Cooling and slowing in high-pressure jet expansions

Pulsed, supersonic beams of pure carbon monoxide and carbon dioxide at stagnation conditions above their critical point have been investigated by time-of-flight measurements as a function of pressure and temperature. Although both molecules form clusters readily in adiabatic expansions, surprisingly large speed ratios above 100 indicative of very low translational temperatures below 0.1 K have been achieved. In particular, the supersonic expansion of CO 2 at stagnation temperatures slightly above the phase transition to the supercritical state results in unprecedented cold beams. This efficient cooling is attributed to the large values of the heat capacity ratio of supercritical fluids in close vicinity of their critical point.

The Journal of Chemical Physics, 2006

Pulsed, supersonic beams of pure carbon monoxide and carbon dioxide at stagnation conditions above their critical point have been investigated by time-of-flight measurements as a function of pressure and temperature. Although both molecules form clusters readily in adiabatic expansions, surprisingly large speed ratios ͑above 100͒ indicative of very low translational temperatures ͑below 0.1 K͒ have been achieved. In particular, the supersonic expansion of CO 2 at stagnation temperatures slightly above the phase transition to the supercritical state results in unprecedented cold beams. This efficient cooling is attributed to the large values of the heat capacity ratio of supercritical fluids in close vicinity of their critical point.

Zeitschrift für Physikalische Chemie, 2011

The experimental realization of accurately defined source conditions, combined with an undisturbed flight path of precisely known length, permits the generation of supersonic molecular beams with an extremely well characterized axial velocity distribution. This achievement can be used for a quantitative assessment of theoretical models of condensation during the jet expansion, based on high accuracy equations of state. For helium-4, which is commonly put on a level with the ideal gas, we contrast two empirical equations of state with respect to enthalpy changes. These can be validated through a comparison with experimentally determined mean terminal flow velocities. At cryogenic temperatures, strong deviations from the ideal gas behavior are observed. Most notably, even the differences between the predictions of the two fundamental equations are large enough to be distinguished by high-resolution time-of-flight measurements.

Chemical Physics, 1999

Experimental investigations on the collisional alignment of the rotational angular momentum, occurring in supersonic seeded beams and in drift tubes, have recently documented a strong dependence of the observed effects on the final molecular velocity. The present investigation aims at elucidating the possible mechanisms at the molecular collision level. Quantum state-to-state differential scattering cross sections, calculated for the prototype system O 2 -He, for an interaction potential previously obtained in this laboratory, exhibit propensities relevant to reveal nature and selective role of the elastic and inelastic scattering events, participating in the overall mechanisms which lead to molecular alignment and cooling. The present analysis shows that the dynamics of such phenomena crucially depends on the initial and final rotational state, on the collision energy, on the involved orbital angular momentum and therefore alternative routes are possible for molecular polarization and relaxation. These routes lead to scattering into specific angular cones and therefore observations from different experiments provide complementary pieces of information which, exploiting studies of various molecular systems under diverse experimental conditions, can be correlated in a single mosaic.

Supersonic molecular beams constitute a very powerful technique in modern chemical physics. They offer several unique features such as a directed, collision-free flow of particles, very high luminosity, and an unsurpassed strong adiabatic cooling during the jet expansion. While it is generally recognized that their maximum flow velocity depends on the molecular weight and the temperature of the working fluid in the stagnation reservoir, not a lot is known on the effects of elevated particle densities. Frequently, the characteristics of supersonic beams are treated in diverse approximations of an ideal gas expansion. In these simplified model descriptions, the real gas character of fluid systems is ignored, although particle associations are responsible for fundamental processes such as the formation of clusters, both in the reservoir at increased densities and during the jet expansion. In this contribution, the various assumptions of ideal gas treatments of supersonic beams and their shortcomings are reviewed. It is shown in detail that a straightforward thermodynamic approach considering the initial and final enthalpy is capable of characterizing the terminal mean beam velocity, even at the liquid-vapor phase boundary and the critical point. Fluid properties are obtained using the most accurate equations of state available at present. This procedure provides the opportunity to naturally include the dramatic effects of nonideal gas behavior for a large variety of fluid systems. Besides the prediction of the terminal flow velocity, thermodynamic models of isentropic jet expansions permit an estimate of the upper limit of the beam temperature and the amount of condensation in the beam. These descriptions can even be extended to include spinodal decomposition processes, thus providing a generally applicable tool for investigating the two-phase region of high supersaturations not easily accessible otherwise.

