The paper shows that adiabatic expansion of certain mixtures of molecular gases gives rise to the condition of population inversion in terms of vibrational levels existing for a certain time. To achieve this the molecules of the mixture should have substantially different vibrational relaxation times and should be capable of exchanging the energy of the vibrational excitation. (Author).

A time-dependent technique for the numerical solution of convergent-divergent, nonequilibrium nozzle flows was used to analyze the rapid, vibrational nonequlibrium, supersonic expansion of CO2-N2-H2O and CO2-N2-He mixtures, wherein the finite rate molecular energy transfer processes can result in a population inversion between the (001) and (100) vibrational energy levels of CO2. Results for such population inversions are presented. Among these, a comparison was made between the present results and the recent results of Basov et al; this comparison indicates that Basov's calculations overestimate the population inversion in an expanding mixture of CO2 and N2. In addition, results are presented from a series of numerical experiments conducted to assess the validity of several simplified methods for computing population inversions. (Author).

This volume contains eight invited papers and seventy-three contributed papers presented at the Fourth International Symposium on Gas Flow and Chemical Lasers. which was held in Stresa. Italy. from September 13 to 17. 1984. The purpose of the Symposium was to record and discuss current research developments and applications related to high power lasers. Papers were solicited from the entire spectrum of activities, includ ing basic physics, aerothermodynamics, new laser media, laser design, diagnostic techniques, laser propagation. interaction phenom ena, applications. The Symposium was an opportunity for scientists and engineers representing all these disciplines to come together to report their recent work, to exchange ideas and to provide an up-to-date account of international progress in these fields. The contributed papers were reviewed by the members of the Scientific Advisory CoEmittee. who also took responsibility for formulating the program of invited lectures. As editor I wish to express my appreciation and my gratitude to people and organizations that made this Symposium a success: the Members of the Scientific Advisory Committee, the Members of the Local Organizing Committee, the Sponsors and the Symposium Secretary. A grateful acknowledgement is expressed to Dr. M. Sandra Oggiano for assistance in the organization of the Symposium and publication of these proceedings.

A time-dependent technique for the numerical solution of convergent-divergent, nonequilibrium nozzle flows was used to analyze the rapid, vibrational nonequlibrium, supersonic expansion of CO2-N2-H2O and CO2-N2-He mixtures, wherein the finite rate molecular energy transfer processes can result in a population inversion between the (001) and (100) vibrational energy levels of CO2. Results for such population inversions are presented. Among these, a comparison was made between the present results and the recent results of Basov et al; this comparison indicates that Basov's calculations overestimate the population inversion in an expanding mixture of CO2 and N2. In addition, results are presented from a series of numerical experiments conducted to assess the validity of several simplified methods for computing population inversions. (Author).