High-resolution translational-energy spectrometry has been applied to study vibrational excitation in neutral C 0 2 in its ground electronic state by 3 keV impact by O+, N', Ne', Ar+ and Xe' projectile ions. Three quanta of asymmetric stretch mode are excited. Bending mode vibrational excitation also manifests itself in translationalenergy spectra obtained with the heavier projectile ions (Ne', Ar+ and Xe'). In the case of the 0' projectiles, simulkneous electronic (0+*2D@2P) and vibrational (CO,, u j = 0-1) excitation is also observed.High-resolution translational-energy spectrometry (TES) has proved to be a most successful probe of the molecular and dynamical properties of small singly and multiply charged gas-phase cations in recent years (see Ref 1, and references therein, for the most recent compilation of pertinent literature). TES has also found application in studies of electronic excitation of neutral diatomic whereby, in conjunction with the Wigner spin conservation rule, judicious choice of projectile ion species enables collisional excitation of optically spin-forbidden transitions. Information on collisional excitation processes is of importance in developing an understanding of the physico-chemical processes occurring in terrestrial and planetary atmospheres. ' Application of TES to studies of vibrational excitation of neutral molecules has hitherto been hampered by experimental constraints of inadequate energy resolution. Only a handful of experimental studies have been and they have been confined exclusively to measurements carried out using Hf projectiles. Moore and D0ering~3~ have measured cross-sections for the excitation of pure vibrations in H2, DZ, and N2 using H+ beams of 100-600eV energy. Herrero and Doerings have studied collisional excitation of the first four vibrational levels of ground state H2 by 5-2500 eV H+ and D + impact. Cross-sections as large as cm2 have been reported for pure vibrational excitation and it has been noted that such a process may be important in determining the loss of energy by protons in the terrestrial atmosphere.We report here the results of what are believed to be the first experiments on collisional excitation of vibrational modes of a triatomic species, COz, by 3 keV impact with heavy ion projectiles N + , O+, Ne+, Ar+ and Xe+. Vibrational excitation of triatomics such as carbon dioxide is known to play an extremely significant role in the science of gas laser systems. For instance, in the case of N,-C02 laser mixtures, very efficient vibrational excitation in N, causes the energy distribution function of electrons in the mixed discharge to be sharply cut-off at electron energies in excess of 2.5 eV. This results in an efficient population of N, vibrational levels with subsequent collisional * Author to whom correspondence should be addressed. transfer of vibrational energy to the upper laser state of COz (which happens to be the asymmetric stretch vibrational mode referred to below).
EXPERIMENTALAll the measurements reported here were performed by a hig...
