Ice Photochemistry Research Articles

Dual Sources of S2 Observed in Comet 67P: Insights from Comparing ROSINA Measurements and Laboratory Simulations

Hydrogen sulfide (H2S) is the fifth most abundant molecule observed in the coma of comet 67P/Churyumov–Gerasimenko (67P). Prior to its incorporation into cometary materials, H2S likely underwent ultraviolet (UV) radiation exposure, which is thought to initiate a complex sulfur chemistry. We present an investigation into the UV photochemistry of H2S ices using infrared, Raman, and mass spectrometry techniques. Our study reveals the production of complex sulfur allotropes ranging from S2 to S6, alongside polysulfanes (H2S n , n = 2–3). Temperature-programmed desorption measurements postirradiation of H2S exhibit two peaks for S2 molecules: a broad peak between 80 and 140 K and a distinct peak at ∼245 K. Notably, larger allotropes S3–S5 exclusively display the 245 K peak. Furthermore, ROSINA measurements of the S2/H2S ratio during dust impact events and previously reported S2/H2S ratios in the undisturbed coma are compared to our laboratory-determined S2/H2S values. This analysis identifies two distinct sources of S2: a volatile S2 potentially sublimated directly from the comet’s surface and a secondary source likely resulting from fragmentation of larger sulfur chains during dust impacts. We determined the ratio of produced S2 to the initial H2S for both the volatile component and the refractory component at 245 K with both measurements conducted at an irradiation incident fluence of 2.25 × 1017 photons cm−2. These laboratory-derived S2/H2S ratios exhibit concordance with ROSINA measurements. When extrapolated to incident fluences anticipated in molecular clouds, this photoprocessing mechanism offers a plausible explanation for the measured S2/H2S ratio in comet 67P.

New insights into the environmental photochemistry of common-use antibiotics in ice and in water: A comparison of kinetics and influencing factors

The photochemistry of organic contaminants present in ice is receiving growing attention, given the wide presence of ice during winter in temperate regions as well as Polar and mountain environments. Differences between ice photochemistry and aqueous photochemistry, however, influence the quantitative fate and transformation of organic chemicals present in freshwater, marine and ice-cap environments and these differences need to be explored. Here we comparatively studied the ice and aqueous photochemistry of three antibiotics [levofloxacin (LVX), sulfamerazine (SM), and chlortetracycline (CTC)] under the same simulated sunlight (λ > 290 nm). Their photodegradation in ice/water followed pseudo-first-order kinetics, whereby the photolytic rates of LVX in ice and water were found to be similar, SM photodegraded faster in ice, while CTC underwent slower photodegradation in ice. Whether individual antibiotics underwent faster photodegradation in ice or not depends on the specific concentration effect and cage effect coexisting in the ice compartment. In most cases, the fastest photodegradation occurred in freshwater ice or in fresh water, and the slowest photolysis occurred in pure-water ice or in pure water. This can be attributed to the effects of key photochemical reactive constituents of Cl−, HA, NO3− and Fe(III), that exist in natural waters. These constituents at certain levels showed significant effects (P < 0.1) on the photolysis, not only in ice but also in water. However, these individual constituents at a given concentration, serve to either enhance or suppress the photoreaction, depending on the specific antibiotic and the matrix type (e.g., ice or aqueous solution). Furthermore, extrapolation of the laboratory findings to cold environments indicate that pharmaceuticals present in ice will have a different photofate compared to water. These results are of particular relevance for those regions that experience seasonal ice cover in fresh water and coastal marine systems.

