Relative Band Strengths Research Articles

On the effective vibrational temperature of the source using (2)3∏ - X3∏ system of GeC molecule

In this work, the experimental potential energy curves for (2)3Π and X3Π electronic states of GeC molecule have been constructed by using Rydberg-Klein-Rees (RKR) method. The radiative transition parameters viz., Franck-Condon (FC) factor, r-centroid, electronic transition moment, band strength, relative band strength, Einstein coefficients, radiative lifetime and oscillator strength for the (2)3Π−X3Π system of GeC molecule have been estimated for the experimentally observed vibrational levels from Rydberg-Klein-Rees (RKR) potential and the estimated values are tabulated. The estimated effective vibrational temperature found as 5628 K for the (2)3Π−X3Π system of GeC molecule. The radiative transition parameters and effective vibrational temperature are evident that the possible presence of GeC molecule in solar and sunspots atmosphere. Further, these parameters are employed in rationalizations of astrochemical and astrophysical observations.

The hunt for formamide in interstellar ices

Context. Although solid-state pathways are expected to dominate the formation mechanisms of many complex organic molecules (COMs), very few COMs have been securely identified in interstellar ices, in stark contrast with the many COM detections in the gas phase. The launch of the James Webb Space Telescope (JWST) and its increase in sensitivity and spectral resolution opens the possibility of identifying more COMs in ices, but additional laboratory data are necessary. Formamide (NH2CHO) is one such COM that is of great interstellar and prebiotic relevance where more laboratory data are needed in the hunt for its presence in interstellar ices. Aims. This work aims to characterize the mid-IR spectra of formamide in its pure form as well as in mixtures of the most abundant interstellar ices via laboratory simulation of such ices, as well as to demonstrate how these laboratory spectra can be used to search for formamide in ice observations. Methods. Mid-IR spectra (4000–500 cm−1/2.5–20 μm) of formamide, both in its pure form as well as in binary and tertiary mixtures with H2O, CO2, CO, NH3, CH3OH, H2O:CO2, H2O:NH3, CO:NH3, and CO:CH3OH, were collected at temperatures ranging from 15–212 K. Results. Apparent band strengths and positions of eight IR bands of pure amorphous and crystalline formamide at various temperatures are provided. Three of these bands are identified as potential formamide tracers in observational ice spectra: the overlapping C = O stretch and NH2 scissor bands at 1700.3 and 1630.4 cm−1 (5.881 and 6.133 μm), the CH bend at 1388.1 cm−1 (7.204 μm), and the CN stretch at 1328.1 cm−1 (7.529 μm). The relative apparent band strengths, positions, and full width half maxima (FWHM) of these features in mixtures at various temperatures were also determined. All of the laboratory spectra are available to the community on the Leiden Ice Database for Astrochemistry (LIDA) for use in the interpretation of both observations (e.g., from JWST) and laboratory spectroscopic data. Finally, the laboratory spectra are compared to observational spectra of a variety of low- and high-mass young stellar objects as well as prestellar cores observed with the Infrared Space Observatory, the Spitzer Space Telescope, and JWST. A comparison between the formamide CH bend in laboratory data and the 7.24 μm band in the observations tentatively indicates that, if formamide ice is contributing significantly to the observed absorption, it is more likely in a polar matrix. Upper limits ranging from 0.35-5.1% with respect to H2O were calculated via scaling the formamide:H2O laboratory spectrum to the observations. These upper limits are in agreement with gas-phase formamide abundances and take into account the effect of a H2O matrix on formamide’s band strengths.

