Bending Mode Band Research Articles

The profile of the bending mode band in solid CO2

Context. Solid carbon dioxide (CO2 ) is one of the most abundant species detected in icy grain mantles in dense molecular clouds. Its identification is based on the comparison between astronomical and laboratory spectra. In the past 30 yr the profile of solid CO2 infrared absorption bands has been extensively studied experimentally, however, the debate on the structure (amorphous versus crystalline) of CO2 samples obtained in laboratory by the thin-film technique is still open.Aims. The aim of this work is to investigate if the presence of the double peak feature in the profile of the CO2 bending mode band is related to the crystalline or amorphous structure of the sample.Methods. We performed new laboratory experiments depositing CO2 under ultra high vacuum (UHV) conditions at 17 K. We investigated, using infrared transmission spectroscopy, the influence of various experimental parameters on the profile of the CO2 bands, namely deposition rate, sample thickness, annealing, and presence of H2 O, CH3 OH or CO co-deposited with CO2 . Results. We found that, within experimental uncertainties, under UHV conditions the profile of the CO2 bands in pure solid samples does not depend on the deposition rate or the sample thickness in the ranges investigated. In all cases the bending mode band profile shows a double peak (at 660 and 655 cm-1 ). The spectra also show the Fermi resonance features that cannot be active in crystalline samples. On the other hand, when a small fraction of H2 O or CH3 OH is co-deposited with CO2 the double peak is not observed while it is observed when a CO2 :CO mixture is considered. Furthermore, we measured the density of solid CO2 and the refractive index (at 543.5 nm) at 17 K and at 70 K: ρ (17 K)= 1.17 g cm-3 , ρ (70K)= 1.49 g cm-3 , n (17K)= 1.285, and n (70K)= 1.372. Conclusions. Our experimental results indicate that the presence of the double peak in the profile of the bending mode band is not an indication of a crystalline structure of the sample and they do not exclude the presence of amorphous solid CO2 in space.

Solid CO2 in quiescent dense molecular clouds

Context. Carbon dioxide (CO2 ) is one of the most abundant species detected in icy grain mantles in star forming regions. Laboratory experiments have shown that CO2 molecules are efficiently formed in the solid state under interstellar conditions. Specifically, solid CO2 can be formed through energetic (e.g. UV photolysis, electron and ion bombardment) and non-energetic mechanisms (atom-addition reactions).Aims. Here we investigate the role of low-energy cosmic-ray bombardment in the formation of solid CO2 in quiescent dense molecular clouds.Methods. We performed laboratory experiments to study the formation of CO2 after ion irradiation with 200 keV H+ of astrophysical relevant ice mixtures. Laboratory spectra are used to fit the profile of the CO2 bending mode band observed at about 15.2 μ m (660 cm-1 ) by the Spitzer Space Telescope in the line of sight to background sources.Results. From a qualitative point of view, good fits to observations are obtained by considering either three or four laboratory components. From a quantitative point of view, a better result is obtained with four components, i.e. when a spectrum of CO2 formed after ion irradiation of CH3 OH ice is added to the fitting procedure.Conclusions. Our results support the hypothesis that energetic processing of icy grain mantles is an efficient formation mechanism of CO2 ice also in quiescent dark cloud regions, and indirectly suggest the presence of CH3 OH in icy grain mantles in interstellar cold regions.

Triangular D3h Symmetry in the Rotation-Vibration Spectrum of 12C

Our recent measurements of new states in 12C including the second 2+ at 10 MeV and the high spin 5− state at 22.4 MeV allow us to study the Rotation-Vibration spectrum of 12C from which evidence for a new (D3h) geometrical symmetry emerges. The data fit very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D3h symmetry characterized by the sequence of states: 0+, 2+, 3−, 4±, 5− with almost degenerate 4+ and 4− (parity doublet) states. Such a D3h symmetry was observed in triatomic molecules, and it is observed in 12C for the first time in nuclear physics. The triatomic like structure in nuclei is reminiscent of the discovery of diatomic α+14C structure in 18O. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1−) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular, the observation (or non observation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.654 MeV in 12C.

