Kinetic temperature effects on 4He dimers in jets

Area of Science:

• * Atomic and Molecular Physics
• * Low-Temperature Physics
• * Gas Dynamics

Background:

• * Understanding cluster formation in free jet expansions is crucial for various scientific fields.
• * Previous models often assumed equilibrium conditions, which may not hold true in expanding gases.

Purpose of the Study:

• * To experimentally measure helium-4 (4He) dimer and trimer mole fractions in a free jet expansion.
• * To develop and validate a theoretical model that accounts for non-equilibrium temperature effects during expansion.
• * To investigate the survival threshold of dimers at low source pressures.

Main Methods:

• * Utilized diffraction from a nanostructured transmission grating to measure cluster mole fractions.
• * Conducted experiments on 4He gas in a free jet cryogenic expansion.
• * Extended existing rate equations to include temperature relaxation effects and extrapolated data to correct for residual gas attenuation.

Main Results:

• * Observed distinct distance ranges where dimer and trimer mole fractions and ambient temperature approach asymptotic values.
• * Identified an apparent threshold at low source pressures for the survival of dimers in the asymptotic beam.
• * Model accurately reproduced the observed dimer survival threshold and showed minimal deviation from isentropic temperature results at higher pressures.

Conclusions:

• * The extended model successfully captures non-equilibrium temperature effects in free jet expansions.
• * A critical low-pressure threshold exists for dimer survival in the asymptotic beam.
• * The findings provide a more accurate understanding of cluster formation dynamics in cryogenic expansions.