Laboratory Measurements of Stretching Band Strengths of Deuterated Quenched Carbonaceous Composites

The observed large variation in the abundance of deuterium (D) in the interstellar medium suggests that a significant fraction of D may be depleted into polycyclic aromatic hydrocarbons (PAHs). Signatures of the deuteration of PAHs are expected to appear most clearly through the C–D stretching modes at 4.4–4.7 μm, whose strengths in emission spectra relative to those of the C–H stretching modes at 3.3–3.5 μm provide the relative abundance of D to hydrogen (H) in PAHs, once we have accurate relative band strengths of both stretching modes. We report experimental results of the band strengths of the C–D stretching modes relative to the C–H stretching modes. We employ a laboratory analog of interstellar carbonaceous dust, Quenched Carbonaceous Composite (QCC), and synthesize deuterated QCC (D-QCC) by replacing the QCC starting gas of CH4 with mixtures of CH4 and CD4 with various ratios. Infrared spectra of D-QCC are taken to estimate the relative band strengths of the stretching modes, while the D/H ratios in the D-QCC samples are measured with a nanoscale secondary ion mass spectrometer. We obtain relative strengths of aromatic and aliphatic C–D to C–H stretches as 0.56 ± 0.04 and 0.38 ± 0.01 per D/H, respectively. The ratio for the aromatic stretches is in good agreement with the results of theoretical calculations, while that for the aliphatic stretches is smaller than that for the aromatic stretches. The present results do not significantly change the D/H ratios in interstellar PAHs that have previously been estimated from observed spectra.

Direct measurements of infrared intensities of HCN and H2O + HCN ices for laboratory and observational astrochemistry

ABSTRACT Hydrogen cyanide (HCN) is found in a wide variety of extraterrestrial environments within and beyond the Solar system, and for that reason laboratory spectroscopists have studied this compound in many spectral regions, including the infrared (IR). However, one aspect that remains to be investigated is the intrinsic IR spectral intensities of solid HCN as opposed to relative band strengths, intrinsic intensities being needed to measure HCN abundances. Here we report measurements of IR absorption coefficients and band strengths, along with supporting refractive indices and densities, of both amorphous and crystalline HCN at two temperatures, one for interstellar work and one more relevant to the outer Solar system. Spectra are presented at both temperatures, along with optical constants that can be used in numerical models. Despite widespread and longstanding interest in and investigations of solid HCN, this is the first time that the properties we are reporting have been measured in a single laboratory, avoiding the need for estimates or to combine results from various authors. We find that our measured band strength of ∼1 × 10–17 cm molecule–1 for the C≡N vibration of HCN, in both amorphous HCN and in an H2O-rich ice, is substantially higher than an earlier estimate. Unless errors of 100 per cent can be tolerated, then our new value requires a rescaling of earlier work. Our results shed light on why HCN and other nitriles have been so difficult to identify in the solid state, in contrast to their many detections in the gas phase.

A SERS Study of Charge Transfer Process in Au Nanorod-MBA@Cu2O Assemblies: Effect of Length to Diameter Ratio of Au Nanorods.

Surface-enhanced Raman scattering (SERS) is a powerful tool in charge transfer (CT) process research. By analyzing the relative intensity of the characteristic bands in the bridging molecules, one can obtain detailed information about the CT between two materials. Herein, we synthesized a series of Au nanorods (NRs) with different length-to-diameter ratios (L/Ds) and used these Au NRs to prepare a series of core–shell structures with the same Cu2O thicknesses to form Au NR–4-mercaptobenzoic acid (MBA)@Cu2O core–shell structures. Surface plasmon resonance (SPR) absorption bands were adjusted by tuning the L/Ds of Au NR cores in these assemblies. SERS spectra of the core-shell structure were obtained under 633 and 785 nm laser excitations, and on the basis of the differences in the relative band strengths of these SERS spectra detected with the as-synthesized assemblies, we calculated the CT degree of the core–shell structure. We explored whether the Cu2O conduction band and valence band position and the SPR absorption band position together affect the CT process in the core–shell structure. In this work, we found that the specific surface area of the Au NRs could influence the CT process in Au NR–MBA@Cu2O core–shell structures, which has rarely been discussed before.

Matrix isolation spectroscopy and spectral simulations of isotopically substituted C60 molecules.

