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 ...

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

The sensitivity limitations of Fourier transform infrared (FTIR) spectroscopy for the detection of molecular clusters formed in rarefied gas expansions can be overcome by synchronizing intense gas pulses at a low duty cycle with rapid interferometer scans. This turns the broadband FTIR approach into a universal cluster spectroscopy tool applicable from the far (200 cm−1) to the near (8000 cm−1) IR. It nicely complements more selective and more restricted laser-based techniques and it provides a gas-phase variant of the matrix-isolation method, the main drawback being substance consumption. A survey over the capabilities, limitations and perspectives of this high-throughput nozzle approach to cluster FTIR spectroscopy is given.

Femtisecond single-mole infrared spectroscopy of molecular clusters.

The submillimeter: A spectroscopist’s view

Large molecular aggregates with sizes ranging from subnanometers to microns are ubiquitous. As atmospheric aerosols they influence our climate, in interstellar space they are discussed as reactive sites, and in medicine small particles are considered as promising candidates to achieve a targeted drug delivery. The present contribution is focused on the characterization of the physical–chemical properties of these particles and on their targeted generation. One of the greatest challenges is to understand the properties of these aggregates on a molecular level. The latter point is discussed in detail focussing on the vibrational dynamics of these particles.

Large molecular aggregates: From atmospheric aerosols to drug nanoparticles.

An experimental system employing a cryopump has been assembled for high-resolution FTIR spectroscopy of species cooled in a supersonic jet expansion. Coupled to a Bruker IFS 120HR interferometer, it involves an external sample chamber and transfer optics allowing 11 optical passes of the infrared beam. Tests with CO and N2O indicate rotational temperatures in the range 6–30 K. By heating the nozzle, significant population of vibrational states can be achieved without sacrificing rotational cooling. The mid-infrared spectrum of Freon-12, CCl2F2, has been recorded and the ν8 C-type band at 1161 cm –1 analysed. Rotational constants for C35Cl2F2 and C35Cl2ClF2 have been calculated and the rotational temperature estimated at 40 K.

High-resolution FTIR–jet spectroscopy of CCl2F2

A coolable gas cell for the measurement of mid‐infrared spectra of gases at cryogenic temperatures as low as 77 K is described. The design of the cell allows it to be operated in collisional cooling (or diffusive trapping) mode to generate significant nonequilibrium pressures of monomer vapor for gases which would normally have vanishingly small vapor pressure at low temperatures. Fourier transform infrared spectra of carbon dioxide (CO2), nitrogen dioxide (NO2), and methyl silane (CH3SiH3) vapors cooled by collisions with helium gas at 77 K are reported. Sufficiently high concentrations of monomer gas may be produced at low total pressure conditions (<10 h Pa) to allow spectra to be recorded at the Doppler resolution limit. The resulting spectroscopic simplification indicates that efficient collisional cooling of the vibrational, as well as rotational, degrees of freedom occurs.

M. Page

Papers

Doppler‐limited spectroscopy at cryogenic temperatures: Application of collisional cooling

A novel cryogenic molecular beam. Application to supersonic jet spectroscopy of gases