Above and below threshold multiphoton dissociation of volcanic ground states. Application to BeH 2+

Abstract

We report the results of ab initio computations of the time-dependent photodissociation of the BeH2+ molecule by a CO2 laser for intensities in the range 1010–1011 W/cm2 and for a trapezoidal pulse of duration of 0.44 ps. Our approach involves expansion of the wavefunction over box-normalized discrete and continuum eigenstates and integration of the resulting time-dependent coupled equations. In spite of the relatively low intensities, the known phenomenon of above threshold dissociation (ATD) is obtained, as well as a new one, which we have named below threshold dissociation (BTD), since it requires fewer photons than expected from energy considerations. The reason for these results is the volcanic shape of the potential energy surface of BeH2+, which supports a relatively small number of vibrational quasi-discrete levels, of which the two near the barrier top mix heavily with the adjacent dissociation continuum.