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 and low-power electric heating keep the mirrors free from perturbing deposits. The cell can be operated in a dynamic buffer-gas flow-cooling mode. A comprehensive review of existing collisional cooling cells is given. The formation of CO clusters from the gas phase was investigated using FTIR spectroscopy. For the isotope mixture consisting of C1613O,13C18O, and C1612O, a conspicuous change in the main spectroscopic structure of the clusters was observed between 20 and 5 K. The cluster bandwidth of the main isotope C1613O triples. This behavior could be interpreted as a change from the crystalline to the amorphous state or as a decrease in size to smaller clusters with relatively larger surfaces. To our knowledge, this is the first IR investigation of molecular clusters obtained by collisional cooling in this temperature range. For CO2 the change from the monomer to crystalline clusters was investigated. The observed spectra vary considerably with temperature. FTIR spectra of CO2 clusters observed previously by other researchers could be reproduced. The system allows us to determine various gases with a FTIR detection limit in the lower ppb range. With these concentrations and at temperatures <10 K the monomers can be supercooled, and small clusters can be obtained.

published as an Advance Article on the web 3rd August 2001 particles covering a vast size range from some nanometres up to some hundred nanometres were N 2 O (CO 2 ) formed by injecting gaseous into a collisional cooling cell at temperatures between 4 and 110 K. N 2 O (CO 2) 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 inÑuence of the experimental conditions on the Ðne 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 Ðne structure are not simple size e †ects ; 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.

Fourier transform infrared (FTIR) spectroscopy combined with a long-path collisional cooling cell was used to investigate the temporal evolution of CO2 nanoparticles and binary H2O/CO2 nanocomposites in the aerosol phase at 80 K. The experimental conditions for the formation of different CO2 particle shapes as slab, shell, sphere, cube, and needle have been studied by comparison with calculated data from the literature. The H2O/CO2 nanoparticles were generated with a newly developed multiple-pulse injection technique and with the simpler flow-in technique. The carbon dioxide nu3-vibration band at 2360 cm(-1) and the water ice OH-dangling band at 3700 cm(-1) were used to study the evolution of structure, shape, and contact area of the nanocomposites over 150 s. Different stages of binary nanocomposites with primary water ice cores were identified dependent on the injected CO2 portion: (a) disordered (amorphous) CO2 slabs on water particle surfaces, (b) globular crystalline CO2 humps sticking on the water cores, and (c) water cores being completely enclosed in bigger predominantly crystalline CO2 nanoparticles. However, regular CO2 shell structures on primary water particles showing both longitudinal (LO) and transverse (TO) optical mode features of the nu3-vibration band could not be observed. Experiments with reversed nucleation order indicate that H2O/CO2 composite particles with different initial structures evolve toward similar molecular nanocomposites with separated CO2 and H2O regions.