1. 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

2. 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

3. 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)

4. 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

5. 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!

6. 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.

7. 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

8. Isomerization of Quadricyclane3 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

9. 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

10. 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.

11. 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.

12. 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

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

14. 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)

15. 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

16. 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).

17. 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).

18. 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.

19. 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.

20. 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