Experimental investigations of diacetylene ice photochemistry in Titan’s atmospheric conditions

Context. A large fraction of the organic species produced photochemically in the atmosphere of Titan can condense to form ice particles in the stratosphere and in the troposphere. According to various studies, diacetylene (C4H2) condenses below 100 km where it can be exposed to ultraviolet radiation. Aims. We studied experimentally the photochemistry of diacetylene ice (C4H2) to evaluate its potential role in the lower altitude photochemistry of Titan’s atmospheric ices. Methods. C4H2 ice films were irradiated with near-ultraviolet (near-UV) photons (λ &gt; 300 nm) with different UV sources to assess the impact of the wavelengths of photons on the photochemistry of C4H2. The evolution of the ice’s composition was monitored using spectroscopic techniques. Results. Our results reveal that diacetylene ice is reactive through singlet-triplet absorption, similar to the photochemistry of other organic ices of Titan (such as dicyanoacetylene C4N2 ice) that we investigated previously. Several chemical processes occurred during the photolysis: the hydrogenation of C4H2 to form other C4 hydrocarbons (vinylacetylene C4H4 to butane C4H10); the formation of larger and highly polymerizable hydrocarbons, such as triacetylene (C6H2); and the formation of an organic polymer that is stable at room temperature. Conclusions. The nondetection of diacetylene ice in Titan’s atmosphere or surface could be rationalized based on our experimental results that C4H2 is photochemically highly reactive in the solid phase when exposed to near-UV radiation that reaches Titan’s lower altitudes and surface. C4H2 may be one of the key molecules promoting the chemistry in the ices and aerosols of Titan’s haze layers, especially in the case of co-condensation with other organic volatiles, with which it could initiate more complex solid-phase chemistry.

Effect of dissolved organic matter on sulfachloropyridazine photolysis in liquid water and ice

Dissolved organic matter (DOM) is an ubiquitous component of environmental snow and ice, which can absorb light and produce reactive species (RS) and thus is of importance in ice photochemistry. The photodegradation of sulfachloropyridazine (SCP) without and with DOM present in liquid water and ice were investigated in this study. The photodegradation rate constants for SCP without DOM present was enhanced by 52.5 % in ice relative to liquid water, likely due to the enhanced role of SCP self-sensitized RS in ice. DOM significantly promoted SCP photolysis in both liquid water and ice, which was mainly attributed to roles of singlet oxygen (1O2) and triplet excited-state DOM (3DOM*) generated from DOM. 1O2 production from DOM was significantly enhanced in ice relative to liquid water. Hydroxyl radical (•OH) production from DOM in ice was similar to those in liquid water. Enhancement in 3DOM* production in ice was observed at low DOM concentrations. Suwannee River Fulvic Acid (SRFA) and Elliott Soil Humic Acid (ESHA) exhibited differences in RS production in liquid water and ice, as well as in enhancement of 1O2 and 3DOM* produced in ice relative to liquid water. DOM induced reaction pathways of SCP different from those without DOM present, and therefore affected toxicity of SCP photoproducts. There were differences in photodegradation pathways of SCP as well as in toxicity of SCP photoproducts between liquid water and ice.

The Formation of Imines and Nitriles during VUV Photoirradiation of NH3:C2H x Ice Mixtures

Nitriles are key reactants in prebiotic synthesis networks of RNA bases and amino acids. The detection of CH3CN and other complex nitriles in planet-forming disks suggests that such molecules are regularly delivered to nascent planets, increasing the likelihood of origins of life outside of Earth. In this paper, we investigate the formation of CH3CN and the closely related imines from the vacuum ultraviolet irradiation of NH3:C2H6, NH3:C2H4, and NH3:C2H2 ice mixtures at 10–50 K. CH3CN is formed in a subset of these experiments, with the highest yield of ∼5% with respect to the initial NH3 abundance achieved at the lowest ice temperatures for the least saturated hydrocarbon ice mixture. We find that the imine CH3CH=NH serves as an intermediate for the production of CH3CN in all ices and its yield generally appears higher than that of CH3CN. If the investigated ice chemistry is an important formation pathway of nitriles, we should observe CH3CH=NH > CH3CN. The opposite is true toward the Galactic Center, while no published constraints on CH3CH=NH exist in disks. Such constraints are needed to distinguish between the formation pathway presented in this work and other possible gas and ice nitrile formation pathways in different astrophysical environments. In the meantime, we conclude that NH3:hydrocarbon ice photochemistry is an excellent candidate for efficient low-temperature interstellar imine production.

