State-resolved Dynamics Research Articles

Photodissociation dynamics in quantum state-selected clusters: A test of the one-atom cage effect in Ar–H2O

High resolution IR overtone pumping with an injection seeded optical parametric oscillator (OPO) is used in conjunction with excimer laser photolysis to investigate the state-resolved dynamics of quantum state-selected van der Waals clusters in a slit supersonic expansion. The narrow band IR light source (160 MHz, 5 mJ) preselects a specific upper state via the internal rotor band of Ar–H2O which correlates to the ‖03〉−←‖00〉+, 000←101 transition in H2O monomer. At fixed UV photolysis and probe wavelength, scanning the high resolution OPO yields the ‖03〉−←‖00〉+, Σ(000)←Σ(101) overtone action spectrum of Ar–H2O complexes. Conversely, at fixed IR pump wavelength, the state distribution of the OH photoproduct from photolysis of quantum state selected Ar–H2O clusters can be probed by laser induced fluorescence on the A 2Σ+←X 2Π (0,0) band. The OH distributions from H2O monomer vs Ar–H2O photolysis from the same internal rotor state are remarkably similar, though significant anomalies are observed for specific K rotational levels.

Rotation–vibration state resolved unimolecular dynamics of highly excited CH3O (X̃ 2E). II. Intramolecular vibrational dynamics of excited ‘‘C–O stretch’’ states

The stimulated emission pumping (SEP) spectra of highly excited CH3O (X̃ 2E) reported in the preceding paper [A. Geers, J. Kappert, F. Temps, and J. Wiebrecht, J. Chem. Phys. 101, 3618 (1994); paper I] are analyzed to extract quantitative data on the collision-free intramolecular vibrational dynamics of the molecules. Attention is focused on the spectra and dynamics of the excited C–O stretch vibration (ν3) states at energies between 3 000 cm−1≤E≤10 000 cm−1. The spectra are found to exhibit a rather sudden transition at E≊5000 cm−1 from the regular, assignable properties at low energies to the ‘‘statistical’’ region at high energies. This IVR ‘‘threshold’’ is approximately 2000 cm−1 below the asymptotic H–H2CO dissociation limit of the molecule. The observed densities of states at these energies approach the full J- and symmetry-sorted rovibronic densities estimated from harmonic state counts. The nearest-neighbor level spacing distribution in the spectra and the magnitude of the inferred root mean square level coupling matrix elements support the assumption of nearly ‘‘ergodic’’ molecular properties in this region. From computed time autocorrelation functions, the IVR lifetimes of the highly excited C–O stretch states are found to be of the order of τ≊0.2–0.3 ps (v3=5 to 6). Evidence is obtained from some spectra for two-tier IVR processes with characteristic time scales of 0.2 and 2 ps, respectively. The highest excited C–O stretch states at energies above ≊8000 cm−1 (v3≥8) appear to be coupled only weakly to neighboring background states. This last observation hints at the existence of quasistable periodic orbits, which are embedded in the quasicontinuum of dissociative states at these energies.

Rotation–vibration state resolved unimolecular dynamics of highly vibrationally excited CH3O (X̃ 2E). I. Observed stimulated emission pumping spectra

Using the technique of stimulated emission pumping (SEP) spectroscopy, highly excited vibration–rotation states of the CH3O (X̃ 2E) molecule were probed up to energies of E≤10 000 cm−1. The highest excitation energies exceed the asymptotic H–H2CO dissociation limit of the molecule [ΔrH00(H–H2CO)≊6900 cm−1]. Work was carried out at different experimental resolutions. First, low resolution survey SEP spectra were found to exhibit persistent vibrational structure up to energies far above the dissociation limit. The observed main features were found to be assignable, in a zero-order picture that leaves aside possible mode-to-mode couplings, to the progression of the excited C–O stretch vibration states (ν3). The widths of the respective features correspond to localized short-time vibrational motion for times of ≥0.3 ps (≥10 C–O vibrational periods). Second, in high resolution scans over the coarse vibrational features, characteristic clumps of individual vibration–rotation eigenstates were revealed. These clumps are ascribed to distinctive Franck–Condon active bright zero-order levels which are mixed with the large number of Franck–Condon inactive dark bath states. Under carefully selected conditions, the clumps could be attributed to states with defined and well known values of the total angular momentum quantum number J, which remains as a good quantum number in different coupling cases. These clump spectra will be analyzed quantitatively in the following paper with respect to their bearing for the intramolecular vibrational dynamics of highly excited CH3O (X̃) as a function of vibrational and rotational excitation. From the observed spectra, quantitative data can be obtained on the rate and extent of collision-free intramolecular vibrational and rovibrational energy redistribution (IVR and IRVR) processes, which would result after coherent ultrashort pulse excitation of the molecules.

Adsorption and desorption dynamics as seen through molecular beam techniques

A description of the history of knowledge about adsorption and desorption dynamics is given. The individual stations include the encounter with non-cosine, non-Maxwellian distributions of adsorbing and desorbing particles; detailed balancing in its development as a tool to relate adsorption and desorption data is described. A further section treats the concept of precursor mediated adsorption and its verification by molecular beam methods. The problem of surface defects is briefly touched. Refinements in the molecular beam techniques finally lead to the possibility to gain state resolved dynamics data for adsorption and desorption processes.

