Surface Of Interstellar Dust Grains Research Articles

Modelling deuterated isotopologues of methanol towards the pre-stellar core L1544

Context. In the extremely cold and dark environments of pre-stellar cores, methanol is formed on the surface of interstellar dust grains and released into the gas phase via non-thermal desorption mechanisms. Gaseous methanol constitutes the starting point for the formation of many massive complex organic molecules and is therefore of utmost importance for the build-up of chemical complexity. Aims. We aim to improve upon a previous model for the prediction of column densities and deuterium fractions of non-deuterated and singly deuterated methanol. Thereby, we try to identify crucial chemical and physical parameters for which the study of deuteration could provide valuable additional constraints. Methods. We employed a gas-grain chemical code to devise a model that is in agreement with the observed column density and deuterium fraction profiles of the innermost region of the pre-stellar core L1544. For that purpose, we developed a new treatment of reactive desorption, deriving an individual reactive desorption efficiency for every product species in a chemical reaction that depends on the reaction enthalpy and type of the underlying surface. Furthermore, we explored several options to promote the diffusion of hydrogen and deuterium atoms over the surface of interstellar dust grains in order to increase methanol formation. Results. Our fiducial model employs diffusion via the quantum tunnelling of hydrogen and deuterium atoms, resulting in CH3OH and CH2DOH column densities that are approximately an order of magnitude lower than the observed values, which is an improvement over the results of the previous model by a factor of 10. The N(CH2DOH)/N(CH3OH) ratio is reproduced within a factor of 1.2 for the centre and 1.8 for the position of the methanol peak. Given the large uncertainties that chemical models typically have, we consider our predictions to be in agreement with the observations. In general, we conclude that a diffusion process with a high diffusion rate needs to be employed to obtain methanol column densities that are in accordance with the observed values. Also, we find that the introduction of abstraction reactions into the methanol formation scheme suppresses deuteration when used in combination with a high diffusion rate.

Hydrogen abstraction reactions in formic and thioformic acid isomers by hydrogen and deuterium atoms

Context.The isomerism of molecules in the interstellar medium and the mechanisms behind it are essential questions in the chemistry of organic molecules in space. In the particular case of simple formic and thioformic acids, the low temperatures found in molecular clouds indicate that cis-trans isomerization in the gas-phase must be impeded. Reactions taking place on top of interstellar dust grains may explain the isomer interconversion at low temperatures.Aims.We studied the isomerization processes of formic and thioformic acid that are likely to take place on the surface of interstellar dust grains after being initiated by H abstraction reactions. Similarly, deuterium enrichment of the acids can occur by the same mechanism. Our objective is to shed light on both topics to expand our understanding of the key precursors of organic molecules in space.Methods.We determined the rate constants for the H abstraction reactions as well as the binding energies for the acids on water ice using ab initio calculations and the instanton method for calculating the rate constants, including quantum tunneling. In addition, we tested the viability of the deuteration of formic acid with tailored experiments and looked for it on the L1544 source.Results.For formic acid, there is a clear dependence of the H abstraction reactions on the isomer of the reactant, with rate constants at ~50 K that differ by five orders of magnitude. Correspondingly, we did not observe the trans-cis reaction in our experiments. In the case of thioformic acid, a very similar cis-trans reactivity is found for abstraction reactions at the thiol (-SH) group in contrast to a preferential reactivity that is found when abstractions take place at the -CH moiety. We found comparable binding energies for both isomers with average binding energies of around −6200 and −3100 K for formic and thioformic acid, respectively. Our binding energy calculations show that the reactions are precluded for specific orientations, affecting the overall isomerization rate. For H abstractions initiated by deuterium atoms, we found very similar trends, with kinetic isotope effects varying in most cases between 13 and 20.Conclusions.Our results support the cis-trans interconversion of cis-formic acid on dust grains, suggesting that such an acid should not withstand the conditions found on these objects. On the other hand, the trans isomer is very resilient. Both isomers of thioformic acid are much more reactive. A non-trivial chemistry is behind the apparent excess of its trans isomer that is found in cold molecular clouds and star-forming regions due to a subtle combination of preferential reactivity and binding with the surface. In light of our results, all the deuterated counterparts of thioformic acid are viable molecules to be present on the ISM. In contrast, only the trans isomer of deuterated formic acid is expected, for which we provide upper bounds of detection. Given the mechanisms presented in this paper, other mechanisms must be at play to explain the tiny fraction of cis-formic acid observed in interstellar cold environments, as well as the current trans-DCOOH and trans-HCOOD abundances in hot-corinos.

