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

Abstract

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.