Electronic and photoelectron spectroscopy of Rg·NO (Rg  rare gas), NO·N 2 and NO·CH 4 molecular complexes  Jonathan D. Barr, John M. Dyke, Paul Mack, David M. Smith 1 , Timothy G. Wright * Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK Received 27 April 1998; accepted 14 May 1998 Abstract A critical review of experimental and ab initio studies of the Rg·NO complexes is presented. The review covers all the work published as of April 1998, and mainly concentrates on electronic [resonance-enhanced multiphoton ionization (REMPI), and laser-induced fluorescence (LIF)] spectroscopy, as well as photoelectron [zero-kinetic-energy (ZEKE)] spectroscopic studies. In addition, the NO·N 2 and NO·CH 4 molecule–molecule complexes are considered.  1998 Elsevier Science B.V. All rights reserved. 1. Introduction One of the peculiarities of research is that some- times ‘easy’ systems have had rather little attention paid to them, whereas more difficult systems were studied in some depth. A good example of this situa- tion is the Rg·NO (Rg  rare gas) molecular complexes, which, until relatively recently, had rather little attention paid to them, despite the fact that they are prototypical examples of a closed-shell atom inter- acting with an open-shell molecule. Further, the NO molecule has one unpaired electron, which may be excited to Rydberg orbitals, which in each case leads to a Rydberg state with a closed-shell cationic core. The interactions of the Rydberg electron with the Rg atom may be interrogated by spectroscopic means. Rg·NO complexes are ‘easy’ to study, since the NO radical may be obtained from a gas cylinder; however, it is only in the last few years that more- detailed studies of these systems were undertaken. In contrast, the Rg·OH and Rg·SH complexes have been the subject of detailed studies for many years [1], and much information is known about their lowest-lying electronic state, despite the fact that the OH and SH radicals must be produced in situ, by laser photolysis. It is the purpose of the present paper to review the information that is available on the electronic spectro- scopy of the Rg·NO complexes, and also to summarize the information that was gleaned from photoelectron spectroscopy. For organizational purposes, the information known on each electronic state will be collected together, and some more general points will be made. Information on the NO·N 2 and NO·CH 4 molecular complexes are then considered. A significant portion of the available information on the Rg·NO complexes has come from work performed at Southampton; however, it is hoped that a more or less complete coverage of work performed elsewhere has also been achieved. Journal of Electron Spectroscopy and Related Phenomena 97 (1998) 159–170 ELSPEC 3549 0368-2048/98/$ - see front matter  1998 Elsevier Science B.V. All rights reserved. PII: S0368-2048(98)00266-7 * Corresponding author; e-mail: tgw@soton.ac.uk.  Dedicated to Professor Neville Jonathan to mark his retirement. 1 Present address: Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, U.K.