An infrared spectroscopy study of the phase transition in solid ammonia

The infrared spectra of solid ammonia at different phases and the existence of a metastable phase have been in controversy for the last fifty years. In order to address this problem, we studied the infrared spectra of solid ammonia in an ultrahigh vacuum chamber at distinct temperatures. Having prepared amorphous ammonia at 10 K, we observed a transition from the amorphous phase to the cubic crystalline phase at 57 K; successive re-cooling from 85 K back to 10 K confirms the presence of crystalline ammonia. No metastable phase has been detected.     

Infrared spectroscopy of large ammonia clusters as a function of size

We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm(-1) for the average sizes n = 29, 80, 212, 447, and 989 and by depletion spectroscopy for n=8. The spectra are dominated by peaks around 3385 cm(-1) which are attributed to the asymmetric nu3 NH-stretch mode. Two further peaks between 3200 and 3260 cm(-1) have about equal intensity for n = 8 and 29, but only about 0.40 of the intensity of the nu3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2nu4 and the symmetric NH-stretch nu1 in the range from 3150 to 3450 cm(-1) is changed between n = 10 and 100, while the asymmetric NH-stretch nu3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.     

Infrared spectroscopy of solid hydrogen sulfide and deuterium sulfide

The infrared spectra of solid hydrogen sulfide (H2S) and deuterium sulfide (D2S) were collected at very low temperatures. Vapor deposition of thin films at the lowest temperature of 10 K produced amorphous solids while deposition at 70 K yielded the crystalline phase III. Infrared interference fringe patterns produced by the films during deposition were used to determine the film thickness. Careful measurement of the integrated absorbance peaks, along with the film thickness, allowed determination of the integrated band intensities. This report represents the first complete presentation of the infrared spectra of the amorphous solids. Observations of peaks near 3.915 and 1.982 microm (ca. 2554 and 5045 cm(-1), respectively) may be helpful in the conclusive identification of solid hydrogen sulfide on the surface of Io, a moon of Jupiter.     

Infrared spectroscopy and optical constants of porous amorphous solid water

Reflection-absorption infrared spectra (RAIRS) of amorphous solid water (ASW) films grown at 20 K on a Pt(111) substrate at various angles (theta(Beam) = 0-85 degrees ) using a molecular beam are reported. They display complex features arising from the interplay between refraction, absorption within the sample, and interference effects between the multiple reflections at the film-substrate and film-vacuum interfaces. Using a simple classical optics model based on Fresnel equations, we obtain optical constants [i.e., n(omega) and k(omega)] for porous ASW in the 1000-4000 cm(-1) (10-2.5 microm) range. The behavior of the optical properties of ASW in the intramolecular OH stretching region with increasing theta(Beam) is shown to be strongly correlated with its decreasing density and increasing surface area. A direct comparison between the RAIRS and calculated vibrational spectra shows a large difference ( approximately 200 cm(-1)) in the position of the coupled H-bonded intramolecular OH stretching vibrations spectral feature. Moreover, this band shifts in opposite directions with increasing theta(Beam) in RAIRS and vibrational spectra demonstrating RAIRS spectra cannot be interpreted straightforwardly as vibrational spectra due to severe optical distortions from refraction and interference effects.     

Infrared spectra of four p-n-alkoxybenzoic acids in the solid,liquid crystalline and isotropic phases*

Infrared spectra of four p-n-alkoxybenzoic acids (C₆C₉) in solid, liquid crystalline and isotropic phases were measured. The intensities of some internal vibrational bands changed abruptly at the various phase transitions. The changes of intensity were found to be dependent on the symmetry of the normal vibrations, and on the mechanism of the phase transitions.     

Infrared spectroscopy and photochemistry of NCCN+ and CNCN+ trapped in solid neon

When a Ne:NCCN sample is codeposited at 4.3 K with neon atoms that have been excited in a microwave discharge, the infrared and near infrared spectra of the resulting deposit include a prominent peak at 1799.5 cm-1, previously assigned to nu3 of NCCN+, and several new absorptions at higher frequencies which are contributed by combination bands of ground-state NCCN+. The exposure of the deposit to near infrared and red light results in the appearance of two new absorptions which are attributed to CNCN+. The reverse isomerization occurs when the sample is exposed to near ultraviolet radiation, but the two new absorptions are regenerated upon subsequent irradiation with near infrared and red light.     

Positronium in solid phases of phenanthrene

Temperature dependence of positronium formation in phenanthrene was investigated. On crossing the solid-solid phase transition point in both directions a sharp decrease of positronium creation probability was observed. The effect is discussed in terms of formation of an unstable phase, and of possible pyroelectricity of phenanthrene.     

Infrared spectroscopy of supercooled gases

Supercooling of gases is achieved by expansion through a simple converging nozzle. In the resulting free jets, high resolution absorption spectra are observed using tunable infrared diode lasers. The simplification of absorption spectra resulting from supercooling is illustrated for the molecules CO₂, SF₆, WF₆ and UF₆.     

Ultraviolet absorption spectra of ammonia and deuterated ammonia in solid argon

The matrix isolated spectrum of the 2100 Å system of ammonia is reexamined and extended to include the dependence of the absorption spectrum on the temperature of the matrix. Assuming that the observed spectra reflect the populations of the O⁺ and O⁻ vibrational levels in the ground electronic state, the observed temperature dependence of the spectrum of ammonia in argon are more satisfactorily explained for trapped ammonia molecules free to rotate but without nuclear spin interconversion.     

Far-infrared spectroscopy of alumina phases.

