Above-threshold multiphoton dissociation of and in intense laser fields

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Abstract
We have studied multiphoton above-threshold dissociation (ATD) of and by a time-independent close-coupling (CC) method over a wide range of intensities for three initial bound states (v = 0, 1, 2 and J = 0) at a laser frequency of . Molecular rotation is taken into account by including 30 channels with J = 0 - 5 for and 20 channels with J = 0 - 7 for and number of absorbed photons n = 1 - 4. All the radiative couplings (including those due to the permanent dipole moment of ) have been considered in a truncated length-gauge form of interaction. At lower intensities the total dissociative decay is exponential, but at high intensity several dressed states with different linewidths contribute to the decay and no single decay rate can be defined. Use of the proper potential surfaces for , including intramolecular nonadiabatic coupling, enables us to obtain the branching ratios between the production of and fragments. The branching ratios for different numbers of photon absorption (n = 1, 2, 3) change dramatically with intensity. For , branching to channels, forbidden in by symmetry, may become dominant over a certain range of intensity. We have tried to interpret these intensity dependences in terms of the motion of the nuclei on the adiabatic field + molecule potential surfaces.