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Femtosecond real‐time observation of wave packet oscillations (resonance) in dissociation reactions

The photodissociation of CH3I at 266 nm is investigated by means of high resolution photofragment spectroscopy. The resolution is sufficient to determine the vibrational population of the CH3 umbrella motion for the I*(2P1/2)+CH3 product channel. An approximate vibrational distribution for the I(2P3/2)+CH3 product channel is also determined. The rotational energy distribution for the CH3+I*(2P1/2) channel is estimated to be on the order of or less than 600 cm−1 wide for each of the CH3 vibrational states. A refined value for the C–I bond dissociation energy of 53.3±0.7 kcal/mole is determined from the energy threshold for the I*+CH3 channel. The vibrational energy distribution for the I*+CH3 channel is discussed in relation to a recent model calculation by Shapiro and Bersohn and possible explanations for the discrepency between the calculated and the measured distributions are considered.

Photofragmentation of CH3I: Vibrational distribution of the CH3 fragment

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.

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

Femtosecond real-time dynamics of photofragment-trapping resonances on dissociative potential energy surfaces

The excitation of the umbrella mode of CH3 and CD3 formed from photodissociation of CH3I and CD3I at 248 nm

Curve crossing and repulsive potentials probed by laser induced half collisions: ICI and IBr

Wavelength dependence of photofragmentation processes of the first excited states of Na and K halides

Extensive photofragmentation studies of three thallium halides TII, TIBr and TICI are presented. At several wavelengths in the near ultraviolet region time of flight spectra and angular distributions of photofragments are measured. They can be interpreted as arising from transitions to excited states of different total electronic angular momentum Ω. It is found that the B state, which correlates with ground state fragments, and the C state, which correlates in the diabatic limit with spin—orbit excited thallium and ground state halogen, give rise to pure perpendicular transitions. This means that these states can be assigned as Ω = 1 states. Curve crossing are observed involving the C state and other Ω = 1 states which correlate with the two lowest dissociation channels. The continuum at shorter wavelengths is found to consist of two contributions. Besides excitation to a state of strong ionic character also transitions to a state of Ω = 1 symmetry dissociating into TI 2P 1 2 + X are of importance. At the shortest wavelength used, 193 nm, the formation of thallium in its first Rydberg state 72S 1 2 becomes energetically possible for TII. As this process has implications for photodissociation lasers, the experiments at this wavelength have been extended to the homologous system Inl. It is found that the quantum yield for the production of the metal in the 2S 1 2 state is 0.53 for TII and 0.25 for Inl. A model involving curve crossing is proposed which can account for this result. In the same experiments bond strengths for the thallium halides and for indium iodide have been determined. The results are for TII/D0 = 2.78 eV, for TIBrD0 = 3.36 eV, for TICID0 = 3.82 eV and for InID0 = 3.29 eV. The uncertainties are 0.02 eV.

Photofragmentation of thallium halides

Curve crossing in two excited states of IBr studied by photofragmentation

A new technique is introduced based on the measurement of kinetic energies and angular distributions of fragments in multiphoton ionization of molecule Both positive and negative ions are measured as well as neutrals. Information is obtained about highly excited repulsive states and about the multiphot ionization process itself. Examples are reported for MPI of I 2 . In a one-laser experiment dissociative autoionization is observed. In a two-laser experiment processes are distinguised, that have different intermediate states. Lifetimes of those states are measured. Information is also obtained ab an ion pair formation process, leading to a reinterpretation of potential curves.

N.J.A. Van Veen

Papers

Curve crossing and repulsive potentials probed by laser induced half collisions: ICI and IBr

Wavelength dependence of photofragmentation processes of the first excited states of Na and K halides

Photofragmentation of thallium halides

Curve crossing in two excited states of IBr studied by photofragmentation

Multiphoton ionization translational spectroscopy (MITS) of I2