Condensed concepts

2009 is the year I started blogging and I thought it would be interesting to see which posts on this blog have attracted the most viewers, partly because I find sometimes the results of such an analysis surprising. Thanks to Google Analytics this is easy to find out. The top 10 posts (and number of views) are below: Am I HOMO- and LUMO-phobic? (476) Mind the gap (230) Spectral density of water (162) 20 key concepts in thermodynamics and condensed matter physics (132) Can we see visons? (107) How can living organisms be so highly ordered? (101) Walter Kauzmann: master of thermodynamics (98) James Bond meets Niels Bohr (96) Electron versus hole transport in molecular materials (92) Quantum coherence in photosynthesis (91) I found the results really encouraging: both the volume and that posts which I think were important and/or original and high on scientific content [except for 8.] attracted the most attention. I was surprised that some of the career advice and "better sci

Paul Krugman was awarded the Nobel Prize in Economics in 2008 . He is also a New York Times columnist. Why mention him here? He is mentioned in Steven Johnson's book Emergence (p. 89-91) for a mathematical model which describes how spatially segregated business centres emerge. This is described in Krugman's book, The Self-Organising Economy. Here are Krugman's "rules for research", taken from a talk, "How I work" 1. Listen to the Gentiles 2. Question the question 3. Dare to be silly 4. Simplify, simplify 1. means "Pay attention to what intelligent people are saying, even if they do not have your customs or speak your analytical language."

Last Christmas holidays I started reading Emergence: The Connected Lives of Ants, Brains, Cities, and Software, by Steven Johnson . But the holidays ended.... and so I just got back into it. It is a really nice book and he is a very gifted writer. I also find it helpful and fascinating because although it is about emergence it virtually never mentions physics or chemistry! Johnson's background is in software but he does a really nice job connecting emergence in software (e.g. genetic algorithms and SimCity) with emergence in ant colonies, slime moulds, formation of neighbourhoods in cities, .... Johnson says (p. 77) there are five fundamental principles to follow "if you're building a system where macro-intelligence and adaptability derive from local knowledge" More is different Ignorance is useful Encourage random encounters Look for patterns in the signs Pay attention to your neighbours

Howard Gardner is a Professor of Education and Psychology at Harvard. He is best known for developing and promoting the concept of Multiple Intelligences. His latest book, Five Minds for the Future , defines five specific cognitive abilities that he claims will be sought and cultivated by leaders. Roughly here is my paraphrase of each of the minds, as applied to scientific research. The Disciplinary Mind: You need to master a specific discipline or research area. This takes about ten years. The Synthesizing Mind: You need to learn to integrate ideas from different disciplines into a coherent whole and to communicate that integration to others. The Creating Mind: You need to develop the capacity to uncover and clarify new problems, questions and phenomena. The Respectful Mind: You need to be aware of and appreciate different approaches and values within your discipline and between disciplines. The Ethical Mind: You need to fulfill your responsibilities as a worker within your instituti

Seth Olsen alerted me to a thought-provoking article in Wired about the importance of failure in science.

What will be your New Year's resolution for your research in 2010? Write more papers? Get more students? Apply for more grants? Get more lab space? Get a paper published in Nature or Science? Learn a new technique? I think mine is going to be: Spend the first half hour of each day thinking and writing in a notebook about the important science questions I am interested in and want to try and answer. And, specifically coming up with multiple alternative hypotheses and devising ways to distinguish them. Where did this come from? Previously I wrote a post about a beautiful Science paper about Strong Inference by John R. Platt . He references a book he published in 1962, The Excitement of Science. Unfortunately, it is out of print and only two universities in Australia have a copy in their libraries. I got a copy on interlibrary loan and just finished reading it. I have scanned a copy of chapter 7 and chapter 8 , which I found the most helpful and challenging.

