seminar seminar seminar

seminar seminar seminar has moved!

2005-01-24T21:39:00.000-08:00

i've merged it with my old blog, caustic.soda. catch seminar summaries as and when i attend them and find the time to write them up on my new page.

[CMP] Kenneth S. Suslick, Conditions during Single Bubble Cavitation

2004-09-27T13:57:00.000-07:00

Prof. Suslick talked about single bubbles acoustic cavitation experiments. By appling a sinusoidal pressure wave of about 1.5 bar with period 20 microseconds, bubbles can be made to expand to 30 microns in diameter before collapsing to submicron diameters (within the resolution limit of the Mie scattering diameter). The bubbles collapse extremely quickly and exhibit several rebounds in size per period. Insterestingly, sonoluminescing bubbles do not rebound. Non-sonoluminescing bubbles also an approximately blackbody spectrum, which suggests (counterintuitively) that the bubbles are at thermodynamic equlibrium.

The experiment is highly reproducible, with a single bubble being able to last millions of cycles, the only major limiting factor being temperature drift. This allows for a constant buildup of chemical products. The amount of work done allows a tremendously large buildup of energy density, allowing extremely rapid high-energy chemistry to take place. at approximately 60% of the maximum diameter, the material in the bubble begin acqure sufficiently energy to dissociate bonds, with typical temperatures of 5000 K. At higher compression ratios, where the bubble is reduced to 20% of its original diameter, the temperature of 15,000 K begins to allow ionization.

The interfacial region is then critical to determining their properties, with one monolayer of liquid being fully capable of saturate a bubble with its vapour. It is interesting to note that sonoluminescence is observed but is quenched nearly completely by a nonvolatile organic molecules. Dissolved gases, solvent vapor and solute vapor also contribute to vapor formation, the least soluble gases eventually concentrate in the bubble. Polyatomics, having many degrees of freedom, generally limit the maximum achievable temperature before fragmentation and so are not used very much. In terms of energies, the work done on a bubble is 300 TeV whereas the energy released by sonoluminescence is 0.6 MeV; the balance of all this energy much therefore go into chemistry.

When water is a cavitated, the dissolved air in it reacts to form nitrites and nitrogen oxides, with a phenol leaving group. The most sensitive conditions of measurement, i.e. minimum volume, minimum temperature and fluorescence translate into a detection limit of 10 nM over 100,000 cycles. The amount of products forms agrees with theoretical Lohse predictions of 10⁷ molecules per cycle, as compared to 4 x 10⁶ from experimental data.

To maximize the energy in any given bubble, the use of polyatomics and other complex molecules is to be minimzed, but no so much that the sonolysis products mix. Reducing the liquid vapor pressure lowers the temperture needed as well. Therefore highly polar liquids, ionic liquids, and aqueous electrolyte solutions are of interest. The last is of especial interest because the colligative properties can be tuned easily. In particular, solutions of sulfuric acid show extremely intense SBSL (single bubble sonoluminescence), the output peaks at 85% sulfuric acid solution and is strong enough to be seen in ambient lighting conditions. Although aqueous solutions are limited to pressures of 1.4 bar due to fragmentation, Xe emission can be seen at much lower pressures that in pure water. Also, emission from dioxygenyl [O₂⁺] was reported for the first time ever.

SBSL in 85% sulfuric acid has an extremely rich spectrum for many dissolved gases. The spectra for dissolved noble gases show an interesting trend. For dissolved xenon, a continuum is observed, together with xenon and oxygenyl lines. For dissolve argon, both the continuum and argon emission lines are seen. The continuum disappears for dissolved neon but in addition to neon lines, sulfoxyl [SO] emission is also observed. For helium, both sulfoxyl and sulphur dioxide lines are seen, being strong evidence of decomposition of the solvent. However, the temperatures suggested from all the species are inconsistent.

Prof. Suslick also mentioned multibubble (about a million bubbles or so) cavitation, using ultrasonic cell destruction from a piezoelectric transducer. This technique is already used commercially to purify coal gob, separating coal from clay particles. Temperature estimates were made for the spectra from multibubble sonoluminescence of various metal carbonyls, which decompose easily to produce atomic species in the bubble. The spectral intensity fits for Cr, Mo, C₂ and Fe disagreed with each other, statistically speaking, while estimating the temperature to be approximately 5000 K. On the other hand, all the spectra agreed to the same pressure broadening fits, namely that the effective pressure felt by the gas was 300 atm. Given that the transducer was operating at 1 GHz, this corresponds to heating/cooling rates well in excess of 10¹⁰K/s.

