Hans J. Jodl

High-pressure phases of solid nitrogen by Raman and infrared spectroscopy

Raman and infrared spectra of solid nitrogen have been collected between 25 K and room temperature up to 41 GPa. A careful analysis of the spectral band transformations occurring across the high pressure transitions among the δ, δloc, e, and ζ phases allowed to define the phase diagram in the whole P-T region investigated. In particular, the transition between the e and ζ phases has been observed in the range 30–230 K and the corresponding phase-boundary drawn. A significant metastability region (spanning about 10 GPa in pressure) hinders the transformation between the e and ζ phases when pressure is varied at low temperature. Group theory arguments suggest a centrosymmetric structure for the ζ phase and the number of Raman and infrared ν1 and ν2 components can be reproduced both with cubic and tetragonal structures. An appreciable coupling among neighboring molecules is observed, at room temperature, only in the e phase where the relative orientations of the molecules are fixed.

High pressure crystal phases of solid CH4 probed by Fourier transform infrared spectroscopy

High pressure infrared spectra of solid CH4 are reported in the range 0.8–30 GPa at room temperature, coupling a Fourier transform infrared spectrometer to a membrane diamond–anvil cell by means of a high efficiency beam condensing optical system. Two crystal phases, A and B, have been investigated. The phase transition is affected by hysteresis and occurs at 9±0.5 GPa during compression and at 7±0.5 GPa during expansion. Due to hysteresis, the transition has been studied as a function of time at higher pressures and found to undergo a first‐order kinetics, with rate constant increasing with pressure. Since our experimental apparatus allows us to perform high pressure Raman measurements too, structural properties of both A and B phases have been proposed from the analysis of the infrared and Raman data. Within the framework of the widely used three‐site model, the A phase structure is consistent with a D4h unit cell symmetry. On the contrary, the analysis of the ω1 infrared and Raman multiplets in phase B...

Pressure Induced Reactivity of Solid CO by FTIR Studies

The pressure induced reactivity of carbon monoxide was investigated in a wide temperature range (100-400 K) completely avoiding any irradiation of the sample with visible or higher frequency light. FTIR spectroscopy was employed to monitor the reaction and infrared sensors for measuring the pressure. With this approach we have been able to separate the effects of the three variables (P, T and hnu) that establish the conditions for the occurrence of the chemical reaction. A new instability boundary, not affected by the photoactivation of the reaction, is provided. The reaction has been studied in three different crystal phases (epsilon, delta, and beta), but the small differences in the reaction products are ascribable to the temperature changes rather than to the crystalline arrangement. For T<300 K the analysis of the IR spectra reveals the formation of an extended amorphous material formed, according to the vibrational assignment and to the kinetic data, by polycarbonyl linear chains containing a large amount of anhydride groups. For T>or=300 K the formation of carbon dioxide and epoxy rings, and the simultaneous decrease of carbonyl species, let suppose a decarboxylation of the extended solid product. Once exposed to the atmosphere, the reaction product readily and irreversibly reacts with water giving rise to carboxylic groups.

Infrared and Raman studies on high pressure phases of solid N2: An intermediate structural modification between ε and δ phases

The vibrational properties of high pressure phases of solid nitrogen have been studied along several isobars between 30 and 300 K by infrared and Raman spectroscopy. The existence of an intermediate phase (δloc) between the high temperature δ and the low temperature e phases has been confirmed. The analysis of the infrared spectrum in the fundamental region, combined to the knowledge of the Raman spectrum both in the fundamental and in the lattice region, furnishes direct information on the structural properties of the δloc phase. In the δloc phase the disklike molecules are ordered in fixed positions while the spherelike units behave as hindered rotors. An insight on the localization dynamics of the nitrogen molecules is given. A different structure from that of the δ phase is obtained from a group theoretical analysis of the observed Raman and infrared components. As to the e phase, a temperature dependent reorganization of the molecules located on the faces of the rhombohedral cell is suggested.