Over the 25 years Robert Liu [from the University of Hawaii!] has emphasized that the "hula twist" is ubiquitious in photoisomerisation reactions [where after a molecule absorbs a photon it undergoes a structural change] and is beautifully summarised in the figure below. This short review by Liu and Hammond [whose address is Aloha, Oregon!] documents this and also argues that the hula twist is driven by steric considerations because it is "volume conserving". However, it turns out that this geometrical change can be preferred purely by an electronic mechanism and does not require steric hindrance. Seth Olsen and I show this [amongst many other results, some of which I have discussed in a previous post ] in a paper just published in Journal of Chemical Physics. We refer to the "hula twist" as disrotatory motion and discuss it briefly on page 12 of our paper.

Earlier this year a Nature paper reported the data below [black squares with error bars] for the spectrum of high-energy cosmic-ray electrons. The peak was interpreted as evidence for 500 GeV particles (dark matter) predicted by generalisations of the standard model that include extra dimensions. However, more recent data [shown as red points] from NASA's Fermi Gamma-ray space telescope was recently published in PRL . The data has much better statistics and shows no peak. More details can be found here . This is a good cautionary tale. There is no substitute for two or more independent experiments to test a hypothesis. I don't think Ronald Reagan was a good U.S. president, but his signature phrase "Trust, but verify" has merits.

Every now and then you go to a seminar which goes horribly wrong for the speaker. Someone asks a question that the speaker answers poorly or cannot answer. Then other people start asking questions or offering critical comments and it gets worse..... Why does this happen? How can the speaker prevent it? I think it may be because the speaker violates the important principle: Never offer undefendable ground. i.e. do not make claims that you cannot back up Speakers will sometimes make claims that are not necessary for the actual talk but will irritate members of the audience, particulary senior people. I think students who "parrot" lines they have learnt from their advisor about justification for their work. A random set of sample claims which you may hear variants of include: -our results will allow the design of new materials -silicon based electronics has no future -density functional theory cannot described electronic correlation effects -molecular dynamics simulations of bi

How big a Hilbert space do I need to describe the electronic properties of a molecule? Specifically, suppose in the molecule there are N valence electrons. One must decide then how many spatial orbitals are required and how many Slater determinants? This issue is brought out in this review paper . Benzene is an illuminating case. McWeeny shows that a "brute force" approach based on molecular orbital theory requires hundreds and sometimes thousands of Slater determinants to obtain results of comparable accuracy to that obtained with a valence bond description.The latter uses just six localised orbitals and two Slater determinants (corresponding to the two Kekule structures). Why does this matter? First, the priority of chemical insight favours the valence bond description over the "black box" approach embodied in the molecular orbital theory approach. The issues are described nicely in this Nature paper , which emphasises that the delocalised molecular orbitals ar

Articles from Nature , ScienceInsider , and The Age newspaper summarise recent problems concerning the strained relationship between the board (chaired by a corporate lawyer) and scientists at the Australian Synchrotron . The International scientific advisory committee has resigned in protest.

It does not take long in science to get a negative referee report that makes ones blood boil. However, as frustrating (and silly) as some reports are I think we can gain a lot by reading them carefully and reflecting on why the referee expressed the view they do. This was brought home to me recently when within a week I had two papers outright rejected. As painful as it was to acknowledge I can now see there is some basis for some of the referees criticisms. I still claim that in both papers the science was both valid and important. However, I now see that the way the papers were written that a quick reading (which I do not begrudge since I do it too) could frustrate a referee and lead to a negative report. So I am now rewriting both papers. I think the end result will be better papers. So, try and put your shoes in the referee [wow what a Freudian slip! ] I mean put yourself in the shoes of the referee and see if you can see what they said and why.

My family continues to enjoy watching the TV show The Big Bang Theory . In the episode we watched tonight Sheldon unsuccessfully tried to give Nobel Laureate George Smoot (appearing as himself) a copy of his latest paper at a conference. Non-equilibrium Green's functions also got a mention, probably the first (and only time) in the history of prime time TV!