Simulation of CH3OH ice UV photolysis under laboratory conditions

Context. Methanol is the most complex molecule that is securely identified in interstellar ices. It is a key chemical species for understanding chemical complexity in astrophysical environments. Important aspects of the methanol ice photochemistry are still unclear, such as the branching ratios and photodissociation cross sections at different temperatures and irradiation fluxes. Aims. This work aims at a quantitative agreement between laboratory experiments and astrochemical modelling of the CH3OH ice UV photolysis. Ultimately, this work allows us to better understand which processes govern the methanol ice photochemistry present in laboratory experiments. Methods. We used the code ProDiMo to simulate the radiation fields, pressures, and pumping efficiencies characteristic of laboratory measurements. The simulations started with simple chemistry consisting only of methanol ice and helium to mimic the residual gas in the experimental chamber. A surface chemical network enlarged by photodissociation reactions was used to study the chemical reactions within the ice. Additionally, different surface chemistry parameters such as surface competition, tunnelling, thermal diffusion, and reactive desorption were adopted to check those that reproduce the experimental results. Results. The chemical models with the code ProDiMo that include surface chemistry parameters can reproduce the methanol ice destruction via UV photodissociation at temperatures of 20, 30, 50, and 70 K as observed in the experiments. We also note that the results are sensitive to different branching ratios after photolysis and to the mechanisms of reactive desorption. In the simulations of a molecular cloud at 20 K, we observed an increase in the methanol gas abundance of one order of magnitude, with a similar decrease in the solid-phase abundance. Conclusions. Comprehensive astrochemical models provide new insights into laboratory experiments as the quantitative understanding of the processes that govern the reactions within the ice. Ultimately, these insights can help us to better interpret astronomical observations.

Resonant infrared irradiation of CO and CH3OH interstellar ices

Context. Solid-phase photo-processes involving icy dust grains greatly affect the chemical evolution of the interstellar medium by leading to the formation of complex organic molecules and by inducing photodesorption. So far, the focus of laboratory studies has mainly been on the impact of energetic ultraviolet (UV) photons on ices, but direct vibrational excitation by infrared (IR) photons is expected to influence the morphology and content of interstellar ices as well. However, little is still known about the mechanisms through which this excess vibrational energy is dissipated, as well as its implications for the structure and ice photochemistry. Aims. In this work, we present a systematic investigation of the behavior of interstellar relevant CO and CH3OH ice analogs following the resonant excitation of vibrational modes using tunable IR radiation. We seek to quantify the IR-induced photodesorption and gain insights into the impact of vibrational energy dissipation on ice morphology. Methods. We utilized an ultrahigh vacuum setup at cryogenic temperatures to grow pure CO and CH3OH ices, as well as mixtures of the two. We exposed the ices to intense, near-monochromatic mid-IR (MIR) free-electron-laser radiation using the LISA end-station at the FELIX free electron laser facility to selectively excite the species. Changes to the ice are monitored by means of reflection-absorption IR spectroscopy combined with quadrupole mass-spectrometry. These methods also allowed us to characterize the photodesorption efficiency. Results. The dissipation of vibrational energy is observed to be highly dependent on the excited mode and the chemical environment of the ice. All amorphous ices undergo some degree of restructuring towards a more organized configuration upon on-resonance irradiation. Moreover, IR-induced photodesorption is observed to occur for both pure CO and CH3OH ices, with interstellar photodesorption efficiencies on the order of 10 molecules cm−2 s−1. This result is comparable to or higher than what is found for UV-induced counterparts. An indirect photodesorption of CO upon vibrational excitation of CH3OH in ice mixtures is also observed to occur, particularly in environments that are rich in methanol. Here, we discuss the astrochemical implications of these IR-induced phenomena.

