Laser-Based Detection of Highly Vibrationally Excited H2 for Plasma Diagnostics

Abstract

It has been suggested that the main source of H- in hydrogen plasmas is dissociative attachment to H2(v=6 to 10)1. In order to test this, it is necessary to measure the vibrational level populations inside the plasma. We have constructed a source of vibrationally excited H2 using a recently discovered hot-wire effect 2,3. Our source consists of a hot tantalum wire in a cooled stainless steel cell. The H2 pressure is 10 mTorr; the residence time in the cell is about 40 ms. Using 2+1 resonantly-enhanced multiphoton ionization (REMPI) through the EF state, we have observed 27 H2 bands from X(v"=4-11) to EF(v'=0-14) at wavelengths from 232 nm to 285 nm. Ions produced by REMPI are separated by mass in a time-of-flight apparatus. Both H+ and H 2 + are produced. Rotational temperatures appear to be 300-650 K, well below that required for thermal excitation of the observed vibrational levels. The population of vibrational levels falls off dramatically between v"=9 and 10. This is the first detection by optical absorption of v"==10 and 11. This source makes possible the development of laser-induced fluorescence techniques for the detection of vibrationally excited H2.