Ladder climbing and multiphoton dissociation of polyatomic molecules excited with short pulses: Basic theory and applications to HCO

We present a numerically efficient algorithm for multiphoton dissociation of molecules with light pulses in the infrared frequency regime. The shape of the external electric field can be arbitrary. The method is essentially based on discretizing the continuum; dissociation is simulated by a coordinate-dependent cutoff function. A substantial reduction of the computer time required for solving the large set of first-order differential equations in time $\left(N\approx 2000\right)$ can be achieved if the interaction with the field can be separated according to $Ŵ\mathrm{}\left(t\right)=E\left(t\right)\mathrm{}{\hat{\mu }}_{\mathrm{eff}},$ where ${\hat{\mu }}_{\mathrm{eff}}$ is an effective coordinate-dependent dipole moment function. We applied this method to the excitation and dissociation of the triatomic molecule HCO including all three vibrational degrees of freedom. Comparison with calculations based on the propagation of a wave packet on a three-dimensional grid yields good agreement, provided the rate of excitation is not too large. The accuracy of classical trajectory calculations is also tested.