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 Ondash;H stretches. Highly excited hyperspherical modes are more efficiently and more selectively populated by twohyphen;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 twohyphen;photon excitation proceeds via one single (lsquo;lsquo;bondndash;bondrsquo;rsquo;) ladder, which to a first approximation is characterized by sequential excitation of the two individual Ondash;H bonds by one photon each. Predicted experimental conditions for selective preparation of highly excited hyperspherical modes call for long (Gt;ps), intense (ge;TW cmminus;2) laser pulses. These requirements are slightly less demanding than corresponding conditions for the preparation of comparably highly excited local modes.
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