I had some great discussions today at ANU with Cedric Simenel and David Hinde about decoherence in nuclear collisions. One of the key issues became clearer to me. Suppose a projectile nucleus in its ground state |P> collides with a target nucleus in its ground state |T>. After the collision one observes the projectile to be in state |P> with probability |a|^2 and in state |P*> with probability |b|^2. Simple scattering theory would say that the state of the whole system is |Psi> = a |P>|T> + b |P*>|T*> and the reduced density matrix for P has non-zero off-diagonal terms which only disappear after the measurement is made by the detectors. However, I suspect that if the nuclei are large enough (i.e., have enough internal degrees of freedom) then the collision itself will decohere the superposition. So, which is the correct picture? Presumably there is a "quantum-classical" crossover as the nuclei get heavier? Are there smoking gun experiments (e.g.,

Heavy nuclei are complex quantum many-body systems with many degrees of freedom. The observation of deep inelastic collisions (DHIC) in the 1970's led to the notion of friction in nuclear physics. This concept can be used to describe the transfer of energy from the relative motion of the nuclei to the internal degrees of freedom. I am eager to quantify this friction because it will also enable us to say something about the role of decoherence in nuclear physics. Today at ANU, Cedric Simenel brought to my attention a recent paper that is relevant. For several nuclear collisions the position dependent friction was recently evaluated from the Dissipative Dynamics version of TDHF (Time-dependent Hartee-Fock) theory. It was found the friction monotonically increases with decreasing inter-nuclear separation, with a value of about 10^{21} s^{-1} at the barrier. For the 40Ca + 40Ca reaction at a centre of mass energy of 100 MeV, about 10 MeV of energy is converted to internal excitation

This week I am in Canberra at ANU working with nuclear physicists on quantum decoherence in nuclear collisions. Discussions today inspired the previous post. But, it is ironic that I am quoting Zurek because he is at UQ right now, and giving a seminar tomorrow.

How long does it take for Schrodingers cat to live or die? For a quantum system interacting with an environment (containing many degrees of freedom) what causes decoherence? What sets the time scale on which it occurs? It turns out the decoherence and dissipation are intimately connected. Decoherence arises from fluctuations in the phase of the system quantum state due to its interaction with the environment. An important calculation was performed by Zurek in the 1980's and elegantly summarised in an expanded and updated version of his famous Physics Today article from 1991). Consider a free particle which is in a superposition state consisting of two Gaussian wave packets a distance Delta x apart. Let gamma be the damping rate associated with friction from interaction with the environment. Then the decoherence time (i.e., the rate at which the off-diagonal parts of the density matrix decay) is given by Thus we see that the same physics that causes friction (energy flow from the

In my talk on academic career advice I mentioned the importance of mental health issues, which struck a chord with many. The anecdotal evidence is that this is a significant problem among people in academia at all career stages, but particularly among graduate students. Academics (on average) tend to be highly gifted, driven, creative, critical, introspective, and sensitive. This makes them more vulnerable to mental illness, particularly depression, than the average person. Here are just a few prominent examples, at the more extreme end of the spectrum. John Nash ( A Beautiful Mind ) was a young faculty member at MIT when he was afflicted with schizophrenia. He never returned to any form of employment. In 1994, he received the Nobel Prize in Economics for work laying founds in game theory, completed in his Princeton Ph.D in mathematics. Ludwig Boltzmann and Paul Ehrenfest both suffered from depression and committed suicide. In 2007, the Times Higher Education Guide listed Michel

Yesterday John Clarke (University of California, Berkeley) gave a nice colloquium at UQ on Applications of SQUID s (Superconducting Quantum Intereference Devices). Clarke is arguably the "father" of the development of SQUIDs in both science and technology. Many of the people currently leading the development of superconducting qubits were at one time his students or postdocs. Tim Duty is to be thanked for bringing Clarke to UQ to give this fascinating talk. The two key physical effects on which the SQUID is based are Josephson tunneling and magnetic flux quantisation. In a DC SQUID the I-V curve is modulated by magnetic flux, and so the device is basically a flux to voltage transducer with noise that can approach the quantum limit. They can detect magnetic fields as small as a femtotesla! Roger Highfield's book, The Science of Harry Potter describes how the sorting hat that Harry used in the first book is based on SQUIDs. [When I told my son that, he said this is not c

Irina Bariakhtar, webmaster for the Division of Condensed Matter Physics of the American Physical Society has established a gallery of images that are fascinating and may be useful for talks and teaching. Some of these images are used in a 2010 calendar .

