Energy selective ladder

This reaction has been carried out with a carbon dioxide laser line tuned to the wavelength of 10.61 p.m, which corresponds to the spacing of the lowest few states of the SF ladder. The laser is a high power TEA laser with pulse duration around 100 ns, so that there is no time for energy transfer by coUisions. This example shows the potential for breakup of individual molecules by a tuned laser. As with other laser chemistry, there is interest in driving the dissociation reaction in selected directions, to produce breakup in specific controllable reaction channels. 

While cyclobutadiene itself yields only a dimer ([3]-ladderane) in such process, some derivatives, such as (2b) or (2c), provide higher members of the series. The difference in reactivities stems from a difference in the HOMO-LUMO energy gap of the reactants. The energy gap is readily calculated, providing a way to select the most suitable cyclobutadiene derivative to undergo oligomerization to a ladder. ... 

Figure 10. Surfaces of eigen-energies (in units of 8) as functions of QP/8 and Us/8 for the case /.] < >.2 < -3, with Ap = 8/2 and As = — 8/2. Paths (a) and (b) (with pulses of the same length and peak amplitude) correspond respectively to the intuitive (transfer to 2)) and counterintuitive (transfer to 13)) pulse sequences in A or ladder systems leading to the selective transfer. Paths (a) and (c) correspond to the selective transfer in V systems (for which the initial population resides in 12)), respectively to 11) and 3).

Very fast energy dissipation after vibrational excitation in the electronic ground state, however, has been inferred from picosecond relaxation measurements on large molecules in the gas phase. Moreover the view has been adopted that in solids even at low temperature rapid energy dissipation prevents multiphoton excitation and vibrational ladder climbing in matrix isolated molecules. On the other hand, in recent experiments surface reactions, such as desorption, evaporation and molecular decomposition, stimulated by vibrational multi-quantum excitation with resonant laser infrared have been observed at moderate threshold laser intensity and with high frequency selectivity. 

Of course, the rapidity of V-V energy exchange also has disadvantages. The promotion of reaction by ladder-climbing can only be mode-selective rather than state-selective. If the goal is to measure reaction (or reaction plus relaxation) rates from a specified vibrational level, it is necessary to choose conditions which effectively eliminate the effects of V-V exchange. 

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