Controlled deceleration of cold molecular ions formed in a supersonic jet using pulsed electric fields

We present a compact electrostatic device for producing velocity-controlled bunches of cold molecular ions. In our implementation, BaF molecules are generated in a supersonic expansion and photoionized inside a stack of electrodes providing >99% field homogeneity. Short voltage pulses applied across the structure decelerate the resulting BaF+ ions by several hundred meters per second over only a few millimeters, enabling control of the mean velocity down to the meV regime while preserving a narrow velocity spread. The setup delivers bunches of thousands of ions and thus provides a versatile source for ion-neutral collision studies, high-resolution spectroscopy, and efficient trap loading. Although the present work does not address rovibrational state preparation, threshold photoionization techniques could be straightforwardly combined with this approach to generate ions in a well-defined internal distribution. The instrument, therefore, offers a simple and general route to the controlled preparation of cold molecular ion beams for a broad range of applications in atomic and molecular physics.