Returning to the problem of temperature measurement, three thermometers can be deduced from the SBSL spectra. The continuum could be fit a to a black body to determine the effective temperature. Alternatively, the fine structures of argon atomic lines and sulfonyl vibronic lines are capable of furnishing their own temperature estimates. One important point to note is that these molecules are unable to measure temperatures above their own dissociation temperatures, which are 15 000 K for [O₂⁺] and CN, 10 000 K for C₂, 5 000 K for SO and 3 000 K for SO₂. All in all, the blackbody data and argon emission lines disagree by 40%. Strangely enough, the apparent temperature was inversely proportional to the thermal conductivity of the medium, a strange and unexplained result. The spectra data are also inconsistent. While SO and SO₂ emission imply temperatures of below 4 000 K (0.25 eV), the Ne lines show energies of 20 eV! The only conclusion is that the emission lines are from different spatial points. Also, [O₂⁺] must have been generated from a non-thermal process as the oxygen molecule will fragment long before it will ionize if thermally excited. The blackbody fit of the continuum also only gives a temperature of 17 000 K, which is approximately the temperature of the shockwave generated by a thermonuclear fireball.

Prof. Suslick then proposed a model for the structure of sonoluminescing bubbles to explain the inconsistencies. By theorizing that a sonoluminescing bubble contains a light-emitting shell surrounding an opaque plasma core, much like a star, this allows for different regions to have different processes giving rise to different temperatures. According to experiments at Los Alamos National Laboratory, temperatures could reach as high as 1 000 000 K, similar to that in a star.

The very high core temperature led some researches to speculate that bubbles generated from acoustic cavitation could be used for thermonuclear fusion. Taleyarkhan at Oak Ridge National Laboratories has reportedly seen neutron emission from cavitating liquids. The evidence for fusion, though, is hotly contested by Saltmarsh and Shapira. The conditions reported were for a 1 064 Nd:YAG line generating 3 ns pulses of 5 mJ each and at cavitation pressure amplitudes of 20 atm. The laser-initiated cavitation stems from nucleation off dirt particles, which Taleyarkhan et. al. claim to be reaction sites for nuclei and neutrons.

Prof. Suslick and collaborators have tried to replicate the results up to 31 atm of cavitation pressure and using the most sensitive neutron detectors available, but were unable to see any evidence for fusion. Prof. Suslick thus remained skeptical about the prospect of "bubble fusion" and he didn't think that "Chain Reaction - The Movie" will happen at Oak Ridge. When asked for his opinion of the fusion data, he also said "I wouldn't trust Taleyarkhan to fix my car."

[Analytical] Joel M. Harris, Raman Spectroscopy of Liquid/Solid Interfaces and Dispersed Particles

2004-09-27T13:08:00.000-07:00

Thursday, September 9, 2004: 8 pm at 116 Roger Adam Laboratory

Prof. Harris's talk centered around using Raman spectroscopy to study the dielectric properties of adsorbent surfaces, particularly the processes involved in preconcentration, surface interactions, and environmental transport. Controlling the extent of binding of molecules to surfaces allows the control of wetting and reactivity of solid surfaces, which have implications for combinatorial synthesis, liquid chromatography, and the development of immobile analytical reagents. Chemical reactions on surfaces are of interest in understanding surface-binding of ligands, and the development of fiber-optics sol-gel sensors and nanostructured materials. At these interfaces, the species present as well as the rates of reactions need to be quantified.

Raman spectroscopy, while well suited to the task, employs nonlinear scattering processes with low optical cross-sections, about 2 x 10^-28 cm²; fluorescence, by comparison, has a cross-section of ~ 10^-14 cm². Several techniques have been developed to address the inherently lower efficiency of Raman scattering, such as substrates with increased surface areas such as porous silica or polystyrene. The exploitation of metal-plasmon resonances has led to the development of SERS. And on the small scale, high numerical aperture confocal microscopy has been successfully employed to perform Raman spectroscopy.

