Introduction

Plasma is an ionized gas which contains equal amounts of positive and negative charges. Positive charges are carried by positive ions. Negative charges are usually supplied by electrons, but in some cases negative ions have a contribution. Plasmas are broadly classified into two classes. One is a hightemperature, fully ionized plasma. The other is a low-temperature, weakly ionized one. This book is concerned with a molecular plasma, i.e., a plasma containing neutral molecules. In most cases, a molecular plasma belongs to the class of low-temperature, weakly ionized plasmas. But, although belonging to the class, plasmas composed only of atoms (e.g., rare gases) are not dealt with in this book. Some of the high-temperature, fully ionized plasmas have a supply of molecules from outside and partly become a molecular plasma. For example, a plasma in a fusion device has a very hot core region, but also contains a large amount of molecules in its boundary region (see Sect. 2.4).