Roaming reactions comprise a new class of reaction in which a molecule undergoes frustrated dissociation to radicals, followed by an intramolecular abstraction reaction. Nitro compounds have long been known to dissociate to give NO as a major product. However, rates based upon isomerization via calculated tight transition states are implausibly slow, so the key dissociation pathway for this important class of molecules remains obscure. Here, we present an imaging study of the photodissociation of nitrobenzene with state-specific detection of the resulting NO products. We observe a bimodal translational energy distribution in which the slow products are formed with low NO rotational excitation, and the fast component is associated with high rotational excitation. High-level ab initio calculations identified a 'roaming-type' saddle point on the ground state. Branching ratio calculations then show that thermal dissociation of nitrobenzene is dominated by 'roaming-mediated isomerization' to phenyl nitrite, which subsequently decomposes to give C(6)H(5)O + NO.

Velocity map ion imaging of the H atoms formed in the photodissociation of vibrationally excited ammonia molecules measures the extent of adiabatic and nonadiabatic dissociation for different vibrations in the electronically excited state. Decomposition of molecules with an excited symmetric N-H stretch produces primarily ground state NH(2) along with a H atom. The kinetic energy release distribution is qualitatively similar to the ones from dissociation of ammonia excited to the electronic origin or to several different levels of the bending vibration and umbrella vibration. The situation is very different for electronically excited molecules containing a quantum of antisymmetric N-H stretch. Decomposition from that state produces almost solely electronically excited NH(2)*, avoiding the conical intersection between the excited state and ground state surfaces. These rotationally resolved measurements agree with our previous inferences from lower resolution Doppler profile measurements. The production of NH(2)* suggests that the antisymmetric stretching excitation in the electronically excited molecule carries it away from the conical intersection that other vibrational states access.

Comparing the recoil energy distributions of the fragments from one-photon dissociation of phenol-d(5) with those from vibrationally mediated photodissociation shows that initial vibrational excitation strongly influences the disposal of energy into relative translation. The measurements use velocity map ion imaging to detect the H-atom fragments and determine the distribution of recoil energies. Dissociation of phenol-d(5) molecules with an initially excited O-H stretching vibration produces significantly more fragments with low recoil energies than does one-photon dissociation at the same total energy. The difference appears to come from the increased probability of adiabatic dissociation in which a vibrationally excited molecule passes around the conical intersection between the dissociative state and the ground state to produce electronically excited phenoxyl-d(5) radicals. The additional energy deposited in electronic excitation of the radical reduces the energy available for relative translation.

Vibrational action spectroscopy of jet-cooled formic acid dimer measures the frequency of the C-H(D) stretching vibration and its coupling to nearby states. The action spectrum of (DCOOH)2 reveals a specific Fermi resonance between the C-D stretch and two antisymmetric combination states formed from the C-O stretch and DCO bend. A three-state deperturbation analysis shows that there is a relatively strong coupling between the fundamental vibration and each of the combination vibrations (mid R:13 cm(-1)mid R:) as well as between the combination states themselves (mid R:7 cm(-1)mid R:). This situation contrasts with that for the action spectrum of (HCOOD)2, where the C-H oscillator is isolated and not strongly coupled to other states.

Velocity map ion imaging of H atoms from the photodissociation of vibrationally excited ammonia molecules measures the angular distribution of the fragments from dissociation of the excited state symmetric N-H stretch (1 1 ) and the antisymmetric N-H stretch (3 1 ). For the symmetric stretching state, the high rotational energy NH 2 fragments recoil with an angular distribution whose maximum is parallel to the polarization direction of the lasers, and the low rotational energy fragments recoil with a perpendicular angular distribution. This behavior is similar to that observed for photodissociation from the origin (0 0 ) but with a larger range in the anisotropy parameter ( 2 ) caused by additional alignment in the two-photon excitation. The dissociation through the excited state antisymmetric N-H stretch produces primarily electronically excited products (NH Ã 2 þ H). The initial alignment of molecules in the antisymmetric stretching state is a combination of perpendicular alignment from the vibrational excitation and parallel alignment from the electronic excitation. Fitting the more complicated angular distributions for the NH Ã 2 channel requires a higher order term ( 4 ) that also varies with recoil energy to describe the angular distribution.