Understanding the impact of diffusion of CO in the astrochemical models

Abstract The mobility of lighter species on the surface of interstellar dust grains plays a crucial role in forming simple through complex molecules. Carbon monoxide is one of the most abundant molecules, its surface diffusion on the grain surface is essential to forming many molecules. Recent laboratory experiments found a diverse range of diffusion barriers for CO on the grain surface, their use can significantly impact the abundance of several molecules. The impact of different diffusion barriers of CO, in the astrochemical models, is studied to understand its effect on the abundance of solid CO and the species for which it is a reactant partner. A gas-grain network is used for three different physical conditions; cold core and warm-up models with slow and fast heating rates. Two different ratios (0.3 and 0.5) between diffusion and desorption barrier are utilised for all the species. For each physical condition and ratio, six different models are run by varying diffusion barriers of CO. Solid CO abundance for the models with the lowest diffusion barrier yields less than 0.1% of water ice for cold clouds and a maximum of 0.4% for slow and fast warm-up models. Also, solid $\textrm{CO}_2$ in dense clouds is significantly overproduced ( ${\sim}140\%$ of water). The abundance of H2CO and $\textrm{CH}_3\textrm{OH}$ showed an opposite trend, and HCOOH, $\textrm{CH}_3\textrm{CHO}$ , $\textrm{NH}_2\textrm{CO}$ , and $\textrm{CH}_3\textrm{COCH}_3$ are produced in lower quantities for models with low diffusion barriers for CO. Considerable variation in abundance is observed between models with the high and low diffusion barrier. Models with higher diffusion barriers provide a relatively better agreement with the observed abundances when compared with the models having lower diffusion barriers.

Hydrogenation of glycolaldehyde to ethylene glycol at 10 K

ABSTRACT Glycolaldehyde, the simplest sugar, is a complex organic molecule detected in many regions of the interstellar medium (ISM). Although its synthetic routes are fairly well known and consistent with many laboratory studies, queries still arise about its reactivity and its role in the complex chemistry of the ISM. This study shows the surface and bulk hydrogenation of glycolaldehyde at 10 K in order to confirm or invalidate the astrophysical models which suggest that CHOCH2OH would be a precursor of ethylene glycol through hydrogenation processes occurring on the surface of interstellar dust grains. By coupling IR spectroscopy and mass spectrometry, we show that the formation of HOCH2CH2OH from CHOCH2OH + H solid state reaction occurs, supporting the existence of a chemical link between these two organics in the ISM. This work suggests that while CHO + CH2OH and CH2OH + CH2OH radical recombination would lead to CHOCH2OH and HOCH2CH2OH, respectively, the presence of H-atoms in the ISM would be a secondary source to favour ethylene glycol over glycolaldehyde. These results are in good agreement with different astronomical observations which show simultaneous detections of glycolaldehyde and ethylene glycol with an abundance ratio HOCH2CH2OH/CHOCH2OH ranged between 1 and 15.

Molecular Dynamics Simulations of Energy Dissipation on Amorphous Solid Water: Testing the Validity of Equipartition.

Many different molecular species have been observed in the interstellar medium. These range from simple diatomic species to saturated organic molecules with several carbon atoms. The latter molecules are assumed to be formed predominantely on the surface of interstellar dust grains. All surface reactions that can proceed under the low interstellar temperatures are exothermic. Their exothermicity can be as high as a few electron volts, which is considerable compared to the thermal energy of the molecules at 10 K. It is postulated that this exothermicity can be used for the desorption of reaction products from the grain. In previous studies, we have shown that translational excitation can lead to desorption, whereas vibrational and rotational excitations are much less efficient in the desorption of surface products. Vibrational excitation is however much more likely upon bond formation than translational excitation. The present study follows energy dissipation upon translational, vibrational, or rotational excitation of admolecules on a surface and its conversion, or lack thereof, to different energy contributions. To this end, thousands of molecular dynamics simulations were performed with an admolecule on top of a surface that received a fixed amount of energy, vibrational, rotational, or translational. Three different surface species have been considered, CO2, H2O, and CH4, spanning a range in binding energies, the number of internal degrees of freedom, and molecular weights. A fast exchange of energy between vibrational stretches is observed, but only very limited exchange to rotational or translation excitation has been found. For the dissipation of energy to the surface, excitation of the surface–admolecule bond is critical. Astrochemical models often assume instantaneous equipartition of energy after a reaction process to estimate the amount of available energy for chemical desorption. Based on the present study, we conclude that this assumption is not justified.

