Bulletin of the American Physical Society
79th Annual Meeting of the APS Southeastern Section
Volume 57, Number 16
Wednesday–Saturday, November 14–17, 2012; Tallahassee, Florida
Session CA: Atomic, Molecular, and Optical Physics I |
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Chair: John Yukich, Davidson College Room: DoubleTree Ballroom |
Thursday, November 15, 2012 10:45AM - 11:15AM |
CA.00001: Controlled Dipole-Dipole Interactions in a Cold Rydberg Gas Invited Speaker: Robert Jones The great spatial extent of highly-excited Rydberg atoms endows them with large dipole moments and electric polarizabilities, making them extremely sensitive to external fields and neighboring atoms. We use cold diffuse ensembles of Rydberg atoms to explore few-and many-body dipole-dipole interactions at long range. Such systems are of potential interest for quantum computing platforms. We focus on time-domain methods and utilize pulsed lasers and electric fields to create specific electronic superposition states and control the evolution of the excited atoms. We have confirmed that electronic coherence can persist for long times ($>$ 10 $\mu$s in our samples and have evidence that the coherence may be extended through the application of the appropriate control fields. We are exploring methods which exploit the dipole-dipole coupling to transport electronic coherence between atoms and to manipulate the relative positions of atoms in a magneto optical trap. [Preview Abstract] |
Thursday, November 15, 2012 11:15AM - 11:27AM |
CA.00002: Measurements of the Polarization of Spectral Lines of Highly Ionized Ions Using a Two-Crystal Technique A.J. Smith, P. Beiersdorfer, K.J. Reed We Have measured the polarization of various spectral lines of highly ionized ion species including the intercombination line of heliumlike vanadium, the resonance lines of heliumlike and lithiumlike sulphur as well as of neonlike and fluorinelike iron. The lines were excited directly by electron impact in the LLNL electron beam ion trap (EBIT-I or EBIT-II) and the polarization measurements carried out using a two-crystal technique. The results of our measurements are important for the diagnostics of astrophysical as well as of laser-produced plasmas. In this presentation we compare our results with theory. [Preview Abstract] |
Thursday, November 15, 2012 11:27AM - 11:39AM |
CA.00003: CO$_{2}$ Laser enhanced lifetime of femtosecond and nanosecond produced air plasmas Jorge Martinez, Charlemagne Akpovo, Dawn Lewis, Staci Brown, Lewis Johnson Air produced plasmas generated from a 20 mJ femtosecond pulse at 800 nm and a 5 mJ nanosecond pulse at 532 nm were orthogonally enhanced with a 3 J defocused CO$_{2}$ pulse to study the effect of interpulse delay on plasma lifetime. When interpulse delay was optimized with respect to the CO$_{2}$ pulse, both the femtosecond and nanosecond plasmas exhibited longer lifetimes. The impact of CO$_{2}$ laser pulse energy was explored by maintaining a fixed CO$_{2}$ laser fluence at the interaction point. The femtosecond/CO$_{2}$ combination exhibited the same relationship as with the defocused 3 J pulse with a substantial lengthening of plasma lifetime and luminance. The effect of the enhancing wavelength was studied by replacing the CO$_{2}$ laser with a 1064 nm nanosecond laser pulse to monitor any differences in the laser generated air plasmas. The femtosecond/nanosecond combination exhibited quenching of the generated plasma with almost no change in plasma lifetime. A study of these interactions will broaden the understanding of the role of pulse duration and wavelength when considering the phenomena that occur in laser generated plasmas such as inverse bremsstrahlung processes, multi-photon ionization, and collisional heating. [Preview Abstract] |
Thursday, November 15, 2012 11:39AM - 12:09PM |
CA.00004: UV Photoprotection of Ammonia and Adenine Studied by Time-resolved Photoelectron and Photofragmentation Spectroscopy Invited Speaker: Susanne Ullrich The UV photostability of molecules is determined by excited state electronic relaxation mechanisms that must operate on ultrafast time scales in order to dominate over competing photochemical processes that potentially lead to destruction of the molecule. Electronic excited states with notable $\sigma $* character, centered at X - H (where X = O or N) bonds, may play a particular important role in efficient photoprotection of many (bio)molecules. We have investigated the photophysics of UV excited ammonia and adenine using three complementary femtosecond (fs) pump-probe techniques: time-resolved photoelectron (TRPES), ion-yield (TRIY) and photofragment translational spectroscopy (TRPTS). Ammonia, a prototypical amine group which appears in a number of organic molecules, is resonantly excited to specific vibrational levels of its first electronic excited state of n$\sigma $* character. Three deactivation paths are available along the N-H stretching coordinate: Non-adiabatic crossing through a conical intersection leads to either repopulation of the NH$_{3}$ ground state or dissociation into ground state NH$_{2}$ and H photoproducts whereas adiabatic avoidance correlates with excited state NH$_{2}$ and ground state H. TRPES spectra give direct spectroscopic evidence of $\sigma $* mediated relaxation in form of combination bands of the umbrella mode and symmetric stretch. TRPTS measurements of H-atom appearance times provide time constants of $<$ 75 fs to 350 fs for the relaxation, which increase with the amount of internal energy partitioned into the NH$_{2 }$co-fragment. Adenine, a purine DNA base, is shown to undergo similarly efficient $\pi \sigma $* mediated relaxation in competition with a ring puckering pathway following 200nm photoexcitation to a bright $^{1}\pi \pi $* state. H-atom photoproducts from the NH-stretching pathway are observed within $<$200fs, whereas deactivation along the ring puckering pathway takes $\sim $700fs. \\[4pt] N. L. Evans, H. Yu, G. M. Roberts, V. G. Stavros, S. Ullrich: Observation of Ultrafast NH$_{3}$ (\~{A}) State Relaxation Dynamics using a Combination of Time-resolved Photoelectron Spectroscopy and Photoproduct Detection, \textit{Phys. Chem. Chem. Phys.}, 2012, \textbf{14}, 10401 \\[0pt] N. L. Evans, S. Ullrich: Wavelength Dependence of Electronic Relaxation in Isolated Adenine Using UV Femtosecond Time-Resolved Photoelectron Spectroscopy,\textit{ J. Phys. Chem. A,} 2010, \textbf{114}, 11225 [Preview Abstract] |
Thursday, November 15, 2012 12:09PM - 12:21PM |
CA.00005: Excited state dynamics in imidazole, pyrazole and pyrrole studied by femtosecond time-resolved spectroscopy Hui Yu, Nicholas Evans, Susanne Ullrich In many biomolecules including nucleic acids and amino acids, ultrafast ($<$200fs) relaxation from an electronically excited state back to the ground state provides a self-protection mechanism against harmful ultraviolet radiation. The excited state dynamics of gas-phase imidazole, a basic five-membered heterocyclic subunit of the above mentioned biomolecules, is thus investigated experimentally to determine its contribution to the photoprotection. The imidazole photophysics are compared to structurally similar pyrrole and pyrazole that are less common in biological chromophores. Time-resolved photofragment translational spectroscopy and H-atom total kinetic release methods have been applied to follow the deactivation dynamics in real time. All three molecules display two competing pathways that operate on femtosecond timescales: A $\pi \sigma $* (N-H dissociation) and a $\pi \pi $* (ring deformation) mechanism are identified based on observation of H-atom and larger photoproducts, with an energetic onset specific to the individual molecule. [Preview Abstract] |
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