Time-Resolved Measurement of Interatomic Coulombic Decay Induced by Two-Photon Double Excitation of Ne_{2}.

The hitherto unexplored two-photon doubly excited states [Ne^{*}(2p^{-1}3s)]_{2} were experimentally identified using the seeded, fully coherent, intense extreme ultraviolet free-electron laser FERMI. These states undergo ultrafast interatomic Coulombic decay (ICD), which predominantly produces singly ionized dimers. In order to obtain the rate of ICD, the resulting yield of Ne_{2}^{+} ions was recorded as a function of delay between the extreme ultraviolet pump and UV probe laser pulses. The extracted lifetimes of the long-lived doubly excited states, 390(-130/+450) fs, and of the short-lived ones, less than 150 fs, are in good agreement with ab initio quantum mechanical calculations.

Probe of Multielectron Dynamics in Xenon by Caustics in High-Order Harmonic Generation.

We investigated the giant resonance in xenon by high-order harmonic generation spectroscopy driven by a two-color field. The addition of a nonperturbative second harmonic component parallel to the driving field breaks the symmetry between neighboring subcycles resulting in the appearance of spectral caustics at two distinct cutoff energies. By controlling the phase delay between the two color components it is possible to tailor the harmonic emission in order to amplify and isolate the spectral feature of interest. In this Letter we demonstrate how this control scheme can be used to investigate the role of electron correlations that give birth to the giant resonance in xenon. The collective excitations of the giant dipole resonance in xenon combined with the spectral manipulation associated with the two-color driving field allow us to see features that are normally not accessible and to obtain a good agreement between the experimental results and the theoretical predictions.

Slow Interatomic Coulombic Decay of Multiply Excited Neon Clusters.

Ne clusters (∼5000 atoms) were resonantly excited (2p→3s) by intense free electron laser (FEL) radiation at FERMI. Such multiply excited clusters can decay nonradiatively via energy exchange between at least two neighboring excited atoms. Benefiting from the precise tunability and narrow bandwidth of seeded FEL radiation, specific sites of the Ne clusters were probed. We found that the relaxation of cluster surface atoms proceeds via a sequence of interatomic or intermolecular Coulombic decay (ICD) processes while ICD of bulk atoms is additionally affected by the surrounding excited medium via inelastic electron scattering. For both cases, cluster excitations relax to atomic states prior to ICD, showing that this kind of ICD is rather slow (picosecond range). Controlling the average number of excitations per cluster via the FEL intensity allows a coarse tuning of the ICD rate.

Non-collinear high-order harmonic generation by three interfering laser beams.

High order harmonic generation (HHG) has shown its impact on several applications in Attosecond Science and Atomic and Molecular Physics. Owing to the complexity of the experimental setup for the generation and characterization of harmonics, as well as to the large computational costs of numerical modelling, HHG is generally performed and modelled in collinear geometry. Recently, several experiments have been performed exploiting non-collinear geometry, such as HHG in a grating of excited molecules created by crossing beams. In such studies, harmonics were observed at propagation directions different from those of the driving pulses; moreover the scattered harmonics were angularly dispersed.In this work we report on a new regime of HHG driven by multiple beams, where the harmonics are generated by three synchronized, intense laser pulses organized in a non-planar geometry. Although the configuration we explore is well within the strong-field regime, the scattered harmonics we observe are not angularly dispersed.

Trajectory-resolved Coulomb focusing in tunnel ionization of atoms with intense, elliptically polarized laser pulses.

In strong-field light-matter interactions, the strong laser field dominates the dynamics. However, recent experiments indicate that the Coulomb force can play an important role as well. In this Letter, we have studied the photoelectron momentum distributions produced from noble gases in elliptically polarized, 800 nm laser light. By performing a complete mapping of the three-dimensional electron momentum, we find that Coulomb focusing significantly narrows the lateral momentum spread. We find a surprisingly sensitive dependence of Coulomb focusing on the initial transverse momentum distribution, i.e., the momentum at the moment of birth of the photoelectron. We also observe a strong signature of the low-energy structure in the above threshold ionization spectrum.

Ultrafast spectroscopy of linear carbon chains: the case of dinaphthylpolyynes.

