Jet expansion

Several instniments have been developed for measuring kinetics at temperatures below that of liquid nitrogen [81]. Liquid helium cooled drift tubes and ion traps have been employed, but this apparatus is of limited use since most gases freeze at temperatures below about 80 K. Molecules can be maintained in the gas phase at low temperatures in a free jet expansion. The CRESU apparatus (acronym for the French translation of reaction kinetics at supersonic conditions) uses a Laval nozzle expansion to obtain temperatures of 8-160 K. The merged ion beam and molecular beam apparatus are described above. These teclmiques have provided important infonnation on reactions pertinent to interstellar-cloud chemistry as well as the temperature dependence of reactions in a regime not otherwise accessible. In particular, infonnation on ion-molecule collision rates as a ftmction of temperature has proven valuable m refining theoretical calculations. 

Some other extremely iisellil spectroscopic teclmiques will only be mentioned here. Probably the most important one is spectroscopy in free jet expansions. Small molecules have often been studied by gas-phase spectroscopy where sharp rotational and vibrational structure gives detailed iufonnation about molecular... 

Plenary 9. J W Nibler et al, e-mail address niblerj chem.orst.edu (CARS and SRS). High resolution studies of high lymg vibration-rotational transitions in molecules excited in electrical discharges and low density monomers and clusters in free jet expansions. Ionization detected (REMPI) SRS or IDSRS. Detect Raman... 

If both alcohol monomers forming a dimer are on average achiral, one may still have chirality synchronization events, where the two monomers match their transient chiral conformations when they bind to each other. A particularly simple example is that of ethanol dimer, where the lowest-energy conformer involves two gauche monomers of the same helicity [80,91]. However, the energy difference to other conformers is so small that efficient isomerizing collisions in a supersonic jet expansion are required to favor the lowest-energy form over the others. A more... 

As became obvious in the preceding section, progress in understanding alcohol clusters very much depends on the ability to generate these clusters in supersonic jet expansions or in other variants of low temperature isolation and to detect their dynamics via spectroscopic methods. Therefore, some important spectroscopic tools employed in this field shall be summarized, with focus on the alcoholic systems that have been addressed by them. Solution [22, 26, 141, 142] and supercritical [24 26] state techniques will not be covered systematically. 

The preparation of single isomers for methanol dimer, trimer, and presumably tetramer [16] in a supersonic jet expansion contrasts the structural diversity that can be prepared and manipulated in cryogenic matrices [34]. It underscores the ability of supersonic jet expansions to funnel all intermolecular isomers down to the global minimum, if there are no major barriers to overcome on the way. 

The O H stretching spectra of ethanol trimers and larger clusters cannot be conformationally resolved in a slit jet expansion [65, 77, 157], VUV-IR spectra [184] are even broader, sometimes by an order of magnitude, and band maxima deviate systematically by up to +50 cm 1 from the direct absorption spectra. We note that ethanol dimers and clusters have also been postulated in dilute aqueous solution and discussed in the context of the density anomaly of water ethanol mixtures [227], Recently, we have succeeded in assigning Raman OH stretching band transitions in ethanol-water, ethanol water, and ethanol water2 near 3550, 3410, and 3430cm, respectively [228],... 

T. Haber, U. Schmitt, and M. A. Suhm, FTIR spectroscopy of molecular clusters in pulsed supersonic slit jet expansions. Phys. Chem. Chem. Phys. 1, 5573 5582 (1999). 

To observe the transient spatiotemporal structure of the WP interference, we have performed the fs pump-fs probe experiment [37], The sample gas was prepared by molecular jet expansion of the mixture of iodine vapor and Ar buffer gas into a vacuum chamber. A continuous gas jet is preferable when we use a high-repetition-rate laser system. The estimated vibrational temperature was 170K[37]. 

Pump-probe diffraction techniques offer exciting new ways to probe transient structures in molecular, nanoscale, and biological systems. For dilute systems, or very small targets, electron diffraction is the preferred tool, because the cross sections for scattering of electrons from molecules are very large. In our research we show that pump-probe electron diffraction is an excellent technique to probe the dynamics of chemical reactions in the rarified environment of jet expansions, and for probing the diffraction signatures of individually excited vibronic states. 

Van der Waals complexes of C2v symmetry between 1,2,4,5-tetrazine and a number of light gases (He, Ar, H2) have been observed and characterized by laser spectroscopic studies of free supersonic jet expansion of the tetrazine in the carrier gas (84CHEC-(3)53l). In these complexes, one equivalent of noble gas sits on top of the aromatic TT-system of the heterocycle. 1,2,4,5-Tetrazine, its 3-methyl, and 3,6-dimethyl derivative as well as aminotetrazine have all been used as heterocycles with noble gases, water, HC1, benzene, and acetylene, playing the role of the second partner. 

The ability to cool (and eventually liquefy) gases by adiabatic expansion underlies industrial gas liquefaction processes. Adiabatic cooling of gaseous nozzle-jet expansions is also an important technique in modem molecular beam and mass spectrometric research. Thermodynamicist John Fenn, winner of the 2002 Nobel Prize in Chemistry, pioneered many of the techniques of adiabatic nozzle-beam cooling. 

Quantum Beats and Dephasing in Isolated Large Molecules Cooled by Supersonic Jet Expansion and Excited by Picosecond Pulses Anthracene, W. R. Lambert, P. M. Felker, and A. H. Zewail, J. Chem. Phys. 75, 5958 (1981). 

---