Multiphoton Dissociation in Formaldehyde

Abstract

Two reports have previously been published on the multiphoton dissociation (MPD) in formaldehyde by an intense pulsed CO2 TEA laser [1,2]. In [1] natural formaldehyde at 20 torr was irradiated by 300 tightly focused laser pulses of 20 MW, yielding a deuterium enrichment factor of approximately 40 in the dissociation products. The main photodissocation reaction is $$HDCO + nh\nu \to HD + CO$$ (1) where n is the total number of photons of energy hv which are absorbed selectively in the HDCO molecules while most of the H2CO molecules are left unexcited by the laser irradiation. In [2] a more systematic investigation was performed, and the MPD of H2CO, HDCO and D2CO was measured as a function of the number of laser pulses, the laser intensity I (or fluence E) and the irradiated gas pressure P. Unexpectedly, the photodissociation results versus I of H2CO, whose absorption bands are far removed from the P(20) line of the CO2 laser used for the irradiation, have very much the same multiphoton nature as those of HDCO and D2CO, whose absorption bands overlap the P(20) line. In the present research this problem is resolved and an answer to the question which of the two parameters, intensity or fluence, is the more important in the MPD of formaldehyde is also given. Note that the latter can not be obtained on the basis of the data of [2] alone.