A straightforward method for Vacuum-Ultraviolet flux measurements: The case of the hydrogen discharge lamp and implications for solid-phase actinometry

Abstract

Vacuum-Ultraviolet (VUV) radiation is responsible for the photo-processing of simple and complex molecules in several terrestrial and extraterrestrial environments. In the laboratory such radiation is commonly simulated by inexpensive and easy-to-use microwave-powered hydrogen discharge lamps. However, VUV flux measurements are not trivial and the methods/devices typically used for this purpose, mainly actinometry and calibrated VUV silicon photodiodes, are not very accurate or expensive and lack of general suitability to experimental setups. Here, we present a straightforward method for measuring the VUV photon flux based on the photoelectric effect and using a gold photodetector. This method is easily applicable to most experimental setups, bypasses the major problems of the other methods, and provides reliable flux measurements. As a case study, the method is applied to a microwave-powered hydrogen discharge lamp. In addition, the comparison of these flux measurements to those obtained by O2 actinometry experiments allow us to estimate the quantum yield (QY) values QY122 = 0.44 ± 0.16 and QY160 = 0.87 ± 0.30 for solid-phase O2 actinometry.

Controlling the emission profile of an H2discharge lamp to simulate interstellar radiation fields

“…In the actinometry case, the O 2 to O 3 conversion efficiency typically used is taken from gas phase experiments performed by Groth (1937). Recent experiments indicate that the quantum efficiency in the solid state is significantly lower (Cottin et al 2003;Fulvio et al 2014) and hint toward a wavelength dependency, although detailed wavelength dependent studies are still lacking in the literature. It is clear that both calibration methods require knowledge of the emission spectrum of the lamp to accurately determine the flux at different photon energies, which strongly depends on the experimental settings.…”

“…the dependence on the experimental settings, influence both the total photon flux and the SED of the impacting radiation at the ice sample. In other situations the flux of the lamp is either directly measured using a National Institute of Standards and Technology (NIST) calibrated photodiode or indirectly by actinometry using for example the conversion of O 2 to O 3 (Gerakines et al 1996;Cottin et al 2003;Islam et al 2014;Fulvio et al 2014). When using a NIST calibrated photodiode for photon flux measurements, the quantum yield is known at different wavelengths.…”

Vacuum ultraviolet photochemistry of solid acetylene: a multispectral approach

“…The photodissociation pathways of C 2 H 2 were analyzed by A83, page 8 of 11 Okabe (1975 The VUV emission profile comprises Lyman-alpha radiation and a broadband component around 160 nm roughly in a 1:2 intensity ratio (Fulvio et al 2014). As dehydrogenation requires less energy than dissociation of the C≡C triple bond (for which the broadband at 160 nm does not provide enough energy), dehydrogenation will be the dominant pathway.…”

Laboratory Experiments on the Low-temperature Formation of Carbonaceous Grains in the ISM

“…VUV spectra of hydrogen discharge lamps are dominated by two spectral features, centered at 122 nm and 160 nm. The lamp flux in our setup, at the sample po- sition, is F 122 = 1.8 ± 0.6 × 10 14 ph cm −2 s −1 and F 160 = 3.1 ± 1.1 × 10 14 ph cm −2 s −1 (Fulvio et al 2014). The internal structure of the carbonaceous condensate was finally investigated by high-resolution transmission electron microscopy (HRTEM) after the evaporation of the ice matrix.…”

“…In this view, we have performed additional experiments where the laser ablation of graphite and deposition in Ar-matrix has been followed by VUV irradiation. The irradiation has been carried out by employing a microwave-powered hydrogen discharge lamp (see Section 2 for technical details) with an emission spectrum dominated by two spectral features, centered at 122 nm and 160 nm (Fulvio et al 2014). Fig.…”