The Rotation-Vibration Structure of 12C

The newly measured high spin Jπ = 5− state at 22.4(2) MeV in 12C reported in this conference, fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D3h symmetry characterized by the sequence of states: 0+, 2+, 3− 4±, 5− with almost degenerate 4+ and 4− (parity doublet) states. Such a D3h symmetry was observed in triatomic molecules, and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1−) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular, the observation (or non observation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.654 MeV in 12 C.

Evidence for triangular D3h symmetry in 12C.

We report a measurement of a new high spin Jπ=5- state at 22.4(2) MeV in 12C which fits very well to the predicted (ground state) rotational band of an oblate equilateral triangular spinning top with a D3h symmetry characterized by the sequence 0+, 2+, 3-, 4±, 5- with almost degenerate 4+ and 4- (parity doublet) states. Such a D3h symmetry was observed in triatomic molecules, and it is observed here for the first time in nuclear physics. We discuss a classification of other rotation-vibration bands in 12C such as the (0+) Hoyle band and the (1-) bending mode band and suggest measurements in search of the predicted ("missing") states that may shed new light on clustering in 12C and light nuclei. In particular, the observation (or nonobservation) of the predicted ("missing") states in the Hoyle band will allow us to conclude the geometrical arrangement of the three alpha particles composing the Hoyle state at 7.654 MeV in 12C.

Highly selective separation of carbon dioxide from nitrogen and methane by nitrile/glycol-difunctionalized ionic liquids in supported ionic liquid membranes (SILMs).

Novel difunctionalized ionic liquids (ILs) containing a triethylene glycol monomethyl ether chain and a nitrile group on a pyrrolidinium or imidazolium cation have been synthesized and incorporated into supported ionic liquid membranes (SILMs). These ILs exhibit ca. 2.3 times higher CO2/N2 and CO2/CH4 gas separation selectivities than analogous ILs functionalized only with a glycol chain. Although the glycol moiety ensures room temperature liquidity of the pyrrolidinium and imidazolium ILs, the two classes of ILs benefit from the presence of a nitrile group in different ways. The difunctionalized pyrrolidinium ILs exhibit an increase in CO2 permeance, whereas the permeances of the contaminant gases rise negligibly, resulting in high gas separation selectivities. In the imidazolium ILs, the presence of a nitrile group does not always increase the CO2 permeance nor does it increase the CO2 solubility, as showed in situ by the ATR-FTIR spectroscopic method. High selectivity of these ILs is caused by the considerably reduced permeances of N2 and CH4, most likely due to the ability of the -CN group to reject the nonpolar contaminant gases. Apart from the CO2 solubility, IL-CO2 interactions and IL swelling were studied with the in situ ATR-FTIR spectroscopy. Different strengths of the IL-CO2 interactions were found to be the major difference between the two classes of ILs. The difunctionalized ILs interacted stronger with CO2 than the glycol-functionalized ILs, as manifested in the smaller bandwidths of the bending mode band of CO2 for the latter.

Solid CO2in low-mass young stellar objects

Context. Solid interstellar CO_2 is an abundant component of ice dust mantles. Its ubiquity towards quiescent molecular clouds, as well as protostellar envelopes, has recently been confirmed by the IRS (InfraRed Spectrograph) aboard the Spitzer Space Telescope. Although it has been shown that CO_2 cannot be efficiently formed in the gas phase, the CO_2 surface formation pathway is still unclear. To date several CO_2 surface formation mechanisms induced by energetic (e.g., UV photolysis and cosmic ray irradiation) and non-energetic (e.g., cold atom addition) input have been proposed. Aims. Our aim is to investigate the contribution of cosmic ray irradiation to the formation of CO_2 in different regions of the interstellar medium (ISM). To achieve this goal we compared quantitatively laboratory data with the CO_2 bending mode band profile observed towards several young stellar objects (YSOs) and a field star by the Spitzer Space Telescope. Methods. All the experiments presented here were performed at the Laboratory for Experimental Astrophysics in Catania (Italy). The interstellar relevant samples were all irradiated with fast ions (30−200 keV) and subsequently annealed in a stainless steel high vacuum chamber (P < 10^(-7) mbar). Chemical and structural modifications of the ice samples were monitored by means of infrared spectroscopy. Laboratory spectra were then used to fit some thirty observational spectra. Results. A qualitative analysis shows that a good fit can be obtained with a minimum of two components. The choice of the laboratory components is based on the chemical-physical condition of each source. A quantitative analysis of the sources with known visual extinction (A_V) and methanol abundances highlights that the solid carbon dioxide can be efficiently and abundantly formed after ion irradiation of interstellar ices in all the selected YSOs in a time compatible with cloud lifetimes (3 × 10^7 years). Only in the case of field stars can the expected CO_2 column density formed upon energetic input not explain the observed abundances. This result, to be confirmed along the line of sight to different quiescent clouds, gives an indirect indication that CO_2 can also be formed in an early cloud stage through surface reactions induced by non-energetic mechanisms. In a later stage, when ices are exposed to higher UV and cosmic ray doses, the CO_2 total abundance is strongly affected by energetic formation mechanisms. Conclusions. Our results indicate that energetic processing of icy grain mantles significantly contribute to the formation of solid phase interstellar CO_2.