Formation of COMs through CO hydrogenation on interstellar grains

Context. Glycoaldehyde, ethylene glycol, and methyl formate are complex organic molecules that have been observed in dark molecular clouds. Because there is no efficient gas-phase route to produce these species, it is expected that a low-temperature surface route existst that does not require energetic processing. CO hydrogenation experiments at low temperatures showed that this is indeed the case. Glyoxal can form through recombination of two HCO radicals and is then further hydrogenated. Aims. Here we aim to constrain the methyl formate, glycolaldehyde, and ethylene glycol formation on the surface of interstellar dust grains through this cold and dark formation route. We also probe the dependence of the grain mantle composition on the initial gas-phase composition and the dust temperature. Methods. A full CO hydrogenation reaction network was built based on quantum chemical calculations for the rate constants and branching ratios. This network was used in combination with a microscopic kinetic Monte Carlo simulation to simulate ice chemistry, taking into account all positional information. After benchmarking the model against CO-hydrogenation experiments, simulations under molecular cloud conditions were performed. Results. Glycoaldehyde, ethylene glycol, and methyl formate are formed in all interstellar conditions we studied, even at temperatures as low as 8 K. This is because the HCO + HCO reaction can occur when HCO radicals are formed close to each other and do not require to diffuse. Relatively low abundances of methyl formate are formed. The final COM abundances depend more on the H-to-CO ratio and less on temperature. Only above 16 K, where CO build-up is less efficient, does temperature start to play a role. Molecular hydrogen is predominantly formed through abstraction reactions on the surface. The most important reaction leading to methanol is H2CO + CH3O → HCO + CH3OH. Our simulations are in agreement with observed COM ratios for mantles that have been formed at low temperatures.

Erratum: The surface reactivity of acrylonitrile with oxygen atoms on an analogue of interstellar dust grains

Experiments designed to reveal the low-temperature reactivity on the surfaces of interstellar dust grains are used to probe the heterogeneous reaction between oxygen atoms and acrylonitrile (C2H3CN, H2C=CH-CN). The reaction is studied at a series of fixed surface temperatures between 14 and 100 K. After dosing the reactants on to the surface, temperature-programmed desorption, coupled with time-of-flight mass spectrometry, reveals the formation of a product with the molecular formula C3H3NO. This product results from the addition of a single oxygen atom to the acrylonitrile reactant. The oxygen atom attack appears to occur exclusively at the C=C double bond, rather than involving the cyano(-CN) group. The absence of reactivity at the cyano site hints that full saturation of organic molecules on dust grains may not always occur in the interstellar medium. Modelling the experimental data provides a reaction probability of 0.007 ± 0.003 for a Langmuir–Hinshelwood style (diffusive) reaction mechanism. Desorption energies for acrylonitrile, oxygen atoms, and molecular oxygen, from the multilayer mixed ice their deposition forms, are also extracted from the kinetic model and are 22.7 ± 1.0 kJ mol−1 (2730 ± 120 K), 14.2 ± 1.0 kJ mol−1 (1710 ± 120 K), and 8.5 ± 0.8 kJ mol−1 (1020 ± 100 K), respectively. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and acrylonitrile could occur on interstellar dust grains on an astrophysical time-scale.

The surface reactivity of acrylonitrile with oxygen atoms on an analogue of interstellar dust grains

Experiments designed to reveal the low-temperature reactivity on the surfaces of interstellar dust grains are used to probe the heterogeneous reaction between oxygen atoms and acrylonitrile (C2H3CN, H2C=CH-CN). The reaction is studied at a series of fixed surface temperatures between 14 and 100 K. After dosing the reactants on to the surface, temperature-programmed desorption, coupled with time-of-flight mass spectrometry, reveals the formation of a product with the molecular formula C3H3NO. This product results from the addition of a single oxygen atom to the acrylonitrile reactant. The oxygen atom attack appears to occur exclusively at the C=C double bond, rather than involving the cyano(-CN) group. The absence of reactivity at the cyano site hints that full saturation of organic molecules on dust grains may not always occur in the interstellar medium. Modelling the experimental data provides a reaction probability of 0.007 ± 0.003 for a Langmuir–Hinshelwood style (diffusive) reaction mechanism. Desorption energies for acrylonitrile, oxygen atoms, and molecular oxygen, from the multilayer mixed ice their deposition forms, are also extracted from the kinetic model and are 22.7 ± 1.0 kJ mol−1 (2730 ± 120 K), 14.2 ± 1.0 kJ mol−1 (1710 ± 120 K), and 8.5 ± 0.8 kJ mol−1 (1020 ± 100 K), respectively. The kinetic parameters we extract from our experiments indicate that the reaction between atomic oxygen and acrylonitrile could occur on interstellar dust grains on an astrophysical time-scale.