The dynamics of excited states in α,ω-dinaphthylpolyyne, a class of linear sp-carbon chains, has been investigated by ultrafast transient absorption spectroscopy and DFT//TDDFT calculations. We show that the role of molecular conformers, in which end-capped naphthalene rings are planar or perpendicular to the polyyne plane, is fundamental for understanding both the steady state properties, such as UV-Vis absorption spectra and vibronic transitions, and the ultrafast transient absorption features. In particular, we observed in one of the conformers the ultrafast formation of a narrow photo-induced absorption band rising within 30 ps. This band can be assigned to an inter-system crossing event leading to the formation of triplet excited states.

Polarization pulse shaping induced by impulsive molecular alignment in optical filamentation and application to high-order harmonic generation.

We investigated theoretically and experimentally the ultrafast birefringence induced by impulsive alignment in a molecular gas during optical filamentation. This phenomenon is able to substantially affect the polarization state of an ultrashort laser pulse that propagates through the aligned medium at suitable delays from a first aligning pulse. We exploited this modulation of the polarization state in order to effectively control the high-order harmonic generation (HHG) process, which is strongly dependent on the driving pulse polarization. These results open new and fascinating perspectives for the tailoring of strong-field phenomena by means of polarization pulse shaping.

Efficient continuum generation exceeding 200 eV by intense ultrashort two-color driver.

A temporal gating on the high-order harmonic emission process is achieved using an intense 20 fs, 1.45 microm pulse (IR) in combination with an intense 13 fs, 800 nm pulse [visible (VIS)]. Exploiting this two-color gating scheme, a coherent continuous emission extending up to 160 eV using Ar gas and 200 eV using Ne gas is efficiently generated. The IR pulse contributes to significantly extending the harmonic emission to higher photon energies, whereas the VIS pulse improves the conversion efficiency of the process. These results indicate the possibility to produce bright attosecond pulses approaching the soft X spectral region.

Adequate connexin-mediated coupling is required for proper insulin production.

To assess whether connexin (Cx) expression contributes to insulin secretion, we have investigated normal and tumoral insulin-producing cells for connexins, gap junctions, and coupling. We have found that the glucose-sensitive cells of pancreatic islets and of a rat insulinoma are functionally coupled by gap junctions made of Cx43. In contrast, cells of several lines secreting insulin abnormally do not express Cx43, gap junctions, and coupling. After correction of these defects by stable transfection of Cx43 cDNA, cells expressing modest levels of Cx43 and coupling, as observed in native beta-cells, showed an expression of the insulin gene and an insulin content that were markedly elevated, compared with those observed in both wild-type (uncoupled) cells and in transfected cells overexpressing Cx43. These findings indicate that adequate levels of Cx-mediated coupling are required for proper insulin production and storage.

High-energy, few-optical-cycle pulses at 1.5 microm with passive carrier-envelope phase stabilization.

We report on a source of ultrabroadband self-phase-stabilized near-IR pulses by difference-frequency generation of a hollow-fiber broadened supercontinuum followed by two-stage optical parametric amplification. We demonstrate energies up to 200 microJ with 15 fs pulse width, making this source suited as a driver for attosecond pulse generation.

Identification of two further gap-junctional proteins, connexin40 and connexin45, in human myometrial smooth muscle cells at term.

The powerful synchronous contractions of the uterus in labor depend on electrical coupling of myometrial smooth muscle cells by gap junctions. In humans and other mammals, gap junctions are scarce in the myometrium of the non-pregnant uterus, but become abundant at term and/or with the onset of labor. Previous work has shown that the gap-junctional protein (connexin) expressed by human myometrial smooth muscle cells is connexin43, the same connexin type that predominates in cardiac muscle. Here we show that two further gap junctional proteins, connexin40 and connexin45, are expressed by the myometrial smooth muscle cells of the human uterus at term. Transcripts encoding the human isoforms of these connexins were demonstrated by Northern blot analysis, and immunoconfocal microscopy enabled precise localization of the corresponding proteins to punctate contact points (i.e., gap junctions) between interacting smooth muscle cells. Double labeling demonstrated that, while some fluorescent spots comprise only connexin43, both connexin40 and connexin45 predominantly colocalize to connexin43-positive fluorescent spots. Triple labeling revealed that where all three connexin types were expressed, they frequently localized to the same gap junction spot. As gap-junctional channels composed of different connexin types have been demonstrated in vitro to have different functional properties, multiple connexin expression may contribute to modulation of gap junction function in human myometrial smooth muscle cells in vivo.

