Control of Bond Excitation and Dissociation in HCN Using Laser Pulses^†

The potential for selectively controlling the excitation and dissociation of the two bonds of the linear triatomic molecule HCN using short laser pulses is studied. We show that intramolecular vibrational redistribution (IVR) has strong influence on the controlling process: at low excitation level, IVR is slow and it is easy to control the excitation of either bond, whereas at higher exitation, IVR causes rapid leakage into the unexcited bond, making control more difficult. Simple linearly chirped pulses are effective in exciting and dissociating the weaker C−H bond. In an attempt to excite the stronger C−N bond, we found that although linearly chirped pulses can transfer energy to the C−N bond, they cannot compete with the much faster internal energy redistribution, and the C−N bond cannot be dissociated because the weaker C−H bond will always break first. To overcome this problem, we construct an optimization scheme and use it to find pulses that do effect C−N dissociation. These pulses however are very short (less than 0.5 ps) and too intense (with brief peaks in the low 1015 W/cm2 range) so that the molecule will be ionized before dissociation into neutral fragments can occur.