Spectroscopic investigation of wave driven microwave plasmas

The study of the emission of “mixed gas” plasmas reveals many surprising results, especially when hydrogen is one of the gases. Anomalous, even extreme, hydrogen line broadening was found in a number of mixed discharge plasmas excited via direct current dc or radio frequency rf electric fields. 1–5 The Balmer line spectra emitted by these discharges have typical multimode behavior, with widely broadened “wings” “fast” hydrogen and a sharp top “slow” hydrogen. The results have usually been explained in terms of Doppler shift and broadening due to the acceleration of charges such as H + ,H 2 , and H3 ions in the high dc electric fields present in the sheath regions of these discharges. The acceleration of hydrogen ions in these dc fields is followed by neutralization and generation of fast excited H atoms. This is the origin of the “wings” in the spectra. Strikingly excessive Balmer- line broadening has been observed in He– H210% microwave discharge, 6 but this has not yet been confirmed by other research groups. On the contrary, measurements of H line profiles emitted by microwave discharges under similar conditions have not revealed excessive broadening. 7–10 Nevertheless, selective hydrogen line broadening has been detected in microwave discharges and their afterglows when there is no significant broadening of noble gas lines or hydrogen molecular lines. 8,9,11 A possible explanation for such selective heating of H atoms may be connected with the main creation processes of excited H atoms, namely, ion conversion and electron impact dissociation. Furthermore, hyperthermal hydrogen atoms have surprisingly been detected at atmospheric pressure Ar– H2 microplasma jets, where the Hn =3 temperatures were found to range from 12,000 to 19,600 K. 12 It is now clear that hydrogen line broadening causes controversy so that more experimental observations are currently needed in order to try to elucidate the mechanisms and processes behind this phenomenon in different types of discharges. The aim of this experimental work is to address some of these problems. This article presents spectroscopic measurements in He– H2 and Ar– H2 low-pressure plasmas generated by a surface wave of frequency / 2 = 2.45 GHz. Results on the line shape and the emission intensities of excited hydrogen and helium atoms, and the emission intensities of the Q-branch of the Fulcher- band d 3 uv =1 →a 3 g v =1 are presented and discussed. Different temperatures are determined from the measured hydrogen and helium emission line shapes and the rotational distribution of hydrogen molecular lines. Furthermore, the population distribution of excited H atoms is determined from measurements of the relative intensities of transitions within the Balmer series.