Tracking ultrafast hot-electron diffusion in space and time by ultrafast thermomodulation microscopy.

The ultrafast response of metals to light is governed by intriguing nonequilibrium dynamics involving the interplay of excited electrons and phonons. The coupling between them leads to nonlinear diffusion behavior on ultrashort time scales. Here, we use scanning ultrafast thermomodulation microscopy to image the spatiotemporal hot-electron diffusion in thin gold films. By tracking local transient reflectivity with 20-nm spatial precision and 0.25-ps temporal resolution, we reveal two distinct diffusion regimes: an initial rapid diffusion during the first few picoseconds, followed by about 100-fold slower diffusion at longer times. We find a slower initial diffusion than previously predicted for purely electronic diffusion. We develop a comprehensive three-dimensional model based on a two-temperature model and evaluation of the thermo-optical response, taking into account the delaying effect of electron-phonon coupling. Our simulations describe well the observed diffusion dynamics and let us identify the two diffusion regimes as hot-electron and phonon-limited thermal diffusion, respectively.

Evidence for a vibrational phase-dependent isotope effect on the photochemistry of vision.

Vibronic coupling is key to efficient energy flow in molecular systems and a critical component of most mechanisms invoking quantum effects in biological processes. Despite increasing evidence for coherent coupling of electronic states being mediated by vibrational motion, it is not clear how and to what degree properties associated with vibrational coherence such as phase and coupling of atomic motion can impact the efficiency of light-induced processes under natural, incoherent illumination. Here, we show that deuteration of the H11-C11=C12-H12 double-bond of the 11-cis retinal chromophore in the visual pigment rhodopsin significantly and unexpectedly alters the photoisomerization yield while inducing smaller changes in the ultrafast isomerization dynamics assignable to known isotope effects. Combination of these results with non-adiabatic molecular dynamics simulations reveals a vibrational phase-dependent isotope effect that we suggest is an intrinsic attribute of vibronically coherent photochemical processes.

Lack of evidence for phase-only control of retinal photoisomerization in the strict one-photon limit.

The concept of shaping electric fields to steer light-induced processes coherently has fascinated scientists for decades. Despite early theoretical considerations that ruled out one-photon coherent control (CC), several experimental studies reported that molecular responses are sensitive to the shape of the excitation field in the weak-field limit. These observations were largely attributed to the presence of rapid-decay channels, but experimental verification is lacking. Here, we test this hypothesis by investigating the degree of achievable control over the photoisomerization of the retinal protonated Schiff-base in bacteriorhodopsin, isorhodopsin and rhodopsin, all of which exhibit similar chromophores but different isomerization yields and excited-state lifetimes. Irrespective of the system studied, we find no evidence for dissipation-dependent behaviour, nor for any CC in the strict one-photon limit. Our results question the extent to which a photochemical process at ambient conditions can be controlled at the amplitude level, and how the underlying molecular potential-energy surfaces and dynamics may influence this controllability.

Principles and Applications of Broadband Impulsive Vibrational Spectroscopy.

We present an experimental setup for recording vibrational coherences and thereby Raman spectra of molecules in their ground and excited electronic states over the 50-3000 cm(-1) spectral range using broadband impulsive vibrational spectroscopy. Our approach relies on the combination of a <10 fs excitation pulse with an uncompressed white light continuum probe, which drastically reduces experimental complexity compared to frequency domain based techniques. We discuss the parameters determining vibrational coherence amplitudes, outline how to optimize the experimental setup including approaches aimed at conclusively assigning vibrational coherences to specific electronic states, and provide a clear comparison with existing techniques. To demonstrate the applicability of our spectroscopic approach we conclude with several examples revealing the evolution of vibrational coherence in rhodopsin and ß-carotene.

Direct observation of the coherent nuclear response after the absorption of a photon.

How molecules convert light energy to perform a specific transformation is a fundamental question in photophysics. Ultrafast spectroscopy reveals the kinetics associated with electronic energy flow, but little is known about how absorbed photon energy drives nuclear motion. Here we used ultrabroadband transient absorption spectroscopy to monitor coherent vibrational energy flow after photoexcitation of the retinal chromophore. In the proton pump bacteriorhodopsin, we observed coherent activation of hydrogen-out-of-plane wagging and backbone torsional modes that were replaced by unreactive coordinates in the solution environment, concomitant with a deactivation of the reactive relaxation pathway.

Vibrationally coherent crossing and coupling of electronic states during internal conversion in ß-carotene.

Coupling of nuclear and electronic degrees of freedom mediates energy flow in molecules after optical excitation. The associated coherent dynamics in polyatomic systems, however, remain experimentally unexplored. Here, we combined transient absorption spectroscopy with electronic population control to reveal nuclear wave packet dynamics during the S2 → S1 internal conversion in ß-carotene. We show that passage through a conical intersection is vibrationally coherent and thereby provides direct feedback on the role of different vibrational coordinates in the breakdown of the Born-Oppenheimer approximation.

Kinetic effects of the concentration-dependent stimulation of soluble guanylate cyclase from rat lung.

Soluble guanylate cyclase (GTP pyrophosphate-lyase (cyclizing), EC 4.6.1.2) from rat lung demonstrated concentration-dependent stimulation, that is, an increase in specific activity with increasing enzyme (protein) concentration. This phenomenon persisted through several steps of enzyme purification and was apparently due to the presence of a macromolecular activator, similar in size to the enzyme. Treatment of partially purified enzyme with N-ethylmaleimide destroyed catalytic activity, but did not effect the ability of the preparation to stimulate activity. Kinetic analysis demonstrated that the stimulation was due to an increased V value with no change in the apparent Km value for MnGTP. Stimulation occurred without a time lag, the activator apparently interacting reversibly with the enzyme to increase catalytic capability. Some nonionic detergents of the Triton series inhibited enzyme activity by decreasing the V value, with no change in the Km value, and also decreased concentration-dependent stimulation. However, the two phenomena were not directly related. While the physiological significance of the activator is unclear, its presence affects estimations of recovery during enzyme purification, V determinations, and determinations of the effect of hormone or drug treatment on the activity of tissue extracts.