Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session H26: Focus Session: Non-adiabatic Molecular Dynamics and Control at Conical Intersections I |
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Sponsoring Units: DCP Chair: Todd Martinez, University of Illinois at Urbana-Champaign Room: Colorado Convention Center 205 |
Tuesday, March 6, 2007 8:00AM - 8:36AM |
H26.00001: Electronically Excited States and Conical Intersections in Cytosine and its Analogs Invited Speaker: Conical intersections between two and three electronic states of the same symmetry have been found to play a key role in nonadiabatic processes. In recent years many studies have shown that conical intersections are important in the photophysics of nucleobases and facilitate radiationless decay to the ground state. Interestingly, there are molecules very similar in structure to the nucleobases which show very different photophysical behavior, i.e., longer excited state lifetimes and high quantum yields of fluorescence. An important question that arises is what causes the different behavior between nucleobases and their fluorescent analogs. In this work we present studies of cytosine and several of its analogs in an effort to correlate the molecular structure to the photophysical behavior. Large scale ab initio multireference configuration interaction methods (MRCI) are being used. Our results show the presence of many seams of two- or three-state conical intersections in both types of systems and energetic differences seem to be the cause of the different photophysical behavior. A mixed quantum mechanical/ classical mechanical (QM/MM) approach where the solute is described with the MRCI method will also be presented as a means to study the effect of the solvent on excited states. [Preview Abstract] |
Tuesday, March 6, 2007 8:36AM - 9:12AM |
H26.00002: Coherent Control of the Photo-isomerization of Retinal in Bacteriorhodopsin Invited Speaker: Proteins are highly evolved structures in which their central role is to direct chemical or solar energy into functions. A central tenet in biology is that proteins have evolved to stabilize the transition states processes directing energy transduction into functions. In the transition state region, the motions are fairly localized such the wave properties of matter can lead to destructive and constructive interferences that have a pronounced effect on transmission probabilities along reaction coordinates. Further, the time scale for motion through a transition state, often involving a conical intersection, is comparable to the currently believed decoherence times for electronic and vibrational degrees of freedom governing this motion. The question arises whether the phases of the underlying matter waves could play a role in directing biological processes. In order to address this question, we exploited coherent control protocols using shaped laser fields to determine whether or not the absolute quantum yield of the photo-isomerization of retinal in bacteriorhodopsin (bR) could be achieved using weak field conditions to probe the natural function. Through feedback-controlled amplitude and phase variation of the spectral components comprising the excitation pulse, we could selectively enhance or suppress the isomerization quantum yield by 20{\%} in either direction. Our experimental observation illustrates that the wave properties of matter, as manifest on vibrational quantum coherences, can play a role in biological processes to the point that they can even be manipulated. [Preview Abstract] |
Tuesday, March 6, 2007 9:12AM - 9:24AM |
H26.00003: Anion Photoelectron Spectroscopy of Deprotonated Cytosine and Thymine at 5.826 eV Terry Yen, Bradley F. Parsons, Sean M. Sheehan, Daniel M. Neumark UV radiation from sunlight is one of the most ubiquitous and lethal forms of environmental carcinogens. Luckily, perhaps as a result of selective pressure by molecular evolution, DNA is remarkably stable to photochemical decay. The solar UV photons are absorbed primarily by the building blocks of DNA, the nucleobases, which undergo ultrafast nonradiative relaxation processes back to the ground state by internal conversion. It has been suggested that the nucleobases' excited singlet states of pi-sigma-star character, which are dissociative along the NH stretch coordinate, play a key role in the relaxation pathways. Studying the dynamics of the excited states of deprotonated nucleobases is therefore of great importance in understanding the photostability of DNA. Excited state dynamics of the deprotonated nucleobases cytosine and thymine has been investigated using anion photoelectron spectroscopy at 5.826 eV. In the experiments, a laser is used to detach an electron from a mass selected deprotonated nucleobase anion and the electron kinetic energy (eKE) is measured using velocity-map imaging (VMI). Through VMI, information is obtained on the nature of the excited states involved in the non-radiative processes of DNA. [Preview Abstract] |
Tuesday, March 6, 2007 9:24AM - 9:36AM |
H26.00004: Comparing the electronic relaxation of pyrimidine bases and nucleosides in aqueous solution Stephen Bradforth, Askat Jailaubekov, Delmar Larsen, Christi Chester The ultrafast deactivation of DNA bases excited in the ultraviolet is known to occur by rapid nuclear motion through conical intersections between different electronic potential energy surfaces. How the intersections between these surfaces and the dynamics over these surfaces are modified by surrounding the base with water is a significant open question in DNA photophysics. Using a broadband transient absorption apparatus with 30 fs time resolution, we observe dispersed spectra from 300 -- 700 nm revealing excited-state dynamics originating for U and T both as nucleobases and nucleosides. New sub-100fs dynamics is observed, including stimulated emission. The deactivation pathways and spectral signatures of the various intermediates are compared to data from gas-phase time-resolved photoelectron spectroscopy and non-adiabatic quantum-classical simulations. [Preview Abstract] |
Tuesday, March 6, 2007 9:36AM - 9:48AM |
H26.00005: ABSTRACT HAS BEEN MOVED TO L26.00002 |
Tuesday, March 6, 2007 9:48AM - 10:00AM |