The synthesis of ion-metal binding chemistry and SERS spectroelectrochemistry has created a new analytical technique known as ion-metal SERS electrochemistry. 8-hydroxyquinoline (8HQ) is the common choice of ligand as it binds most metals and is needed only in milligram quantities. The binding of metals affects the rate of 8HQ tautomerization; the azo form binds metals while the hydrazone form is preferred under acidic conditions. The tautomerization can be monitored quantitatively by SERS.

Accurate quantitation, however, requires accounting for local acitivity due to interface effects by introducing the Boltzmann factor corresponding to the surface potential relative to its zeta-potential. The charged interface generates the well-known electric double layer, which generates a large local concentration of products, but this can be controlled by changing the metal surface; however, this removal of charge buildup also reduces the capacitance of the surface.

The extent of this electric double-layer can be studied with self-assembled monolayers (SAM) containing pendant nitrile groups, which can be monitored using the vibrational Stark effect. This is a local electric field effect whereby the nitrile dipole is strengthened when placed in an external field, causing it to stiffen and exhibit a higher vibrational frequency. Experiments with alkanethiols with pendant nitriles on a silver surface were carried out. The potential is increased to improve ion activity in the double layer. The field was found to drop almost linearly in the SAM region immediately above the electrode to reach 3 V nm^-1 at the SAM-liquid interface, whereby beyond that the field decays with a Debye length of 0.3 nm to a constant long-range value. However, in low ioinic strength solutions, the Debye length is greatly increased so the electric field effect is lower. The results of the SAM experiment shows that continuum theory breaks down at distance scales comparable to typical hydrated ionic radii, ~0.4 nm. Discrete ion-size effects play an important role.

SERS depends on surface selection rules, which are applied to the spectrum of SAM-thiol on silica. Interestingly, the 8HQ tautomers have vastly different dipole moments, with the hydrozone having a 5.8 Debye while the aza form has only a 0.8 D moment. This implies that adjusting the applied electric field can tune the conformationm of 8HQ and therefore its activity.

The next part of Prof. Harris's talk was on the analytical chemistry of dispersed particles. The need for such technology was highlighted, with applications in chemical preparation of organic compounds in water, suspension solid delivery of organic compounds such as drugs, monitoring the environmental impact of organic/inorganic colloids, and also studying biochemical systems such as cells, bacteria and vesicles. The study of these particles pose many measurement challenges, as simply knowing the size and shaped of particles is insufficient. Identifying the chemical species present require optical methods. Importantly, material is usually bound to a single phase and does not partition well between phases, and therefore single-particle analyses are often needed, particularly when it comes to studying the chemistyr of suspensed particles.

The major limitation of optical methods is in the fast diffusion rate of particles in suspension. Happily, this can be circumvented by optical trapping using focussed lasers on the size scale of 1 micron or so, even with low power lasers on the order of just a few milliwatts. The optical tweezer effect makes use of the momentum change of light as it refracts through and out of an optically transpoarent particles. This makes the particle behave as a lens, with the change in optical index related to its polarizability. The concept of optical trapping is analogous to a ping-pong ball trapped in an air stream generated from a blow dryer.

Immobilized reagents for biochemical studies are now an important class of analysis reagents. For example, aminated silanol surfaces reacted with FMOC can leave amino acid residues bonded to a silanol oxygen. Such a surface is particularly useful for phospholipids, which have no multilamellar structure that many protein exhinite. SERS on immobilized aminao acids can be used to study the rate of anion exchange and other leakages in phosphololipids.

[CMP] Philip W. Phillips - The Elusive Bose Metal

2004-09-11T15:26:00.000-07:00

Thursday, September 9, 2004: 4 pm at 151 Loomis Laboratory

Phase diagram showing the destruction of superconductivity: 1) The yellow region represents the ordered phase in which all the electron pairs share the same phase (all arrows pointing up), 2) The elusive Bose metal is in blue in which all the phases are disordered but form a glass, and 3) Beyond the electron pairs fall apart and form an insulator. The vertical axis represents temperature and the in-plane axes any of the tuning parameters that destroy superconductivity such as defects or an external magnetic field. From UIUC News Bureau.

Today's talk was largely based on Prof. Phillips' ex-student's work, Denis Dalidovich on investigating the possibility of a bosonic condensate possessing a finite resistance at absolute zero. The conventional theory results in the two-dimensional localization principle, stating that when noninteracting electrons interact with random scatterers, they always return to their origin. This implies that electrons are not free under such circumstances, but are bound to some potential. Therefore, a metallic state in two dimensions and below is impossible. However, this idea was refuted with the discovery of so-called novel metallic states that contain both strongly correlated electrons as well as disorder sites. Cooper pairs of electrons can behave as bosonic particles and condense to have a well-defined phase. Two well defined eigenstates of such a Bose-Einstein condensate are known, with zero and infinite conductivity respectively.

