) technique was used by Rizzo and Ctim [ 31, and Sherer et al. [ 41 for detec- tion of the O-H radical after the excitation of tert- butylhydroperoxide t-C&,OOH and hydrogen peroxide H,O, molecules, respectively. The LIF method was also used by Foy et al. [ 51 in an overtone excitation experiment on HNz. To extend such direct measurements to other reac- tions, it is necessary to solve the product detection sensitivity problem. The overtone transitions are char- acterized by very small absorption cross-sections [ 61. This is the reason why it is not possible to obtain a high concentration of the dissociation product. We believe, 0301-0104/94/$07.00 8 1994 Elsevier Science B.V. All rights reserved SSD10301-0104 (94)00076-M