H26.00006: Photoisomerization selectivity in conjugated $\pi$-bond systems through local microenvironment Aaron Virshup, Todd Martinez Photoisomerization represents one of the simplest means to convert light energy into mechanical motion on the molecular scale. Theoretical models of photobiology often require description of not only a small photochemically active chromophore, but also the effects of the much larger solvent or protein environment containing the chromophore. We have recently developed a program for carrying out excited state QM/MM studies of photodynamics using \textit{ab initio} quantum chemistry techniques for the QM region, and modeling the time evolution of the system with the Full Multiple Spawning method for molecular dynamics. With this method, we show how local charge environments can be used to manipulate and enhance the photoisomer selectivity of small conjugated molecules. [Preview Abstract] |
Tuesday, March 6, 2007 10:00AM - 10:12AM |
H26.00007: Excited State Dynamics of DNA and RNA bases Hanneli Hudock, Benjamin Levine, Todd Martinez Recent ultrafast spectroscopic experiments have reported excited state lifetimes for DNA and RNA bases and assigned these lifetimes to various electronic states. We have used theoretical and simulation methods to describe the excited state dynamics of these bases in an effort to provide a mechanistic explanation for the observed lifetimes. Our simulations are based on ab initio molecular dynamics, where the electronic and nuclear Schrodinger equations are solved simultaneously. The results are further verified by comparison to high-level ab initio electronic structure methods, including dynamic electron correlation effects through multireference perturbation theory, at important points along the dynamical pathways. Our results provide an explanation of the photochemical mechanism leading to nonradiative decay of the electronic excited states and some suggestions as to the origin of the different lifetimes. Comparisons between pyrimidines illustrate how chemical differences impact excited state dynamics and may play a role in explaining the propensity for dimer formation in thymine. [Preview Abstract] |
Tuesday, March 6, 2007 10:12AM - 10:24AM |
H26.00008: Probing solvation effects at conical intersections by ultrafast photoelectron imaging Benoit Soep, Lionel Poisson, Kevin Raffael, Jean Michel Mestdagh The electronic excitation of polyatomic molecules is generally followed by relaxation of the electronic energy to the ground state or to metastable, low lying states such as triplet states in hydrocarbons. It can be extremely rapid whenever conical intersections between the surfaces are at play, owing to their structural changes. Since, in general, relaxation is observed in condensed phases, it is essential to conduct the relevant experiments in the presence of a perturbing medium, here the surface of an argon cluster. We address the coupling of two excited configurations in a molecule possessing charge transfer intermediates thus prone to medium effects. We shall compare here the observation of the free and deposited molecule at the surface of argon clusters. The effect of the cluster and the possibility to record significant photoelectron spectra is thus described that represents an innovation for large systems. We made use of the anisotropy of the photoelectron angular distribution of the electrons to unravel the dynamics of the several excited configurations that are traversed during the electronic relaxation. [Preview Abstract] |
Tuesday, March 6, 2007 10:24AM - 10:36AM |
H26.00009: Azobenzene optical switch controlled by external force Martin Konopka, Nikos Doltsinis, Dominik Marx, Ivan Stich External pulling force can be employed to manipulate optical switching properties of azobenzene molecule which is a promising system for molecular electronics devices. We perform density functional treatment of azobenzene terminated by S-H groups with pulling force applied via restraints on the sulfur atoms. We treat the system both at zero and room temperature and among other quantities focus on separation between ground (S0) and first excited (S1) singlet states. The separation is crucial for cis $\leftrightarrow$ trans reisomerization rate. For trans isomer we observe decrease of the S1-S0 separation with increased stretching force and the molecule length. For cis isomer we find opposite: the separation increases thus lowering photoisomerization rate. Another interesting point is mechanically induced cis $\rightarrow$ trans inversion in the ground state which occurs for extensions above 5~\AA. [Preview Abstract] |
Tuesday, March 6, 2007 10:36AM - 10:48AM |
H26.00010: Control of molecular fragmentation using shaped femtosecond pulses Marcos Dantus, Vadim Lozovoy The ability to tailor the excitation laser pulse by pulse shaping has inspired a number of scientist to search for special pulses that would be capable of inducing selective bond fragmentation or specific molecular rearrangements. This presentation will summarize results from a comprehensive look at the interaction of shaped laser pulses with para-nitrotoluene molecules. We have performed exhaustive experimental evaluation over very different pulse shaping strategies such as chirp, sinusoidal modulation, sinusoidal phase, binary phase, binary amplitude, binary phase and amplitude. For all these methods we recorded hundreds of mass spectra together with the integrated second harmonic generation (SHG) as an independent parameter related to pulse complexity or pulse duration. As expected, the integrated SHG of a given laser pulse correlates linearly with the total amount of ions detected. We were surprised, however, that the fragmentation patterns observed varied simply and predictably with the integrated SHG regardless of pulse shaping strategy. This implies that the integrated SHG fraction compared to a transform limited pulse is an excellent predictor of the fragmentation pattern for a given molecule. The implications of our findings for this and other molecules will be discussed from the fundamental point of view of bond selective chemistry. The development of applications for molecular recognition will also be discussed. [Preview Abstract] |
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