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

Collisional cooling and supersonic jet expansion both allow us to perform i nfrared spectroscopy of supercooled molecules and atomic and molecular clus ters. Collisional cooling has the advantage of higher sensitivity per molec ule and enables working in thermal equilibrium. A new powerful method of co llisional cooling is presented in this article. It is based on a cooling ce ll with integrated temperature-invariant White optics and pulsed or continu ous sample-gas inlet. The system can be cooled with liquid nitrogen or liqu id helium and operated at gas pressures between <10(-5) and 13 bar. Tempera tures 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 homoge neous 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 hea ting keep the mirrors free from perturbing deposits. The cell can be operat ed in a dynamic buffer-gas flow-cooling mode. A comprehensive review of exi sting collisional cooling cells is given. The formation of CO clusters from the gas phase was investigated using FTIR spectroscopy. For the isotope mi xture consisting of (CO)-C-13-O-16,(CO)-C-13-O-18, and (CO)-C-12-O-16, a co nspicuous change in the main spectroscopic structure of the clusters was ob served between 20 and 5 K. The cluster bandwidth of the main isotope (CO)-C -13-O-16 triples. This behavior could be interpreted as a change from the c rystalline to the amorphous state or as a decrease in size to smaller clust ers with relatively larger surfaces. To our knowledge, this is the first IR investigation of molecular clusters obtained by collisional cooling in thi s temperature range. For CO2 the change from the monomer to crystalline clu sters was investigated. The observed spectra vary considerably with tempera ture. 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 an d at temperatures <10 K the monomers can be supercooled, and small clusters can be obtained. (C) 2001 American Institute of Physics.