Extraterrestrial prebiotic molecules: photochemistry vs. radiation chemistry of interstellar ices.

In 2016, unambiguous evidence for the presence of the amino acid glycine, an important prebiotic molecule, was deduced based on in situ mass-spectral studies of the coma surrounding cometary ice. This finding is significant because comets are thought to have preserved the icy grains originally found in the interstellar medium prior to solar system formation. Energetic processing of cosmic ices via photochemistry and radiation chemistry is thought to be the dominant mechanism for the extraterrestrial synthesis of prebiotic molecules. Radiation chemistry is defined as the "study of the chemical changes produced by the absorption of radiation of sufficiently high energy to produce ionization." Ionizing radiation in cosmic chemistry includes high-energy particles (e.g., cosmic rays) and high-energy photons (e.g., extreme-UV). In contrast, photochemistry is defined as chemical processes initiated by photon-induced electronic excitation not involving ionization. Vacuum-UV (6.2-12.4 eV) light may, in addition to photochemistry, initiate radiation chemistry because the threshold for producing secondary electrons is lower in the condensed phase than in the gas phase. Unique to radiation chemistry are four phenomena: (1) production of a cascade of low-energy (<20 eV) secondary electrons which are thought to be the dominant driving force for radiation chemistry, (2) reactions initiated by cations, (3) non-uniform distribution of reaction intermediates, and (4) non-selective chemistry leading to the production of multiple reaction products. The production of low-energy secondary electrons during radiation chemistry may also lead to new reaction pathways not available to photochemistry. In addition, low-energy electron-induced radiation chemistry may predominate over photochemistry because of the sheer number of low-energy secondary electrons. Moreover, reaction cross-sections can be several orders of magnitude larger for electrons than for photons. Discerning the role of photochemistry vs. radiation chemistry in astrochemistry is challenging because astrophysical photon-induced chemistry studies have almost exclusively used light sources that produce >10 eV photons. Because a primary objective of chemistry is to provide molecular-level mechanistic explanations for macroscopic phenomena, our ultimate goal in this review paper is to critically evaluate our current understanding of cosmic ice energetic processing which likely leads to the synthesis of extraterrestrial prebiotic molecules.

Efficient photochemistry of coronene:water complexes

The photochemistry of ices with polycyclic aromatic hydrocarbons (PAHs) has been extensively studied, but to date no investigation has been made of PAHs in interaction with low numbers (n&lt; 4) of molecules of water. We performed photochemical matrix isolation studies of coronene:water complexes, probing the argon matrix with FTIR spectroscopy. We find that coronene readily reacts with water upon irradiation with a mercury vapour lamp to produce oxygenated PAH photoproducts, and we postulate a reaction mechanism via a charge transfer Rydberg state. This result suggests that oxygenated PAHs should be widely observed in regions of the ISM with sufficiently high water abundances, for example near the edges of molecular clouds where water molecules begin to form, but before icy layers are observed, that is at AV&lt; 3. In order to explain the low derived observational abundances of oxygenated PAHs, additional destruction routes must be invoked.

Photochemistry and Astrochemistry: Photochemical Pathways to Interstellar Complex Organic Molecules.

The interstellar medium is characterized by a rich and diverse chemistry. Many of its complex organic molecules are proposed to form through radical chemistry in icy grain mantles. Radicals form readily when interstellar ices (composed of water and other volatiles) are exposed to UV photons and other sources of dissociative radiation, and if sufficiently mobile the radicals can react to form larger, more complex molecules. The resulting complex organic molecules (COMs) accompany star and planet formation and may eventually seed the origins of life on nascent planets. Experiments of increasing sophistication have demonstrated that known interstellar COMs as well as the prebiotically interesting amino acids can form through ice photochemistry. We review these experiments and discuss the qualitative and quantitative kinetic and mechanistic constraints they have provided. We finally compare the effects of UV radiation with those of three other potential sources of radical production and chemistry in interstellar ices: electrons, ions, and X-rays.