Multimedia material for teaching physics (search, evaluation and examples)

Finding multimedia material for teaching physics worldwide would seem to be easy at first glance, since there is now a lot of material available. But on closer inspection it becomes obvious that (apart from standard topics) it is very difficult, if not impossible, to find excellent teaching materials. Several representative databases are collected and described here. Only a few of them use an evaluation scheme to judge their content. If the material is evaluated, every organization uses its own list of criteria with its own weaknesses and strengths. Therefore we want to initiate a discussion to finalize a standardized evaluation scheme and to make this available to physicists and physics teachers.This paper finishes with two multimedia examples we have produced ourselves: about diffraction and about Michelson interferometry.

Pressure tuning of Fermi resonance in crystal CO2

A solid state theory on pressure tuning of Fermi resonance in crystals is presented and applied to the CO2 case. The theory relies on consideration of delocalized ω2+ω2 two‐phonon states anharmonically coupled to ω1 and it is developed along the formalism of the Green function technique. This gives a straightforward method to split bound states (or biphonons) out of the continuum. Explicit expressions for their energy separation and intensity are obtained. Calculations are performed on the pressure dependence of the main parameters governing the Fermi resonance and discussed in terms of the crystal approach. In contrast with the more limited molecular treatment, W is seen to decrease with pressure. The effect is specifically related to the spread of the two‐phonon continuum over a finite energy interval. Other quantities, for instance the intensity ratio between bound states, do not suffer such a big discrepancy between the two models. Limits of applicability of our approach are also sketched.

Multimedia in physics education: a?video for the quantitative analysis of the Reynolds number

A video of the Reynolds transition experiment, developed for physics teaching, shows the continuous transition from laminar to turbulent flow. Additionally, the critical Reynolds number of the experimental set-up is determined approximately. By looking at it, the user of the video can measure all necessary data and then calculate a result.

The spectroscopy and relaxation dynamics of three-phonon bound states in crystal CO2

The high‐resolution Fourier‐transform infrared spectrum of a single CO2 crystal has been studied in the (ω1+ω2);3ω2 Fermi resonance region. The spectrum shows three types of vibrational excitation, one where the three ω2 vibrons propagate freely (P+P+P) into the crystal, a second with two (out of three) coupled on the same molecule (BP+P), and the last with all three ω2 modes linked on one molecule and travelling as a single excitation (TP). Fermi resonance may split off the three‐phonon continuum (ω2+ω2+ω2) both the biphonon+phonon (BP+P) band and the sharp triphonon (TP) peak. A theoretical analysis of triphonons following a Green‐function approach is proposed. The temperature dependence of triphonon bandwidths has been measured in the temperature range 20–200 K. This gives information on the relaxation mechanisms of bound crystal states. Our results emphasize the role of dephasing through bending mode in the vibrational relaxation of the upper triphonon. The lower triphonon has a behavior with temperature which may be explained as due only to dephasing by lattice phonons. Triphonon depopulation is treated theoretically in a highly simplified form to enlighten the main channels available for decay.

From optical spectra to phase diagrams—the binary mixture N2–CO

We investigate the T–c% phase diagram of the binary system N2–CO. From changes in IR spectra of all kinds of mode excitations (phonons, vibrons) we were able to determine the temperature of phase transitions (solid-solid, solid-liquid). The improvements in comparison to structural investigations by x-rays or electrons are the following: sample growing and handling with perfect optical and thermodynamic quality; determination of actual concentration (N2)x(CO)y from optical spectra; reduction of thermal hysteresis by careful cooling-heating cycles of the samples.

Multimedia in physics education: a video for the quantitative analysis of the centrifugal force and the Coriolis force

A digital video on forces occurring in rotating frames of reference is presented. It allows students to observe and to analyse the trajectory of one and the same moving ball in both systems, the laboratory system at rest and the rotating system. An application of the video for physics exercises in the first year of physics study is outlined.