Volume 77, number 3 CHEMICAL PHYSlCS LETTERS 1 February 1981 LASER-INDUCED FLUORESCENCE SPECTROSCOPY OF AN N;-ION B:AM A DING,K RICHTER Halrrl-dlelrrler-ltlstrlllr fur KernJorsdr~rng Bcrlm CmbH Berc~ch StralAwciremrr, D-1000 Berlm 39, West German) and M MENZINGER Departmetlr of Chem!str~ , zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA i3rr11 ernr_t of Toronto. Toronto. Omarro. Carlada M.5S I.41 Recelred 18 August 1980, in fiial zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA form 10 h’ovembcr 1980 The mternal state dwrlbution of an N;-ion beam whuzh has been produced in a phsma eon source has been determmed b) Liser-Induced fluorescence The results show a marked devlatlon from a thermal Boltzmann dlsrrlbutlon Ion-beam den- SIU~Sof less than 500 tons/cm 1x1 n pxtlcular wbratron-rotation state have been measured I. Introduction hlolecular eons play an important role m collusion processes m the gas phase, particularly m the upper armosphere or m a plasma The dlfferentlal colhslon cross sections of such processes often depend sense- tlvely on the Internal excitation of the partners whch IS usually not known Laboratory experiments show that It IS hardly pos- sible to produce an ion beam consisting of only one Internal state. Non-spectroscopic techruques to meas- ure the Internal dlstnbutlon are not unique and m gen- eral rather inaccurate [ 1] A variety of optlcal methods have been devlsed to determme the excitation of molecular ions Conven- tlonai methods of enusslon spectroscopy m the vlslble [2] , infrared [3] and mlcrowave range [4] are lumted by the mavoidable space charge effects of molecular ions Laser-Induced evcltatlon [S] and fluorescence methods [6-91 have been recently developed, and seem to combme highest resolution with sufficient senutlvity. The euperlments, which are presented here, de- scribe the measurement of the ground-state vlbratlon- rotation dlstrlbutlon of an Ni-ion team In spite of the fact that Nq molecular Ions have been observed m bulk by laser-induced fluorescence and one example of a true ion-beam measurement (for CO+ Ions) has been pubhshed the combmatron of both has, to our knowledge, not been performed yet Because of the totaLly different method of productIon of molecular ions in a gas discharge, our results differ slgruficantly from those published by Alhson et al [7] and Dagdlglan and Doermg [8] m that we find slgruflcant devlatlons from a thermal Boltzmann dlstrlbutlon 2. Experimental Fig 1 &plays a schematic view of the experiment Ni ions are formed In a plasma ion source [lo] The ions pass a three-element ekctrostatlc emzellens, are deflected around 90” in a magnetic mass analyzer (eqmpped with permanent magnets) and then are lead - coavlally to the laser beam - throtigh another elec- trostatlc lens mto the observation range where the laser-mduced fluorescence could be detected Finally, they were deflected onto an Ion collector where they were monitored durmg the course of the euperunent. The purpose of the magnet was not only to mass se- lect the Ion beam, but also to let the laser beam,vlhlch runs in a &rectlon opposite to the ions, pass freely through the chamber. For uutlal adjustment a mov- able ion detector could be shd into the beam Just be- 0 009-26 14/8 l/0000-0000/$02 50 0 North-Holland F’ubhshmg Company 523