Far-infrared spectroscopy (FIR) has been used to distinguish alumina phases boehmite, diaspore, gibbsite and bayerite. The pellets of samples were prepared by mixing alumina phases with polyethylene at a ratio of 1:50, and the spectra were recorded between 50 and 400 cm(-1). The spectrum of boehmite resembles that of diaspore in the 300-400 cm(-1) region. Boehmite has two characteristic FIR bands at 366 and 323 cm(-1), while diaspore has five at 354, 331, 250, 199 and 158 cm(-1). The spectrum of gibbsite resembles that of bayerite in the 230-300 cm(-1) region. Gibbsite shows three characteristic FIR bands at 371, 279 and 246 cm(-1), whereas bayerite shows six at 383, 345, 326, 296, 252 and 62 cm(-1). The overlapping bands were resolved, and the spectra were manipulated appropriately using band analysis techniques. The FIR spectra are in harmony with the FT-Raman spectra. Far-infrared spectroscopy allows the study and differentiation of the stretching of AlO4 units to characterize these four alumina phases. Far-IR spectroscopy complements the mid-IR and near-IR for distinguishing alumina phases in bauxites.     

Morphological study into the temperature dependence of solid ammonia under astrochemical conditions using vacuum ultraviolet and Fourier-transform infrared spectroscopy

The authors present the results of a morphological study of solid ammonia using both Fourier-transform infrared and vacuum ultraviolet (VUV) spectroscopy. Dramatic changes in the VUV and infrared spectra at temperatures between 65 and 85 K provide a deeper insight into the structure of ammonia ice particularly with the observation of an exciton transition at 194 nm (6.39 eV) in the VUV spectrum, revealing a structure that is composed of crystallites. A complementary structure is observed in the IR spectrum at 1100 cm(-1) which is assigned to the symmetric deformation of ammonia molecules at the surfaces of the crystallites. Such spectral signatures may be used to identify the environment within which the ammonia ice is formed and provide a new route for obtaining information on the physical and chemical conditions occurring within the interstellar medium, on the surfaces of planetary bodies, and in Kuiper belt objects.     

Molecular motion in solid ammonia trimethylalane

Solid state ²H NMR has been used to study molecular motion in deuterated ammonia trimethylalane (CH₃)₃AlND₃. From analysis of the ²H NMR lineshape between 123 and 298 K, reorientation of the -ND₃ group about the molecular Al-N axis is shown to occur at a rate higher than 10⁸ s^-1, and simulation of partially relaxed ²H NMR lineshapes shows that the reorientation can be described as a 3-site 120° jump motion. From the temperature dependence of the ²H spin-lattice relaxation time, the activation energy for this motion is estimated to be 9.3±0.3 kJ mol^-1. There is no evidence from either ²H or ²⁷Al NMR data for any site-exchange between the sites occupied by the -ND₃ and -CH₃ groups. The anisotropy of the dynamics of (CH₃)₃AlND₃ indicates that the orientation of the Al-N bond is highly constrained, presumably by a strong interaction between the electric dipoles of neighboring molecules.     

Positronium in solid phases of long-chain paraffins

The study of positronium intensity rise in long-chain alkanes shows two kinds of electron traps: the first, which are discharged at low temperature (≈200 K); the other, which remain populated up to the transition point to the rotator phase; e.g. in C₃₀H₆₂ they are still observed at 328 K.In the rigid phase of even-numbered alkanes o-Ps lifetime is shorter than that of odd-numbered ones, due to the difference in the width of gap between the molecular lamellae.     

Infrared spectroscopy of single-walled carbon nanotubes

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.     

Infrared laser spectroscopy of jet-cooled cyclopropane

The v₁₁ band of cyclopropane between 850 and 880 cm^?1 has been recorded in a molecular beam source using a tunable infrared diode laser. The beam from a 50 μm nozzle was chopped at about 1 kHz and the absorption synchronously detected. Preliminary analysis yields a value of 868.357(1) cm^?1 for the band centre.     

Infrared spectroscopy of hydrated sulfate dianions

We report the first infrared spectra of multiply-charged anions in the gas phase. The spectra of SO(4) (2-)(H(2)O)(n), with n=3-24, show four main bands assigned to two vibrations of the dianionic core, the water bending mode, and solvent libration. The triply degenerate SO(4) (2-) antisymmetric stretch vibration probes the local solvent symmetry, while the solvent librational band is sensitive to the hydrogen bonding network. The spectra and accompanying electronic structure calculations indicate a highly symmetric structure for the n=6 cluster and closure of the first solvation shell at n=12.     

Infrared spectroscopy of oxygen bond systems

The vibrational spectra of various types of oxygen-bonded systems with other elements are interpreted in terms of factor group analysis and normal vibration analysis. The stereochemical structures of pseudo-octahedral (MO_nL_6-n) and pseudo-tetrahedral (MO_nL_4-n) compounds with terminal oxygen atoms and of M₂OL_n compounds with oxygen bridge bonding are discussed on the basis of our own data and those from the literature.The present work also contains certain general statements concerning p_π-d_π interactions in oxy compounds of metals having d and p electrons.     

Infrared laser spectroscopy of SH+

Many components of the fundamental band of ³²SH⁺ have been detected and measured in absorption using infrared diode laser spectroscopy. The transitions are characterized by the triplet fine-structure splitting expected for the X ³∑⁻ state of the cation and their positions are in good agreement with predictions obtained from an analysis of the A ³Π-X ³∑⁻ system. Some lines with opposite phase to the cation signals are tentatively assigned to SH⁻.