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Probing transition states in photodissociation reactions by femtosecond XUV transient absorption

HT-03

Probing transition states in photodissociation reactions by femtosecond XUV transient absorption


A. R. Attar1*, A. Bhattacherjee1, S. R. Leone1

1Department of Chemistry, University of California, Berkeley and Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

Femtosecond extreme ultraviolet (XUV) pulses produced by high harmonic generation are used to probe photodissociation reactions of methyl iodide and allyl iodide by the real-time evolution of core-to-valence transitions near the iodine N edge. The gaseous alkyl iodide molecules are photoexcited by an ultraviolet (266 nm) pump pulse, accessing repulsive valence-excited states, which dissociate along the C-I bond to form I(2P3/2) or I*(2P1/2) and the corresponding alkyl radical. On a sub-100 fs timescale, during the process of C-I bond breaking, new core-to-valence electronic states appear in the iodine N pre-edge absorption spectra, indicative of transition states, which decay concomitantly with the rise of the characteristic atomic iodine resonances. The transient features observed during dissociation are assigned to repulsive valence-excited transition states that connect with core-electron transitions and represent the evolution of the valence electronic structure during the bond-breaking reactions. The dissociation times are measured by monitoring the rise of sharp atomic iodine resonances.[1] In the case of allyl iodide, the transition state resonances peak after ~50 fs and decay to complete C-I dissociation in the major I* product channel in ~70 fs.

(a) Differential absorption spectra of allyl iodide near the atomic iodine pre-edge resonances, at 50 and 500 fs following 266 nm excitation. (b) Time-resolved differential absorption lineouts of allyl iodide.

(a) Differential absorption spectra of allyl iodide near the atomic iodine pre-edge resonances, at 50 and 500 fs following 266 nm excitation. (b) Time-resolved differential absorption lineouts of allyl iodide.

References:

[1] A. R. Attar, L. Piticco, and S. R. Leone, J. Chem. Phys. 141, 164308 (2014).