Fluorescence from VUV excitation of formaldehyde

Abstract

The photoabsorption and fluorescence cross sections of H2CO were measured in the 105–180 nm region using synchrotron radiation as a light source. Vacuum ultraviolet (VUV) emission from excited photofragments was dispersed and identified to be the CO(A 1Π→X 1Σ+) system. The vibrational population of CO(A) was determined from the fluorescencespectrum and was used to study the photodissociation mechanism. The threshold for the production of the VUV emission is at 140.3 nm. UV emission from HCO* was observed below a threshold of 147.5 nm. The upper limit of the dissociation energy,D 0(H–HCO), determined from the HCO emission threshold is 3.61±0.03 eV. The quantum yield for the production of either CO* or HCO* is a smooth function of the excitation wavelength, from which two dissociative states with vertical energies at 8.69 and 10.7 eV are derived. The VUV and UV fluorescences have maximum quantum yields at 116 nm of about 1.6% and 0.23%, respectively. The photodissociation process of H2CO in VUV is discussed.