Strongly coupled organic systems are characterized by unusually large Rabi splittings, even in the vacuum state. They show the counter-intuitive feature of a lifetime of the lower polariton state longer than for all other excited states. Here we build up a new theoretical framework to understand the dynamics of such coupled system. In particular, we show that the non-Markovian character of the relaxation of the dressed organic system explains the long lifetime of the lower polariton state.

We have observed a Bose-Einstein condensate in a dilute gas of 4He in the (3)2S(1) metastable state. We find a critical temperature of (4.7+/-0.5) microK and a typical number of atoms at the threshold of 8 x 10(6). The maximum number of atoms in our condensate is about 5 x 10(5). An approximate value for the scattering length a = (16+/-8) nm is measured. The mean elastic collision rate at threshold is then estimated to be about 2 x 10(4) s(-1), indicating that we are deeply in the hydrodynamic regime. The typical decay time of the condensate is 2 s, which places an upper bound on the rate constants for two-body and three-body inelastic collisions.

(<250 words). Femtosecond fluorescence up-conversion, UV-Vis and IRtransient absorption spectroscopy are used to study the photo-isomerization dynamics of a new type of zwitterionic photoswitch based on a N-alkylated idanylidene pyrroline Schiff base framework (ZW-NAIP). The system is biomimetic, as it mimics the photophysics of retinal, in coupling excited state charge translocation and isomerisation. While the fluorescence lifetime is 140 fs, excited state absorption persists over 230 fs in form of a vibrational wavepacket according to twisting of the isomerising double bond. After a short "dark" time window in the UV-visible spectra, which we associate to the passage through a conical intersection (CI), the wavepacket appears on the ground state potential energy surface, as evidenced by the transient mid-IR data. This allows for a precise timing of the photoreaction all the way from the initial Franck-Condon region, through the CI and into both ground state isomers, until incoherent vibrational relaxation dominates the dynamics. The photo-reaction dynamics remarkably follow those observed for retinal in rhodopsin, with the additional benefit that in ZW-NAIP the conformational change reverses the zwitterion dipole moment direction. Last, the pronounced low-frequency coherences make these molecules ideal systems for 3 investigating wavepacket dynamics in the vicinity of a CI and for coherent control experiments.4

Single molecules that act as light-energy transducers (e.g., converting the energy of a photon into atomic-level mechanical motion) are examples of minimal molecular devices. Here, we focus on a molecular switch designed by merging a conformationally locked diarylidene skeleton with a retinal-like Schiff base and capable of mimicking, in solution, different aspects of the transduction of the visual pigment Rhodopsin. Complementary ab initio multiconfigurational quantum chemistry-based computations and time-resolved spectroscopy are used to follow the light-induced isomerization of the switch in methanol. The results show that, similar to rhodopsin, the isomerization occurs on a 0.3-ps time scale and is followed by <10-ps cooling and solvation. The entire (2-photon-powered) switch cycle was traced by following the evolution of its infrared spectrum. These measurements indicate that a full cycle can be completed within 20 ps.CASPT2//CASSCF ͉ mid-IR ͉ photochemical switch ͉ time resolved spectroscopy ͉ UV-vis M olecular switches based on photochemical E/Z isomerizations have been used in different contexts to convert light energy into ''mechanical'' motion at the molecular level (1-3). For instance, switches based on azobenzene have been used to control ion complexation (4, 5), electronic properties (6), catalysis (7), and the folding of peptides (8-13) whereas diarylidenes have provided the framework for the construction of rotary motors and transmissions (14). The computer modeling of switches that differ in size, polarity, and isomerization mechanism represents an attractive research target (15) yielding building blocks to be used in diverse molecular environments. However, this cannot be limited to the computation of equilibrium properties but requires the description of the entire photocycle. In other words, one needs to compute the potential energy surfaces controlling the switch E 3 Z and Z 3 E excited-state evolution, its decay and ground state relaxation, and the competing thermal E/Z isomerization in the proper environment (e.g., in solution or in a biomolecule backbone). The complexity of these calculations impedes the study of candidates that are intractable with accurate quantum chemical methods (allowing comparison with spectroscopic data) or that feature, as for azobenzene and diarylidenes (16), more than 1 low-lying excited state, leading to a plethora of reaction paths to be computed.The retinal protonated Schiff-base chromophore of rhodopsins (17-19) constitutes an example of an E/Z switch shaped by biological evolution that can be modeled with quantitative computations (20). In bovine rhodopsin (Rh), a selective photoisomerization of the 11-cis chromophore (PSB11) occurs via evolution of a single 3 * excited state (S 1 ) that survives for only 150 fs and yields, upon decay, the all-trans ground state (S 0 ) product with a 67% quantum yield (16,20). Although these properties make Rh an excellent reference for the design of E/Z switches, irradiation of PSB11 in solution (26, 27) features an unselec...