Direct measurement of desorption and diffusion energies of O and N atoms physisorbed on amorphous surfaces

Context. Physisorbed atoms on the surface of interstellar dust grains play a central role in solid state astrochemistry. Their surface reactivity is one source of the observed molecular complexity in space. In experimental astrophysics, the high reactivity of atoms also constitutes an obstacle to measuring two of the fundamental properties in surface physics, namely desorption and diffusion energies, and so far direct measurements are non-existent for O and N atoms.

A stochastic model and Monte Carlo algorithm for fluctuation-induced H2 formation on the surface of interstellar dust grains

A stochastic algorithm for simulation of fluctuation-induced kinetics of H2 formation on grain surfaces is suggested as a generalization of the technique developed in our recent studies [1] where this method was developed to describe the annihilation of spatially separate electrons and holes in a disordered semiconductor. The stochastic model is based on the spatially inhomogeneous, nonlinear integro-differential Smoluchowski equations with random source term. In this paper we derive the general system of Smoluchowski type equations for the formation of H2 from two hydrogen atoms on the surface of interstellar dust grains with physisorption and chemisorption sites. We focus in this study on the spatial distribution, and numerically investigate the segregation in the case of a source with a continuous generation in time and randomly distributed in space. The stochastic particle method presented is based on a probabilistic interpretation of the underlying process as a stochastic Markov process of interacting particle system in discrete but randomly progressed time instances. The segregation is analyzed through the correlation analysis of the vector random field of concentrations which appears to be isotropic in space and stationary in time.

Conversion of HI Molecule to H2 Molecule

In this article I review the historical development and conversion of atomic to molecular hydrogen in astronomy. I discuss how the discoveries of HI and H2 in the interstellar medium were followed by studies of the relative abundance of atomic and molecular gas. Understanding this led to increasingly sophisticated theoretical models for H2 formation on the surface of interstellar dust grains. In certain situations, astronomical data can be used to constrain the formation rate of H2 molecules. Finally, I use the reasonably well-determined chemistry of HI and H2 to determine the overall timescale of star formation. The Himalayan Physics Vol. 5, No. 5, Nov. 2014 Page: 82-86

Laboratory surface astrochemistry experiments.

Although several research groups have studied the formation of H2 on interstellar dust grains using surface science techniques, few have explored the formation of more complex molecules. A small number of these reactions produce molecules that remain on the surface of interstellar dust grains and, over time, lead to the formation of icy mantles. The most abundant of these species within the ice is H2O and is of particular interest as the observed molecular abundance cannot be accounted for using gas-phase chemistry alone. This article provides a brief introduction to the astronomical implications and motivations behind this research and the requirement for a new dual atomic beam ultrahigh vacuum (UHV) system. Further details of the apparatus design, characterisation, and calibration of the system are provided along with preliminary data from atomic O and O2 beam dosing on bare silica substrate and subsequent temperature programmed desorption measurements. The results obtained in this ongoing research may enable more chemically accurate surface formation mechanisms to be deduced for this and other species before simulating the kinetic data under interstellar conditions.

H2FORMATION IN DIFFUSE CLOUDS: A NEW KINETIC MONTE CARLO STUDY

We used the continuous-time random-walk Monte Carlo technique to study anew the formation of H2 on the surfaces of interstellar dust grains in diffuse interstellar clouds. For our study, we considered three different grain materials, olivine (a polycrystalline silicate), amorphous silicate, and amorphous carbon, as well as a grain temperature that depends on granular size. For some runs, we included temperature fluctuations. Four different granular surfaces were used, one with one type of binding site due to physisorption, one with five different types of physisorption binding sites due to lateral forces, and two with sites for chemisorption, one in which chemisorption sites are entered through precursor physisorption sites, and one in which chemisorption is direct but occurs with a barrier for the adsorption of the first hydrogen atom. We found that on flat and rough olivine surfaces, molecular hydrogen is formed at low efficiencies, with smaller grains contributing very little despite their large numbers due to high temperatures. For flat amorphous carbon and amorphous silicate surfaces, the efficiency increases, reaching unity for the largest grains. For models with barrierless chemisorption, the efficiency of formation of H2 is near unity at all grain sizes considered, while for direct chemisorption via a barrier, we found efficiencies of 0.13-0.6 depending upon the barrier, but independent of grain size. Treating the flat olivine and amorphous silicate surfaces with temperature fluctuations increases the efficiency of H2 formation.

