Dynamics of unimolecular reactions induced by monochromatic IR radiation

Experiment and theory for CnFmHkI → CnFmHk + I probed with hyperfine-, doppler- and uncertainty-limited time resolution of iodine-atom IR absorption

Yabai He*, Jörg Pochert, Martin Quack, Roland Ranz, Georg Seyfang

*Corresponding author for this work

Research output: Contribution to journalArticle

25 Citations (Scopus)

Abstract

Coherent multiphoton excitation of polyatomic molecules with pulsed CO2 lasers leads to unimolecular reactions induced by monochromatic infrared radiation (URIMIR). We report a detailed study of the dynamics of dissociation of trifluoroiodomethane (CF3I), 1,1,1,2-tetrafluoro-2-iodoethane (CF3CHFI) and pentafluoroiodobenzene (C6F5I). The primary dissociation after ro-vibrational excitation in the electronic ground state results in iodine atoms I(2P3/2), which are detected by diode laser IR absorption on the (2P3/2-2P1/2) magnetic dipole transition with about 1 MHz frequency resolution and up to 1 ns time resolution, essentially bounded by the uncertainty principle. This allows us to detect the product-state distribution over nuclear hyperfine levels in I atoms, and product translational-energy distributions from Doppler lineshapes combined with quantitative, time-resolved kinetic analysis under conditions of irradiation with shape-controlled CO2 laser pulses of well defined fluence and intensity. The kinetic results for absolute rates are analysed in terms of the laser chemical rate coefficient kI(st) and compared to theoretical calculations based on the case B/C master equation including non-linear intensity effects, which are found to be important only for CF3I. The results for relative rates are analysed in terms of a simple theoretical model for the centre-of-mass product translational-energy distribution P(Et). The results are discussed in relation to the foundations of IR laser chemistry.

Original languageEnglish
Pages (from-to)275-300
Number of pages26
JournalFaraday Discussions
Volume102
DOIs
Publication statusPublished - 1995
Externally publishedYes

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