Isotopically enriched (3.5% 13C) and depleted (0.5% 13C) fullerene C60 molecules are isolated in parahydrogen (pH2) solids at cryogenic temperatures and studied by high resolution (0.01-0.1 cm-1) infrared (IR) absorption measurements. Spectra of natural isotopic abundance (1.1% 13C) C60 molecules isolated in solid pH2, orthodeuterium (oD2), and Ne matrix hosts serve to identify the relatively minor spectral perturbations due to the trapping environments. Spectral features observed for the four IR-active T1u modes of threefold degeneracy in Ih symmetry, namely, T1u(1) at 529.77 cm-1, T1u(2) at 578.24 cm-1, T1u(3) at 1184.7 cm-1, and T1u(4) at 1432 cm-1, are assigned to the superpositions of matrix perturbed vibrational-mode spectra of a number of 13Cn 12C60-n isotopologues. New molecular orbital calculations show the symmetry lowering effects of 13C substitution, namely, split vibrational frequencies and modified IR intensities. IR spectral patterns calculated for the 328 distinct isotopomers of 13Cn 12C60-n up to n = 3 are used to satisfactorily simulate most of the observed absorption features. For the T1u(4) mode at 1432 cm-1, the observed splitting is insensitive to the 13C abundance, indicating spectral perturbations due to Fermi resonance. Weak absorption features at 1545 cm-1 are assigned to a combination of lower frequency modes. We discuss relative and absolute band strengths for the astrophysical application of estimating C60 abundances in planetary nebulae.

Synchrotron spectroscopy of the CSH-bending and CH3-rocking bands of methyl mercaptan

The CSH-bending and CH3-rocking infrared bands of CH3SH have been investigated in Fourier transform spectra recorded at 0.001 cm−1 resolution employing synchrotron radiation at the Canadian Light Source in Saskatoon. The relative band strengths and structures are quite different from those of the CH3OH methanol analogue, with the CSH bend being very weak and both the in-plane and out-of-plane CH3 rocks being strong with intensities greater than the C–S stretch. The CSH bend has parallel a-type character with no detectable b-type component. The out-of-plane CH3 rock is a purely c-type perpendicular band, whereas the in-plane rock is of mixed a/b character. Sub-bands have been assigned for A and E torsional species up to about K = 10 for each vibrational mode, providing upper-state term values from which K-reduced substate origins were determined. For the CSH bend, the origins follow the customary oscillatory torsional pattern as a function of K with a sharply reduced amplitude of 0.362 cm−1 compared to the 0.653 cm−1 for the ground state. The torsional energy curves for the out-of-plane rock are also well-behaved but have inverted ordering of the A–E torsional splitting components and a larger amplitude of 1.33 cm−1. In contrast, the substate origins for the in-plane rock are scattered over a range of about 2 cm−1 without clear oscillatory structure. Several instances of coupling among the lower modes and the ground state via resonances between accidentally near-degenerate levels have been observed and characterized. Ab initio calculations are found to give a good representation of the frequency and relative intensity patterns for the lower vibrational modes.

STAR FORMATION AND THE INTERSTELLAR MEDIUM IN NEARBY TIDAL STREAMS (SAINTS):SPITZERMID-INFRARED SPECTROSCOPY AND IMAGING OF INTERGALACTIC STAR-FORMING OBJECTS

