Ultrafast Dynamics in Rydberg States of Aliphatic Amines

Ultrafast Dynamics in Rydberg States of Aliphatic Amines Application of Rydberg Fingerprint Spectroscopy International Symposium on Molecular Spectroscopy June 21 st 2005 Michael P. Minitti, Jaimie L. Gosselin and Peter M. Weber Brown University Theis Sølling – University of Copenhagen

Outline

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:

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 CH 2 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 k. Hz rep. rate • 150 fsec pulse e- MCPs • 418 nm & 209 nm • 3 x 1012 W/cm 2 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 3 p 7. 25 3 s 5. 4 4. 5 5. 93 Q • PES spectra reveal mechanism for distribution of the excitation energy

Summary for DMIPA Fast Internal Conversion from 3 p → 3 s (≤ 1 ps) Fragmentation occurs in the ion and NOT in the Rydberg state since the QD doesn’t change Occurs on a rate greater than 1 ps

Preliminary TR-Structural Analysis Application of RFS to observe folding dynamics in ‘floppy’ molecules in the gas phase N, N, N’-Tetramethyl-ethylene-diamine (TMEDA)

Preliminary Results Observation of line narrowing in the 3 s peak of TMEDA at increasing pump-probe delay times

Peak narrowing in TMEDA Time FWHM (ps) (e. V) (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