- Time-resolved infrared-spectroscopic observation of relaxation and reaction processes during and after infrared-multiphoton excitation of 12CF3I and 13CF3I with shaped nanosecond pulses
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We have produced shaped infrared laser pulses of several kinds ranging from about 2-100 ns duration using a line tuned CO2 laser combined with intracavity absorbers and CdTe electro-optical switch.The time-dependent infrared absorption of 12CF3I and 13CF3I during and after infrared-multiphoton excitation with these pulses was followed by means of a line tuned continuous wave-CO2 laser and a fast HgCdTe infrared detector (time resolution about 1 ns).The effective time-dependent absorption cross section shows fluence-dependent decay at large fluence with an effective exponential decay constant k1,? ca. 1.12 cm2J-1.This can be interpreted by first generation and then decay by further radiative pumping of highly excited levels of CF3I.The results have been analyzed by master equation modeling using a nonlinear case B/C master equation for multiphoton excitation and very simple models for the absorption properties of highly excited molecules.After nanosecond excitation to very high levels, one finds unimolecular decay CF3I --> CF3 + I with distinct rate constants (2+/-1) * 108 and (5+/-4) * 106 s-1, which corresponds to ensembles of molecules differing by one CO2-laser quantum of energy, in agreement with unimolecular rate theory and master equation models.The most striking observation is a slow, collision-free intramolecular rovibrational redistribution process observed by real time spectroscopy on the nanosecond time scale for molecules excited by modest fluence corresponding to typical average energies of five CO2 laser quanta and somewhat more.
- Quack, Martin,Schwarz, Rene,Seyfang, Georg
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p. 8727 - 8740
(2007/10/02)
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