Xinxin Cheng, Yao Zhang, Michael P. Minitti, Sanghamitra Deb, James Budarz, Peter M. Weber

Chemistry DepartmentBrown University

66th OSU International Symposium on Molecular Spectroscopy

06/21/2011

Ultrafast Structural Dynamics of Tertiary Amines upon Electronic

Excitation

h

Response to Electronic Excitation: Ammonia

h

Ground State

David M. Bishop, J. C. Leclerc and Y. Takahata, Journal of Molecular Spectroscopy, 1971, 39, 79-89

Halpern, A. M.; Roebber, J. L.; Weiss, K. J. Chem. Phys. 1968,49, 1348–1357

Excited State

laser pulse

Tertiary Amines

Ground State

Halpern, A. M.; Roebber, J. L.; Weiss, K. J. Chem. Phys. 1968,49, 1348–1357

h ?

Issues in tertiary amines:• Steric hindrance• Conformers• Intramolecular vibrational redistribution(IVR)

• What is the structural response of tertiary amines to electronic excitation?

• Can we measure it?• Observe the structure as a function of time.

Question to Ask and Our Goal

Rydberg Fingerprint Spectroscopy

Xiao Liang, Michael G. Levy, Sanghamitra Deb, Joseph D. Geiser, Richard M. Stratt*, Peter M. Weber,*Journal of Molecular Structure 978 (2010) 250–256

EB Ry

(n )2

n: principle quantum #δ: quantum defect

BE Structure

5 KHz Rep. RateTwo color experiment

• 4ω+2ω208 nm pump (5.96 eV)416 nm probe (2.98 eV)

Jie Bao, Dissertation, 2011, page 10

Experimental Settings

PES 4ω+2ω

• A typical two-color PES of amine

Jaimie L. Gosselin,† Michael P. Minitti,† Fedor M. Rudakov,† Theis I. Sølling,‡ and Peter M. Weber*,† J. Phys. Chem. A 2006, 110, 4251-4255

(DMIPA)

3p

3s

Proposed Structural Response

h

IVR

• Tertiary amine changes from an initial pyramidal structure to a planar structure.

• Relaxation from a local energy minimum to a global minimum: an IVR process.

The Process to Become Planar Structure

~

465.05cm 1

~

352.59cm 1

,1BE

,2BE

,2 ,1B BE E

Fit Results

• Linear peak center shift model• Exponential peak center shift model

Linear shift

Exponential shift

(DMIPA)

Preliminary Data

compond shift amplitude (mev) shift time constant (fs)

 

DEMA 74(1) 95(2)

 

DMIPA 24(2) 133(12)

 

DABCO 47(1) 111(3)

 

CENNA 52(2) 94(6)

Summary

• The structural response of polyatomic molecules can be complex because more degree of freedom are included.

• Though complicated, the responses are observable, for instance, using Rydberg Fingerprint Spectroscopy.

• The initial motion of tertiary amines is on the order of 100fs. The response involves intramolecular vibrational redistribution and structure changes that make the amine planar and rotate the methyl groups.

Acknowledgement

Prof. Peter Weber Yao Zhang Dr. Sanghamitra Deb Dr. Michael Minitti Dr. Jie Bao Dr. Joseph Geiser

Chrishtine Buehler Xiao Liang

James Budarz Yan Gao Sonia Vignale

Thank You!