University of Missouri – Kansas City Molecular Structure and Dynamics Probed by Photoionization out of Rydberg States Fedor Rudakov University of Missouri – Kansas City 72nd Meeting – June 19th-23rd, 2017 Champaign-Urbana, Illinois

A → B Ultrafast Chemistry Molecular structure Spectroscopic properties Conventional (not ultrafast) Structural Chemistry Ultrafast Chemistry Mechanisms of chemical reaction A → B What is the nuclear motion? How fast is this motion? Need another coordinate – time

Ultrafast Chemistry To observe chemical bond breakage on a femtosecond time scale it is required to pump very large amounts of energy in the molecule. A technique that is sensitive to molecular structure, yet insensitive to vibrational motion is required. R.C. Dudek, P.M. Weber J. Phys. Chem. 105(17), 4167 (2001)

Dimethyl isopropyl amine Rydberg States Hydrogen atom Electron is moving in Coulomb potential created by proton Energy levels: Ry=13.6 eV - Rydberg Constant, n – principal quantum number Potential: Dimethyl isopropyl amine Molecular Rydberg states In molecules, Rydberg states are analogous to the states of hydrogen atom. The Rydberg electron is moving in the potential created by the molecular ion. Energy levels: quantum defect accounts for the shielding created by the molecular ion

Photoionization out of Rydberg States Transition intensity between two electronic energy levels is given by the square of Frank- Condon factor. e- Ry Ion Ground state ψi and ψj are vibrational eigenfunctions of the initial and final electronic state. Energy The potential energy surfaces and the vibrational eigenfunction in Rydberg and ion state are nearly identical and are nearly orthogonal to each other. Frank- Condon factor: Nuclear Coordinate No vibrational quantum number change upon photoionization!

Photoionization out of Rydberg States Linear perturbation shifts energy levels of a harmonic oscillator by a constant value. No peak broadening will take place if oscillations are harmonic (even if potential energy surface of ion state is not identical to that of Rydberg state). Ion Rydberg Large amplitude anharmonic motion results in Rydberg peak broadening.

Structural dynamics in N-methyl-morpholyne 404 nm probe pulse pump pulse 226 nm Ground state Yao Zhang Oscillation period=650(13) fs Chair-equatorial Chair-axial Damping time constant =750(90) fs

Isomerization of Quadricyclane 3 4 6 7 2 1 5 0.5 eV Energy QD Norbornadiene (NB) Rydberg NB 1 2 3 4 5 6 7 Ground state 1.47 eV QD Quadricyclane (QD) NB 1.5 1.7 2.3 C5-C7=C4-C6 distance, Å

Isomerization of Quadricyclane 416 nm 3p QD 3p QD 3s 416 nm 208 nm 3s Electron Binding Energy, eV NB 3s 0.5 eV QD Time Delay, fs v NB Ground state QD -quadricyclane 1.47 eV QD NB NB -Norbornadiene

Isomerization of Quadricyclane: dynamics Rydberg electron binding energy (∆Eb) and FWHM (∆W) as a function of time delay between laser pulses State ΔE time constant, fs ΔW time constant, fs 3s 936(146) 812 (446) 3p (2.29 eV) 271(20) 304(38) 3p (2.24 eV) 452(31) 787(495) The time constants for the peak shift and the peak narrowing are equal within the uncertainty intervals. Both processes originate from the same phenomena – formation and evolution of the wavepacket on the Rydberg surface.

Trace chemical sensing UV/VIS absorption spectra Rydberg spectra of C7H8 isomers Rydberg states are extremely sensitive to the structure of the molecules and can be used for molecular fingerprinting. Complexity of Rydberg spectrum does not scale with molecular size.

Trace Chemical Sensing How to acquire Rydberg spectra remotely? Standard techniques (photoelectron spectroscopy, mass spectrometry) require ultrahigh vacuum and are not suited for the standoff detection. Novel approach: Utilize microwave scattering from plasma produced by photoionization. Typical microwave scattering signal from plasma produced by photoionization Experimental Setup

Photoionization scheme 213 nm Ion state Rydberg states Expected: 3s UV pulse Ionization Efficiency Ground state Photon Energy

Standoff Detection of DABCO Expected: Actual: Photon Energy Ionization Efficiency 1,4-dizobicyclooctane (DABCO) Red line – Rydberg spectrum acquired in air Black line – Rydberg spectrum acquired in vacuum F. Rudakov, Z. Zhang, Opt. Lett., 37(2), 145, (2012)

Rydberg Ion-Dip Spectroscopy N,N-dimethylisopropylamine UV Ion State VIS/IR Vibrational States Rydberg states 3s Lifetimes of the 3s Rydberg state as a function of pump photon energy. UV Ground state Very low laser power densities are required

Rydberg Ion-Dip Spectroscopy Ry=13.6 eV - Rydberg Constant, n – principal quantum number quantum defect F. Rudakov, Y. Zhang, X. Cheng, P.M. Weber Optics Letters, 38 (21), 4445 (2013).

Detection of methyl radical Microwave backscattering intensity for methane, propane, and hexane flames as a function of VIS/IR photon wavelength. Microwave scattering as a function of the VIS/IR photon wavelength and the distance from the surface of the burner for a) a methane flame and b) a propane flame. F. Rudakov, Y. Gao, F., X. Cheng, P.M. Weber Combustion and Flames, 171, 162–167 (2016).

Detection of tetramethyethylene in tetramethylethane flame Detection of tetramethyethylene. (a) photoionization scheme, (b) Rydberg spectra of pure tetramethylethylene (green line), and spectrum of tetramethylethane flame (blue line). The signals were scaled to provide roughly the same intensities.

Summary Photoionization out of Rydberg states reveals highly resolved and very characteristic spectra which are largely insensitive to vibrational motion. Rydberg spectra can be utilized for probing molecular structure and dynamics. Photoionization through Rydberg states coupled with detection of laser induced plasma via microwave radiation can be utilized for standoff trace chemical sensing and detection of combustion intermediates.

Acknowledgements Dr. Jerome. B. Hastings Dr. David. H. Dowell Dr. John. F. Schmerge Professor Peter Weber Dr. Job Cardoza Dr. Mike Minitti Prof. Zhili Zhang Dr. Yehuda Braiman Dr. Jacob Barhen Dr. Thomas Thundat Dr. Bo Liu