Ab Initio and Experimental Studies of the E Internal Rotor State of He-CH 3 F Kelly J. Higgins, Zhenhong Yu, and William Klemperer, Department of Chemistry and Chemical Biology, Harvard University

Outline Introduction Intermolecular Potential Energy Surface Bound States Experimental Methods A Internal Rotor State Modified Potential E Internal Rotor State

Introduction – Helium Complexes Sample an appreciable amount of the entire potential energy surface Lower bound states tend to be localized, but with some overlap Intermolecular potentials are qualitatively similar to heavier rare gas complexes Accurate He-molecule potentials useful for helium nanodroplet spectroscopy Excellent test of computational methods

Introduction – He-CH 3 F CH 3 F exists as two essentially non-interconverting species –I Htot = 3/2 and K = 0, 3n (A state He-CH 3 F) –I Htot = 1/2 and K = 1, 3n ± 1 (E state He-CH 3 F) Low temperature helium pressure broadening studies of CH 3 F by De Lucia and coworkers and Willey and coworkers ( ) One study of CH 3 F in helium nanodroplets reported in A. Conjusteau’s Ph.D. thesis at Princeton (2002) SAPT potential by Bussery-Honvault et al (2003)

Study Flow Calculate ab initio potential and bound states Observe A-state transitions Morph ab initio potential to fit observed A- state transitions Predict E-state bound states and transitions Observe E-state transitions

He-CH 3 F Intermolecular Potential MP4/7s5p3d2f(C,F) 6s3p2d(H,He) + 3s3p2d(bond) Counterpoise corrected

Bound States Calculated using Hutson’s BOUND or Cohen and Saykally’s collocation program T-shaped ground state and linear excited state Nearly free internal rotation

A-State Energy Levels

Experimental Methods Fraser-type electric resonance spectrometer with liquid-He cooled bolometer detector –Broadband frequency range from 10 MHz to  –Simple to perform multiple resonance experiments to find and assign lines –Fast scanning compared to FTMW FTMW in Pat Thaddeus’ Lab –Higher resolution to resolve hyperfine structure

Observed A-state Transitions

Morphed Potential Morph only the correlation energy –Deepens the well and moves it in radially at the same time –Calculation is at the basis set limit for SCF but not for electron correlation –Requires fewer parameters than morphing the entire interaction potential Morph to best reproduce fitted A-state constants –Constants: c 0 = , c 1 = , c 2 = , and c 3 = reduce relative rms deviation from 0.42 to 0.022

Morphed Potential

Predicted E-state Levels

Observed E-state Transitions

E-state 0 1  1 1 Transition (FTMW)

E-state 1 2  0 1 Transition (FTMW)

E-state 2 2  1 1 Transition (MBER)

E-state 1 3  0 1 Transition (MBER)

Measured E-state Frequencies Frequency (MHz)MP4Morphed 0 1  (5)  (5)  (5)  (30)  (5)  (30)  (30)  (30)  (30)  (30)  (30)  (30)  (30)  (30)  (30)

E-state Summary 15 transitions corresponding to 9 energy levels observed, but no T-to-linear yet Morphed potential much better at predicting lines E-state calculated to be more tightly bound than A- state: / cm -1 for unmorphed/morphed potential T-to-linear gap greater in E-state by / cm -1 for unmorphed/morhped potential Hyperfine structure needs to be analyzed and verified for the high frequency lines

Acknowledgements National Science Foundation Mike McCarthy and Pat Thaddeus

Thank You