A spectroscopic analysis of 10 intergalactic star forming objects (ISFOs) and a photometric analysis of 67 ISFOs in a sample of 14 interacting systems is presented. The majority of the ISFOs have relative polycyclic aromatic hydrocarbon (PAH) band strengths similar to those of nearby spiral and starburst galaxies. In contrast to what is observed in blue compact dwarfs (BCDs) and local giant HII regions in the Milky Way (NGC 3603) and the Magellanic Clouds (30 Doradus and N 66), the relative PAH band strengths in ISFOs correspond to models with a significant PAH ion fraction (<50%) and bright emission from large PAHs (~100 carbon atoms). The [NeIII]/[NeII] and [SIV]/[SIII] line flux ratios indicate moderate levels of excitation with an interstellar radiation field that is harder than the majority of the Spitzer Infrared Nearby Galaxies Survey and starburst galaxies, but softer than BCDs and local giant HII regions. The ISFO neon line flux ratios are consistent with a burst of star formation < 6 million years ago. Most of the ISFOs have ~million solar masses of warm molecular hydrogen with a likely origin in photo-dissociation regions (PDRs). Infrared Array Camera photometry shows the ISFOs to be bright at 8 um, with one third having [4.5] - [8.0] > 3.7, i.e., enhanced non-stellar emission, most likely due to PAHs, relative to normal spirals, dwarf irregulars and BCD galaxies. The relative strength of the 8 um emission compared to that at 3.6 um or 24 um separates ISFOs from dwarf galaxies in Spitzer two color diagrams. The infrared power in two thirds of the ISFOs is dominated by emission from grains in a diffuse interstellar medium. One in six ISFOs have significant emission from PDRs, contributing ~30 % - 60 % of the total power. ISFOs are young knots of intense star formation.

Ab initio study of breakdown of the Condon approximation in the F2Σ+–A2Π electronic transition of AlO

Breakdown of the Condon approximation in the F2Σ+v′–A2Πv″ transition of AlO is studied from the ab initio band strengths and Franck–Condon factors (FCFs) for bands with vibrational quantum numbers v′=0–18 and v″=0–18. The band strength distribution is clearly different from the FCF distribution, indicating a breakdown of the Condon approximation. The breakdown is characterized by the small value (below 10−1) of the ratio of the relative band strength to the relative FCF relating to the FCF maximum bands in each v″-progression for v′=5–16. We analyzed the effect of the internuclear distance (R) dependence of the electronic transition moment on the band strengths for the FCF maximum bands for v′=1–16 by using a model of transition matrix element. The analysis shows that the variation of the electronic transition matrix element μ(R) with R in the regions represented by the R-centroids for the bands makes the greatest contribution to the breakdown. The variation of μ(R) around 4.1 a0, where μ(R) changes sign, is crucial to the large scale breakdown in the F–A transition.

The electronic transition moment function of the E 1Π u -X 1Σ g + system of Ag2

Measurement of relative band strengths of 10 absorption bands of the E^1Π_u-X^1Σ_g^+ system of diatomic silver, ^(107,109)Ag_2, was performed for the first time. Theoretical analysis of the experimental data, based on Rydberg-Klein-Rees potential energy curves, revealed that assumption of the r-centroid approximation is valid for this system. Comparison of the measured and computed band strength ratios for 5 pairs of bands having common lower levels led to the following linear relative electronic transition moment function for the ^(107,109)Ag_2 E-X band system: R_e(r_(V'V"))=236r_(V'V")−564, in arbitrary units, over the 2.65–2.73 A of internuclear distance.

Anharmonic resonances in the CH chromophore overtone spectra of CHBrF2

The gas-phase Fourier transform infrared spectra of bromodifluoromethane, CHBrF2, have been recorded up to 10,100 cm−1 with a resolution of 0.5 cm−1. All the most relevant absorptions in the overtone region have been ascribed to the CH chromophore. Twenty-six bands involving either the CH-stretching (ν 1) and bending (ν 2, ν 7) modes have been assigned and their relative band strengths have been determined. Vibrational analysis of the corresponding polyads (up to N = 7/2) has been performed by means of an effective Hamiltonian that takes into account anharmonic resonances, thus leading to the determination of Fermi and Darling–Dennison coupling terms. The obtained effective spectroscopic parameters were able to quantitatively describe both the position and intensity of all the absorption bands, and also to rationalize the effects and the magnitudes of the various resonances in each polyad. The results obtained using this approach have been compared with those computed using high-quality ab initio electronic structure calculations employing second-order vibrational perturbation theory (VPT2). Overall, satisfactory agreement between the observed and the ab initio computed values was obtained by properly taking into account in the final calculations the relative magnitude of the Fermi and Darling–Dennison resonance effects in each polyad.