Following up on my previous post on impedance spectroscopy I came across two helpful reviews. The first is a "Colloquium" article by D.L. Sidebottom in a recent Reviews of Modern Physics. It shows how in glasses which have conducting ions the rms motion of the ions can be extracted directly from the frequency dependence of the conductivity. Understanding ion motion in disordered solids from impedance spectroscopy scaling The second article is a tutorial review applying impedance spectroscopy to conducting polymers, especially when they are used as electrodes in biomedical applications

If you are applying for a job: read the advertisement very carefully and note answers to the following questions what sort of person are they looking for? is the job to work -with a specific person, with a specific group, or to work independently? - on a specific project? - in a specific research area? is the job supported by a specific grant or program? After you have gleaned answers to these questions answer: Do I want this specific job? Would should they be specifically interested in me? Then if you apply make sure your cover letter specifically tells them why you want THIS specific job and why they should interview specifically YOU.

An important step in modelling complex materials is writing down effective Hamiltonians which capture the essential physics. One approach to estimating model parameters for a specific material is to calculate them using methods from electronic structure theory. About ten years ago, following earlier work by Kino and Fukuyama, I wrote a review arguing that the relevant Hamiltonian for the kappa-(BEDT-TTF)2X family of superconducting organic charge transfer salts was a Hubbard model on the anisotropic triangular lattice at half filling. A recent PRL by a group from Goethe Universitat Frankfurt is of particular interest to me because it describes state-of-the-art calculations based on density functional theory. For three different anions X and two different pressures, the authors calculate the parameters t and t' which define the band structure. Figure 4 from the paper gives a nice summary of the results. [Similar results were obtained at the same time by a group in Japan and report

They are the spin-1/2 quasi-particle excitations associated with a spin liquid ground state of a quantum antiferromagnet. Perhaps the easiest way to understand them is in contrast to the low-lying excitations in an antiferromagnet with an ordered ground state. Spontaneously broken symmetry is the key concept behind understanding the nature of these excitations. Specifically, for infinite systems the ground state is usually degenerate and is not invariant under the samesymmetries as the system Hamiltonian. This family of ground states is described by an `` order parameter '' which describes the extent of the symmetry breaking. For example, quantum antiferromagnets can be described by a Heisenberg model Hamiltonian which describes a lattice of spins which interact with their nearest (and sometimes next-nearest) neigbours on the lattice. The model Hamiltonian is invariant under rotations of all the spins and under lattice translations. However, both these symmetries can be brok

A good (and painful) question to ask when evaluating research, both our own and others, is: What does the scientific community know now that we did not know when you began the research? There is a similar probing question to ask yourself before you start a project (or a new sub-project). Suppose everything goes as well as can be hoped (i.e., you are able to complete the calculation, do the measurement, get the new technique to work, or make the compound). Then will you be able to say something new? If not, is it worth even trying? The before question is a good one for both students and supervisors to contemplate. It is too easy for supervisors (including me) to say do this extra calculation (or make this extra compound and measure all its properties) without considering enough the time cost to the student or postdoc.

Hopefully, the title got your attention. This is mostly directed at people applying for postdocs. The cover letter is key. I believe most postdoc (and many faculty) applications live or die [i.e., get to the long short list] based on the quality of the cover letter. You need to specifically answer the following specific questions: Why are you interested in this specific job? Why are you interested in this specific research? Why should they hire specifically you for this specific job? Most cover letters I receive are generic. People tend to write the same letter for every job they apply for. Furthermore, the research achievements and research interests they list are usually generic. So to be specific ! Write something like: "One of the scientific questions I am most interested in is "What is the physical mechanism for XXXX in material YYY? I recently read your nice paper "blah blah" in journal YY and I have been wondering if a similar approach mi

Here are the slides of the fascinating talk that Joel Gilmore gave at the Careers session organised by the Australian Institute of Physics (Qld branch) last wednesday.