The breakup of the condensate can be achieved either by pair-breaking (decreasing the amplitude of the wavefunction) or by phase fluctuations (destroying the coherence over the entire state). The latter may be achieved simply by exploiting the uncertainty relationship between phase and particle number. This duality results in the two extreme cases of insulator, with well-defined particle number; and semiconductor, with well-defined phase due to entanglement of many number configurations. The insulating state is finitely correlated and hence has a nonvanishing mass associated with the state; in contrast, the superconducting state is completely uncorrelated and hence has a massless mediating particle.

The so-called phase-only model of the superconductor Hamiltonian contains a term which is the product of the charging energy and sum of squares of the conjugate momentum to the phase, and another term that is the product of the Josephson coupling strength and a cosine term describing phase change due to boson hopping. The latter term shows that phase locking maximizes the fluctuation in the energy due to Josephson coupling.

The insulating and superconducting states are diametrically opposed and therefore a phase transition can be observed between them. A particularly neat experimental demonstration is by starting with a perfectly aligned optical lattice created in an intense laser beam. Decreasing the laser intensity lowers the barrier height and therefore increases the probability of trapped escaping from the electromagnetic well. When the laser beam is sufficiently diffuse, the particles then behave as a superfluid.

The onset of criticality in a bosonic system is seen as a lack of change in some quantity of interest as a function of some tuning parameter, say g. For example, a transition to superconductivity is seen by a sudden drop in resistivity as a function of decreasing grain boundary size as a sign if lower disorder (as well as being a function of temperature). The standard theory states that below the critical value, g < g_c, the resistivity vanishes to give a superconductor. Above the critical value, g > g_c, the resistivity is infinite and an insulating state is observed. Also, the resistivity diverges to either 0 or ∞ as the temperature tends to zero, except as the critical point g = g_c, where the resistivity tends to a finite value of ρ_c = h/4e² = 6.5 kΩ. (Note that the charge carriers are electron pairs, hence the charge of 2e.)

Prof. Phillips is of the opinion that the critical resistivity ρ_c is not a universal quantity, and cites experimental data taken of critical resistivities of Bi(Ge), Al(Ge), Pb(Ge) and Mo(Ge), i.e. various metal layers deposited on top of a germanium substrate. A large scatter was observed. In addition experimental data with magnetic systems also suggest that there are many g values that lead to ρ leveling off at and below 2K. In fact, there are cross-over points due to intersection of curves plotted at different temperatures, and also hysteresis effects, both of which are totally forbidden by the standard theory.

The data therefore point to the presence of two critical points between the superconducting and insulating states, with an intermediate phase that is stable to disorder and has intact Cooper pairs. Heuristically, it must also have a finite resistivity is seen to have a power-law dependency on proximity to the superconducting state. This implies the existence of some scattering mechanism, i.e. dissipation, and therefore transport properties near critical points become important. In fact, it could be shown that conductivity must be a monotonic function of frequency divided by temperature.

A toy system of Josephson junctions and superconducting rings was considered. Semiclassical theory gives the conductivity as the product of the number of scattering centers and the mean lifetime between collisions. Quasiparticle excitation of the order parameter regularize the conductivity in this case and cancel out the effect of scattering, but the cancellation is destroyed with a slight perturbation, quickly forming an insulating state. This does not occur because of disorder parameters, describing a random distribution of orientations as well as Josephson coupling magnitudes.

Consideration of disorder leads to the random spin model, which exhibits frustration. This generates a new glassy phase, called a phase glass, that exhibits a non-collinear sum of orientations. This model is characterized by the Edwards-Anderson order parameter, with a correlation function that contains a long-range interaction term in τ^-2 describing dissipation (due to the Fourier transform). Such spin glasses have many degenerate states, with many low-lying excitation states that can lead to dissipation. This gives rise to system with superconducting, metallic and insulating phases, and a theory built upon such a model predicts the possibility of finite conductances.

