Photoionization and Absorption Spectrum of Formaldehyde in the Vacuum Ultraviolet

Abstract

Absorption and photoionization coefficients have been measured for H2CO in the 600–2000-Å region. Integrated oscillator strengths were determined for a number of strong Rydberg transitions above 1200 Å. From the photoionization curve the first adiabatic ionization potential was found to be 10.87±0.01 eV. As an aid in interpreting the absorption spectrum, theoretical calculations were made using a single-configuration self-consistent field procedure for the Rydberg states and a model which included mixing between the Rydberg and valence states. On this basis, weak absorption features between 1340 and 1430 Å have been assigned to the B11(σ → π *) valence state. The 1A1(π → π *) valence state is deduced to be strongly autoionized just above the 2B2 ionization limit.

Equations of motion method: Excitation energies and intensities in formaldehyde

“…These results include transition energies and oscillator strengths for transitions below 2000 A which allow us to explain some unusual features recently observed in the optical absorption 6 and electron scattering spectrum. 7 These studies were stimulated by the recent discovery of formaldehyde in interstellar dust cl0uds 8 and provided accurate absorption coefficients between 2400 and 912 A.…”

“…2 which is larger than typical values for pure intravalence transitions, i.e., 2-3 (a. u. ) 2 6 we can expect strong mixing between the n-11* state and the 1 A 1 (2b 2 ndb 2 ) states. In the valence [3s 2p /1s] basis 2 the 1 A (7T-n*) state has a large f value of about 0.…”

“…This state is a valence-like state with sorr.e admixture of Rydberg character through its interaction with nearby Rydberg states. 6 For example the change in the average value of Li(x7+y~) for the transition is about 15 (a. u. ) 2 which is larger than typical values for pure intravalence transitions, i.e., 2-3 (a. u. )…”

“…2 eV for the 1 A 1 -1 B 1 (a-7T*) transition is close to the value of 9 eV inferred from the observed spectrum by Mental! et al 6 The oscillator strength of this transition is only 0. 002 in agreement with the assignment of weak absorption features between 1340 and 1430 A (8.…”

“…The 1 A 1 (11-rr*) state is one of the puzzling aspects in the interpretation of the spectrum of formaldehyde. No feature was observed in either the absorption 6 or electron scattering 7 spectrum which could be related to the IT-11* excitation. We predict an excitalion energy of 10.…”