Molecular beams supersonic expansion

As with most methods for studying ion-molecule kinetics and dynamics, numerous variations exist. For low-energy processes, the collision cell can be replaced with a molecular beam perpendicular to the ion beam [106]. This greatly reduces the thennal energy spread of the reactant neutral. Another approach for low energies is to use a merged beam [103]. In this system the supersonic expansion is aimed at the tluoat of the octopole, and the ions are passed tluough... 

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. 

Supersonic molecular beam (SMB) mass spectrometry (SMB-MS) measures the mass spectrum of vibra-tionally cold molecules (cold El). Supersonic molecular beams [43] are formed by the co-expansion of an atmospheric pressure helium or hydrogen carrier gas, seeded with heavier sample organic molecules, through a simple pinhole (ca. 100 p,m i.d.) into a 10 5-mbar vacuum with flow-rates of 200 ml. rn in. In SMB, molecular ionisation is obtained either through improved electron impact ionisation, or through hyperthermal surface ionisation... 

The molecular beam is formed by the supersonic expansion of gas through a pulsed nozzle. It is then collimated by two skimmers, and enters... 

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. 

One important feature of the spectroscopy in molecular beams is nicely illustrated in this example, namely the very high resolution of absorption and emission spectra. This results from the low effective temperatures which can be reached through supersonic expansion. 

The method of choice for the generation of vdW clusters utilizes supersonic expansions. In this technique, the species to be clustered are allowed to expand from a high pressure to a low pressure region through a molecular beam nozzle. The basic principles of adiabatic expansion have been the focus of a number of reviews (Hagena 1974, 1987 Scoles 1988) and only the pertinent aspects will be described here. 

Theory quite naturally gives us the initial and final state resolved probabilities, but in experiment this is not always so. The internal state populations in a molecular beam are determined by the temperature of the nozzle used to produce the supersonic expansion. More than one state is present in such a beam. This has been partially overcome in recent years by Raman pumping of the incident molecular beam [66-69]. Laser beams intercept the molecular beam moving a fraction of the molecules into a particular ro-vibrational state (determined by the laser properties). With careful timing of the firing of the probe lasers, it is possible to measure changes in this fraction of molecules and measure some of the final states populated by the scattering process. 

Dutton et examined the efficiency of rotational relaxation of SO2 by various gases by measuring the terminal translational and rotational temperatures of SO2 seeded in various supersonic molecular beams. They noted that an increase in relaxation efficiency was obtained with increasing temperature in each case. Rotationally cold SO2 was prepared in a supersonic expansion of SO2 seeded in Ar and excited to the first electronically allowed state, A A2, by pulsed laser radiation in the range 300-320 Time-resolved measure-... 

The advent of supersonic molecular beam expansion techniques and high spectral-and time-resolved laser spectroscopic probing has transformed experimental... 

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