Toward the use of Rydberg states for state-selective production of molecular ions David Grimes, Timothy J Barnum, Stephen Coy, Robert Field Department of Chemistry, MIT

Outline Why state selected ensembles of molecular ions would be useful Brief introduction to Rydberg states and autoionization Explanation of Multichannel Quantum Defect Theory (MQDT) Results from experiments on CaF and resulting predictions Description of a simpler, more general method 2

Uses for state selected molecular cations 3 Cold/Ultracold Chemistry Fundamental Physics Quantum Information Ng, Lewandowski, Ye, Jin, Hudson Cornell, Ye Hudson, Odom,

Refresher on Rydberg states E: Energy IP: Ionization potential : Rydberg constant n: Principal quantum number δ: Quantum defect v + : Vibration of ion-core N + : Rotation of ion-core μ: Dipole moment q: Charge d: Distance d ≈ 100 nm for n = 45 d -

What is autoionization? 5 v + = 0, N + = 0 v + = 0, N + = 1 v + = 0, N + = 2 v + = 1, N + = 0 Occurs on a sub-ns timescale (widths of 0.1 – 0.5 cm -1 ) Collision of Rydberg electron with ion core scrambles angular momentum of resulting fragments Useful for highly efficient ion generation for detection of resonances above the ionization potential Positioned at the intersection of spectroscopic theory (resonance with a continuum) and scattering theory (long lived scattering resonance)

Enter Multichannel Quantum Defect Theory (MQDT) † H (1) Asymptotic Regime Born-Oppenheimer Regime Scattering Regime Ψ e (r) r

A calculable mess 7 Born- Oppenheimer Regime Asymptotic Regime Scattering Regime Frame Transformation Scattering Transformation

Fits to experiment 8 Required ~1000 cm -1 of spectra taken at 1 GHz resolution Chirped Pulse spectroscopy provides the same information at 50 kHz resolution across ~10 cm -1 of spectra Kay, J. J., et. al.(2011). JCP, 134(11),

Results from our fit 9 (Term Energy – 47000)*100 cm -1 Normalized Ionization Oscillator Strength Rovibrational Branching Ratio Normalized Ionization Oscillator Strength

An easier way? 10 N N+N+ ℓ N + is now a good quantum number, as ℓ is decoupled from the motion of the ion core. A small field is required to ionize this state, not large enough to mix states of different rotational quantum numbers. In principle, works for any molecular system. ℓRℓR

Conclusions Rydberg states are a promising avenue for gently preparing molecular ions in a single rovibrational state MQDT is necessary for determining ionization branching ratios for easily accessible Rydberg states Our CPmmW spectrometer can provide significantly more accurate fit models. Core nonpenetrating states (the topic of the next talk) are the easiest way to prepare molecular ions in a single rovibrational state. 11

Acknowledgements Prof. Robert Field Yan Zhou Tim Barnum Dr. Steve Coy Ethan Klein Dr. Tony Colombo Dr. Jeff Kay Dr. Carrie Womack Barratt Park Jun Jiang Dr. Josh Baraban Bryan Changala 12 Prof. John Doyle Dr. Dave Patterson Prof. Dave DeMille Dr. John Berry Prof. Brooks Pate Dr. Justin Neill Prof. John Muenter