  The predissociation dynamics of the vibrationless level of the first Rydberg
state 6s (B 2E) state of CH3I has been studied by femtosecond-resolved velocity
map imaging of both the CH3 and I photofragments. The kinetic energy
distributions of the two fragments have been recorded as a function of the
pump-probe delay, and as a function of excitation within the umbrella and
stretching vibrational modes of the CH3 fragment. These observations are made
by using (2+1) Resonant Enhanced MultiPhoton Ionization (REMPI) via the 3pz
2A2" state of CH3 to detect specific vibrational levels of CH3. The vibrational
branching fractions of the CH3 are recovered by using the individual
vibrationally state-selected CH3 distributions to fit the kinetic energy
distribution obtained by using nonresonant multiphoton ionization of either the
I or CH3 fragment. The angular distributions and rise times of the two
fragments differ significantly. These observations can be rationalized through
a consideration of the alignment of the CH3 fragment and the effect of this
alignment on its detection efficiency. Two extra dissociation channels are
detected: one associated with Rydberg states near 9.2 eV that were observed
previously in photoelectron studies, and one associated with photodissociation
of the parent cation around 15 eV.
