High rovibrationally-excited H2, HD and D2 molecules in an expanding plasma studied by VUV-LIF spectroscopy on the Lyman band

  • J.J.A. van den Dungen

Student thesis: Master

Abstract

Rovibrationally-excited molecules play a prominent role in plasma chemistry, fusion research and astrophysics. Rovibrationally-excited molecules (Xrv), are molecules with additional internal energy, distributed over vibrational and/or rotational states. Due to this additional internal energy, numerous reaction rates can be increased by orders of magnitude compared to non-rovibrationally-excited molecules. This work focuses on production and detection of rovibrationally-excited H2, HD and D2 molecules, as well as their implications on the plasma chemistry inside a H2/D2 expanding thermal plasma. To produce an H2/D2 expanding thermal plasma, H2 and D2 mixtures are in- jected into a so-called cascaded arc. The plasma supersonically expands from the cascaded arc into the expansion vessel, leading to a stationary plasma expansion. Within several millimeters into the expansion HDrv molecules are measured. It is expected that these HDrv molecules together with Hrv 2 and Drv 2 molecules, are very e±ciently created inside the arc. Several production mechanisms are investigated and atom{molecule and/or surface desorption mechanisms are believed to be the dominant production processes of the HDrv molecules. With use of a technique based on laser induced °uorescence in the vacuum UV, part of the Lyman band of H2, HD and D2 is measured. Using a spectrum simulation model and the measured Lyman spectrum, 401 Lyman transitions of rovibrationally- excited H2, HD and D2 molecules were determined. Over 70 HD and D2 Lyman transitions of rovibrationally-excited molecules with rotational states higher than J = 10 were measured and used to extend the spectroscopic data base with. From the assigned Lyman transitions, also density distributions of the internal energy of the three molecules were determined, close to the exit of the arc. Although the expanding plasma is in ¯rst order identical to a hot gas expansion, the H2, HD and D2 density distributions cannot be described by a Boltzmann distribution. Instead, each distribution is described by two Boltzmann distributions, with di®erent temperatures. The ¯rst temperature describes the distribution for the low rotational states and the second temperature describes the distribution for the high rotational states. These temperatures are determined to be 700 K and 3800 K for H2, 500 K and 3400 K for HD and 300 K and 3000 K for D2. One of the most prominent reactions in which ro-vibrational excitation plays a decisive role is the dissociative attachment reaction, in which H¡ or D¡ is produced from Hrv 2 or Drv 2 (or HDrv) by collision with an electron. With the density distri- butions as measured in the plasma expansion, the production rate of these negative ions is increased by up to 10 orders of magnitude as compared to the production rate from non rovibrationally-excited molecules. This makes rovibrationally-excited molecules an interesting source for negative ions.
Date of Award30 Jun 2008
Original languageEnglish
SupervisorO.G. Gabriel (Supervisor 1) & Richard A.H. Engeln (Supervisor 2)

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