Consider a material in which there are two distinct charge carriers (e.g., electrons and protons or electrons and oxygen vacancies). A measurement of the conductivity of a sample will just yield the sum of the conductivities of the two individual charge carriers. Given that the physical conduction mechanism for the two carriers may be distinctly different (e.g., small polaron hopping vs. vacancy diffusion) the temperature, pressure, and composition dependence of the two components may be completely different. Is there a way to extract for each of the carriers the conductivity, density of carriers, and mobility? A few weeks ago I thought this was hopeless, but I was wrong. So my favourite paper for this week has the weighty title Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria by Wei Lai and Sossina Haile from Caltech. The paper is in a journal I have never looked at before, Journal of the American Ceramic Society (n.

Understanding lattice models for strongly correlated electron systems is a major challenge. Widely studied (and still poorly understood) models include the Hubbard and Heisenberg models. But some insight can be gained from studying model Hamiltonians on small clusters such as four lattice sites. Although, such a small system is far from the thermodynamic limit, these models can illustrate some of the essential physics associated with the interplay of strong electronic correlations, frustration, and quantum fluctuations. They illustrate the quantum numbers of important low-lying quantum states, the dominant short-range correlations, and how frustration changes the competition between these states. Furthermore, understanding these small clusters is a pre -requisite for cluster extensions of dynamical mean-field theory and rotationally invariant slave boson mean-field theory which describes band selective and momentum space selective Mott transitions. See for example my earlier pos

My wife brought to my attention an article in the New York Times about the consequences of California's budget woes for the University of California system, and especially Berkeley. People interviewed include Bob Birgeneau (famous for inelastic neutron scattering studies of strongly correlated electron materials, now Chancellor at Berkeley) and Richard Mathies (pioneer in femtosecond spectroscopy, now Dean of the College of Chemistry, Berkeley)

Dr Richard Walding gave some insights into the current state of high school Physics teaching in Queensland at the AIP Careers seminar on wednesday. He was Head of Science for the past 20 years at Moreton Bay College and now tutors Senior Physics student teachers at Griffith Uni. Richard said tha the demand for physics teachers was very strong, not only here in Queensland but everywhere in the word. Teacher recruiting companies have an unmet demand for physics teachers and it would seem they can place you at countries all over the world. Richard said the starting wage for a graduate was about $51,000 rising to $72,000 after 7 years or so. He said there were 7000 Senior Physics students in Queensland in Yr 11 and 12 from 176 schools and taught by about 220 physics teachers. Most of the teachers had BSc degrees but, surprisingly enough, only a handful majored in physics at university . To a question about whether you'd have to teach other subjects beside Physics and Jun

More great quotes from Bob Laughlin , A Different Universe: Reinventing Physics from the Bottom Down “The great power of science is its ability, through brutal objectivity, to reveal to us truth we did not anticipate.” (p. xvi) ``mythologies are immensely powerful things, and sometimes we humans go to enormous lengths to see the world as we think it should be, even when the evidence says we are mistaken.’’ (p. 114) “ideologies preclude discovery. All of us see the world as we wish it were rather than as it actually is.” (p. 116). There are similarities to the cautions of Walter Kauzmann, in his Reminiscences of a Life in Protein Chemistry.

Here is a copy of the talk I gave tonight on academic careers. Some of the feedback included: * the importance of mental health issues (I will try and do a few future posts on this). * dogged perseverance is often a key component to success It would be good to get some discussion going on some of the issues I raise in the talk.

Self-esteem, stress, and depression among graduate students. Kreger DW . Wright Institute, Berkeley, CA 94704, USA. Psychological Reports 1995 Feb;76(1):345-6. In a study of 29 graduate students, self-ratings of stress correlated with low scores on self-esteem but were not related to an objective indicator of actual stress . Both self-rated stress and low self-esteem scores were related to scores on depression, with a weak interaction effect.

Tomorrow evening I am giving a talk on career advice for people who want to pursue a career in scientific research for a careers night of the local branch of the Australian Institute of Physics. Currently, points I am planning to make include: There is more to life than a research career. Be realistic and consider alternative careers. Learn to write, to get along with other people, .... plus previous career advice I have posted, especially this advice to Ph.D students.

