Radical channel photodissociation dynamics of aliphatic aldehydes: the nascent state distribution of the HCO photoproduct

摘要

Abstract: The photodissociation dynamics of jet-cooled formaldehyde, acetaldehyde, and propionaldehyde have been investigated at wavelengths of 308 nm or 309 nm by monitoring the resultant nascent HCO fragments by laser induced fluorescence excitation spectroscopy (LIF). For acetaldehyde and propionaldehyde, the distribution of energy in the HCO fragment was deduced directly from intensities in the rotationally resolved spectrum while the Doppler widths of selected rotational lines provided an estimate of the translational energy release. This has revealed that for both reactions, at these excitation wavelengths, most of the excess energy is deposited as translational energy in the two fragments (80 - 90%) and rotation of the HCO fragment (10 - 20%) with apparently minimal internal energy deposited in the alkyl fragment. The experimental observations indicate that the radical channel photodissociation mechanisms for acetaldehyde and propionaldehyde are similar. The reaction occurs following intersystem crossing onto the lowest lying triplet surface. On this surface there is a loose transition state above a relatively high, late barrier. The fixed energy of the exit channel appears to be converted almost solely into translation and HCO rotation. Very preliminary results are also presented for the 309 nm photolysis of formaldehyde. For this system, the HCO spectrum demonstrates clearly a dominance of K$-a$/ equals 1 states over K$-a$/ equals 0 states. This is consistent with ab initio calculations which place the departing hydrogen atom almost perpendicularly above the carbon atom at the triplet transition state. However the results are inconsistent with the singlet transition state geometry which is planar. !38