Energy dependence of projectiles on ion formation in electrospray droplet impact SIMS

Charged electrospray droplets are introduced in vacuum and are allowed to impact the metal target. The secondary ions generated are measured by an orthogonal time-of-flight mass spectrometer. For the self-assembled monolayer as a sample, the ionization/desorption of monolayer molecules is observed without fragmentation. The secondary ion abundances for various samples deposited on the metal substrate are measured as a function of the acceleration voltage of the projectiles. There are found two threshold behaviors for secondary ion formation, i.e., low-energy and high-energy regimes. In the low-energy regime, desorption of the samples deposited on the metal target starts but no ionization takes place. In the high-energy regime, kinetic energy of the projectile is converted to internal energies for the system taking part in the collision and molecules deposited on the metal substrate are ionized/desorbed. There are found two modes of fragmentation for rhodamine B and crystal violet in the high-energy regime. When N2 was deposited on the metal substrate at ∼17 K, ion signals decreased suddenly. This is due to the drastic change of energy dissipation processes in the high-momentum collision.