Femtosecond real-time probing of reactions. II. The dissociation reaction of ICN

Experimental results obtained for the dissociation reaction ICN^*→[I⋅⋅⋅CN]^(‡*)→I+CN using femtosecond transition‐state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a "dissociation" time of 205±30 fs was measured and was related to the length scale of the potential. The transition states live for only ∼50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecondcoherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES

Femtosecond real-time probing of reactions. I. The technique

When a chemical bond is broken in a direct dissociationreaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10^(−13) s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as "transition states." Femtosecond transition‐state spectroscopy (FTS) is a real‐time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real‐time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s)

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

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

Real-time femtosecond probing of "transition states" in chemical reactions

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Femtosecond real-time probing of reactions. I. The technique

When a chemical bond is broken in a direct dissociationreaction, the process is so rapid that it has generally been considered instantaneous and therefore unobservable. But the fragments formed interact with one another for times on the order of 10^(−13) s after the photon has been absorbed. On this time scale the system passes through intermediate transition configurations; the totality of such configurations have been, in the recent literature, designated as "transition states." Femtosecond transition‐state spectroscopy (FTS) is a real‐time technique for probing chemical reactions. It allows the direct observation of a molecule in the process of falling apart or in the process of formation. In this paper, the first in a series on femtosecond real‐time probing of reactions, we examine the technique in detail. The concept of FTS is explored, and the interrelationship between the dynamics of chemical reactions and molecular potential energy surfaces is considered. The experimental method, which requires the generation of spectrally tunable femtosecond optical pulses, is detailed. Illustrative results from FTS experiments for several elementary reactions are presented, and we describe methods for relating these results to the potential energy surface(s)

Femtosecond real-time probing of reactions. II. The dissociation reaction of ICN

Experimental results obtained for the dissociation reaction ICN^*→[I⋅⋅⋅CN]^(‡*)→I+CN using femtosecond transition‐state spectroscopy (FTS) are presented. The process of the I–CN bond breaking is clocked, and the transition states of the reaction are observed in real time. From the clocking experiments, a "dissociation" time of 205±30 fs was measured and was related to the length scale of the potential. The transition states live for only ∼50 fs or less, and from the observed transients we deduce some characteristics of the relevant potential energy surfaces (PES). These FTS experiments are discussed in relation to both classical and quantum mechanical models of the dynamical motion, including features of the femtosecondcoherence and alignment of fragments during recoil. The observations are related to the radial and angular properties of the PES