Supersonic cooling

The intensely developing technique of high-resolution IR-spectroscopy of dimers composed of two different molecules in supersonic cooled jets offers a new promising approach to the quantum dynamics of reaction complexes. In essence, this is a unique possibility of modelling low-temperature chemical reactions. 

Fig. 6. Doppler profile of H and D fragments from the photolysis of C2H2 and C2D2, respectively. The narrow doublet at the center is the Doppler profile of a supersonically-cooled H-atom beam.

Cold, with well-defined internal energy. This is normally ensured with supersonic cooling. This requirement is important for high resolution and state-resolved studies, and it can remove the spectroscopy congestion of the radicals and enhance the interpretation of the results in photochemistry experiments. 

Ernsting, N.R, Asimov, M., and Shaefer, F.R (1982) The electronic origin of the p-p absorption of amino coumarins studied in a supersonically cooled free jet. Cbem. Pbys. Lett. 91, 231. 

A universal method that satisfies both requirements is supersonic cooling. For this reason, this method is a cornerstone of modern low-temperature studies. The results described in this chapter are mainly obtained with this method. For special reviews devoted to applications of supersonic cooling in molecular spectroscopy we refer the reader to the papers by Levy [1980] and Cohen and Saykally [1992]. Technical details of the method can be found in the papers we cite in this chapter. 

Recently, there has been a surge in applying gas phase spectroscopic techniques for studies of biologically relevant species [1-9]. Laser desorption with supersonic cooling has fundamentally solved the problem of vaporization of non-volatile species [5, 7], Electrospray ionization coupled with ion trapping and cooling has... 

The experimental apparatus, as shown in Figure 11-1, was a standard molecular beam machine with a heated pulsed valve for vaporization of the non-volatile species and for supersonic cooling. Samples of 1-methyluracil, 1,3-dimethyluracil and thymine were purchased from Aldrich Co. and used without further purification. The sample 1,3-dimethylthymine was synthesized from thymine following a literature procedure [33], and its purity was checked by nuclear magnetic resonance (NMR) and infrared absorption (IR) spectroscopy. The heating temperatures varied for different samples 130°C for DMU, 150°C for MU, 180°C for DMT, and 220°C for thymine. No indication of thermal decomposition was observed at these... 

Figure 11-1. Experimental apparatus. The sample is supersonically cooled and intercepted by counter-propagating laser beams. Both fluorescence and ion signals can be observed...

An important parameter for comparison with theory as well as for understanding many properties would be relative binding energies or stabilities. Unfortunately those are hard to assess in the gas phase. One of the few experiments to report thermodynamic binding energies between base pairs is the work by Yanson et al. in 1979, based on field ionization [25], Relative abundances of nucleobase clusters in supersonic beams are an unreliable measure of relative stability for a two reasons First, supersonic cooling is a non-equilibrium process and thus comparison with thermal populations is tenuous at best. Secondly, ionization probabilities may be a function of cluster composition. The latter is certainly the case for multi photon ionization, as will be discussed in detail below. 

The Si — So Resonant Two-Photon Ionisation Spectrum of Supersonically Cooled Triptycene... 

Note added in proof. Very recently, A. Furlan, M. J. Riley, and S. Leutwyler [J. Chem. Phys. 96, 7306 (1992)] have reported a resonant two-photon ionization spectrum of the S, vibronic transitions of supersonically cooled triptycene. The excited state exhibits E (8>e Jahn-Teller activity with linear and quadratic vibronic coupling in a single, degenerate, benzene wagging mode. 

It is not just loosely bound complexes that can be specified with profit by spectroscopic analysis of a supersonic jet. Of this there is no more thrilling example than the transient organometallic radical VCH (39), formed when vanadium atoms generated by laser ablation are entrained in a pulse of high-pressure helium containing 5-10% CH4 prior to expansion to give a supersonic, cooled molecular beam. High-resolution studies of the molecular fluorescence near 800 nm excited by a tunable probe laser reveal extensive vibrational and... 

In order to further examine the dependencies of the branching ratio S( P)/S( D) and the photodissociation cross section on the internal energy distributions of SCH3 and HS, it would be interesting to compare the cross sections and branching ratios S( T)/S( D) for processes 55 and 56 measured using supersonically cooled SCH3 and HS beams. 

Instead of gas discharges, electrons emitted from hot cathodes can be used for ionization (Fig. 4.34c). With several cathodes arranged cylindrically around a cylindrical grid acting as an anode, a large electron current can be focused into the cold molecular beam. Because of the low electron mass, electron impact ionization at electron energies closely above ionization threshold does not much increase the rotational energy of the ionized molecules, and rotationally cold molecular ions can be formed from cold neutral molecules. Rotational temperatures of about 20 K have been reached, for instance, when supersonically cooled neutral triacetylene molecules were ionized by 200 eV electrons in a seeded free jet of helium [484]. 

Fig. 8.24 Schematic diagram of the crossed-jet direct-absorption reactive scattering experiment. Fluorine atoms produced in a discharge puised jet expansion are intersected at 90° 4.5 cm downstream with a pidse of supersonically cooled H2. Tunable single-mode IR laser light is multipassed perpendicular to the collision plane and probes HF(v, J) products by direct absorption [1061]...

B. Friedrich, Slowing of supersonically cooled atoms and molecules by time-varying non-resonant dipole forces. Phys. Rev. A 61, 025403 (2000)... 

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