Cooling by supersonic expansion

A series of n-alkylbenzenes, cooled by supersonic expansion and excited to what are initially well localized ring distortion vibrations within the first excited singlet states, show fluorescence spectral behaviour that is dependent upon the alkyl chain length. For the first three members of the series (toluene to n-propylbenzene) resonance fluorescence from the initially pumped mode in 5, was observed, but for... 

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). 

A variety of means has been developed recently for the preparation of molecular beams containing sufficient concentrations of free radicals for spectroscopic analysis by MPI-MS or LIF detection. For larger species, it is particularly important to prepare them in such a manner that they can also be cooled to low temperatures by supersonic expansion to relieve the considerable spectral congestion typical of hot radical sources. To accomplish this, several devices have been developed which combine radical production with supersonic nozzle sources. 

ABSTRACT. Three new techniques are presented which allow rate coefficient measurements for ion molecule reactions in the temperature range of interstellar cloudy. With an ion trap and a static drift tube cryogenic cooling by liquid helium is used. These techniques are therefore restricted to molecular hydrogen as neutral reactant. With the CRESU method the low temperature is obtained by supersonic expansion. This last technique has a high chemical versatility. 

With the CRESU method, devetoped at the Laboratoire d A rothermique in Meudon, a comptetety different approach to obtaining the very low temperature was used. A buffer gas in which reactbns take place is cooled down by supersonic expansion. This means of oooting a gas appears quite obvtous and has been used widely as a source of low temperature for spectroscopic and cluster studies (Hermann et al,... 

We use laser photofragment spectroscopy to study the vibrational and electronic spectroscopy of ions. Our photofragment spectrometer is shown schematically in Eig. 2. Ions are formed by laser ablation of a metal rod, followed by ion molecule reactions, cool in a supersonic expansion and are accelerated into a dual TOE mass spectrometer. When they reach the reflectron, the mass-selected ions of interest are irradiated using one or more lasers operating in the infrared (IR), visible, or UV. Ions that absorb light can photodissociate, producing fragment ions that are mass analyzed and detected. Each of these steps will be discussed in more detail below, with particular emphasis on the ions of interest. 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

Although this book is devoted to molecular fluorescence in condensed phases, it is worth mentioning the relevance of fluorescence spectroscopy in supersonic jets (Ito et al., 1988). A gas expanded through an orifice from a high-pressure region into a vacuum is cooled by the well-known Joule-Thomson effect. During expansion, collisions between the gas molecules lead to a dramatic decrease in their translational velocities. Translational temperatures of 1 K or less can be attained in this way. The supersonic jet technique is an alternative low-temperature approach to the solid-phase methods described in Section 3.5.2 all of them have a common aim of improving the spectral resolution. 

The desorption of ions and neutrals into the vacuum upon irradiation of a laser pulse onto a surface proceeds as a jet-like supersonic expansion. [38] a small, but initially hot and very rapidly expanding plume is generated. [49] As the expansion is adiabatic, the process is accompanied by fast cooling of the plume. [38]... 

An example of an alternative use of FT technology in the UV/VIS is our work on the X2Z" B2Z+ emission spectrum of jet-cooled CN [21], These experiments were made possible by the development of the corona-excited supersonic expansion source by Engelking [20]. The Engelking source creates radicals in a continuous discharge, followed by immediate cooling in the expansion. A high number density of rotationally and translationally cold radicals in excited electronic and vibrational states is produced. As a result, excited vibronic states of reactive species can be studied with a minimum of rotational congestion. 

Most investigations of photoinduced electron transfer have been performed in condensed phases. Much less is known about conditions that permit the occurrence of intramolecular ET in isolated (collision-free) molecular D-A systems. A powerful method for this kind of study is the supersonic jet expansion teehnique (which was originally developed by Kantrowitz and Grey in 1951 [66]) combined with laser-induced fluorescence (LIF) spectroscopy and time-of-flight mass spectrometry (TOF-MS). On the other hand, the molecular aspects of solvation can be studied by investigations of isolated gas-phase solute-solvent clusters which are formed in a supersonic jet expansion [67] (jet cooling under controlled expansion conditions [68] permits a stepwise growth of size-selected solvation clusters [69-71]). The formation of van der Waals complexes between polyatomic molecules in a supersonic jet pro-... 

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