A point of consideration is whether rotating the phase changes the conductivity. It is possible that the stiffness of the phase results in symmetry breaking when shifted, generating massless modes. However this turns out not to be the case, as any phase rotation velocity produces diffusive modes. Stiffness is a transient quantity, with contributions from twisting and limited range of allowable phases. Such stiffness leads to well-confined phases. On the other hand, if all of phase space is allowed, the resistivity vanishes because glasses of this nature have no stiffness.

When an external magnetic field is applied to a superconductor with intrinsic disorder, a vortex glass is produced due to entanglement of magnetic flux lines through the material. The cosine term describing boson hopping then acquires a random gauge term in its argument, giving rise to fluctuations in the Josephson junction coupling strength (by the cosine addition formula). The question is then whether a vortex glass can be a superconductor. If so, a nonlinear IV curve (non-Ohmic behavior) should be obtained below the glass transition temperature, but this is not observed. The reason is due to untrimmed, pointlike defects, as demonstrated by completely Ohmic behavior of yttrium barium cuprates. This suggests that a vortex glass is a manifestation of a Bose metal.

Plotting normal v. critical resistivities shows that they are equal and therefore suggests that the second transition results from the breakup of electron pairs. This follows from the Bardeen relationship, which states that the magnetic field applied associated with breaking up the pairs is proportional to the energy bandgap between the paired and unpaired states.

[PChem] James E. Patterson, Ultrafast Molecular Dynamics at a Shock-Compressed Metal-Liquid Interface

2004-09-10T15:47:00.000-07:00

Thursday, September 9. 2004: 2pm at A410 CLSL

The idea behind today's talk is to pulse a metal coated with some adsorbate with ultrafast laser pulses, so as to create mechanical shock waves that propagate through the adsorbate. The advantage of using lasers is the very short (< 4ps) rise time and moderate compression density changes (~10 - 20%) achievable, which results in compression pressures well in the nonlinear response regime. These are great advantages over traditional methods such as gas-driven pulse generators. The short rise time makes this technique particularly suitable for investigating the response of thin (nm scale) layers to the shock front.

The molecules chosen for this study were n-pentadecanethiol (PDT), n-octadecanethiold (ODT) and benzenemethanethiol (BMT) as these form well-defined, close-packed self-assembled monolayers (SAM) of ~2 nm easily on gold surfaces. The most interesting comparison was between PDT and ODT because the parity difference (odd- vs. even-numbered carbon chain) leads to differing interfacial properties due to different orientations of the methyl terminus..

Vibrational sum-frequency generation was used to probe the conformational changes, if any, that resulted from the compression of the monolayer surface during the shock wave propagation. One of the frequencies of laser light was chosen to lie in the infrared, resonant with a vibrational excitation. The other frequency was chosen to be red light, so that the sum frequency would still be in the visible region. This is advantageous for detection purposes since visible-spectrum CCD detectors are far more sensitive than in the infrared region. SFG has a response that exponentially decays with a time constant of 0.7 ps.

Another advantage of SFG is that the methylene (CH₂) units, being locally centrosymmetric, do not contribute to the hyperpolarizability tensor and therefore probing the CH stretching region gives data only on the terminal methyl group. The specific frequencies of interest are 2877 cm^-1 (symmetric stretch), 2937 cm^-1 (Fermi resonance between symmetric stretch and bending mode) and 2964 cm^-1 (antisymmetric stretch). An important difference is that the intensity of the asymmetric stretch relative to the symmetric stretch is higher in PDT than in ODT.

The components of the hyperpolarizability tensor can be calculated for the symmetric and asymmetric stretching modes as a function of tilt of the methyl group from the penultimate carbon symmetry axis. The symmetric stretch has maximum intensity at 0° and vanishes at 90°. The antisymmetric stretch has no intensity at 0° and goes through a maximum at around 60°. Therefore, the intensity ratios of these two stretching modes can be monitored to determine the orientation angle of the methyl terminus. This can be used then to distinguish between the trans and gauche conformers of the terminal ethyl group.

The SAM-coated gold plate was dipped into perdeuterated polyethylene glycol, which has no CH background signal and does not decompose under mechanical shock. 1500-2000 pulses were bombarded on the surface; the plate was translated each time because of the inherently destructive nature of this spectroscopy. This is because of plasma generation, which vaporizes the SAM locally about 500 ps after shock wave propagation. The plasma also luminesces, contributing to noise.

