S. Bauerecker

Liquid-helium temperature long-path infrared spectroscopy of molecular clusters and supercooled molecules

Collisional cooling and supersonic jet expansion both allow us to perform infrared spectroscopy of supercooled molecules and atomic and molecular clusters. Collisional cooling has the advantage of higher sensitivity per molecule and enables working in thermal equilibrium. A new powerful method of collisional cooling is presented in this article. It is based on a cooling cell with integrated temperature-invariant White optics and pulsed or continuous sample-gas inlet. The system can be cooled with liquid nitrogen or liquid helium and operated at gas pressures between <10−5 and 13 bar. Temperatures range from 4.2 to 400 K and can be adjusted to an accuracy of ±0.2 K over most of the useable range. A three-zone heating design allows homogeneous or inhomogeneous temperature distributions. Optical path lengths can be selected up to values of 20 m for Fourier transform infrared (FTIR) and 40 m for laser operation. The cell axis is vertical, so optical windows are at room temperature. Diffusive trapping shields ...

High resolution infrared spectroscopy and global vibrational analysis for the CH3D and CHD3 isotopomers of methaney

We report infrared spectra of CH3D and CHD3 in the range 2900 to 9000 cm−1 measured with the Zürich high resolution Fourier transform infrared (FTIR) interferometer Bruker IFS 125 prototype (ZP 2001) at 80 K in a collisional-cooling cell with optical paths ranging from 5 to 10 m. In all, 57 new ro-vibrational bands of CH3D and 40 for CHD3 were assigned and analysed. Using a strategy of the direct assignment of the J = 0 states of excited vibrational levels, precise experimental values of the band centres with uncertainties in the range of about 0.0001 to 0.0003 cm−1 were obtained. Including 15 previously known band centres of CH3D and 12 previously known band centres of CHD3, these data were used as the initial information for the determination of the harmonic frequencies, anharmonic coefficients, and vibrational resonance interaction parameters in an effective hamiltonian. A joint set of 64 parameters reproduces the 124 experimental vibrational energies of both molecules up to 6500 cm−1 with a root mean deviation of d rms = 0.73 cm−1. The results are discussed in relation to intramolecular dynamics on a global potential hypersurface of methane, intramolecular vibrational redistribution, and the spectroscopy of the atmospheres of the earth and planetary systems.

Formation and growth of ice particles in stationary ultrasonic fields

The formation of ice particles in stationary ultrasonic fields (SUSFs) from ice aerosol was observed. Under suitable experimental conditions aerosol particles gather in stable ellipsoidal systems at temperatures down to <50 K below ambient. We call these systems cold gas traps. The aerosol concentrates in the pressure node areas of the SUSF and conglomerates to form larger particles and snowflakes with diameters up to 10 mm. The resulting shapes are shown. Most of them are similar to those occurring in nature. An attempt is made to explain the effect. Three different processes of ice particle formation are visualized and discussed. Here the existence of a quasiliquid layer (QLL) on the particle surfaces seems to be essential because both the formation of ice particles and the QLL occur only at temperatures higher than about −25 °C. The observed trapping and generation process can be used, for example, in atmospheric physics for the study of particle aggregation, precipitation formation, and water and radi...

Trapping of heavy gases in stationary ultrasonic fields

We have observed heavy-gas trapping in stationary ultrasonic fields. This effect as well as the recently observed cold-gas traps have been investigated. Both can be explained by the difference in mass density between the trapped gas and ambient media. A first theoretical approach is given.

Survey of the high resolution infrared spectrum of methane (12CH4 and 13CH4): Partial vibrational assignment extended towards 12 000 cm−1

We have recorded the complete infrared spectrum of methane (12)CH4 and its second most abundant isotopomer (13)CH4 extending from the fundamental range starting at 1000 cm(-1) up to the overtone region near 12,000 cm(-1) in the near infrared at the limit towards the visible range, at temperatures of about 80 K and also at 298 K with Doppler limited resolution in the gas phase by means of interferometric Fourier transform spectroscopy using the Bruker IFS 125 HR prototype (ZP 2001) of the ETH Zürich laboratory. This provides the so far most complete data set on methane spectra in this range at high resolution. In the present work we report in particular those results, where the partial rovibrational analysis allows for the direct assignment of pure (J = 0) vibrational levels including high excitation. These results substantially extend the accurate knowledge of vibrational band centers to higher energies and provide a benchmark for both the comparison with theoretical results on the one hand and atmospheric spectroscopy on the other hand. We also present a simple effective Hamiltonian analysis, which is discussed in terms of vibrational level assignments and (13)C isotope effects.