Review of Scientific Instruments, 1995

A general formalism is presented for the production, characterization, and use of uniform supersonic flows for the study of structural and collisional molecular properties. These flows make possible the generation of thermally equilibrated gaseous environments at temperatures generally extending from near 10 to above 300 K at pressures between 0.1 and 10 Ton: In addition, the invariance of flow conditions for distances of many nozzle diameters (5-30) beyond the exit makes the flows ideal for collisional or temporal studies. A concise outline for the design of the Lava1 nozzles necessary to produce specified density and temperature conditions is presented. Pulsed operation of these expansions is demonstrated, and a variety of useful characterization tools providing information on flow uniformity and thermal characteristics is considered. Recent applications of these flows for the study of low-temperature chemical reactions are reviewed. 0 1995 American Institute of Physics.

Plasma Sources Science and Technology, 2001

The transport of neutral argon atoms in an expanding thermal argon/hydrogen plasma is studied by means of laser-induced fluorescence spectroscopy around 811 nm, on the long living Ar[4s] atoms. Although the Doppler shifted laser-induced fluorescence measurements are performed on argon atoms in the metastable Ar * (3 P 2) and resonant Ar * (3 P 1) states, it is argued that in the plasma jet the velocity distribution function of these Ar[4s] atoms images the velocity distribution functions of the ground-state argon atoms. From the results it is inferred that the velocity behaviour of the supersonically expanding argon gas can be predicted from the momentum balance, and the temperature from the adiabatic relation between density and temperature. However, the adiabatic constant is found to be 1.4 ± 0.1, smaller than the adiabatic constant of a neutral argon gas expansion which is 5 3. Both in the axial and in the radial directions the velocity distributions measured in the shock region show clear departures from thermodynamic equilibrium. From the radial velocity distribution it is concluded that background gas invades the supersonic part of the expanding plasma jet. The results on temperature and velocity in the subsonic region show that the radius of the plasma jet hardly increases after the stationary shock front, indicating that the flow pattern is geometrically determined.

The Journal of Physical Chemistry A, 2011

We introduce a straightforward experimental approach for determining the mean flow velocity of a supersonic jet with very high precision. While time measurements easily can achieve accuracies of Δt/t e 10-4 , typically the absolute flight distances are much less well-defined. This causes significantly increased errors in calculations of the mean flow velocity and mean kinetic energy. The basic concept to improve on this situation is changing the flight distance in vacuo by precisely defined increments employing a linear translation stage. We demonstrate the performance of this method with a flight path that can be varied by approximately 15% with a tolerance of setting of 50 μm. In doing so, an unprecedented accurate value for the mean flow velocity of Δv/AEvae < 3 Â 10-4 has been obtained without prior knowledge of the total distance. This very high precision in source pressure, temperature, and particle speed facilitates an improved energetic analysis of condensation processes in supersonic jet expansions. The technique is also of broad interest to other fields employing the strong adiabatic cooling of supersonic beams, in particular, molecular spectroscopy. In the presented case study, a thorough analysis of arrival time spectra of neutral helium implies cluster formation even at elevated temperatures.

Supersonic expansions are a useful tool for cooling molecules in the gas phase. While translational and rotational degrees of freedom can generally be cooled to low temperatures (&lt;30 K) using this technique, there has been conflicting evidence concerning the cooling of vibrational degrees of freedom. This cooling is particularly important for gas-phase absorption spectroscopy of large molecules with many vibrational degrees of freedom, such as buckminsterfullerene (C60). We have attempted gas-phase infrared spectroscopy of C60 by producing C60 vapor in a hot (˜900 K) oven and seeding the vapor in an argon supersonic expansion, but have been unable to observe any absorption signal. We attribute this to insufficient cooling of vibrational degrees of freedom in the expansion. In contrast, we have performed a similar experiment with pyrene (C16H10) heated to ˜430 K, which yielded rotationally-resolved spectra and an estimated vibrational temperature of 25-90 K. We will discuss these ...

The European Physical Journal D, 2006

Rotational cooling and collisional alignment of ethylene molecules is induced by seeding effects in supersonic expansions with lighter gas carriers such as He and Ne. The dependencies of the degree of alignment on the rotational state, on the final speed of the molecules and on the diffusion angular cone have been characterized by coupling two different experimental methodologies. An application to surface scattering is then demonstrated by measuring stereo-dynamical effect in the adsorption on metallic surfaces.

