Photoabsorption and photoionization cross sections for formaldehyde in the vacuum-ultraviolet energy range

Abstract

We report a theoretical-experimental investigation on the interaction of vacuum-ultraviolet radiation with formaldehyde (H2CO) in the gas phase. Experimentally, the absolute photoabsorption cross sections and the photoionization quantum yields were measured in the (11.0–21.5) eV range using the double-ion chamber technique. Also, the absolute photoionization and neutral-decay cross sections were derived from these data. In addition, in the same energy region, the dissociation pattern was obtained with a time-of-flight mass spectrometer using the photoelectron-photoion coincidence technique, and the absolute photoionization cross sections were derived for each ionic fragment observed. Moreover, theoretical photoionization cross sections were calculated for the ionization of the four outermost molecular valence orbitals (2b2, 1b1, 5a1, and 1b2) from the threshold to 35 eV. The calculations were performed using the iterative Schwinger variational method to solve the Lippmann-Schwinger equation in the exact static-exchange level of approximation. In general, there is a good agreement between our experimental and previous data reported in the literature. Our theoretical results show a fair qualitative agreement with the experimental data and with previous theoretical results. Above 20 eV, a better quantitative agreement with the experimental data is also observed.

Vacuum Ultraviolet Photodesorption and Photofragmentation of Formaldehyde-Containing Ices

“…On the contrary, Rydberg states in the 12–14 eV region enhance the production of vibrationally excited ions; i.e., there could be autoionization from these Rydberg states, − and a strong and large resonance is seen in the gas phase spectrum (Figure ). Gas phase studies in the VUV show that when exciting H 2 CO at 13 eV, the production of ions is favored relatively to that of neutrals and H 2 CO + is the major ion detected, HCO + being much smaller …”

“…Gas phase studies in the VUV show that when exciting H 2 CO at 13 eV, the production of ions is favored relatively to that of neutrals and H 2 CO + is the major ion detected, HCO + being much smaller. 92 The fact that the electronic states of H 2 CO in the gas phase are predissociative, the large width of the features observed in the photodesorption spectrum ( Figure 2), and the observation of photodesorbing fragments point to the dissociative nature of the electronic states of solid H 2 CO. Another finding is that photodesorption yields lie in the 4 − 8 × 10 −4 molecule/photon in the 7-13.6 eV range, and increase slightly and continuuously above 10 eV, which is expected to correspond more or less to the ionization energy (the ionization energy of solid H 2 CO is not known, but is expected about 1 eV below the gas phase value of 10.88 eV). 88 Besides, it seems that no sign of an autoionized state (which is at around 13 eV in the gas phase) is observed in the photodesorption spectrum, so that autoionization is not correlated to the photodesorption of neutral molecules.…”

Cross Sections and Asymmetry Parameters for Formic Acid in the Vacuum-Ultraviolet Energy Range

“…This methodology enables us to perform measurements only above the first ionization threshold. The experimental setup and procedure have been already described in previous works − and will be briefly presented here. The photoabsorption cross section (σ a ) as a function of photon energy ( E ) is given by where i 1 and i 2 are the ion currents detected using two ion collectors of the same length ( l ) and n is the molecular number density.…”

Photoabsorption and Photoionization Cross Sections of Pyridine in the Vacuum-Ultraviolet Energy Range

“…The photoabsorption cross sections ( σ a ) and the ionization quantum yield (η) measurements were performed above the ionization threshold using a double-ion-chamber spectrometer . The experimental apparatus has already been used in our previous works − and consists, basically, of a photoabsorption cell containing two subsequent ion collector electrodes of the same length ( L = 66.5 ± 0.1 mm) separated by a gap of 0.5 mm. An ion repeller electrode is positioned 8 mm above the ion collectors.…”