KINETIC MONTE CARLO STUDIES OF H2FORMATION ON GRAIN SURFACES OVER A WIDE TEMPERATURE RANGE

We have used the continuous-time random-walk Monte Carlo technique to study the formation of H{sub 2} from two hydrogen atoms on the surface of interstellar dust grains with both physisorption and chemisorption sites on olivine and carbonaceous material. In our standard approach, atoms must first enter the physisorption site before chemisorption can occur. We have considered hydrogen atom mobility due to both thermal hopping and quantum mechanical tunneling. The temperature range between 5 K and 825 K has been explored for different incoming H fluxes representative of interstellar environments with atomic hydrogen number density ranging between 0.1 cm{sup -3} and 100 cm{sup -3} and dust grain sizes ranging from 100 sites to 10{sup 6} sites, the latter corresponding roughly to olivine grains of radius 0.2 {mu}m. In addition, we have also considered rough surfaces with multiple binding sites. Tunneling is found to dominate the surface chemistry at low temperature, but as the temperature increases, the scenario changes. The inclusion of chemisorption sites can provide a meaningful efficiency for H{sub 2} production up to temperatures as high as 700 K depending upon the depth of the chemisorption well. We found that over virtually the entire temperature range studied, the use ofmore » rate equations overestimates the H{sub 2} formation rate to some extent. This overestimate is large at high temperatures, due to very low surface residence times. We have also considered models in which chemisorption sites are entered directly and diffusion proceeds only to other chemisorption sites.« less

COMPETING MECHANISMS OF MOLECULAR HYDROGEN FORMATION IN CONDITIONS RELEVANT TO THE INTERSTELLAR MEDIUM

The most efficient mechanism of the formation of molecular hydrogen in the current universe is by association of hydrogen atoms on the surface of interstellar dust grains. The details of the processes of its formation and release from the grain are of great importance in the physical and chemical evolution of the space environments where it takes place. The main puzzle is still the fate of the 4.5 eV released in H2 formation and whether it goes into internal energy (rovibrational excitation), translational kinetic energy, or heating of the grain. The modality of the release of this energy affects the dynamics of the ISM and its evolution toward star formation. We present results of the detection of the rovibrational states of the just-formed H2 as it leaves the surface of a silicate. We find that rovibrationally excited molecules are ejected into the gas phase immediately after formation over a much wider range of grain temperatures than anticipated. Our results can be explained by the presence of two mechanisms of molecule formation that operate in partially overlapping ranges of grain temperature. A preliminary analysis of the relative importance of these two mechanisms is given. These unexpected findings, which will be complemented with experiments on the influence of factors such as silicate morphology, should be of great interest to the astrophysics and astrochemistry communities.

Kinetic Monte Carlo method for simulating astrochemical kinetics: Hydrogen chemistry in diffuse clouds

We perform numerical simulations of the molecular hydrogen production on the surface of interstellar dust grains and its dissociation by the ultraviolet background in conditions typical for the interstellar medium. The kinetic version of the Monte Carlo method is used for the modeling of the catalytic chemical reactions on the surface of the dust fraction and in the surrounding medium. Our simulations show the importance of the interstellar dust particles for hydrogen chemistry in diffuse molecular clouds.

Astroelectrochemistry: the role of redox reactions in cosmic dust chemistry

We propose that redox reactions on the surface of interstellar dust grains contribute to the synthesis of some polyatomic species that have been identified by spectroscopic signatures. Most of the dust is found in clouds along with a rich abundance of molecular and atomic species, creating a thermodynamically distinct region in the interstellar medium (ISM) where chemistry can be supported. Using knowledge of redox process at the solid/liquid interface, a hypothesis is presented for processing mechanisms involving electron transfer between surface adsorbed species and the solid dust grains found in the ISM. The hypothesis is based on the interaction of dust grains with electromagnetic radiation and plumes of ionised gas, which electrostatically charge dust grains leading to an adjustment of the Fermi energy of electrons on the surface of individual grains. This process is equivalent to applying an external electrochemical potential to an electrode, to drive redox chemistry on an electrode surface in electrolysis or dynamic electrochemistry. Here the individual grains act as 'single electrode' electrochemical reactors in the gas phase. In this paper we highlight a gap in understanding of redox reactions at the solid/gas interface, which is potentially a very fruitful and interesting area of research.