Physics of Fluids, 2022

A method to design de Laval nozzles to generate uniform supersonic flows for gas-phase molecular studies at very low temperature is presented. The nozzle design is optimized for the flows in argon, helium, or nitrogen, up to Mach 5 and down to a few kelvin. Experimental results have shown that flows exhibit a good uniformity in terms of speed, temperature, and density, with the length of the uniformity of the supersonic flows up to 50 cm which corresponds to a kinetic time of about 1 ms in nitrogen for nozzles with a throat of about 1 cm in diameter. The design of the de Laval nozzles is concentrated at the diverging section. The method is based on the calculation of an isentropic core as described in Owen's work [J. M. Owen, "An improved method of supersonic nozzle design for rarefied gas flows," Ph.D. thesis (University of California, 1950)] of supersonic nozzle design for rarefied gas flows. The determination of the isentropic nozzle wall is carried out by the method of characteristics following Cronvich's algorithm [L. Cronvich, "A numerical-graphical methods of characteristics for axially symmetric isentropic flow," J. Aeronaut. Sci. 15, 156-162 (1948)]. The laminar boundary layer is corrected by employing Michel's integral method [R. Michel, "A erodynamique: Couches limites, frottement et transfert de chaleur" (ENSAE, 1963)]. This approach has already largely shown its potency and had been widely used for 30 years in the field of experimental molecular physics or laboratory astrophysics [sometimes known under the french acronym CRESU for Cinetique de R eaction en Ecoulement Supersonique Uniforme (reaction kinetics in uniform supersonic flow)]. Based on this approach, an inhouse computer program with graphical user interface to design de Laval nozzles for kinetic studies is published for the first time.

We describe a generally applicable method for the experimental determination of stationary flow conditions in pulsed supersonic beams, utilizing time-resolved electron induced fluorescence measurements of high pressure jet expansions of helium. The detection of ultraviolet photons from electronically excited helium emitted very close to the nozzle exit images the valve opening behavior—with the decided advantage that a photon signal is not affected by beam-skimmer and beam-residual gas interactions; it thus allows to conclusively determine those operation parameters of a pulsed valve that yield complete opening. The studies reveal that a " flat-top " signal, indicating constant density and commonly considered as experimental criterion for continuous flow, is insufficient. Moreover, translational temperature and mean terminal flow velocity turn out to be significantly more sensitive in testing for the equivalent behavior of a continuous nozzle source. Based on the widely distributed Even-Lavie valve we demonstrate that, in principle, it is possible to achieve quasi-continuous flow conditions even with fast-acting valves; however, the two prerequisites are a minimum pulse duration that is much longer than standard practice and previous estimates, and a suitable tagging of the appropriate beam segment.

37th AIAA Thermophysics Conference, 2004

The direct simulation Monte Carlo (DSMC) method is used to simulate the flow created prior to formation of a Z-pinch in the Nuclear Weapons Effects (NWE) Decade facility at the Arnold Engineering Development Center (AEDC). The simulation domain and the inflow boundary conditions are determined on the basis of a continuum-based Computational Fluid Dynamics (CFD) analysis. The DSMC simulation shows that the rapid expansion of the argon gas in the dual shell, concentric annular nozzle leads to large supersaturated regions. A homogeneous condensation model is then proposed for the DSMC simulation to include the effects of the nucleation and condensation, which explains the principal differences between the simulation results and available experimental data.

Chemical Physics Letters, 1999

Photoionisation spectra of the 'hot' 6 1 1 2 16 1 and 6 0 1 3 bands of the benzene molecule were used to estimate the 0 0 1 1 0 rotational temperature of benzene in He, Ne, Ar, Kr, N , or CH buffer gas, cooled in a supersonic molecular beam. All the 2 4 temperature estimates fall within the 4.5-19 K range. For the 6 1 level, the vibrational angular momentum of the 1 0 degenerate 6 1 state was found to play a significant role in system cooling. q 1999 Elsevier Science B.V. All rights 1 0 reserved. ) Corresponding author. Present address: Chemical Department,

Astronomy and Astrophysics

While it is generally thought that molecular outflows from young stellar objects (YSOs) are accelerated by underlying stellar winds or highly collimated jets, the actual mechanism of acceleration remains uncertain. The most favoured model, at least for low and intermediate mass stars, is that the molecules are accelerated at jet-driven bow shocks. Here we investigate, through high resolution numerical simulations, the efficiency of this mechanism in accelerating ambient molecular gas without causing dissociation. The efficiency of the mechanism is found to be surprisingly low suggesting that more momentum may be present in the underlying jet than previously thought. We also compare the momentum transferring efficiencies of pulsed versus steady jets. We find that pulsed jets, and the corresponding steady jet with the same average velocity, transfer virtually the same momentum to the ambient gas. The additional momentum ejected sideways from the jet beam in the case of the pulsed jet ...