Hyperinsulinemia-induced hypoglycemia is enhanced by overexpression of connexin 43.

To assess whether cell to cell communications via connexins (Cx) participate to insulin secretion in vivo, we studied insulinoma cells (INS1) implanted in rats after stable transfection with connexin 43 (Cx43). We found that compared to wild-type and transfected cells, which in vivo express modest levels of Cx43 and junctional communication, cells overexpressing Cx43 communicated extensively, featured decreased growth, and induced a much higher hyperinsulinemia. As a result, rats with insulinomas made of these cells became more severely hypoglycemic than rats implanted with either wild-type, neomycin-transfected cells or cells transfected with a Cx43 antisense complementary DNA. Rats implanted with transfected cells that expressed modest level of Cx43 showed levels of circulating insulin similar to those in rats implanted with wild-type INS1 cells. The data show that overexpression of Cx43 influences the growth and secretion of the implanted insulinoma cells, providing evidence for a contribution of Cx-mediated cell to cell communication in the functioning of insulin-producing cells in vivo.

Thyrotropin-releasing hormone causes direct excitation of dorsal vagal and solitary tract neurones in rat brainstem slices.

The effect of thyrotropin-releasing hormone (TRH) on neurones in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract was studied using extracellular single-unit recordings from brainstem slices of the rat. About one third of vagal neurones were excited by TRH. The remaining neurones were unaffected. The lowest effective peptide concentration was around 10 nM and a half maximal effect was achieved at about 100 nM. The action of TRH persisted in a low-calcium, high-magnesium solution which blocks synaptic transmission. The biologically inactive compound, TRH-free acid, was without effect. In the nucleus of the solitary tract, one fourth of the neurones were excited by TRH; none were inhibited by this peptide. Part of the vagal TRH-responsive neurones were also excited by oxytocin and some of the solitary tract neurones sensitive to TRH also responded to vasopressin. We conclude that a fraction of neurones located in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract possess functional TRH receptors. TRH may thus act as a neurotransmitter or neuromodulator in the dorsal brainstem and may participate in the regulation of autonomic functions.

Prospective randomized study of two cryopreservation policies avoiding embryo selection: the pronucleate stage leads to a higher cumulative delivery rate than the early cleavage stage.

OBJECTIVE: To compare the cumulative live birth rates obtained after cryopreservation of either pronucleate (PN) zygotes or early-cleavage (EC) embryos. DESIGN: Prospective randomized study. SETTING: University hospital. PATIENT(S): Three hundred eighty-two patients, involved in an IVF/ICSI program from January 1993 to December 1995, who had their supernumerary embryos cryopreserved either at the PN (group I) or EC (group II) stage. For 89 patients, cryopreservation of EC embryos was canceled because of poor embryo development (group III). Frozen-thawed embryo transfers performed up to December 1998 were considered. MAIN OUTCOME MEASURE(S): Age, oocytes, zygotes, cryopreserved and transferred embryos, damage after thawing, cumulative embryo scores, implantation, and cumulative live birth rates. RESULT(S): The clinical pregnancy and live birth rates were similar in all groups after fresh embryo transfers. Significantly higher implantation (10.5% vs. 5.9%) and pregnancy rates (19.5% vs. 10.9%; P< or = .02 per transfer after cryopreserved embryo transfers were obtained in group I versus group II, leading to higher cumulative pregnancy (55.5% vs. 38.6%; P < or = .002 and live birth rates (46.9% vs. 27.7%; P< or = .0001. CONCLUSION(S): The transfer of a maximum of three unselected embryos and freezing of all supernumerary PN zygotes can be safely done with significantly higher cumulative pregnancy chances than cryopreserving at a later EC stage.

Extracranial carotid angioplasty and stenting. Initial results and short-term follow-up.

