Research on the neutralization of ion beams by electron-capture collisions with atomic particles has largely been within the domain of atomic and high-energy physics with the emphasis on understanding the fundamentals of ion-atom interactions. This account presented a chemist's perception of the possibilities to obtain new information about molecules formed by neutralization of beams of molecular ions. Only reactions in which a molecule is formed in an electron-capture process are discussed. In this article neutralized ion beam spectroscopy was defined as a technique for investigating modes of energy disposal in species formed when a fast beam of ions is neutralized in the electron-transfer reaction AB/sup +/ + M(g) ..-->.. AB* + M*/sup +/ where AB/sup +/ is any diatomic or polyatomic ion and M is a thermalized target atom. The target atom series M = Cs, K, Na, Ca, Mg, and Zn that covers an ionization potential range of 3.9 to 9.4 eV. The most significant technical advance from neutralized ion beam studies is the capability of producing beams of stable or metastable molecules that would be difficult, if not impossible, to obtain by other methods. 5 figures, 2 tables. (DP)

Neutralized ion-beam spectroscopy

This article is a review of the mechanisms responsible for collisional activation of ions in mass spectrometers. Part I gives a general introduction to the processes occurring when a projectile ion and neutral target collide. The theoretical background to the physical phenomena of curve-crossing excitation (for electronic and vibrational excitation), impulsive collisions (for direct translational to vibrational energy transfer), and the formation of long-lived collision intermediates is presented. Part II highlights the experimental and computational investigations that have been made into collisional activation for four experimental conditions: high (>100 eV) and intermediate (1-100 eV) center-of-mass collision energies, slow heating collisions (multiple low-energy collisions) and collisions with surfaces. The emphasis in this section is on the derived post-collision internal energy distributions that have been found to be typical for projectile ions undergoing collisions in these regimes.

The mechanisms of collisional activation of ions in mass spectrometry

Total cross sections for the reaction of H+2(v′) with He were calculated for v′=0 to 5 over a range of total energies between 1 and 5 eV using classical mechanics. The potential surface used was an appropriate diatomics‐in‐molecules function adjusted to fit existing ab initio points. In agreement with experiment, reagent vibrational excitation is very effective in promoting formation of HeH+ or dissociation, a fact which we explain in terms of the increased range of the attractive forces when the H+2 bond is extended. This effect is not properly described by comparison collinear calculations.

Trajectory calculation of the effectiveness of reagent vibration in the H+2+He→HeH++H or He+H+H+ reactions

A new apparatus is developed and used to obtain nascent vibrational and rotational distributions in the ground electronic state of CO/sup +/ formed from the charge transfer reactionN/sup +/(/sup 3/P)+CO (X /sup 1/..sigma../sup +/)..-->..N(/sup 4/S)+CO/sup +/ (X /sup 2/..sigma../sup +/,v,J)+0.52 eV,at approximately thermal energies. The device utilizes a flow tube for the production of thermal N/sup +/ ions in a helium buffer and a large diameter sampling orifice which delivers the ions via a mild free jet expansion into a low pressure chamber. The expansion is crossed by a stream of reactant CO molecules and the CO/sup +/ product states are probed by laser-induced fluorescence. Although the energy available is sufficient to populate CO/sup +/ vibrational states up to v'' = 2, the major vibrational channel in the CO/sup +/ product is v'' = 0. The relative vibrational distribution is found to be: N/sub v/ = 0>0.81 (observed under single collision conditions), N/sub v/ = 1<0.15 (not observed), and N/sub v/ = 2roughly-equal0.04 (observed only under nonsingle collision conditions). The rotational distribution in the v'' = 0 state is characterized closely by a Boltzmann distribution with a temperature of 410 +- 50 K. This represents a fractional energy disposal intomore » rotation of only 2%. Nearly all of the reaction exothermicity is therefore released into translational recoil. These results are considered in terms of simple dynamical models of the charge transfer process.« less

Single collision ion–molecule reactions at thermal energy: Rotational and vibrational distributions from N++CO→N+CO+

A novel experimental technique couples a flowing afterglow ion source with a supersonic nozzle expansion in order to deliver high densities of relatively low kinetic energy ions into a low pressure chamber. The technique is used to study the charge transfer reaction Ar+(2P3/2)+N2(v=0) →Ar(1S0)+N+2 (v+0,1;N)+ΔE=0.18 eV under single collision conditions at 0.24 eV c.m. Nascent rotational and vibrational state distributions are obtained by the method of saturated laser‐induced fluorescence probing. It is found that a substantial fraction of the available energy is partitioned into internal excitation of the N+2 product molecule. The higher of the two energetically accessible vibrational levels N+2 (v=1) is populated in 89±9% of the charge transfer collisions. The nascent rotational distribution in N+2 (v=1) is characterized by a Boltzmann distribution with Trot=700±50 K. The results are compared with a number of recent state‐selected experiments on charge exchange in ArN+2 , ArH+2 , and NCO+ systems. It is s...

Laser-induced fluorescence measurement of nascent vibrational and rotational product state distributions in the charge transfer of Ar++N2→Ar+N+2 (v=0,1) at 0.2 eV

J.J. Leventhal

Papers

Dynamic model for vibronic excitation in low energy atom-molecule collisions

Classical superposition phenomena in H+2 (v = O) - He reactive collisions

Rydberg excitation of NO IN H+NO collisions near threshold