Knowledge Bank

A pulsed, doubled, dye laser is used to excite individual lines in the $A\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}Au-X\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}\Sigma g\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$ (i.e. $S1-S0$) system. Lines occuring in vibrational bands with 1 to 4 quanta of trans-bending vibration have been studied. Quantum beats and anticrossing spectra enable natural lifetime limited resolution in measurements on individual, room temperature. molecules. Anticrossings are seen as dips in the total fluorescence as the applied magnetic field is scanned and nearby triplet levels are tuned into near resonance with the excited singlet. The data reveal interactions of each excited $S1$ revibronic level with nearby triplet levels (T) and extremely highly vibrationally excited levels of the ground electronic state. Our results shed light on some of the important properties of a molecule with a chemically interesting' amount of energy. Densities of vibrational states for the (optically inaccessible) T and $S_{0}$ states at this energy can be estimated. The range in size of interaction matrix elements between levels belonging to the (at least) three electronic states involved can be found. These properties determine energy transfer within the molecule by the processes of intersystem crossing ($S_{1}$ to T) and internal conversion ($S_{1} - S_{0}$). Our promoter' model explains clusters of small anticrossings as being a three-way interaction where a large $S1-T$ anticrossing displaces the $S1$ level so that it traverses several nearby $S0$ levels, anticrossing with each of them. What we have yet to understand is why anticrossings suddenly display this cluster structure at $V3+=3$ as the number of quanta excited in this trans-bending mode is increased. Sicne SEP experiments (1) have shown that $S0$ accetylene displays behavior suggestive of quantum ergodicity at much lower energies, a small increment in energy should not profoundly affect the vibrational character of $S0$ levels at 45,000 $cm-1$: they should eachbe filling most of the energetically accessible phase space. Moreover, the density of $S0$ states is not increasing rapidly: neither the small, fractional increase in their vibrational energy nor dissociation channel effects can produce significant changes here. Thus, $S0$ interactions with an $S1-T$ anticrossing should not change rapidly with energy, yet they seem to do so.