A new measurement of the photodissociation of CH3I at 193 nm is reported in which we use a combination of vacuum ultraviolet photoionization and velocity map ion imaging. The iodine photofragments are probed by single-photon ionization at photon energies above and below the photoionization threshold of I((2)P(3/2)). The relative I((2)P(3/2)) and I*((2)P(1/2)) photoionization cross sections are determined at these wavelengths by using the known branching fractions for the photodissociation at 266 nm. Velocity map ion images indicate that the branching fraction for I((2)P(3/2)) atoms is non-zero, and yield a value of 0.07 ± 0.01. Interestingly, the translational energy distribution extracted from the image shows that the translational energy of the I((2)P(3/2)) fragments is significantly smaller than that of the I*((2)P(1/2)) atoms. This observation indicates the internal rotational/vibrational energy of the CH3 co-fragment is very high in the I((2)P(3/2)) channel. The results can be interpreted in a manner consistent with the previous measurements, and provide a more complete picture of the dissociation dynamics of this prototypical molecule.

Zero-kinetic-energy-photoelectron spectroscopy (ZEKE-PES) and time-of-flight mass spectrometry are used to investigate the near-threshold, two-color photoionization of NO via the A 2~+, v=O level. Pulsed-field ionization of Rydberg states within 20 cm-1 of the ionization threshold is shown to proceed via a diabatic mechanism. Particular emphasis is given to the effect of predissociation on the signal produced by delayed field ionization. A small ( -3 V / cm) dc electric field is shown to produce a dramatic decrease in the field ionization signal in both the NO+ -ion and ZEKE-electron channels. This decrease is thought to be due to an increase in the predissociation rate caused by the dc electric field. The implications of these experiments for mass-analyzed threshold ionization are discussed.

The predissociation dynamics of the vibrationless level of the 6s (B (2)E) Rydberg state of CH(3)I was studied by femtosecond-resolved velocity map imaging of photoelectrons. By monitoring the decay of the CH(3)I(+) produced by photoionizing the B state, the predissociation lifetime was measured to be 1310 ± 70 fs. Photoelectron spectra were recorded as a function of the excitation scheme (one or two photons to the B state), and as a function of the ionizing wavelength. All of these photoelectron spectra show a simple time dependence that is consistent with the decay time of the CH(3)I(+) ion signal. The photoelectron angular distributions for the ionization of the B state depend on the excitation scheme and the ionizing wavelength, and show a strong dependence on the vibrational modes excited in the resulting CH(3)I(+). At long delays, the photoelectron spectra are characterized by photoionization of the I((2)P(1/2)) fragment formed by predissociation of the B state.

The rotationally resolved, zero kinetic energy, pulsed-field ionization (ZEKE-PFI) spectrum of the HI+ X 2Π1/2, v+=0 level, obtained by double-resonance excitation via the HI F 1Δ2, v=0 level, is reported. The rotational and Λ-doubling constants for the HI+ X 2Π1/2, v+=0 level obtained from the experiment are close to those estimated theoretically by Mank et al. [J. Chem. Phys. 95, 1676 (1991)]. At higher pressures, the dissociative charge transfer reaction HI*+HI→HI++H+I− represents a potentially serious loss mechanism for the high Rydberg states that give rise to the ZEKE-PFI signal. This result is of more general applicability, because it provides evidence that collisions of the Rydberg electron with neighboring molecules can play a significant role in ZEKE-PFI experiments.

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