A pi-extended [2-(2-nitrophenyl)propoxy]carbonyl (NPPOC) derivative has been prepared as an efficient UV and near-IR photolabile protecting group for glutamate. This glutamate cage compound exhibits efficient photorelease upon one-photon excitation (epsilonPhi=990 M(-1) cm(-1) at 315 nm). In addition, it also shows efficient photorelease in activation of glutamate receptors in electrophysiological recordings. Combined with a high two-photon uncaging cross-section (deltaPhi=0.45 GM at 800 nm), its overall properties make this new cage-3-(2-propyl)-4'-methoxy-4-nitrobiphenyl (PMNB)-for glutamate a very promising tool for two-photon neuronal studies.

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene-pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited- and ground-state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far-red-detuned and rapidly blue-shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double-bond twisting and ring inversions, already populated during the excited-state reactive motion, induced periodic changes in π-conjugation that modulate the ground-state absorption after the non-adiabatic decay. This prediction further supports that the observed ground-state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode-selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.

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We produce giant helium dimers by photoassociation of metastable helium atoms in a magnetically trapped, ultracold cloud. The photoassociation laser is detuned red of the atomic 2 3 S1−2 3 P0 line and produces strong heating of the sample when resonant with molecular bound states. The temperature of the cloud serves as an indicator of the molecular spectrum. We report good agreement between our spectroscopic measurements and our calculations of the five bound states belonging to a 0 + u purely long-range potential well. These previously unobserved states have classical inner turning points of about 150 a0 and outer turning points as large as 1150 a0.PACS numbers: 34.20. Cf, 32.80.Pj, 34.50.Gb In the purely long-range molecules first proposed by Stwalley et al.[1], the binding potential depends only on the long-range part of the atom-atom interaction, and the internuclear distance is always large compared with ordinary chemical bond lengths. Theoretical description of these molecules involves only the leading C 3 /R 3 terms of the electric dipole-dipole interaction and the fine structure inside each atom. These well-known interactions allow precise calculation of potential wells and rovibrational energies. Previous experimental studies of such spectra in alkali atoms have utilized the technique of laser-induced photoassociation (PA) in a magneto-optical trap (MOT) [2,3]. In addition to testing calculations of molecular structure, that work has produced precise measurements of excited-state lifetimes [4,5,6,7] and has led to accurate determinations of s-wave scattering lengths for alkali systems, which are of interest for studies of Bose Einstein condensates (BECs) [8,9]. This letter reports novel spectroscopic measurements and calculations for extraordinarily long-range molecules that are produced when two 4 He atoms in the metastable 2 3 S 1 state absorb laser light tuned close to the 2 3 S 1 − 2 3 P 0 (D 0 ) atomic line at λ = 1083 nm. As compared with the alkali dimers considered previously, the potential wells are shallower, and molecules are much more tenuous, with an internuclear distance reaching values as large as 1150 a 0 (a 0 ≃ 0.53Å, the Bohr radius). At such large distances, retardation clearly influences the dipole-dipole interaction. Moreover, these purely longrange molecular states of metastable helium are distinctive in that each atom carries a high internal energy (the 2 3 S 1 state lies 20 eV above the ground state). While one normally expects Penning ionization to destabilize such * Electronic address: leonard@lkb.ens.fr † Permanent address: Calvin College, Grand Rapids, MI, USA. ‡ Permanent address: FOM instituut voor plasmafysica Rijnhuizen, and University of Twente, The Netherlands. § Present address: Institut für Quantenoptik, Universität Hannover, Germany.energetic molecules, we note that purely long-range interactions might rather suppress this process, since the atoms are effectively held apart by the same potential that binds them together. The absence of purely longrange resonances ...