Spin-orbit Distributions Research Articles

Rotational and spin-orbit distributions of photochemically desorbed molecules.

Rotational and spin-orbit distributions of photochemically desorbed molecules.

NO(X 2Π) product state distributions in molecule–surface collision-induced dissociation: Direct inelastic scattering of n,i-C3F7NO from MgO(100) at Eincident≤7.0 eV

Molecule–surface collision-induced dissociation (CID) has been studied for n-C3F7NO and i-C3F7NO molecular beams scattered from MgO(100) at incident kinetic energies (Eincident) up to 7 eV. The NO fragment was detected state selectively using two-photon, two-frequency ionization, and rotational and spin–orbit distributions are reported for several Eincident values. State and angle-resolved signals were integrated to give CID yields, which increased sharply with Eincident . In most cases, rotational excitation could be described by separate temperatures for each spin–orbit state. The upper 2∏3/2 state was underpopulated relative to statistical predictions (e.g., for n-C3F7NO at Eincident =5.0 eV, the spin–orbit temperature was ∼170 K, while Trot was ∼500 K). The CID results are compared to NO state distributions derived from the photodissociation of expansion-cooled molecules under collision-free conditions, at different energies (E°) above D0. These distributions were measured for both n-C3F7NO and i-C3F7NO up to E°∼4500 cm−1, and rotational excitation within each spin–orbit state was statistical, except at E°≥3000 cm−1. As with CID, a low [2∏3/2]/[2∏1/2] ratio was observed, and the reaction mechanism is probably unimolecular decomposition via the lowest triplet surface T1 with little or no exit channel barrier. The pronounced similarities between the CID and photodissociation results suggest that common reaction mechanisms may be operative. All of the CID results are compatible with direct inelastic scattering followed by unimolecular reaction on the S0 and/or T1 potential surfaces.

The unimolecular reaction of t-BuNO on singlet and triplet surfaces: Spectroscopy, real-time rate measurements, and NO energy distributions

The predissociation of jet-cooled (CH3)3CNO (t-BuNO) following laser excitation in the à 1A″←X̃ 1A′ system has been studied in both the energy (frequency) and time domains. Unlike the smaller nitroso compounds whose predissociation has been already examined, unimolecular reaction is the rate determining step for predissociation near threshold. Consequently, it has been possible to separately measure radiationless transition rates and unimolecular reaction rates in real time. Dissociation on both the ground state (S0) and the first triplet state (T1) has been identified. At threshold, dissociation proceeds only on S0, with lifetimes >3.5 μs, but for E°≥650 cm−1, fast (<10 ns) predissociation via T1 becomes progressively the dominant dissociative route. Nascent NO photofragments have been characterized in detail using one-photon LIF. The rotational and spin-orbit distributions of NO following dissociation on S0 are statistical, depending only on E°. The NO derived from dissociation on the T1 surface is not at all statistical, consistent with a sizable (∼650 cm−1) exit channel barrier on this surface. Most notably, the [NO(2Π3/2)]/[NO(2Π1/2)] ratios are much smaller, and rotational distributions are colder than predicted by a statistical model. In order that some of the predissociating vibrational states could be assigned, the 1A″←1A′ spectrum of t-BuNO has been analyzed and the electronic origin assigned as 13 911 cm−1. The ground state dissociation energy, D0, is found to be 13 930±30 cm−1, i.e., 39.8±0.1 kcal mol−1.

Vibrational predissociation of the nitric oxide dimer: Total energy distribution in the fragments

Rotational, spin-orbit, lambda doublet, and kinetic energy distributions were measured by laser-excited fluorescence techniques for the nitric oxide fragments formed from the vibrational predissociation of nitric oxide dimers in a free jet expansion. The NO fragments, produced following excitation in the dimer ν1 fundamental, were described by a rotational ‘‘temperature’’ of TR≂100 K, with full equilibration of lambda doublet states, and approximately equal populations in the two spin-orbit states. The velocity distributions were isotropic with an average fragment kinetic energy of 400 cm−1. Time-resolved measurements placed a 15 ns upper limit on the predissociation lifetime.

NO( ν = 1) scattering from Ag(111)

Application of laser excitation and detection techniques has allowed us to perform state-specific studies of the scattering of a molecular beam of vibrationally excited NO molecules from a Ag(111) surface. State-specific angular and velocity distributions have been obtained which indicate a direct-inelastic type of scattering mechanism. This information is combined with rotational and electronic (spin-orbit) distributions to determine the probability for survival of vibrational excitation, SP = 0.9.