I am getting tired of hearing talks and reading reports which state, "The first Bose-Einstein condensate was observed in 1995." I think a more accurate statement would be "The first BEC in a dilute atomic gas was observed in 1995." Many would argue that superfluid 4He is a BEC. This new phase of matter was first observed around 1930. In 1932 Fritz London proposed that this was a BEC. (BTW, this is the same London as in the Heitler-London wave function, the London penetration depth, and London dispersion forces...). But it should be noted that the case of a BEC in superfluid He is not as clear cut as in dilute atomic gases. Nevertheless, I dont think these subtleties validate ignoring 80 years of research on superfluid helium. A very useful summary of the history and the associated physical issues is contained in this nice article by Sebastian Balibar. I think that people who are supervising Ph.D students on BEC's should be familiar with these issues, make sure

Just how efficient are biomolecular systems? For a long time it was claimed that fireflies had a quantum efficiency close to 100 per cent. However, it was recently found this is not the case, the efficiency being about 40%. A nice summary of the work is in this News and Views article in Nature Photonics last year. Besides measuring the quantum efficiency Ando et al. find that there are three components to the light emission and all are pH dependent. One component is of unknown origin. Clearly there is a correlation between the colour of the emission and the protonation state of the chromophore. Only in a 2006 Nature paper was a structural basis for two different emission states proposed. I am curious as to how much quantum chemistry has been done on these issues. This may help address questions such as: What determines the quantum efficiency of emission? How are non-radiative decay channels suppressed? What role does the protein environment play?

I just went to a session in the department where 6 new staff members each had 5 minutes to introduce themselves and their research. If you have such an opportunity I would advise trying to just answer the following questions. What is the scientific question you want to answer in the next few years? Why is this important? Why are you excited about it?

Americans are known for scientific prowess but not their knowledge of geography! A colleague recently told me that he now begins all his talks overseas with a map of Australia and shows where Brisbane is. On my last trip to Europe I also did this. However, it took me a while to find the pictures I really wanted, those which I had seen on postcards. I eventually found them and will include one in all my future talks overseas. They really put the size of Australia in perspective.

Subir Sachdev has a Physics article which provides a background to recent work using tensor networks (inspired by quantum information theory) to find the ground state of quantum spin lattice models. I really like the following succinct summary of the problem: The simplest of these problems involve only the spin operators S i of electrons residing on the sites, i , of a regular lattice. Each electron can have its spin oriented either up or down, leading to a Hilbert space of 2 N states, on a lattice of N sites. On this space acts the Heisenberg Hamiltonian H = ∑ i < j J i j S i ⋅ S j , (1) where the J i j are a set of short-range exchange interactions, the strongest of which have J i j > 0 , i.e., are antiferromagnetic. We would like to map the ground-state phase diagram of H as a function of the J i j for a variety of lattices in the limit of N → ∞ . Note that we are not interested in obtaining the exact wave function of the ground state: this is a hopeless task in dim

This follows up with the earlier post about a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, which been accepted for publication in Journal of Chemical Physics. An important issue is after a organic molecule absorbs a photon what conformational change will occur. Below are several options involving bond twists. A second issue is how the charge distribution in the molecule changes upon twisting. This kind of physics is at the heart of how your eye works. When retinal absorbs a photon it undergoes a conformational change which produces a charge separation which eventually leads to an electrical signal in your brain. It is also at the heart of designing better organic solar cells. We considered a model Hamiltonian for a large class of dyes. The figure below shows contour plots for the first excited state potential energy surface for several parameter values. The lower pa

I have not read the book but found the title very helpful and challenging!

If you were going to an isolated island to do computational chemistry and you could take this year’s computers and 10-year-old algorithms or this year’s algorithms and 10-year-old computers, which would choose to take? This is a question that Donald Truhlar asks in a JACS Editorial for a Select issue of 23 papers on Molecular Modeling of Complex Chemical Systems. How would you answer the question? You can look in the article to see how most computational chemists would answer the question. The article is a very nice read to a physicist because it provides a very helpful and concise summary of historical landmarks in the computational modeling of large chemical systems. However, I disagree and am concerned with one of the opening statements in the article: Almost all modern theoretical chemistry is computational chemistry, because most of the progress that can be made with pencil and paper without a computer has been already made. Computations on complex systems are, in my o

In 1998 Max Tegmark wrote a paper with the great title, The Interpretation of Quantum Mechanics: Many worlds or many words . He suggested that the validity of different interpretations of quantum theory cannot be decided empirically, but are: “purely a matter of taste, roughly equivalent to whether or not one believes mathematical language or human language to be more fundamental.” But it is interesting that he is now writing articles about multiverses...

