Index of content:
Volume 111, Issue 1, 01 July 1999
Femtosecond time-resolved photoelectron–photoion coincidence imaging studies of dissociation dynamics111(1999); http://dx.doi.org/10.1063/1.479248View Description Hide Description
We present the first results using a new technique that combines femtosecond pump–probe methods with energy- and angle-resolved photoelectron–photoion coincidence imaging. The dominant dissociativemultiphoton ionization (DMI) pathway for at 375.3 nm is identified as three-photon excitation to a repulsive potential surface correlating to followed by one-photon ionization to Dissociation along this surface is followed on a femtosecond timescale.
111(1999); http://dx.doi.org/10.1063/1.479249View Description Hide Description
Photofragment translational spectra at 3(HD), and have been obtained for and at 157 nm excitation. Analysis of the time-of-flightspectra reveals two different atomic H loss channels: hydroxyl H elimination, and methyl H elimination. While the hydroxyl H elimination seems to be a single fast process, the methyl H loss exhibits clearly two significantly different mechanisms: one fast and one slow. Experimental results also show two molecular hydrogen elimination channels: three-center elimination from the methyl group, which displays two different micropathways, and four-center elimination involving hydrogen atoms on both the C and O sites. The relative branching of the atomic versus molecular hydrogen elimination channels was found to be 1:0.15. These results present a uniquely clear picture of methanol photodissociation at 157 nm, and thus provide an excellent case for quantitative theoretical investigations.
111(1999); http://dx.doi.org/10.1063/1.479250View Description Hide Description
The effect of ionic drift caused by small constant electric field on autocatalytic reaction fronts of ionic species is studied both theoretically and numerically. Besides varying the velocity of propagation, the electric field parallel to the direction of propagation may induce lateral instability in planar fronts resulting in the emergence of cellular structures. The difference in the diffusivities at the onset of instability are lowered when the electric field tends to separate the species spatially. The predictions of the linear stability analysis based on a thin-front approximation are confirmed by the numerical integration of the full two-dimensional system.
Surface vibrational coherence at the /air interface: Vibrational wave packet dynamics as a probe of interface inhomogeneity111(1999); http://dx.doi.org/10.1063/1.479251View Description Hide Description
Vibrational wave packet dynamics from a monolayer-covered surface are reported. These dynamics reflect surface vibrational coherence in a monolayer of amphiphilic molecules deposited at the /air interface. The induced macroscopic polarization following coherent excitation of adsorbate vibrational modes displays quantum interferenceeffects (quantum beats) and decays on a time scale dependent on the nature of the interfacial environment. These observations provide a link between the degree of interfacial order and the vibrational coherence lifetime and demonstrate that monitoring interfacial wave packet dynamics represents a new method for characterizing these important chemical regions.