RYDBERG ELECTRONS International Symposium on Molecular Spectroscopy 17 June 2008 Michael P. Minitti Brown University STEALTHY SPIES OF MOLECULAR STRUCTURE

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: Rydberg States Rydberg I

Experimental Setup YLF Pump Ti:Sapphire Regenerative Amplifier CPU Timing Electronics Ion MCPs e - MCPs 4ω4ω2ω2ω BBO upconversion Molecular beam 5 kHz rep. rate 209 nm pump / 418 nm probe ~230 fs 4ω pulse width

Structural Dispersion in Flexible Molecules SD I RFS spectra of molecules w/ various internal rotation DOFs show multiple structures are populated Well resolved even in the presence of large vibrational temperatures M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, (2006) 1,4-Dimethyl-piperazine (DMPZ) N,N-Dimethyl-isopropanamine (DMIPA) N,N-Dimethyl-2-butanamine (DM2BA) N,N-Dimethyl-1-butanamine (DM1BA) N,N-Dimethyl-3-hexanamine (DM3HA)

Near time zero 30 ps delay Intensity (arbitrary units) DMPZ DMIPA DM2BA DM3HA DM1BA Intensity (arbitrary units) SD II

Vibrational Temperatures M.P. Minitti, J.D. Cardoza and P.M. Weber, JPCA, 110, (2006) SD III

History says... Spectral line shape “Electronic transitions consist of a series of bands, each band corresponding to a transition between a given pair of vibrational levels.” -Ira N. Levine, Physical Chemistry 1. Linewidth due to vibrational congestion {ν’}{ν}{ν} Intensity BEBE

History says... Spectral line shape 2. Linewidth due to lifetime of the state Long lifetimes = sharp lines Short lifetimes = broad lines How is it then that we see sharp lines in the presence of large vibrational energies in addition to very fast intermediate lifetimes?

Time-Dependent Structural Dispersion TD I Time-dependent structural dispersion observed in 3s Rydberg peak of DM2BA M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, (2007) 0 ps 150 ps70 ps

TD II Experimentally determined fractional populations (area under the curves) Two dominant conformeric forms, A and B, in equilibrium via opposing first order reactions M.P. Minitti and P.M. Weber, Phys. Rev. Lett, 98, (2007)

DFT Calculation M.P. Minitti and P.M. Weber, Phys. Rev. Lett., 98, (2007) Observed t = 0 fractional ground state population: 0.67/0.33 Calculated t = 0 fractional ground state population: 0.65/0.35 (using RT distributions) Observed t = ∞ fractional excited state population: 0.78/0.22 Calculated t = ∞ fractional excited state population: 0.70/0.30 (using previously estimated* vibrational temperature of 950 K) * M.P. Minitti, J.D. Cardoza and P.M. Weber, J. Phys. Chem. A., 110, (2006) TD III

Structural Dispersion Spectra are insensitive towards vibrational excitation and provide a purely electronic spectrum dependent on the coordinates of all electrons and nuclei and therefore the molecular structure What other spectral features can our Rydberg electron spies tell us about the molecular structure? NEXT MISSION: N,N,N’,N’ - TMEDA

fs-resolved TMEDA PES

5 ps 40 ps 200 ps Pump-Probe Delay (ps) 3s Rydberg Peak - Full Width Half Maximum (cm -1 ) 3s linewidth as a function of pump-probe delay

Linewidth comparisons to similar tertiary amines N,N-Dimethyl-1- butanamine N,N,N’,N’-TMEDA 1,4 -Dimethyl-piperazine

What’s the cause? TMEDA condenses in a minimum q hνhν The molecule contains vibrational energies that are significant to the barriers in its energy landscape As vibrational energy dissipates, the molecule condenses in a minimum on its surface configuration coordinate

Dimer Paren t Presence of Noble gas clusters and multimers TMEDA Mass Spectra Parent + He

Closing Remarks Rydberg Fingerprint Spectroscopy has been proven to be sensitive to an array of molecular properties Coupled with mass spectroscopy, RFS has multiplexing advantages Chemically relevant systems can be investigated

Acknowledgement s Prof. Peter Weber Dr. Job Cardoza Fedor Rudakov Joe Bush Sanghamitra Deb Brad Taylor Jie Bao Brian Bayes $$$ DOE - Basic Sciences