Model preparation of H2O hyperspherical modes by visible versus infrared multiphoton excitation

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Abstract
Multiphoton excitation of hyperspherical modes of H2O is simulated using the quasiresonant approximation (QRA) applied to a model of coupled Morse oscillators representing the two O–H stretches. Highly excited hyperspherical modes are more efficiently and more selectively populated by two‐photon (vis) than by multiphoton (IR) transitions. The IR excitation mechanism is not dominated by a single ladder of exclusively hyperspherical levels, as in the case of a weakly bound ABA model system, but includes interfering pathways via nonhyperspherical modes, which often provide the dominant contributions. Vis two‐photon excitation proceeds via one single (‘‘bond–bond’’) ladder, which to a first approximation is characterized by sequential excitation of the two individual O–H bonds by one photon each. Predicted experimental conditions for selective preparation of highly excited hyperspherical modes call for long (≫ps), intense (≥TW cm−2) laser pulses. These requirements are slightly less demanding than corresponding conditions for the preparation of comparably highly excited local modes.