Intensity dependence of multiphoton dissociation in formaldehyde

A new intensity-dependent measurement of multiple-photon dissociation (MPD) in H₂CO, HDCO, and D₂CO gases by the use of an intense pulsed CO₂TEA laser is reported. In this measurement the energy and duration of the laser pulses are kept constant, and the intensity is varied by irradiating the sample using concave mirrors of different focal lengths. A model calculation is used to analyze and fit the present and previous experimental MPD data of HDCO and D₂CO. In this model it is assumed that dissociation is obtained by a repeated mechanism in which coherent multiphoton excitation (CME) of the molecule to high vibration-rotation states | v, J> is followed by intramolecular transfer of the excitation energy (ITEE) to the other modes of the molecule. In the calculations the CME is described in the framework of the density matrix formalism with relaxations, and is used to calculate absorption from the ground state as well as absorption from excited states reached by the energy redistribution in the molecule. The ITEE process is assumed to be intensity independent and to cause a random energy distribution in each transferring process. It is found that the experimental results are consistent with the absorption of 14±4 and 17±5 photons per molecule for HDCO and D₂CO, respectively, and this is sufficient to cause their dissociation.