Direct Images of the Virtual Source in a Supersonic Expansion
详细信息 查看全文
- 作者：Thomas Reisinger ; Gianangelo Bracco ; Stefan Rehbein ; Gü ; nter Schmahl ; Wolfgang E. Ernst ; Bodil Holst
- 刊名：Journal of Physical Chemistry A
- 出版年：2007
- 出版时间：December 13, 2007
- 年：2007
- 卷：111
- 期：49
- 页码：12620 - 12628
- 全文大小：645K
- 年卷期：v.111,no.49(December 13, 2007)
- ISSN：1520-5215
文摘
Direct images of the virtual source in a supersonic expansion of helium are presented. The images wereobtained using a Fresnel zone plate with free-standing zones, 540 m in diameter and with an outermostzone width of 50 nm. The general method can be extended to other beams, including seeded beams.Measurements were carried out at absolute source pressures ranging from 11 to 171 bar using a 10 m nozzlewith a source temperature of 320 K. The size of the virtual source was found to be strongly dependent onpressure, changing from a diameter of 67 ± 6 m at an absolute nozzle pressure of 11 bar to 180 ± 9 mat 171 bar. The virtual-source brightness displays a maximum at an absolute nozzle pressure of around 30bar. This phenomenon occurs because of two competing effects: As the pressure increases, the total flux alsoincreases, but at the same time the virtual source broadens. We modeled the expansion process by calculatingthe velocity distribution with solutions from the Boltzmann equation to estimate the location of the quittingsurface where the frequency of interatomic collisions is assumed negligible. Realistic potentials have beenused to calculate the cross section for atomic collisions and, for the velocity distribution perpendicular to thecenter streamline, a proper scaling with distance derived from the continuum expansion model has beenintroduced. A good agreement between experiments and model has been found and we discuss its approximationlimits. For instance, backscattering effects are not included in the calculations and at present we cannotexclude that they also contribute to a broadening of the virtual-source size for the highest pressure regime.The results presented here are important for improving the understanding of the supersonic expansion process.The experimental method might eventually be used as a new way to test molecular and atomic interactionpotentials.