It was found that ODT showed viscoelastic response while PDT showed elastic response in this experimental domain. This is in stark constrast to hydrostatic pressure experiments, which show elastic behavior for both ODT and PDT. This means therefore that ODT undergoes a conformational change that is ultimately reversible. ODT also showed different time evolution of the signal for each of the three different modes being monitored. The data showed that PDT was almost completely found in the trans conformation while ODT had a significant population of gauche conformers. Also, no frequency shifts were observed, which meant that the decrease in SFG signal was unlikely to be due to other effects such as vibrational coherence dephasing.

An important factor to consider was whether the observed irreversibility effect in the subnanosecond regime could be simply a thermal effect. However, experiments conducted at elevated temperatues showed an increase in the methylene signal over time with increasing temperature, which was due to partial desorption allowing methylene rotation and local symmetry breaking. Since this was not observed in the laser pulse-driven compression experiments, this possibility was ruled out.

Computational studies were also carried out to model the compressive effects at the molecular level using Cerius² modeling software. Phase space plots of the terminal and penultimate methylene bond angles were computed for ODT and PDT. It was found that ODT can access a greater configuration space as the penultimate methylene bond can partially swivel, whereas the methylene unit in PDT is sterically confined. In fact, the 2-deep gauche configuration was found to be a likely deformation-preferred conformation.

An important limitation of the modeling process was the neglect of the gold surface entirely. Not only was the gold buckling process not modeled, the surface roughness of 4 nm was not modeled at all. Local rough spots could lead to populations of conformationally free molecules which exhibit more diverse conformers. On the other hand, SAMs are known to form domains of about 5-7 nm in size, even on an ideal flat surface. Another limitation was the assumption of linear response of the molecule, due to the limited capabilities of the modeling software.

The results show that ODT is explained pretty well by a linear deformation to a variety of conformers involving gauche and trans relative conformations on the methyl terminal and penultimate methylene. The PDT response is overestimated due to the probably incorrect assumption of free rotation about the methylene group. Finally, BMT's response is underestimated, the reason most likely being that the benzene ring was assumed to be rigid under deformation. Therefore the breaking of ring symmetry plays in important role in the deformation of BMT under nonlinear coompression.

Future work would be to study the same SAMs on silver surfaces as the adsorption sites are closer together. Stiffer responses are expected. Varying the liquid medium used and investigating other SAMs are in the works.

[nanotech] Karl Hess, Nanotechnology: Large Systems: From Evolution to Quantum Computing

2004-09-09T23:52:00.000-07:00

September 8, 2004: 4 pm in B02 Coordinated Sciences Laboratory

"Can we make nanostructures as cheaply as nature does? If not as pleasurably. You know, production in nature comes from sex." - Prof. Karl Hess

"There are still limits to what quantum computation can do: you cannot kill a cell with quantum computation." - Prof. Karl Hess

Prof. Hess explained that his talk was motivated from the union of ideas from two influential essays. The first was R. P. Feynman's famous article entitled There's Plenty of Room At The Bottom, explaning how in principle the ultimate end of scaling will require the manipulation of single atoms, and how such technology will allow vast information storage densities. The second is by M. Iansiti and J. West, entitled From physics to function: an empirical study of research and development performance in the semiconductor industry. This article documents the scaling history of semiconductor elements and predicts an inevitable limit to the current paradigm of scaling: the 'tyranny of bulk matter' will end once components become small enough for tunneling currents to dominate the electrical properties of components such as diodes and transistors. In addition, interfacial effects, such as hydrogen passivation of silicon domains will play an increasingly greater role in determining the lifetime of computer chips.

Prof. Hess emphasized that scaling laws such as Moore's law are not laws of nature, but of economics; each generation of scaling research cost the same amount, of approximately $6 billion. According to him, Pharoah Sneferu was the first person to discover that scaling laws were not laws of nature. The Pharoah supposedly ordered the construction of a huge pyramid, five times larger than the then-largest pyramid existing. However, the architecture was unable to sustain the massive weight and collapsed. Prof. Hess also hints that the name of Sneferu found its way into the neologism snafu, but I think the link is dubious at best. A quick google search reveals that Pharoah Sneferu's monument is best known as the Bent Pyramid and was thought to have collapsed partially, forcing a change of gradient in the pyramid's constructuon.