FORMATION OF N 3 , CH 3 , HCN, AND HNC FROM THE FAR-UV PHOTOLYSIS OF CH 4 IN NITROGEN ICE

The irradiation of pure solid N2 at 3 K with far-ultraviolet light from a synchrotron produced infrared absorption lines at 1657.7, 1655.6, and 1652.4 cm−1 and an ultraviolet absorption line at 272.0 nm, which are characteristic of the product N3. The threshold wavelength at which N3 was generated was 145.6 ± 2.9 nm, corresponding to an energy of 8.52 ± 0.17 eV. The photolysis of isotopically labeled 15N2 at 3 K consistently led to the formation of 15N3 with the same threshold wavelength of 145.6 ± 2.9 nm for its formation. The photolysis of CH4 in nitrogen ice in low concentrations also led to the formation of N3, together with CH3, HCN, and HNC, with the same threshold wavelength of 145.6 ± 2.9 nm. These results indicate that N3 radicals may play an important role in the photochemistry of nitrogen ices in astronomical environments.

Simulation of Titan’s atmospheric photochemistry

We studied the photochemistry of frozen ice of a polar Titan’s atmospheric molecule cyanodiacetylene (HC5N) to determine the possible contribution of this compound to the lower altitude photochemistry of haze layers found on Titan. We used infrared analysis to examine the residue produced by irradiation of solid HC5N at > 300 nm. The resulting polymer is orange-brown in color. Based on theoretical analysis and the general tendency of HC5N and C4N2 to undergo similar ice photochemistry at longer wavelengths accessible in Titan’s lower atmosphere, we conclude that Titan’s lower atmosphere is photochemically active in the regions of cloud, ice, and aerosol formation. C4N2 is a symmetric molecule with no net dipole moment whereas, HC5N has a large dipole moment of 4 D. Consequently, though both these molecules have very similar molecular weight and size, their sublimation temperatures are di erent, HC5N subliming around 170 K compared to 160 K for C4N2. Based on our studies we conclude that in Titan’s atmosphere the cyanoacetylene class of molecules (HCN, HC3N, HC5N, etc.) would condense first followed by the dicyanoacetylenes (C2N2, C4N2, C6N2, etc.), leading to fractionation of di erent class of molecules. From the fluxes used in the laboratory and depletion of the original HC5N signals, we estimate Titan’s haze ice photochemistry involving polar nitriles to be significant and very similar to their non-polar counterparts.

Methane ice photochemistry and kinetic study using laser desorption time-of-flight mass spectrometry at 20 K

The ice photochemistry of pure methane (CH4) is studied at 20 K upon VUV irradiation from a microwave discharge H2 flow lamp. Laser Desorption Post-Ionization Time-Of-Flight Mass Spectrometry (LDPI TOF-MS) is used for the first time to determine branching ratios of primary reactions leading to CH3, CH2, and CH radicals, typically for fluences as expected in space. This study is based on a stable end-products analysis and the mass spectra are interpreted using an appropriate set of coupled reactions and rate constants. This yields clearly different values from previous gas phase studies. The matrix environment as well as the higher efficiency of reverse reactions in the ice clearly favor CH3 radical formation as the main first generation photoproduct.

Role of dissolved organic matter in ice photochemistry.

In this study, we provide evidence that dissolved organic matter (DOM) plays an important role in indirect photolysis processes in ice, producing reactive oxygen species (ROS) and leading to the efficient photodegradation of a probe hydrophobic organic pollutant, aldrin. Rates of DOM-mediated aldrin loss are between 2 and 56 times faster in ice than in liquid water (depending on DOM source and concentration), likely due to a freeze-concentration effect that occurs when the water freezes, providing a mechanism to concentrate reactive components into smaller, liquid-like regions within or on the ice. Rates of DOM-mediated aldrin loss are also temperature dependent, with higher rates of loss as temperature decreases. This also illustrates the importance of the freeze-concentration effect in altering reaction kinetics for processes occurring in environmental ices. All DOM source types studied were able to mediate aldrin loss, including commercially available fulvic and humic acids and an authentic Arctic snow DOM sample isolated by solid phase extraction, indicating the ubiquity of DOM in indirect photochemistry in environmental ices.