Kinetic Monte Carlo method for simulating astrochemical kinetics: Test calculations of molecular hydrogen formation on interstellar dust particles

Molecular hydrogen formation on the surfaces of interstellar dust grains has been studied numerically. Different stochastic methods (analogue Monte Carlo methods, direct solutions of the chemical master equation, continuous-time random-walk simulations, etc.) have been recently applied to the astrochemistry of hydrogen formation; however, the computational efficiency of these approaches is usually low because they are extremely time consuming (Herbst and Shematovich, 2003). A kinetic version of the Monte Carlo method based on splitting by physical processes is presented in the study. Each of the basic physical processes—adsorption of atomic hydrogen on the surface, thermal diffusion, formation of molecular hydrogen, and hydrogen desorption from the surface-are considered as independent random Markovian-type processes and are simulated using stochastic algorithms. The suggested numerical model is computationally efficient allowing calculations to be held on model surfaces with up to 106 active centers, which corresponds to interstellar grains of up to 0.4 μm in diameter. Test calculations of the efficiency of molecular hydrogen formation were held for the two models of the surfaces of interplanetary grains, namely, for the homogeneous surfaces of olivine and amorphous carbon. It was confirmed that the effective formation of molecular hydrogen in diffuse molecular clouds is possible only within a narrow range of dust-particle temperatures. A comparison with the numerical results of other authors is presented.

Mobility of D atoms on porous amorphous water ice surfaces under interstellar conditions

Aims. The mobility of H atoms on the surface of interstellar dust grains at low temperature is still a matter of debate. In dense clouds, the hydrogenation of adsorbed species (i.e., CO), as well as the subsequent deuteration of the accreted molecules depend on the mobility of H atoms on water ice. Astrochemical models widely assume that H atoms are mobile on the surface of dust grains even if controversy still exists. We present here direct experimental evidence of the mobility of H atoms on porous water ice surfaces at 10 K. Methods. In a UHV chamber, O2 is deposited on a porous amorphous water ice substrate. Then D atoms are deposited onto the surface held at 10 K. Temperature-Programmed Desorption (TPD) is used and desorptions of O2 and D2 are simultaneously monitored. Results. We find that the amount of O2 that desorb during the TPD diminishes if we increase the deposition time of D atoms. O2 is thus destroyed by D atoms even though these molecules have previously diffused inside the pores of thick water ice. Our results can be easily interpreted if D is mobile at 10 K on the water ice surface. A simple rate equation model fits our experimental data and best fit curves were obtained for a D atoms diffusion barrier of 22(+-)2 meV. Therefore hydrogenation can take place efficiently on interstellar dust grains. These experimental results are in line with most calculations and validate the hypothesis used in several models.

Millimeter and Submillimeter Emissions from Galactic and Extragalactic Photodissociation Regions

The galactic and Extragalactic photodissociation regions are primarily heated by photoelectrons ejected from the surface of interstellar dust grains by Far-ultraviolet (FUV) photons. But there is no direct mechanism to measure the photoelectric heating efficiency. To understand the role of dust grains in processing the Interstellar Radiation Field (ISRF) and heating the gas, we compare the intensities ICII, ICO and IFIR for ( 2 P3/2 → 2 P1/2 )&( J = 1 → 0) line emis- sion of CII & CO at 158 μm & 2.6 mm and integrated far- infrared from number of photodissociation regions, HII re- gions, planetary nebulae, reflection nebulae and high latitude translucent clouds (HLCs). It is found that ICII is linearly cor- related with IFIR. In the cold medium where cloud is exposed to weak radiations temperature is low and most of the cool- ing is due to (CII) emissions. As a result the ratio of ICII/IFIR provide indirect method to evaluate the photoelectric heat- ing efficiency. For the neutral cold medium it is evaluated to be ∼0.028. The FUV radiation field G0 are estimated through the model calculation of ICII and ICO for different galactic and photodissociation regions. The intensity of FIR radiation IFIR are well represented as 1.23 × 10 −4 G0(ergs cm −2 s −1 sr −1 ) almost same as estimated for HLCs by Ingalls et al. (2002). Hydrogen density for each source has also been estimated.