Carotid percutaneous transluminal angioplasty, with or without stent implantation, is becoming another therapeutic option for carotid revascularization. To evaluate the feasibility and effectiveness of the technique, from October of 1995 to March of 1997, we performed 24 percutaneous transluminal angioplasty procedures in 22 patients with severe extracranial carotid artery stenosis. Three common carotid and 21 internal carotid arteries were treated, and 19 procedures included stent implantation using nonarticulated PALMAZ stents (P154 and P204). Twelve patients were asymptomatic and 10 patients were symptomatic; 2 of the symptomatic patients had complete obstruction of the internal carotid artery that was successfully recanalized. Technical and angiographic success was achieved in 23 of 24 procedures, with the carotid artery obstruction diminishing from 85.6% +/- 8.5% to 5.7% +/- 3.2% (P < 0.001). Average stenosis length was 12.5 +/- 3.1 mm, and mean time of carotid occlusion during balloon inflation was 11.5 +/- 2.5 seconds. Three patients experienced transitory seizures during the procedure prior to dilation, 1 patient had a minor stroke with complete recovery within 72 hours, and 1 patient had a major stroke and died 45 days after the procedure. Clinical follow-up was achieved in all patients (mean, 10.5 +/- 7.2 months) and angiographic follow-up in 16 patients (mean, 6.3 +/- 1.2 months). The results obtained in this initial experience provide adequate support to continue further evaluation of this new therapeutic strategy.

High-order harmonic spectroscopy for molecular imaging of polyatomic molecules.

High-order harmonic generation is a powerful and sensitive tool for probing atomic and molecular structures, combining in the same measurement an unprecedented attosecond temporal resolution with a high spatial resolution of the order of an angstrom. Imaging of the outermost molecular orbital by high-order harmonic generation has been limited for a long time to very simple molecules, like nitrogen. Recently we demonstrated a technique that overcame several of the issues that have prevented the extension of molecular orbital tomography to more complex species, showing that molecular imaging can be applied to a triatomic molecule like carbon dioxide. Here we report on the application of such a technique to nitrous oxide (N(2)O) and acetylene (C(2)H(2)). This result represents a first step towards the imaging of fragile compounds, a category which includes most of the fundamental biological molecules.

Molecular rotovibrational dynamics excited in optical filamentation.

The rotovibrational dynamics excited by optical filamentation in molecular gases is studied in the temporal domain. Two time-delayed replicas of the same laser pulse have been used to generate a first filament, for the rotovibrational excitation of the sample, and a second collinear filament probing the Raman dynamics. The Fermi doublet structure in CO(2) as well as the very fast stretching mode of H(2) were clearly resolved.

Rotational Raman effects in the wake of optical filamentation.

The spatiotemporal effects generated in the wake of a laser filament propagating in nitrogen are investigated. At suitable time delays, a probe light pulse propagating along the wake experiences a strong spatial confinement and a noticeable spectral broadening at the same time. Numerical simulations, well reproducing the experimental findings, show the key role of the impulsive rotational Raman response in the observed phenomena.

Hollow-fiber compression of visible, 200 fs laser pulses to 40 fs pulse duration.

We demonstrate the use of a very simple, compact, and versatile method, based on the hollow-fiber compression technique, to shorten the temporal length of visible laser pulses of 100-300 fs to pulse durations shorter than approximately 50 fs. In particular, 200 fs, frequency-doubled, Nd:glass laser pulses (527 nm) were spectrally broadened to final bandwidths as large as 25 nm by nonlinear propagation through an Ar-filled hollow fiber. A compact, dispersive, prism-pair compressor was then used to produce as short as 40 fs, 150 microJ pulses. A very satisfactory agreement between numerical simulations and measurements is found.

Millijoule-level phase-stabilized few-optical-cycle infrared parametric source.

Ultrabroadband self-phase-stabilized near-IR pulses have been generated by difference-frequency generation of a filament broadened supercontinuum followed by two-stage optical parametric amplification. Pulses with energy up to 1.2 mJ and duration down to 17 fs are demonstrated. These characteristics make such a source suited as a driver for high-order harmonic generation and isolated attosecond pulse production.