ENANTIOMERIC EXCESSES INDUCED IN AMINO ACIDS BY ULTRAVIOLET CIRCULARLY POLARIZED LIGHT IRRADIATION OF EXTRATERRESTRIAL ICE ANALOGS: A POSSIBLE SOURCE OF ASYMMETRY FOR PREBIOTIC CHEMISTRY

The discovery of meteoritic amino acids with enantiomeric excesses of the L-form (ee L) has suggested that extraterrestrial organic materials may have contributed to prebiotic chemistry and directed the initial occurrence of the ee L that further led to homochirality of amino acids on Earth. A proposed mechanism for the origin of ee L in meteorites involves an asymmetric photochemistry of extraterrestrial ices by UV circularly polarized light (CPL). We have performed the asymmetric synthesis of amino acids on achiral extraterrestrial ice analogs by VUV CPL, investigating the chiral asymmetry transfer at two different evolutionary stages at which the analogs were irradiated (regular ices and/or organic residues) and at two different photon energies (6.6 and 10.2 eV). We identify 16 distinct amino acids and precisely measure the L-enantiomeric excesses using the enantioselective GC × GC-TOFMS technique in five of them: α-alanine, 2,3-diaminopropionic acid, 2-aminobutyric acid, valine, and norvaline, with values ranging from ee L = –0.20% ± 0.14% to ee L = –2.54% ± 0.28%. The sign of the induced ee L depends on the helicity and the energy of CPL, but not on the evolutionary stage of the samples, and is the same for all five considered amino acids. Our results support an astrophysical scenario in which the solar system was formed in a high-mass star-forming region where icy grains were irradiated during the protoplanetary phase by an external source of CPL of a given helicity and a dominant energy, inducing a stereo-specific photochemistry.

Using Singlet Molecular Oxygen to Probe the Solute and Temperature Dependence of Liquid-Like Regions in/on Ice

Liquid-like regions (LLRs) are found at the surfaces and grain boundaries of ice and as inclusions within ice. These regions contain most of the solutes in ice and can be (photo)chemically active hotspots in natural snow and ice systems. If we assume all solutes partition into LLRs as a solution freezes, freezing-point depression predicts that the concentration of a solute in LLRs is higher than its concentration in the prefrozen (or melted) solution by the freeze-concentration factor (F). Here we use singlet molecular oxygen production to explore the effects of total solute concentration ([TS]) and temperature on experimentally determined values of F. For ice above its eutectic temperature, measured values of F agree well with freezing-point depression when [TS] is above ∼1 mmol/kg; at lower [TS] values, measurements of F are lower than predicted from freezing-point depression. For ice below its eutectic temperature, the influence of freezing-point depression on F is damped; the extreme case is with Na2SO4 as the solute, where F shows essentially no agreement with freezing-point depression. In contrast, for ice containing 3 mmol/kg NaCl, measured values of F agree well with freezing-point depression over a range of temperatures, including below the eutectic. Our experiments also reveal that the photon flux in LLRs increases in the presence of salts, which has implications for ice photochemistry in the lab and, perhaps, in the environment.

