atoms Article 16 + Two-Photon Vibrational Transitions in O2 as Probes of Variation of the Proton-to-Electron Mass Ratio Ryan Carollo , Alexander Frenett and David Hanneke * Physics & Astronomy Department, Amherst College, Amherst, MA 01002, USA;
[email protected] (R.C.);
[email protected] (A.F.) * Correspondence:
[email protected]; Tel.: +1-413-542-5525 Received: 31 October 2018; Accepted: 18 December 2018; Published: 20 December 2018 Abstract: Vibrational overtones in deeply-bound molecules are sensitive probes for variation of the proton-to-electron mass ratio m. In nonpolar molecules, these overtones may be driven as two-photon 16 + transitions. Here, we present procedures for experiments with O2 , including state-preparation through photoionization, a two-photon probe, and detection. We calculate transition dipole moments 2 2 between all X Pg vibrational levels and those of the A Pu excited electronic state. Using these dipole moments, we calculate two-photon transition rates and AC-Stark-shift systematics for the overtones. We estimate other systematic effects and statistical precision. Two-photon vibrational 16 + transitions in O2 provide multiple routes to improved searches for m variation. Keywords: precision measurements; fundamental constants; variation of constants; proton-to-electron mass ratio; molecular ions; forbidden transition; two-photon transition; vibrational overtone 1. Introduction Even simple molecules contain a rich set of internal degrees of freedom. When these internal states are controlled at the quantum level, they have many applications in fundamental physics [1,2] such as searches for new forces [3], the investigation of parity [4,5] and time-reversal [6,7] symmetries, or searches for time-variation of fundamental constants [8–10].