Femtosecond real-time probing of reactions. IV. The reactions of alkali halides

Abstract

The photodissociation dynamics of some alkali halides are explored via the method of femtosecond transition-state spectroscopy (FTS). The alkali halide dissociation reaction is influenced by the interaction between the covalent and the ground state ionic potential energy surfaces (PES), which cross at a certain internuclear separation. Depending upon the adiabaticity of the PES, the dissociating fragments may be trapped in a well formed by the avoided crossing of these surfaces. Here, we detail the FTS results of this class of reactions, with particular focus on the reaction of sodium iodide: NaI*→[Na---I]°* →Na+I. As in our first report [T. S. Rose, M. J. Rosker, and A. H. Zewail, J. Chem. Phys. 88, 6672 (1988)], we observe the dynamical motion of the wave packet along the reaction coordinate and the crossing between the covalent and ionic surfaces. The studies presented here characterize the effects of various experimental parameters, including pump and probe wavelengths, on the dynamics of the dissociation and its detection. Comparisons of the results with classical and quantum mechanical calculations are also presented.

Pump-Probe Photoionization Study of the Passage and Bifurcation of a Quantum Wave Packet Across an Avoided Crossing

“…A typical example, and the one of interest here, is intramolecular electron transfer induced by vibrational motion in the excited state of alkali halides such as the NaI molecule. Pump-probe studies of this system have demonstrated the decrease of wave packet population on the excited adiabatic surface due to dissociation at the avoided crossing [3,4]. Oscillations in the population of the dissociative products (Na and I) due to the interference of wave packets on the covalent and ionic potentials merging at the avoided crossing have also been observed [3].…”

“…Pump-probe studies of this system have demonstrated the decrease of wave packet population on the excited adiabatic surface due to dissociation at the avoided crossing [3,4]. Oscillations in the population of the dissociative products (Na and I) due to the interference of wave packets on the covalent and ionic potentials merging at the avoided crossing have also been observed [3]. Although bifurcation of wave packets must be invoked to explain these observations, no real-time evidence of the instance of wave packet bifurcation has yet been experimentally observed.…”

“…The pump-probe spectroscopy of NaI has been studied extensively [3,4,[15][16][17][18][19][20][21]. Figure 1 depicts the pump-probe scheme and the relevant potential curves for femtosecond photoelectron spectroscopy of the excited-state wave packet dynamics.…”

Anatomy of an Elementary Chemical Reaction

“…3). The wavepacket oscillates back and forth several times in the well formed by the covalent and ionic surfaces, and a small fraction is lost at each oscillation (18).…”

Time-Resolved Laser Spectroscopy

“…The same group also studied the pre-dissociation of NaI by resolving the molecular vibrations when probing the dissociation products. [10,11] There were of course also more complicated reactions studied during this early era of femtoseconds. The Diels-Alder reaction, for example, which is of great importance in organic chemistry, [12] since it is stereospecific.…”