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Ultrafast Dynamics in Rydberg
States of Aliphatic Amines
Application of Rydberg Fingerprint
Spectroscopy
International Symposium on Molecular Spectroscopy
June 21st 2005
Michael P. Minitti, Jaimie L. Gosselin and Peter M. Weber
Brown University
Theis Sølling – University of Copenhagen
Outline
Rydberg
states
and
previous
Rydberg Fingerprint Spectroscopy
(RFS) results
Time-resolved RFS dynamics results


DMIPA (energy distribution)
TMEDA (structural fluctuations)
Conclusion
Rydberg States
State of an ion or molecule where an
excited electron has a high principal
quantum number
Hydrogenic in nature, with a binding energy
given as:
EB 
Ryd
(n   )
2
Previous RFS results
• Richly structured spectra
• Unique quantum defects observed for azulene
and naphthalene
N. Kuthirummal and P.M. Weber, Chem. Phys. Lett., 378, 647 (2003)
• Rydberg peaks associated
with n=3 states
• Different fingerprint spectra
obtained
• Addition of one CH2 group
alters phase shift affecting
the quantum defect
J.L. Gosselin and P.M. Weber, J. Phys. Chem. A. , 22, 4899 (2005)
Previous results cont.
RFS Conclusions
Unique technique that is isomer specific
Rydberg states are sensitive probes of
molecular structure
The size of Rydberg orbitals allows for
characterization of larger molecules
Time-resolved energy distribution and
structural analyses
Experimental Setup
Ti:Sapphire
BBO
upconversion
Regenerative Amplifier
YLF Pump
4ω
2ω
• 50 kHz rep. rate
• 150 fsec pulse
e- MCPs
• 418 nm & 209 nm
• 3 x 1012 W/cm2
CPU
Timing Electronics
Molecular beam
Ion MCPs
Impetus to use amines
Interest began in charge transfer studies
in 2-phenylethyl-N,N-dimethylamine
(PENNA)
Relatively low IPs – easily accessible via
MPI
Attractive signal to noise ratio
Stepping stone to larger bio-relevant
molecules
W. Cheng, N. Kuthirummal, J.L. Gosselin, T.I. Sølling, R. Weinkauf and P.M. Weber, J. Phys. Chem. A., 109, 1920 (2005)
TR-MS of DMIPA
• Confirms α C-C bonds
cleave at sufficient internal
energies
• Fragment is dominate peak
at most delay times
• Rates differ but trend is
similar
compared
to
previous work
T.I. Sølling, C. Kötting and A.H. Zewail, J. Phys. Chem. A., 107, 10872 (2003)
TR-PES of DMIPA
Ion
3p
7.25
3s
5.4
4.5
5.93
Q
• PES spectra reveal mechanism for distribution of the
excitation energy
Summary for DMIPA
Fast Internal Conversion from 3p → 3s
(≤1ps)
Fragmentation occurs in the ion and
NOT in the Rydberg state since the
QD doesn’t change
Occurs on a rate greater than 1ps
Preliminary TR-Structural Analysis
Application of RFS to observe folding dynamics
in ‘floppy’ molecules in the gas phase
N,N,N’,N’-Tetramethyl-ethylene-diamine (TMEDA)
Preliminary Results
Observation of line narrowing in the 3s peak of TMEDA
at increasing pump-probe delay times
Peak narrowing in TMEDA
Time
FWHM
FWHM
(ps)
(eV)
(cm-1)
+2
0.086
694
+20
0.074
597
+40
0.065
524
+60
0.066
532
+80
0.056
451
243 cm-1 difference in FWHM of similar time
dependent Rydberg peak
Instrument Limit (~350 cm-1)
Summary for TMEDA
Narrowing suggests a reduction in
the amplitude of vibrational motions
Possibly attributed to a structural
folding
Believed structural altering event
occurs on the order of 80 ps
Acknowledgements
Prof. Peter Weber
Prof. Theis Sølling, University of Copenhagen
Dr. Narayanan Kuthirummal
Jaimie Gosselin
Job Cardoza
Dr. Wei Cheng
Fedor Rudakov
Joe Bush
Funding Organizations
Army Research Office
Department of Energy