Rovibrational analysis of the 2ν 3, 3ν 3 and ν 1 bands of CHCl2F measured at 170 and 298 K by high-resolution FTIR spectroscopy

We report the infrared spectrum of fluorodichloromethane (CHCl2F, HCFC21) measured using a Bruker IFS 125 HR Zurich Prototype (ZP) 2001 interferometric Fourier transform infrared (FTIR) spectrometer at room temperature and at 170 K in a collisional and enclosive flow cooling cell with adjustable temperature designed with White-type multi-reflection optics and effective optical path lengths of 10–17.5 m. The spectra were recorded with effective resolutions of 0.0011–0.0017 cm−1 (maximum optical path difference (MOPD) 10 m) in the range 1900–3600 cm−1. The spectrum was analysed using an effective Hamiltonian in the 2v 3 region of CH35Cl2F (band center cm−1) and CH35Cl37ClF ( 2139.991 8 cm−1), the 3ν3 region of CH35Cl2F ( cm−1) and CH35Cl37ClF ( cm−1) and in the ν1 region of CH35Cl2F ( cm−1). About 14,000 rovibrational lines were assigned overall. These are tabulated separately in a supplementary publication. Local perturbations were identified in most of the bands. A dark state coupled through a c-type Cori...

Water ice nanoparticles: size and temperature effects on the mid-infrared spectrum

Mid-infrared spectra have been measured for cubic ice (I(c)) nanoparticles (3-150 nm diameter) formed by rapid collisional cooling over a wide range of temperatures (5-209 K). Spectral diagnostics, such as the ratio of surface related dangling OH to interior H-bonded OH stretch bands, reveal the manner in which particle size depends on bath gas temperature and density, and on water molecule concentration. For particles smaller than 5 nm strained intermolecular bonds on the surface and subsurface cause the predominant OH stretch peak position to be dramatically blue shifted by up to 40 cm(-1). In the size regime of 8-200 nm the position of the OH stretch absorption band maximum is relatively unaffected by particle size and it is possible to measure the temperature dependence of the peak location without influences from the surface or scattering. The band maximum shifts in a linear fashion from 3218 cm(-1) at 30 K to 3253 cm(-1) at 209 K, which may assist with temperature profiling of ice particles in atmospheric clouds and extraterrestrial systems. Over the same temperature range the librational mode band shifts very little, from 870 to 860 cm(-1). In the water stretching and bending regions discrete spectral features associated with the surface or sub-surface layers have been detected in particles as large as 80 nm.

The formation of N2O nanoparticles in a collisional cooling cell between 4 and 110 K

N2O (CO2) particles covering a vast size range from some nanometres up to some hundred nanometres were formed by injecting gaseous N2O (CO2) into a collisional cooling cell at temperatures between 4 and 110 K. FTIR spectroscopy was used to study the vibrational dynamics of the nanoparticles between about 600 and 4000 cm−1. For the spectra of the bigger particles formed at temperatures around 78 K, we have studied the influence of the experimental conditions on the fine structure of the strong absorption bands and its temporal behavior. From an estimate of the particle sizes, we conclude that the observed changes in the fine structure are not simple size effects; they are likely to be due to a change in the particle shape or in the molecular order. From a comparison with infrared spectra of clusters generated in a supersonic jet expansion, we estimate the size of the particles produced at temperatures below 10 K to lie around one nanometre.

SIZE AND TEMPERATURE DEPENDENCE IN THE FAR-IR SPECTRA OF WATER ICE PARTICLES

Spectra of water-ice aerosol particles have been measured in the far-IR region using synchrotron radiation. The particles in the nanoscale size regime of 1-100 nm were formed by rapid collisional cooling at temperatures ranging from 4 to 190 K. The spectra show the characteristic bands centered near 44 μm (230 cm–1) and 62 μm (160 cm–1) associated with the intermolecular lattice modes of crystalline ice at all temperatures, in contrast to previous studies of thin films formed by vapor deposition where amorphous ice is generated below 140 K. The bands shift to higher wavenumber values as the temperature is reduced, consistent with the trend seen in earlier studies, but in our experiments the actual peak positions in the aerosol particle spectra are consistently higher by ca. 4 cm–1. This finding has implications for the potential use of these spectral features as a temperature probe. The particle sizes are small enough for their spectra to be free of scattering effects, and therefore provide a means to assess imaginary refractive index values obtained through Kramers-Kronig analyses of thin film spectra.

Calorimetric determination of purity by simulation of dsc curves

This paper presents further results of the purity determinations by simulated DSC curves. The instrumental parameters of a heat-flux DSC are now determined at two temperatures whereby the accuracy of the results is improved. In addition the calorimeter model is adapted to another calorimeter. The method also allows a more exact determination of fusion temperature.