ANGULAR-DEPENDENCE OF INTERNAL ENERGY-DISTRIBUTIONS OF ACTIVATED FE(CO)5.+ AND W(CO)6.+ IONS - THE CONTRIBUTIONS OF VIBRATIONAL AND ELECTRONIC EXCITATION MECHANISMS IN KILOELECTRON VOLT COLLISIONS

The average internal energy deposited into Fe(CO)5.+and W(CO)6.+upon kiloelectron volt collision-induced dissociation (CID) using Ar as a target gas increases with both the collision energy (E) and the angle (θ) through which the collision products are collected. An average energy of 7-8 eV is deposited into the Fe(CO)5.+ion as it is scattered through a laboratory scattering angle of 2° in a 3-keV collision with Ar. Similar average internal energies are deposited into W(CO)6*+ at corresponding laboratory scattering angles, the smaller center-of-mass collision energy Ecmfor W(CO)5.+being compensated for by a larger value of 0cm, making the scattering parameter, (Eθ)cm, similar for both systems. Collision activation experiments performed with He and C9F20as target gases show that the effective mass of the latter is much less than its molecular weight, suggesting that the target is internally excited and/or that parts of the target are spectators during collisional activation. The distribution P(c) of internal energies acquired upon collisional activation is dependent upon the collision energy, the scattering angle, and the nature of the collision gas. The distributions are typically Gaussian in shape at 1.5- and 2-keV collision energies for scattering angles in the range 1 to 2° for the title projectiles. In contrast, distributions for ions scattered through similar angles upon 3-keV collisions are distinctly bimodal in shape. The lower energy component in the bimodal distribution is thought to be due to electronic excitation, while the higher energy component is associated with a vibrational excitation mechanism, as suggested by comparison with energy distributions obtained for electron impact, low-energy (electron volt) collision-activation, and surface-induced dissociation (SID). An approximately linear relationship is found between the energy deposited and the reduced scattering parameter Eθ. These data confirm earlier suggestions that angle-resolved mass spectrometry (ARMS) can be used to select the degree of activation of a mass-selected ion. They also provide evidence that direct vibrational excitation is a significant activation mechanism in collisional activation of polyatomic ions in the keV energy range. © 1990, American Chemical Society. All rights reserved.