Wednesday, June 25, 2014

Condensed phase dynamics in Telluride

Last night I was stranded at Denver airport en route to the bi-annual Condensed phase dynamics meeting at the Telluride Science Research Center.  This is the third time I have been to this wonderful meeting. Getting there can be a real hassle. But, then you look at the scenery and enjoy the science and it seems worth it.

Unfortunately, due to the travel delays I missed the first two talks, by Joe Subotnik and Nandini Ananth.

Dominika Zgid gave a chemist's perspective on "How to make dynamical mean theory quantitative". Some of her work was discussed in a my last post. Today she mostly discussed a generalisation of iterative perturbation theory as an "impurity solver" for DMFT problems with multiple orbitals. See this preprint.

Peter Rossky discussed quantum chemical simulations of exciton dynamics in conjugated polymers.

This was motivated by an experiment reported in Science that claimed evidence for quantum coherent transport of excitons along a polymer chain at room temperature. Several oscillations were seen in the fluorescence polarisation anisotropy  as it decays in about a picosecond. These oscillations were identified with quantum inference [Rabi oscillations] between different exciton states delocalised over the polymer chain.

It turns out the experimental results have a much more mundane explanation.
The simulations of Adam Willard and Rossky are of classical dynamics on the adiabatic excited state potential energy surface calculated from a parameterised PPP [Pariser-Parr-Pople] model [basically a Hubbard model with long-range Coulomb interactions. They see oscillations similar to those in the experiment and can identified simply with classical nuclear motion associated with the polymer backbone stretching [phonons] in response to photo-excitation.

Much-hyped experiments claiming to show quantum coherence in photosynthetic complexes, probably also have a similar classical explanation in terms of nuclear dynamics rather than electronic coherences. A concrete interpretation in terms of vibrational coherences is in this PNAS paper. My skepticism of these "quantum biology" experiments has been expressed in many earlier posts.

Hopefully, tomorrow I will blog about talks from Eran Rabani, Todd Martinez, and Dvira Segal.

1 comment:

  1. Dear Ross, as a frequent reader and big admirer of your blog, I take the opportunity of contributing on this occasion, if only to present a more positive view on what you call "quantum biology".
    I definitely agree with you on the need to fully scrutinize the results obtained using 2D spectroscopy, given the complexity of the spectral response of molecular aggregates. However, I disagree with you on the conclusions that can be drawn from current experiments. In fact, the article you quote by Jonas' group does not argue that electronic coherence is absent neither it advocates a purely classical interpretation. I take the opportunity to advertize a recent work by our group in JCP:
    Origin of long-lived oscillations in 2D-spectra of a "Quantum Vibronic Model: Electronic vs Vibrational coherence"
    M.B. Plenio, J. Almeida, S.F. Huelga, J. Chem. Phys. 139, 235102 (2013)
    where you can see how different contributions (ground state vibrational coherence a la Jonas and electronic coherence) coexist and can actually become of the same order in certain parameter regimes.
    In other words, the complete 2D signal contains electronic coherence but not only that. This result is in agreement with recent theoretical group by our group and others that identify non-trivial spectral structures of the environmental fluctuations and particularly discrete vibrational modes as key to understand the generation and sustenance of both oscillatory energy transport and electronic coherence on timescales that are comparable to excitation energy transport (see Nature Physics 9, 113 - 118 (2013)).
    I hope this may help to convince you that things may be more interesting than what you initially thought.
    Best regards from Ulm
    Susana Huelga