DETECTION OF THE SIMPLEST SUGAR, GLYCOLALDEHYDE, IN A SOLAR-TYPE PROTOSTAR WITH ALMA

Glycolaldehyde (HCOCH2OH) is the simplest sugar and an important intermediate in the path toward forming more complex biologically relevant molecules. In this paper we present the first detection of 13 transitions of glycolaldehyde around a solar-type young star, through Atacama Large Millimeter Array (ALMA) observations of the Class 0 protostellar binary IRAS 16293-2422 at 220 GHz (6 transitions) and 690 GHz (7 transitions). The glycolaldehyde lines have their origin in warm (200-300 K) gas close to the individual components of the binary. Glycolaldehyde co-exists with its isomer, methyl formate (HCOOCH3), which is a factor 10-15 more abundant toward the two sources. The data also show a tentative detection of ethylene glycol, the reduced alcohol of glycolaldehyde. In the 690 GHz data, the seven transitions predicted to have the highest optical depths based on modeling of the 220 GHz lines all show red-shifted absorption profiles toward one of the components in the binary (IRAS16293B) indicative of infall and emission at the systemic velocity offset from this by about 0.2" (25 AU). We discuss the constraints on the chemical formation of glycolaldehyde and other organic species - in particular, in the context of laboratory experiments of photochemistry of methanol-containing ices. The relative abundances appear to be consistent with UV photochemistry of a CH3OH-CO mixed ice that has undergone mild heating. The order of magnitude increase in line density in these early ALMA data illustrate its huge potential to reveal the full chemical complexity associated with the formation of solar system analogs.

N‐(2‐Aminoethyl)glycine and Amino Acids from Interstellar Ice Analogues

Interstellar ices were simulated by condensing and UV irradiating molecules such as H2O, CH3OH, and NH3 at 80 K. Multidimensional gas chromatography analyses allowed for the identification of 26 amino and diamino acids (see graph). The results support the suggestion that potentially prebiotic molecules originating from the photochemistry of interstellar ices could have been incorporated in cometary dust and delivered to the early Earth.

COMPLEX MOLECULES TOWARD LOW-MASS PROTOSTARS: THE SERPENS CORE

Gas-phase complex organic molecules are commonly detected toward high-mass protostellar hot cores. Detections toward low-mass protostars and outflows are comparatively rare, and a larger sample is key to investigate how the chemistry responds to its environment. Guided by the prediction that complex organic molecules form in CH3OH-rich ices and thermally or non-thermally evaporate with CH3OH, we have identified three sight-lines in the Serpens core - SMM1, SMM4 and SMM4-W - which are likely to be rich in complex organics. Using the IRAM 30m telescope, narrow lines (FWHM of 1-2 km s-1) of CH3CHO and CH3OCH3 are detected toward all sources, HCOOCH3 toward SMM1 and SMM4-W, and C2H5OH not at all. Beam-averaged abundances of individual complex organics range between 0.6 and 10% with respect to CH3OH when the CH3OH rotational temperature is applied. The summed complex organic abundances also vary by an order of magnitude, with the richest chemistry toward the most luminous protostar SMM1. The range of abundances compare well with other beam-averaged observations of low-mass sources. Complex organic abundances are of the same order of magnitude toward low-mass protostars and high-mass hot cores, but HCOOCH3 is relatively more important toward low-mass protostars. This is consistent with a sequential ice photochemistry, dominated by CHO-containing products at low temperatures and early times.

2-Nitrobenzaldehyde as a chemical actinometer for solution and ice photochemistry

2-Nitrobenzaldehyde (2NB) is a convenient, photochemically sensitive, and thermally robust actinometer. Although 2NB has been used in a number of solution and ice experiments in the laboratory, the quantum efficiencies and molar absorptivities of 2NB have not been critically evaluated, especially on ice. Using a series of laboratory and field measurements we have measured the light absorbance and photochemical properties of 2-nitrobenzaldehyde in solution (water and/or acetonitrile) and in/on water ice. Our results show that the molar absorptivities of 2NB are only weakly dependent upon temperature and that the quantum yield is independent of temperature in water; the quantum yield is also independent of wavelength, as shown by past reports. Furthermore, we find that the photochemistry of 2NB in/on water ice is the same as in liquid water. While most studies employing 2NB cite and use a quantum yield of 0.50, based on a review of the literature, and on our new experimental data, we recommend a quantum yield of 0.41 for 2NB photolysis for both solution and water ice.