Rotational Cooling of Trapped SiO$^{+}$ by Optical Pumping

We present results for preparation of trapped molecular ions with a high degree of internal state purity by optical pumping with a broadband pulse-shaped femtosecond laser. The highly diagonal Franck-Condon factors of the dipole-allowed B-X transition in SiO$^{+}$ make optical pumping on the P-branch of this transition favorable for fast stepwise pumping through the tens of rotational levels populated in a room-temperature distribution. By using resonance enhanced multiphoton dissociation (1+1' REMPD) to state-selectively form Si$^{+}$, we probe the internal states of our SiO$^{+}$ sample and determine the degree of rotational cooling.