Some rotations like it hot: selective energy partitioning in the state resolved dynamics of collisions between CO2 and highly vibrationally excited pyrazine

The collisional quenching of highly vibrationally excited pyrazine by CO2 molecules has been studied with high resolution diode laser spectroscopy. The vibrationally hot pyrazine molecules are formed by 248 nm excimer laser pumping, followed by rapid radiationless transitions to the ground electronic state. The nascent rotational population distributions in the 0000 and 0001 vibrational levels of CO2 produced by collisions with hot pyrazine were probed at short times following excitation of pyrazine by the excimer laser pulse. In addition, the CO2 translational recoil velocity was measured for a number of rotational levels in each vibrational state. The results of these experiments reveal that very little rotational and translational excitation accompanies the energy transfer from hot pyrazine to excited vibrational levels of CO2. In contrast, rotational excitation of the CO2 ground state due to collisions with highly excited pyrazine is significant and is accompanied by a substantial enhancement in the CO2 translational energy. These results are consistent with a picture in which vibration-vibration (V → V) energy transfer processes, leading to vibrational excitation of the bath, are dominated by long range attractive forces, and vibration-translation/rotation (V → T/R) energy transfer, which leaves the bath vibrations unexcited, is dominated by short range repulsive forces.

A crossed-beam study of the state-resolved dynamics of CH(X 2Π) + D2. II. The isotopic exchange channel

The state-to-state integral cross sections for the isotopic exchange reaction of CH(X 2Π) with D2 to produce CD(X 2Π) have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. To understand some of the finer details of the reaction dynamics similar experiments were also carried out on the reaction CD(X 2Π) with H2 to give CH(X 2Π). For the isotopic exchange channel, the cross section decreased rapidly with increasing translational energy, signifying a complex formation reaction mechanism. The CD(CH) product rotational level distributions are substantially colder than a statistical expectation and are interpreted as the result of a multiple-impact collision between the receding products in the exit channel. A novel frequency-locking mechanism, similar to that proposed to understand the dynamics of inelastic collisions between CH and D2 (the preceding paper), is suggested to explain anomalous peaks in the product rotational level distribution for the isotopic exchange channel. However, the fine-structure state distributions for the reactive process, which are very different from that for the inelastic channel, cannot be rationalized by conventional interpretation. Some basic concepts about fine-structure selectivity in chemical reactions have been developed. With these concepts, it is conjectured that the fine-structure state distributions of open-shell molecules arising from reactive encounters could provide a ‘‘fingerprint’’ of the electronic wave function at the transition state.

A crossed-beam study of the state-resolved dynamics of CH(X 2Π)+D2. I. The inelastic scattering channel

The state-to-state integral cross sections for the inelastic scattering of CH(X 2Π) and D2 to produce rotationally excited CH(X 2Π) product have been measured in a crossed-beam apparatus by the laser-induced fluorescence method. Two types of measurements were performed: (1) the translational energy dependence of an individual quantum state of the product and (2) the state distribution of the products at fixed and well-defined translational energy. For the inelastic scattering channel, the cross sections gradually increased from a dynamical threshold to a broad maximum and then slowly decreased as the translational energy increases. Evidence for multiple-impact rotational rainbows was found and a possible frequency-locking phenomenon between the two receding rotors resulted. Moderate orbital alignment was observed except for the highest rotational levels of the product. By comparing and contrasting the kinematically similar system CH(X 2Π)+He, the influence of a strongly attractive potential energy surface on the inelastic scattering of CH+D2 was inferred. Combining the results of the inelastic scattering and the isotopic exchange channels (the following paper) provide an unprecedented look into the dynamics of collisions between CH(X 2Π) and D2.

Crossed-beam investigations of state-resolved collision dynamics of simple radicals

Abstract A critical survey is given of new developments in the study of collision dynamics of diatomic radicals. The impetus for this survey comes from new experimental results on near state-to-state scattering dynamics of radicals with simple collision partners that have been obtained using the crossed-beam technique. These results, on both inelastic and reactive scattering, provide a unique opportunity to compare experimental observations with formal quantum scattering theory. Photodissociation is also briefly discussed because of its relevance to our fundamental understanding of fine-structure effects associated with radicals arising in all three processes. A few basic concepts pertaining to the dynamics of open-shell systems are outlined. The emphasis is on the underlying physical principles rather than on detailed theoretical treatments. To illustrate the strong interaction between theory and experiment a few case studies of systems investigated under crossed-beam conditions are highlighted. These new results also prompt an appraisal of what an experimentalist should be most anxious to measure and indicate specific areas in which further theoretical developments can be most profitable in the future.

State-resolved photofragmentation dynamics of Fe(CO)5 at 193, 248, 266, and 351 nm

The photofragmentation dynamics of Fe(CO)5 in a supersonic molecular beam have been studied for photolysis wavelengths of 193, 248, 266, and 351 nm. The CO photofragments formed under these collision-free conditions were detected by vacuum ultraviolet laser-induced fluorescence (VUV LIF). This allowed for the determination of the rotational and vibrational distributions for the CO products and, by using Doppler spectroscopy, the translational energy distributions were also determined. These data are presented along with the details of a statistical model calculation which reproduces the experimental data very precisely. The statistical model for the photodissociation dynamics, which works at all photolysis wavelengths studied, is based on sequential elimination of CO ligands, with complete energy randomization in the intermediate ground state Fe(CO)n fragments between elimination steps. The detailed mechanism for the ultraviolet photochemistry of Fe(CO)5 is discussed in light of these results and previous experimental and theoretical work.

State-resolved dynamics of chemical reactions at surfaces

State-resolved dynamics of chemical reactions at surfaces