The structure of the Hoyle state and its 2+partner state in12C

We have measured the 12C(γ, 3α) reaction with an Optical Time Projection Chamber (O-TPC) detector operating with the CO2(80%) + N2(20%) gas mixture and gamma-ray beams from the HIγS facility of the TUNL at Duke. We measured complete angular distributions (between 9.1 – 10.7 MeV) from which we determine the cross section yield curve and E1 – E2 relative phases leading to an unambiguous identification of the second 2+ state in 12C at 10.03(11) MeV. The observed spectrum of 12C below 12 MeV including the 22+ state observed in this work resembles the rotation-vibration spectrum predicted for a triangular shape oblate spinning top in which the Hoyle state is the first vibrational breathing mode of the triangular three alpha-particle system. We also observed a hint of the 23+ state which is predicted by the U(7) model as a member of the 1- bending mode band, but the existence of this 23+ is yet to be confirmed. The predicted rotation-vibration spectrum of a triangular shape oblate spinning top (with a D3h symmetry) allows us to compare the moment of inertia of the predicted Hoyle rotational band to the ground state rotational band and in this way extract the (large) rms radius of the Hoyle state of 3.22(8) fm. We compare the deduced rms radius with recent ab-initio theories and cluster models as well as the radius extracted from 12C(p, p′) data.

Formation of interstellar solid CO2after energetic processing of icy grain mantles

Context. Space infrared observations with ISO-SWS and Spitzer telescopes have clearly shown that solid carbon dioxide (CO2 )i s ubiquitous and abundant along the line of sight to quiescent clouds and star forming regions. Due to the CO2 low gas-phase abundance, it is suggested that CO2 is synthesized on grains after energetic processing of icy mantles and/or surface reactions. Aims. We study quantitatively the abundance of carbon dioxide synthesized from ice mixtures of astrophysical relevance induced by ion irradiation at low temperature. We compare the CO2 stretching and bending-mode band profiles observed towards some young stellar objects (YSOs) for which infrared spectra exist. Methods. Using a high vacuum experimental setup, the effects induced by fast ions (30−200 keV) on several ice mixtures of astrophysical interest are investigated. Chemical and structural modifications of the ice samples that form new molecular species are analyzed using infrared spectroscopy. The formation cross section of solid CO2 is estimated from the increase in column density as a function of the dose fitting of experimental data with an exponential curve. Results. Our laboratory experiments showed that carbon dioxide is formed after irradiation of ice mixtures containing C- and O-bearing molecules. Furthermore, when the same amount of energy is released into the icy sample, a larger amount of CO2 is formed in H2O-rich mixtures in agreement with previous studies. We also found that the CO2 stretching and bending mode band profiles depend on the mixture and temperature of the ice sample. We found that the amount of carbon dioxide formed after ion irradiation can account for the observed carbon dioxide towards YSOs. Furthermore, we discovered that laboratory spectra are a good spectroscopic analogue of the interstellar features. Conclusions. Even if the comparison between laboratory and observed spectra presented here cannot be considered unique and complete, our results quantitatively support the hypothesis that interstellar solid CO2 forms after ion irradiation and UV photolysis of icy mantles.

