Precision Mass Spectrometry and Polarizability Shifts with Two Ions in a Penning Trap*

We have implemented a technique for precision mass spectrometry with two ions simultaneously confined in a Penning trap in which each ion is alternately positioned at the center of the trap - where the cyclotron frequency is measured - or else parked in a large cyclotron orbit [1]. We have resolved previous systematics and have been able to exploit the improved statistical precision available with this technique. We have now used this technique to measure the mass ratios $^{31}$P$^{+}$/$^{28}$SiH$_{3}$$^{+}$, $^{28}$Si$^{+}$/ $^{13}$C$_{2}$H$_{2}$$^{+}$ and $^{28}$Si$^{+}$/$^{12}$C$_{2}$H$_{4}$$^{+}$ to obtain new values for the atomic mass of $^{28}$Si and $^{31}$P. In addition we have studied the ratio $^{31}$PH$^{+}$/$^{16}$O$_{2}$$^{+}$ to measure shifts in the cyclotron frequency of $^{31}$PH$^{+}$ due to the interaction between the ion's polarizability and the motional electric field. This has led to a measurement of the polarizability and dipole moment of $^{31}$PH$^{+}$. [1] M. Redshaw, J. McDaniel, W. Shi and E. G. Myers, Int. J. Mass Spec. \bf251\rm, 125 (2006).