Hydrogen bonding interactions between biological chromophores and their surrounding protein and solvent environment significantly affect the photochemical pathways of the chromophore and its biological function. A common first step in the dynamics of these systems is excited state proton transfer between the noncovalently bound molecules, which stabilizes the system against dissociation and principally alters relaxation pathways. Despite such fundamental importance, studying excited state proton transfer across a hydrogen bond has proven difficult, leaving uncertainties about the mechanism. Through time-resolved photoelectron imaging measurements, we demonstrate how the addition of a single hydrogen bond and the opening of an excited state proton transfer channel dramatically changes the outcome of a photochemical reaction, from rapid dissociation in the isolated chromophore to efficient stabilization and ground state recovery in the hydrogen bonded case, and uncover the mechanism of excited state proton transfer at a hydrogen bond, which follows sequential hydrogen and charge transfer processes. DOI: 10.1103/PhysRevLett.117.163002 Light-driven processes in biochemistry are initiated by absorption at a central molecular chromophore which is often linked to its surrounding solvent and/or protein environment via a network of hydrogen bonds (HBs). HBs have a profound effect on the chromophore structure and which parts of the potential energy landscape can be explored following photoexcitation. A famous example is the absence of fluorescence in the chromophore of the green fluorescent protein (GFP) when it is removed from its native protein environment [1,2]. Time-and frequency-resolved measurements on GFP have provided some remarkable results, highlighting the importance of excited state proton transfer (ESPT) [3], but the mechanisms through which this occurs, and how the protein-chromophore interactions influence the dynamics, remain unclear [4]. To circumvent the complexity of entire protein systems, bottom-up approaches have centered on studies of isolated chromophores [5][6][7][8][9][10][11][12]. However, these studies do not account for the effects of the protein-chromophore interactions on the dynamics, which can dramatically alter the local electronic structure and potential energy surface. We present an alternative approach and concentrate specifically on the dynamics around a single HB in a model molecular complex. This allows us to study in detail interactions that are inaccessible in isolated chromophores and to highlight the dramatic changes in dynamics and functionality caused by HBs. Using this approach we study ESPT in the ammonia dimer, a small molecular complex and model of the -NH-N-HB network present, e.g., between nucleobases in the backbone of a DNA double helix. Using time-resolved photoelectron imaging [13,14], we uncover the two-step mechanism of ESPT across the HB and disentangle the hydrogen transfer (HT) and charge transfer (CT) processes, and how these lead to ground state recovery on...

New ab-initio surface hopping simulations of the excited state dynamics of CS 2 including spinorbit coupling are compared to new experimental measurements using a multiphoton ionisation probe in a photoelectron spectroscopy experiment. The calculations highlight the importance of the triplet states even in the very early time dynamics of the dissociation process and allow us to unravel the signatures in the experimental spectrum, linking the observed changes to both electronic and nuclear degrees of freedom within the molecule.

A completely new evaluation of the fundamental atomic constants by the method of least squares is presented. A number of new and highly precise experiments have been taken into account, including the measurement of: (1) the velocity of light by an exceptionally ingenious and precise method due to Hansen and Bol, (2) the absolute proton moment, (3) the ratio of magnetic moments of proton and electron, (4) the proton moment in Bohr magnetons, (5) the hyperfine structure separation of ground-state hydrogen, (6) the ratio of cyclotron to precession frequency of the proton, (7) h/mc using annihilation radiation, and (9) h/e from the x-ray high frequency limit with improved precision.The results of the critical survey of Dorsey on the velocity of light plus the more recent measurements of c are combined to obtain a weighted-mean value: c=299790.0±0.7 km/sec. The results of the experiments listed above have been combined by several writers to compute a, e/m, and F. All of the published values, including those of certain older experiments have been corrected for the new value of c and combined in a least-squares calculation. The results are presented in a complete table of the fundamental constants and are compared with the experimental values in an isometric consistency chart. Six of the experimental results: F, N, mN, a, e/rn, and h/e are found to show excellent agreement with the least-squares values. In particular the new experimental value of Ve= =s (l-37928±0.00004)X10 17 erg sec/esu which precipitated the present work agrees well with the leastsquares value, Ve=(l-379300±0.000016)X10-17 erg-sec/esu.

The dissociation dynamics of the Ã-state of ammonia have been studied using a resonant multiphoton ionisation probe in a photoelectron spectroscopy experiment.

This corrects the article DOI: 10.1103/PhysRevLett.117.163002.