Infrared spectral classification of normal stars

Moderate resolution (400) 2.38-45.2m infrared spectra of stars without dust features were obtained with the Short Wavelength Spectrometer (SWS) on board the Infrared Space Observatory (ISO). The observations are part of a larger program with the objective to extend and refine the current infrared classification schemes. In particular, our data provide the basis for a more detailed classification of the 1.N-1.NO sources (ordinary and oxygen rich naked stars) as defined by Kraemer et al. (2002) in a comprehensive classification of the ISO-SWS spectra. For our analysis, the continuum was determined by fitting Engelke's function (Engelke 1992) to the SWS data. The stellar angular diameters derived from these estimates of the continuum are in good agreement with values obtained by other methods. Analysis of the equivalent widths of the CO fundamental and first overtone molecular bands, the SiO fundamental and first overtone, as well as the H2O bending mode band as a function of MK class, reveals that there is sucient information in the SWS spectra to distinguish between hot (B, A, F) and cool stars. Furthermore, it is possible to determine the spectral type for the G, K and M giants, and subtype ranges in a sequence of K and M giants. The equivalent widths of the CO and SiO bands are found to be well correlated in K and M stars, such that the equivalent widths of the CO fundamental, the SiO first overtone and the SiO fundamental can be reasonably well extrapolated from the depth of the CO first overtone. We have identified two stars, HR 365 and V Nor, whose mid-infrared spectrum does not correspond to their respective optical classification. HR 365 may have a late M companion, which dominates the observed infrared spectrum while V Nor is a late type giant that was included because its spectrum was classified as featureless under the IRAS LRS scheme. According to Kraemer et al. (2002), V Nor has a thin dust shell, which distorts the analysis of its mid-infrared absorption bands.

Density functional and infrared spectroscopy studies of bonding and vibrations of NH species adsorbed on the Ru(001) surface: a reassignment of the bending mode band

The electron energy-loss feature at about 1350 cm −1 often observed on the Ru(001) surface (and on other transition-metal surfaces) with N- and H-containing adsorbates is commonly assigned to the bending mode of the adsorbed NH species. Since it is widely used in the literature as a fingerprint for tilted NH species adsorbed on metals, we have inspected this assignment with the help of density functional model cluster calculations and infrared spectroscopy experiments. Adsorption in three-fold hollow sites with the NH axis oriented perpendicular to the surface is computed to be energetically favored. Consequently, the RuNH bending mode is dipole-forbidden in this adsorption complex, but this mode should be dipole-allowed for the tilted NH moiety calculated to be weaker bound in the on-top position of Ru(001). The vibrational frequencies of the RuNH bending mode for both conceivable structures of the adsorption complexes are calculated to be lower by more than 500 cm −1 than the experimental value assigned to this mode. Newly recorded infrared spectra of the reactive N + O + H(D) Ru(001) system could easily have detected NH (ND) stretching modes of adsorbed NH (ND) species. However, no maxima were found in the region from 700 to 2000 cm −1 which can be related to adsorbed NH, in full agreement with the density functional cluster-model results. Therefore, the electron energy-loss feature at about 1350 cm −1 has to be due to a different NH-related surface species.

Influence of the alloying on the vibrational properties of amorphous silicon in the frequency range 200–450 cm−1: Raman studies

Raman spectra of a large variety of amorphous silicon–carbon films prepared by magnetron sputtering at different technological conditions are studied. Great emphasis is given to the bands in the region 200–450 cm−1. The position of the well-defined dip between the bending mode band and the bands in the central region is used as a quantitative measure of the position of the bands in the central region. The relative carbon content in the films is estimated by infrared spectroscopy. It is shown that the changes in the central region of the Raman spectrum with the carbon atoms incorporation are due to the formation of a new bond type, but not due to increased disorder. It was found that the position of the above-defined dip could be used for comparison of the alloying atom content of nonhydrogenated amorphous silicon alloys. This is also applicable for hydrogenated samples having approximately one and the same hydrogen content.