High‐ResolutionFTIRand Diode Laser Spectroscopy of Supersonic Jets

Abstract

Fourier transform infrared (FTIR) spectrometers and tunable diode lasers in combination with a supersonic molecular beam expansion are a perfect tool for the investigation of molecules, ions, and radicals at low temperatures. The internal degrees of freedom of the molecules are adiabatically cooled to very low temperatures and thus only low-lying energy levels are populated. The reduction of the number of populated levels at low temperatures makes the assignment of the spectra much easier as compared to the congested room-temperature spectra. Under certain conditions, the Doppler linewidths are greatly reduced, corresponding to very low effective translational temperatures. Supersonic expansion also provides a suitable method for producing and investigating van der Waals clusters and hydrogen-bonded complexes. Unstable species such as radicals and ions can be efficiently produced and studied in a molecular beam. The low rotational temperature allows for the study of nuclear spin symmetry conservation or conversion between nuclear spin isomers. A molecular beam expansion can be obtained by expanding gas through a slit or a circular nozzle. Both expansion geometries can be used in combination with a multipass optical setup and with cavity ring down spectroscopy, which enhances the effective absorption path length. Cooling of the molecules can be promoted by seeding in noble gases. This article summarizes the general aspects of the experimental technique as well as current developments. To demonstrate how powerful the combination of a molecular beam expansion with tunable diode laser and FTIR spectroscopy can be, we report results on some important current examples.

High‐Resolution Fourier Transform Infrared Spectroscopy

“…There was an improvement by more than a factor of 10 in resolution at that time, made possible by the invention of dynamic alignment (Kendall et al 1982, Buijs 1979. The availability of these instruments led to early developments in Doppler-limited high-resolution FTIR-supersonic jet techniques , Amrein et al 1987a,b, 1988a) (see review (Quack 1990) and Snels et al 2011: High-resolution FTIR and Diode Laser Spectroscopy of Supersonic Jets, for further references). It also contributed importantly to the high-resolution spectroscopic approach to short-time (as-fs-ps) intramolecular quantum dynamics and kinetics (Marquardt and Quack 2001, Quack 1990, 2001, 2003: Fundamentals of Rotation-Vibration Spectra, this handbook).…”

“…In addition, spectral analysis is an excellent approach for understanding the IR laser chemistry of these compounds (Lupo and Quack 1987, Quack 1989a, 1995 (and references therein). Analyses were possible in the past for many of these molecules only by using advanced FTIR and laser spectroscopic techniques in combination with supersonic jet cooling (Quack 1990, Snels and Quack 1991, Bauder et al 1997, Snels et al 2011: High-resolution FTIR and Diode Laser Spectroscopy of Supersonic Jets, this handbook) (and references therein). Now it is possible to record the spectra of fluorochlorohydrocarbons at low temperatures (120-170 K) in combination with high-resolution FTIR spectroscopy.…”

“…For a given molecule with mass m and spectral line position ν 0 , the Doppler broadening can be reduced by lowering the temperature. Recent very successful techniques are nonequilibrium cooling in supersonic jets (Quack 1990) or (Snels et al 2011: High-resolution FTIR and Diode Laser Spectroscopy of Supersonic Jets, this handbook) spectroscopy in collisional cooling cells operated either under equilibrium or nonequilibrium conditions (Albert et al 2007 and references therein). 2.…”

“…An optical path difference of 1/d MOPD = 0.005 cm −1 was used. Smaller sections of the icosad region were measured and analyzed using supersonic jet cavity ring down laser spectroscopy Quack 2002 andSnels et al 2011) …”

“…We draw from a recent brief review (Albert and Quack 2007a), which we follow in some aspects quite closely but which has been updated and extended for the present purpose. We also refer to other articles in this handbook, which report on results from FTIR spectroscopy (see also Snels et al 2011: High-resolution FTIR and Diode Laser Spectroscopy of Supersonic Jets, Flaud and Orphal 2011: Spectroscopy of the Earth's Atmosphere and Herman 2011: High-resolution Infrared Spectroscopy of Acetylene: Theoretical Background and Research Trends, this handbook).…”