But anyway, Prof. Hess went on to discuss how biological systems and quantum mechanics provide new paradigms for prediction nanoscale scaling laws, and also how the power of quantum computation should be taken into account. While biological systems have been able to produce a vast diversity of nanoscale components, many of them multifunctional, and for next to nothing, they have the disadvantages of not being algorithmic, slow speed, lack of reliability, and the issue of lifetime switching cycles to be considered. Imitation of biological systems was exemplified by DNA-based colloidal assembly demonstrated by Wiltzius, Lu and Brown.

The most interesting biological systems to Prof. Hess are ion channels, membrane proteins that regulate the flow of ionic currents in cells. They act as both pumps and switches, analogous to actuators and transistors respectively. However, the great advantage of membrane proteins as a switch is its infinite on- to off-current ratio; transistors experience tunneling currents that do not permit such clean switching. Strictly speaking, however, I feel that Prof. Hess has overgeneralized this, since it is still possible (albeit much less likely) for ions to tunnel through closed ion channels. This is particularly true of protons tunneling 'through' the ubiquitous hydrogen atoms in aquaporins to poke through the other side.

Prof. Hess then went into detail about how carbon nanotubes (CNTs) could be engineered to imitate ion channels. In particular, his dream applications studied by his postdoc, Slava V. Rotkin, were to have a light-operated molecular switch, composed of a dye somehow linked to a carbon nanotube; and of a CNT METFET. The latter application was conceived based on the sensitivity of the electrical characteristics of CNTs depends on their helicity, becoming either semiconducting or metallic in nature. Therefore it seemed possible to him that a scanning tunneling microscope (STM) tip could be placed over a metallic CNT, itself suspended over a metal surface. Turning on the tip would then force electric field lines into the CNT, disrupting the "symmetry of the CNT" to become a semiconductor, thus creating a MET field effect transistor. However, I find this notion dubious at best, simply because if an external electric field were strong enough to perturb the CNT electronic distribution, it would also short-circuit the CNT to the metal surface.

Prof. Hess then switched topics to quantum computation and briefly touched on entanglement issues, stressing that while entanglement is the key to the power of quantum computation, it is not all-powerful. He cites the example of using entangled qubit channel to play a parity-guessing game, a dumbed-down version of the number-guessing game described by Andrew M. Steane and Wim van Dam in Physics Today. He also states that a classical solution exists to win the game, namely to use the parity of a clock to decide on guessing even or odd numbers, the convention of which having been fixed prior to the game. Thus, he concluded by stating that quantum computation is just a mixture of digital and analog computation since all is needed is extra information determined a priori.

Digging into this ingenious classical solution, I am shocked to discover that Prof. Hess did not cite the source of the solution, which was presented in a letter to Physics Today by a Bob Knauer. In addition, Prof. Hess is probably unaware that he is practically claiming that a quantum Turing machine (QTM) is nothing more (and nothing less) than a classical Turing machine (TM)! The stark differences are discussed at length in any textbook on quantum computation, such as the by now famous text by Nielsen and Chuang, as is even the subtle difference between a probablisitic Turing machine (PTM) and a quantum Turing machine (QTM). This leads me to believe that he has fallen hook, line and sinker for the populist conception of a quantum computer but has yet to actually figure out what it really is.

Finally, Prof. Hess concluded by mentioning Bell's Theorem being the key to the power and unintuitiveness of quantum computation and quotes that:

It wasn't Einstein, and it wasn't Heisenberg or dear old Schrödinger who drove the last nail into the coffin of common sense. It was J. S. Bell who published his memorable theroem in 1964. - R. A. Wilson, from a play (unverified)

Prof. Hess stated that Bell reduced the Einstein-Bohr debate to proving a theorem using measure theory and Kolmogorov axioms. However, he claimed that the underlying assumptions are unphysical, most importantly not having properly treated the "equality of all terms with equal settings" (which I interpret as a lack of certainty of the concept of simutaneity). He takes issue with this in a recent paper (Breakdown of Bell's theorem for certain objective local parameter spaces) which I have yet to read and therefore have no comment on.

[PChem] Oliver Kühn, Ultrafast Laser-Driven Quantum Molecular Dynamics

2004-09-09T23:36:00.000-07:00

August 19, 2004: 2 pm at A410 CLSL

I walked into my office to see Prof. Kühn seated at my desk. Okay, so it was the day I was going to move in. But that was how I knew he was in UIUC to give a talk. He has been a regular visitor here and will continue to be, I imagine.