Rotationally Resolved C2 Symmetric Conformers of Bis-(4-hydroxyphenyl)methane: Prototypical Examples of Excitonic Coupling in the S1 and S2 Electronic States

Rotationally resolved microwave and ultraviolet spectra of jet-cooled bis-(4-hydroxyphenyl)methane (b4HPM) have been obtained using Fourier-transform microwave and UV laser/molecular beam spectrometers. A recent vibronic level study of b4HPM [Rodrigo, C. P.; Müller, C. W.; Pillsbury, N. R.; James, W. H., III; Plusquellic, D. F.; Zwier, T. S. J. Chem. Phys. 2011, 134, 164312] has assigned two conformers distinguished by the orientation of the in-plane OH groups and has identified two excitonic origins in each conformer. In the present study, the rotationally resolved bands of all four states have been well-fit to asymmetric rotor Hamiltonians. For the lower exciton (S(1)) levels, the transition dipole moment (TDM) orientations are perpendicular to the C(2) symmetry axes and consist of 41(2):59(2) and 34(2):66(2)% a:c hybrid-type character. The S(1) levels are therefore delocalized states of B symmetry and represent the antisymmetric combinations of the zero-order locally excited states of the p-cresol-like chromophores. The TDM polarizations of bands located at ≈132 cm(-1) above the S(1) origins are exclusively b-type and identify them as the upper exciton S(2) origin levels of A symmetry. The TDM orientations and the relative band strengths from the vibronic study have been analyzed within a dipole-dipole coupling model in terms of the localized TDM orientations, μ(loc), on the two chromophores. The out-of-the-ring plane angles of μ(loc) are both near 20° and are similar to results for diphenylmethane [Stearns, J. A.; Pillsbury, N. R.; Douglass, K. O.; Müller, C. W.; Zwier, T. S.; Plusquellic, D. F. J. Chem. Phys. 2008, 129, 224305]. The in-plane angles are, however, rotated by 14 and 18° relative to DPM and, in part, explain the smaller than expected exciton splittings of these two conformers.

Radiative transition parameters of the [formula omitted] band system

The relative band strengths of several absorption bands of the Ag 2 C 1 Π u – X 1 Σ g + system are measured for the first time. Ratios of observed strengths for bands with common lower vibronic levels are compared with theoretically predicted absorption band oscillator strengths, which we computed by numerically solving the radial Schrödinger equation for the vibrational wave functions, ψ υ ′ and ψ υ ″ of the Ag 2 107 , 109 C 1 Π u – X 1 Σ g + band system, using Rydberg–Klein–Rees potential energy curves and a suitable choice for the electronic transition moment function. This resulted in a useful set of radiative transition parameters for a large matrix of Ag 2 C– X bands. The functional form of the electronic transition moment required by the observed band strength ratios forces good agreement between the observations and diatomic molecular theory. Since the function is highly nonlinear, the commonly applied r-centroid approximation is invalid, and numerical integration of ∫ ψ υ ′ * R e ( r ) ψ υ ″ dr had to be performed instead. As a final check on the reliability of our results, we find good agreement between the experimentally measured displacements of band heads for the Ag 2 107 , 107 and Ag 2 109 , 109 isotopologues and the corresponding isotopic shifts of our calculated band origins.

