A multifunctional setup to record FTIR and UV-vis spectra of organic molecules and their photoproducts in astronomical ices

Abstract

This article describes a new, multi-functional, high-vacuum ice setup that allows to record the in situ and real-time spectra of vacuum UV (VUV)-irradiated non-volatile molecules embedded in a low-temperature (10 K) amorphous solid water environment. Three complementary diagnostic tools-UV-visible (UV-vis) and Fourier Transform Infrared (FTIR) spectroscopy and temperature-programmed desorption quadrupole mass spectrometry-can be used to simultaneously study the physical and chemical behavior of the organic molecules in the ice upon VUV irradiation. The setup is equipped with a temperature-controlled sublimation oven that enables the controlled homogeneous deposition of solid species such as amino acids, nucleobases, and polycyclic aromatic hydrocarbons (PAHs) in ice mixtures prepared from precursor gases and/or liquids. The resulting ice is photo-processed with a microwave discharge hydrogen lamp, generating VUV radiation with a spectral energy distribution representative for the interstellar medium. The characteristics, performance, and future potential of the system are discussed by describing three different applications. First, a new method is introduced, which uses broadband interference transmission fringes recorded during ice deposition, to determine the wavelength-dependent refractive index, n, of amorphous solid water. This approach is also applicable to other solids, pure and mixed. Second, the UV-vis and FTIR spectroscopy of an VUV-irradiated triphenylene:water ice mixture is discussed, monitoring the ionization efficiency of PAHs in interstellar ice environments. The third and final example investigates the stability of solid glycine upon VUV irradiation by monitoring the formation of dissociation products in real time.

The Refractive Index of Amorphous and Crystalline Water Ice in the UV–vis

“…The experiments were done using a recently constructed setup, described in detail in Kofman et al (2018). We will briefly describe the experimental features that are relevant to this study.…”

A new technique for determining the refractive index of ices at cryogenic temperatures

“…The outputs of the analysis code are the In order to test the theory and the code, test data with known values of n 1 and k 1 were used as a benchmark. The values of n 1 were those of Kofman et al 66 determined for amorphous solid water (ASW). Figure 2 plots n as a function of wavelength determined by Kofman et al 66 The data are fitted with a polynomial function in order to determine n across the full wavelength range used in this work.…”

“…The values of n 1 were those of Kofman et al 66 determined for amorphous solid water (ASW). Figure 2 plots n as a function of wavelength determined by Kofman et al 66 The data are fitted with a polynomial function in order to determine n across the full wavelength range used in this work. It should be noted that the wavelength range has an upper limit of 591 nm due to the availability of the HOPG optical parameters.…”

“…It should be noted that the wavelength range has an upper limit of 591 nm due to the availability of the HOPG optical parameters. 66 The blue line is a fit to the data to give n across the wavelength range used to test the analysis method.…”

“…Kofman et al 66 did not determine wavelength dependent values of k for ASW, therefore a constant value of 0.01 was assumed. This value was chosen, as previously reported values in the relevant wavelength region tend to be small (<< 0.5) and positive for water.…”