Prof. Kühn's talk focused on achieving laser-assisted control of quantum molecular dynamics. One example was to achieve bond selective chemistry through coherent IR ladder climbing. The major problem with blasting the molecule with resonant radiation is of course, intramolecular vibrational relaxation (IVR) and repartitioning of the vibrational quanta to other bonds in the molecule. This may result in a significant probability that other bonds might break instead. This makes bond-selective chemistry very difficult to achieve with continuous-wave (cw) lasers since IVR is an ultrafast (picosecond regime) process.

The aim of coherent IR ladder climbing is to dissociate a molecule while remaining in the electronic ground state manifold, creating fragments of lower energy. One subtlety to this technique is that toward the dissociative regime of the molecular potential, the potential surface becomes strongly anharmonic and no longer resonates at the ground-state transition energy. Therefore, shaped pulse excitation through pulse chirping becomes necessary to take care of the red-shifted energy level spacings.

Quantum molecular dynamics simulations were used to model the reactions undergone by the molecule under laser bombardment. Normal mode analysis allowed the study of only the relevant part of the molecular Hamiltonian. The potential studied was a Morse potential (to describe the dissociating bond) bilinearly coupled to a harmonic potential (describing a non-dissociating bond). The non-dissociating bond studied is potentially able to siphon energy away from the dissociating bond due to IVR and therefore of interest in the simulation. The reaction surface Hamiltonian that is thus formed has a kinetic energy term in the reaction coordinates, plus a Hessian force-field describing the substrate normal modes. The dynamics were then studied using time-dependent multi-configuration Hartree Fock.

The idea of laser control is based on optimal control theory, which aims to maximze overlap with the (target) product wavefunction subject to constraints. Analytical pulses are then optimized parametrically to achieve the desired overlap. In practice, however, feedback techniques are used to shape pseudo-analytical pulses. Several control strategies are useful: pulse chirping, resonant climbing and multiphoton excitation. In particular, resonant climbing is needed because of wavepacket dispersion in an anharmonic well.

The standard technique to achieve quantum control uses the Ehrenfest equation to calculate an optimal field for the damped oscillator (Controllability of molecular systems). However, this needs very large spikes in the optical electric field, which would lead to undesired nonlinear complications. The model used to achieve optimization instead was the local competitive control model, were a cost functional was minimized variationally.

Two molecules whose dissociations are of chemical interest are chromium(0) hexacarbonyl and diazomethane (Motzkus group, Controlling molecular ground-state dissociation by optimizing vibrational ladder climbing, and Getting ahead of IVR: A demonstration of mid-infrared induced molecular dissociation on a sub-statistical time scale). QMD simulations were able to calculate the reorganization energy needed to relax the mode to equilibrium, an important quantity for laser control. In Cr(CO)₆, the dissociation of a CO moiety is hampered by an IVR channel that redistributes energy into the an umbrella mode. The control strategy is therefore to damp motion in this umbrella mode and to antidamp the reactive mode.

The chromium carbonyl study led to consideration of a biological problem, that of the dissociation of carboxymyoglobin (MbCO). The idea was to excite the carbonyl bond and take advantage of IVR to redistribute the energy to the (toxic) iron-carbon bond. Simple laser irradiation leads to a plethora of changes: in addition to the desired photodissociation, there are also unwanted structural changes and many relaxation and recombination channels that interfere. In short, the QMD study concluded that laser control using the local competitive strategy does not work because IVR to the iron-carbon bond was simply too slow to work with.

The analogous carboxyhemoglobin (HbCO) dissociation problem was studied by another group (Coherent vibrational climbing in carboxyhemoglobin) using ladder climbing techniques. They found that pulse chirping of 16 ps pulses excites but does not break the iron-carbon bond. QMD studies showed that IVR channeled too much energy into a doming mode that is a collective motion of the porphyrin ring in heme. Due to the strongly anharmonic nature of both modes the excitation wavepacket breaks up and the desired mode coupling turns out to be too weak, with probability of dissociation, i.e. barrier crossing, hitting a maximum of only 1.2 × 10^-6.

In conclusion, QMD theory can explain experimental results, despite ignoring other effects such as other normal modes, dephasing, relaxation, and singlet-multiplet transitions. This suggests that these other effects are unimportant. Future work would be to see if the pulse shape itself can be optimized, as well as investigating nonlinear responses.