Radiative transition parameters for the [formula omitted] band system of 107,109Ag 2

Radiative transition parameters are tabulated for the A 1 Σ u + - X 1 Σ g + band system of the silver dimer 107,109Ag 2 for bands involving vibrational levels up to v ′ = 54 and v ″ = 98 , which amply covers the band transitions empirically studied in the seminal work of Kleman and Lindkvist [B. Kleman, S. Lindkvist, Arkiv för Fysik 9 (1955) 385–390]. The tabulated parameters include band-origin wavelengths and wavenumbers, Franck–Condon factors, r-centroids, electronic transition moments, Einstein coefficients (spontaneous emission rates) and absorption band oscillator strengths. Rydberg–Klein–Rees potential energy curves for both electronic states, based on the latest available spectroscopic constants, were calculated, and then used to solve the radial Schrödinger equation for the vibrational wave functions, ψ v ′ and ψ v ″ . The constant electronic transition moment function R e ( r ) = 3.05 a.u. was fitted to previously reported relative absorption band strength measurements and the radiative lifetime of the A 1 Σ u + state. A numerical integration of ∫ ψ v ′ ∗ R e ( r ) ψ v ″ d r was performed in computing the Einstein coefficients and absorption band oscillator strengths, and these were found to differ negligibly from similar results obtained with the r-centroid approximation, as expected for a constant R e ( r ) function.

Gas and dust spectra of the D' type symbiotic star HD 330036

Aims. We present a comprehensive and self-consistent modelling of the D’ type symbiotic star (SS) HD 330036 from radio to UV. Methods. Within a colliding-wind scenario, we analyse the continuum, line, and dust spectra by means of SUMA, a code that simulates the physical conditions of an emitting gaseous cloud under the coupled effect of ionisation from an external radiation source and shocks. Results. We find that the UV lines are emitted from high-density gas between thestars downstream of the reverse shock, while the optical lines are emitted downstream of the shock propagating outwards from the system. As regards the continuum SED, three shells are identified in the IR, at 850 K, 320 K, and 200 K with radii r = 2.8 × 10 13 cm, 4 × 10 14 cm, and 10 15 cm, respectively, after adopting a distance to Earth of d = 2.3 kpc. Interestingly, all these shells appear to be circumbinary. Analysis of the unexploited ISO-SWS spectrum reveals that both PAHs and crystalline silicates coexist in HD 330036, with PAHs associated to the internal shell at 850 K, and crystalline silicates stored in the cool shells at 320 K and 200 K. Strong evidence that crystalline silicates are shaped in a disk-like structure is derived on the basis of the relative band strengths. Finally, we suggest that shocks can be a reliable mechanism for activating the annealing and the consequent crystallisation processes. Conclusions. We show that a consistent interpretation of gas and dust spectra emitted by SS can be obtained by models that account for the coupled effect of the photoionising flux and of shocks. The VLTI/MIDI proposal recently accepted by ESO aims to verify and better constrain some of our results by means of IR interferometric observations.

The Mid‐Infrared Absorption Spectra of Neutral Polycyclic Aromatic Hydrocarbons in Conditions Relevant to Dense Interstellar Clouds

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe and are expected to be present in dense interstellar clouds. In these environments, some PAHs may be present in the gas phase, but most should be frozen into ice mantles or adsorbed onto dust grains and their spectral features are expected to be seen in absorption. Here we extend our previous work on the infrared spectral properties of the small PAH naphthalene (C10H8) in several media to include the full mid-infrared laboratory spectra of 11 other PAHs and related aromatic species frozen in H2O ices. These include the molecules 1,2-dihydronaphthalene, anthracene, 9,10-dihydroanthracene, phenanthrene, pyrene, benzo[e]pyrene, perylene, benzo(k)fluoranthene, pentacene, benzo[ghi]perylene, and coronene. These results demonstrate that PAHs and related molecules, as a class, show the same spectral behaviors as naphthalene when incorporated into H2O-rich matrices. When compared to the spectra of these same molecules isolated in inert matrices (e.g., Ar or N2), the absorption bands produced when they are frozen in H2O matrices are broader (factors of 3-10), show small position shifts in either direction (usually <4 cm-1, always <10 cm-1), and show variable changes in relative band strengths (typically factors of 1-3). There is no evidence of systematic increases or decreases in the absolute strengths of the bands of these molecules when they are incorporated in H2O matrices. In H2O-rich ices, their absorption bands are relatively insensitive to concentration over the range of 10 < H2O/PAH < 200. The absorption bands of these molecules are also insensitive to temperature over the 10 K < T < 125 K range, although the spectra can show dramatic changes as the ices are warmed through the temperature range in which amorphous H2O ice converts to its cubic and hexagonal crystalline forms (T > 125 K). Given the small observed band shifts cause by H2O, the current database of spectra from Ar matrix-isolated neutral PAHs and related molecules should be useful for the search for these species in dense clouds on the basis of observed absorption band positions. Furthermore, these data permit determination of column densities to better than a factor of 3 for PAHs in dense clouds. Column density determination of detected aromatics to better than a factor of 3 will, however, require good knowledge about the nature of the matrix in which the PAH is embedded and laboratory studies of relevant samples.

