Simultaneous UV- and ion processing of astrophysically relevant ices

Abstract

Context. Interstellar ices are known to be simultaneously processed by both cosmic-ray bombardment and UV photolysis. Our knowledge of the effects of energetic processing on relevant icy samples is mainly based on laboratory investigations. In the past 35 years many experiments have been performed to study these effects separately but, to the best of our knowledge, never simultaneously. Aims. The aim of this work is to study the effects of simultaneous processing of ices by both cosmic rays and UV photons to investigate to what extent the combined effect of ion bombardment and UV photolysis influences the chemical pathways. Methods. We carried out the simultaneous processing of CH 3 OH:N 2 ice held at 16 K by 200 keV H + ions and Lyman-alpha 10.2 eV UV photons. The samples were analyzed by in situ transmission infrared spectroscopy. The un-combined processes of UV irradiation and bombardment by H + ions of CH 3 OH:N 2 ice were also studied. This mixture was chosen because the effects of ion bombardment and UV photolysis on methanol and nitrogen have been extensively studied in previous investigations. This mixture enables one to investigate whether simultaneous processing (a) influences the destruction of original species; (b) influences the formation of new species; or (c) causes synergistic effects since Lyman-alpha photons have a very low efficiency in breaking the dinitrogen bond because N 2 is almost transparent at Lyman-alpha wavelengths. Results. After processing a CH 3 OH:N 2 sample, the intensity of the methanol bands was observed to decrease at the same rate in all cases. After ion bombardment, species such as CO 2 , CO, H 2 CO, CH 4 , N 2 O, HNCO, and OCN − are formed in the ice mixture. After UV photolysis, species such as CO 2 , CO, H 2 CO, and CH 4 are formed, but no N-bearing species are detected. Spectra of ices processed by both UV photons and ions were compared with spectra of ices bombarded only by ions. We find that there are no differences in the band area and profile of N-bearing species for the two types of experiment at the same ion fluence; therefore, the addition of UV irradiation to ion bombardment does not affect the abundance of N-bearing species. The initial formation rate of CH 4 , within the experimental uncertainties, is the same in all cases studied, while the saturation value of CH 4 is higher for UV photolysis than for ion bombardment when they act separately. In the case of simultaneous processing, when the dose (eV/16u) given by UV photons is similar to the dose given during ion bombardment, the saturation value of CH 4 reaches a value intermediate between the value obtained after UV photolysis and ion bombardment separately. Conclusions. Our results confirm that when UV photolysis and ion bombardment act separately, their effects are very similar from a qualitative point of view, while significant quantitative difference may exist. In the case of simultaneous processing we did not detect any synergistic effect, but in some instances the behavior of newly formed spec...

Laser desorption time-of-flight mass spectrometry of vacuum UV photo-processed methanol ice

“…IR spectroscopy (transmission and RAIRS) have been used in combination with TPD/QMS. This has led to secure detections of small species, such as formaldehyde, carbon monoxide, carbon dioxide, and methane (Gerakines et al 1996;Baratta et al 2002;Bennett & Kaiser 2007;Islam et al 2014). This is in line with the results presented in this paper.…”

“…Typically, atom-addition reactions or recombination reactions of fragments formed upon vacuum UV (VUV) irradiation, cosmic ion bombardment, or electron-impact result in the formation of new species. These processes have been simulated in different laboratories (Gerakines et al 1996;Baratta et al 2002;Bennett & Kaiser 2007;Öberg et al 2009;Hama & Watanabe 2013;Islam et al 2014;Boamah et al 2014;Maity et al 2014Maity et al , 2015Linnartz et al 2015;Chuang et al 2016;Mrad et al 2016). These studies apply infrared (IR) spectroscopy (transmission or reflection) and temperature programmed desorption (TPD) combined with either quadrupole mass spectrometry (QMS), single photon ionisation time-offlight mass spectrometry (SPI TOF-MS), or gas chromatography coupled to mass spectrometry (GC-MS).…”

“…Several recent studies have shown how new products form upon processing of CH 3 OH rich ices. In Baratta et al (2002), Bennett & Kaiser (2007), Islam et al (2014), Boamah et al (2014), Maity et al (2014Maity et al ( , 2015, Henderson & Gudipati (2015) the role of electron or ion bombardment has been studied and in Gerakines et al (1996), Hudson & Moore (2000), Baratta et al (2002), , Öberg et al (2009), Islam et al (2014), Henderson & Gudipati (2015), Mrad et al (2016) VUV photolysis has been characterised. An overview of species detected in recent laboratory studies focusing on methanol ice processing is shown in the first four columns of Table 1.…”

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

“…For the last two decades, microwave discharge hydrogen-flow lamps (MDHLs;Warneck 1962;Davis & Braun 1968) have been widely used as vacuum-ultraviolet (VUV) light sources in laboratory experiments to simulate interstellar radiation fields in astrochemical studies (Westley et al 1995;Gerakines et al 1996;Baratta et al 2002;Muñoz Caro et al 2002, 2010Cottin et al 2003;Leto & Baratta 2003;Loeffler et al 2005;Watanabe et al 2007;Öberg et al 2007;Islam et al 2014;Cook et al 2014;Henderson & Gudipati 2015). These experiments aim to simulate and ultimately unravel the complex photoprocesses (photodesorption, photodissociation, and photochemistry) taking place in interstellar ice analogues.…”

“…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.…”