Nonlinear effects in desorption of valine with fast incident molecular ions

Fast molecular ions as primary particles have been used to study secondary-ion desorption from organic layers. The secondary molecular-ion yield of the amino acid valine (molecular weight, 117) has been measured as a function of the velocity of primary atomic and molecular incident ions. The primary ions used were ${\mathrm{C}}^{+}$, ${\mathrm{O}}^{+}$, ${\mathrm{Ar}}^{+}$, ${\mathrm{C}}_2^{+}$, ${\mathrm{O}}_2^{+}$ , ${\mathrm{CO}}^{+}$, ${\mathrm{CO}}_2^{+}$, ${\mathrm{CH}}^{+}$, ${\mathrm{CH}}_3^{+}$, ${\mathrm{CF}}^{+}$, ${\mathrm{CF}}_3^{+}$, ${\mathrm{C}}_3$ ${\mathrm{F}}_5^{+}$, and ${\mathrm{C}}_4$ ${\mathrm{F}}_7^{+}$ in the energy range 600 keV–3.7 MeV. The secondary molecular-ion yields, when compared to yields for atomic constituents, unambiguously show that collective effects exist in desorption with incident molecular ions. Results are discussed in the framework of enhancement in the electronic stopping power per atom for molecular ions due to the vicinage of the fast-moving charges in the material. The resulting high-yield enhancements, especially with the use of large incident ions such as ${\mathrm{C}}_3$ ${\mathrm{F}}_5^{+}$ and ${\mathrm{C}}_4$ ${\mathrm{F}}_7^{+}$, are very encouraging for the future of mass spectrometry of large organic molecules.