Spectroscopic studies of the I 2/O 3 photochemistry : Part 2. Improved spectra of iodine oxides and analysis of the IO absorption spectrum

Based on multivariate statistical separation techniques spectra of iodine oxides formed in the photolysis of I 2 + O 3 have been obtained. Data was recorded by time resolved UV–vis absorption spectroscopy. Overlapped spectra of ground state IO( ν′ ← 0), vibrationally excited IO( ν′ ← ν″) with ν″ > 0, and OIO have been separated from each other and from underlying absorptions of further iodine oxides. The uncertainty due to uncorrected foreign absorptions is of no more than ±3%. Relative errors of spectra are of the order of a few percent for the main parts of the spectra. The up to now unknown continuous parts of spectra are thereby determined. Previous uncertainties in differential absolute absorption cross-section are removed. By the separation of the ground state IO spectrum from that of vibrationally excited IO spectroscopic measurements under non-equilibrium conditions are enabled. Three further absorber spectra have been extracted in the 200–600 nm window. Two of them are most likely caused by I 2O 2 and possibly I 2O 3 thereby providing part of the missing link between IO and OIO consumption and the formation of higher oxides and possibly aerosol. All spectra are available as supplementary data. Given the unprecedented quality of extracted spectra for the first time an analysis of band strength of the IO(A 2Π 3/2 ← X 2Π 3/2) transition could be made. The continuum absorption of the ground state IO spectrum was resolved into overlapped bands of bound–bound transitions and two bound-free transitions. From the bound-free transitions found the existence of two optically active repulsive states intersecting with the IO(A 2Π 3/2) potential has been inferred and tangents to the two repulsive potentials have been determined. Correlating the tangents to probable dissociation products gives a plausible rough picture of the shape of the repulsive states which is in good agreement with previous observations on predissociation of states. There is evidence that one of the continuous absorptions observed could have its origin in the IO( 2Π 1/2) sub-system. Relative band strength was determined for absorption bands of IO(A 2Π 3/2 ← X 2Π 3/2). They are in good agreement with literature and with simple calculations based on a Morse approximation. An anomalous behaviour of IO(2 ← 0) in time resolved measurements was observed and studied. It could be explained by a hypothetical partial population inversion of the IO(A 2Π 3/2) state, but the source of it remains unclear.