In 1983 Austria introduced this bank note featuring Erwin Schroedinger. Note the Psi! But where is the cat? It is interesting they also introduced a 50 Schilling note with Sigmund Freud and a 5000 Schilling note for Mozart. Is this a relative measure of their contributions to culture and society?

I am very happy that a paper, Conical Intersections, Charge Localization and Photoisomerization Pathway Selection in a Minimal Model of a Degenerate Monomethine Dye by Seth Olsen and I, has been accepted for publication in Journal of Chemical Physics. A key question concerning optically active molecules is what is dynamics of the excited states? Specifically, what are the predominant non-radiative decay mechanisms. The schematic below shows several options for the energy of the potential energy surfaces versus some configurational co-ordinate. On the left both S1 and S2 excited states decay to a conical intersection with the ground state. In contrast, on the right they have distinctly different decay paths. But how does one go beyond such schematics. It turns out that for a broad class of dyes one can justify from high level quantum chemistry calculations a description in terms of just three valence bond states (see below). The description in terms of the three diabatic states allow

I am thinking more about photophysical properties of ketocyanine dyes. I have puzzled through the basics of the molecular orbitals. Some material on this site is useful, including a visualisation of the molecular orbitals of formaldehyde. A key point is that the HOMO is a non-bonding orbital centred on the O atom and lying perpendicual to the C=O bond. Thinking in the alternative resonating valence bond picture there will be three alternative Lewis structures O || C -R | L O- | C -R | L+ O- | C -R+ | L The extent to which the lower two structures contribute will increase the C-O bond length and reduce the C-O stretch frequency. It should be possible to describe the low lying excited states in terms of a complete active space with 4 electrons in 3 orbitals (a pi* orbital on the C=O bridge, and a pi

P. W. Anderson , “ Emergence, Reductionism and the Seamless Web: When and Why Is Science Right ,” Current Science 78:6 (2000), 1. [Based on the Pagels lecture, Aspen, 1999]. Anderson suggests that emergence is the mechanism for consilience (the unifying of disparate pieces of knowledge) and reduction is the evidence for it. Theories may be under-determined, i.e., there may be many possible theories that can explain what is actually known. Hence, a successful theory may not actually correspond to what is happening. If there are only a few constraints (hypotheses, observations) that a theory must satisfy it has sometimes been the case that more than one theory can satisfy the constraints. However, as the number of constraints increases, acceptance of a theory is more likely and it becomes hard to conceive of alternative theories that could satisfy these constraints. Reduction can greatly increase the number of conditions that a theory must satisfy. For example, any alternative to quantu

It is wonderful that last year the US Postal Service issued new stamps featuring four prominent scientists. Pauling and Bardeen were indeed masters in unravelling emergent phenomenon using quantum many-body theory. Update (2016). I just discovered that in 2005 there was one for Josiah Willard Gibbs   and Feynman.

Ketocyanine dyes are of considerable interest. An example is shown above. What are the essential ingredients that determine their photophysical properties? The figure below is from a nice paper which compares essential differences between cyanines (CY), ketocyanines (KCY), and squarenes (SQ). The difference between the upper and lower panels (A and B) relates [I think] to whether one has an even (A) or odd (B) number of p-electron centres on each of the molecular units on the left and right side of the central C=O bridge. Apparently, this is following a "composite molecule" approach in a book by Fabian and Hartmann. A complementary approach to describing optical properties of these materials is within a resonating valence bond approach. There will be three dominant resonant structures, similar to those advocated by Pauling for urea. [I will try and get a picture]. Such an approach will naturally lead to two low-lying singlet excited states. This type of resonance is of gre