The Mid‐Infrared Laboratory Spectra of Naphthalene (C10H8) in Solid H2O

Polycyclic aromatic hydrocarbons (PAHs) are common throughout the universe. Gas-phase PAHs are easily detected in radiation-rich environments by the characteristic infrared (IR) emission bands they produce when they are excited by higher energy radiation. In contrast, in dense interstellar clouds, where the PAHs could be present as unexcited gas-phase molecules or dust grains or frozen into ice mantles, they are expected to be seen in absorption. We present full mid-IR spectra of the PAH naphthalene (C10H8) in several media to facilitate searches for PAHs in absorption in dense clouds. The positions, widths, and strengths of the absorption bands of naphthalene are significantly altered from the gas phase when it is placed in a matrix, with the extent of the alteration depending on the nature of the matrix interaction. The infrared features of gas-phase naphthalene show P, Q, and R branches characteristic of rovibrational transitions, while naphthalene in condensed forms does not. When condensed in a relatively inert matrix (e.g., Ar or N2), the IR features produced by naphthalene are narrow (generally FWHM < 2 cm-1) and may show multiple components associated with different matrix sites. In more interactive matrices (e.g., pure naphthalene and H2O), the bands become considerably broader (factors of 3-10), show small band position shifts (0-10 cm-1 in either direction), and show variable changes in relative band strengths (typically factors of 1-3). There is little systematic increase or decrease in the band strengths (perhaps none). In H2O-rich ices, naphthalene bands are relatively insensitive to concentration and temperature, with the exception that all the bands show dramatic changes as the ices are warmed through the temperature range in which amorphous H2O ice converts to its cubic and hexagonal crystalline forms. Given the small observed band shifts (<10 cm-1; <0.04 μm at 6 μm), the current database of spectra from Ar matrix-isolated PAHs should be useful for the search for PAHs in dense clouds on the basis of observed absorption band positions. Furthermore, these data permit determination of column densities to better than a factor of 3 for PAHs in dense clouds. Column density determination of detected aromatics to better than a factor of 3 will, however, require good knowledge about the nature of the matrix in which the PAH is embedded and laboratory studies of relevant samples.

Interstellar dust: What do space observations tell us?

Space infrared observations from cooled telescopes have been providing significant information on our understanding of interstellar dust grains because they are sensitive to “diffuse emission” from the interstellar medium of the entire Galaxy, which is difficult to observe with warm telescopes. This paper reviews the latest observations of interstellar dust grains by the Infrared Telescope in Space (IRTS) and the Infrared Space Observatory (ISO) missions. Particular emphasis is placed on the observations of the mid-infrared excess, which was discovered by Infrared Astronomical Satellite (IRAS). IRTS and ISO observations have unambiguously revealed the presence of the unidentified infrared (UIR) emission bands in the diffuse infrared galactic light as well as in cirrus clouds. The correlation of the UIR band distribution with the far-infrared intensity indicates that the UIR-emitting material is well mixed with the submicron sized dust grains. Further studies of the UIR bands in different environments show that the band strengths are linearly proportional to the intensity of incident radiation field as long as the incident intensity is in the range from unity to 100 times the solar vicinity value. The relative band strengths do not vary appreciably in this range. These results are in agreement with the prediction of the temperature-fluctuating dust emission or infrared fluorescence for the excess emission. Under radiation fields stronger than 1000 times the solar vicinity value, a possible deviation from linearity is indicated, suggesting that the properties of the UIR band emitters may vary under harsh conditions. On the other hand, the excess emission in the range 25–60 μm varies more rapidly with the incident radiation field, indicating the different nature of this component. Prospects of the observations of dust grains by future missions are also discussed.

Photoelectron spectroscopy of liquid water, some alcohols, and pure nonane in free micro jets

The recently developed technique of accessing volatile liquids in a high vacuum environment by using a very thin liquid jet is implemented to carry out the first measurements of photoelectron spectra of pure liquid water, methanol, ethanol, 1-propanol, 1-butanol, and benzyl alcohol as well as of liquid n-nonane. The apparatus, which consists of a commercial hemispherical (10 cm mean radius) electron analyzer and a hollow cathode discharge He I light source is described in detail and the problems of the sampling of the photoelectrons in such an environment are discussed. For water and most of the alcohols up to six different electronic bands could be resolved. The spectra of 1-butanol and n-nonane show two weakly discernable peaks from which the threshold ionization potential could be determined. A deconvolution of the photoelectron spectra is used to extract ionization potentials of individual molecular bands of molecules near the surface of the liquid and shifts of the order of 1 eV compared to the gas phase are observed. A molecular orientation for water molecules at the surface of liquid water is inferred from a comparison of the relative band strengths with the gas phase. Similar effects are also observed for some of the alcohols. The results are discussed in terms of a simple “Born-solvation” model.