13 Search Results

We propose the generation of attosecond entangled biphotons in the extreme-ultraviolet regime by two-photon decay of a metastable atomic state as a source similar to spontaneous parametric down-conversion photons. The 1s2s ¹S₀ metastable state in helium decays to the ground state by the emission of two energy-time entangled photons with a photon bandwidth equal to the total energy spacing of 20.62 eV. This results in a pair correlation time in the attosecond regime making these entangled photons a highly suitable source for attosecond pump-probe experiments. The biphoton generation rate from a direct four-photon excitation of helium at 240 nm ismore » calculated and used to assess some feasible schemes to generate these biphotons. Possible applications of entangled biphotons in attosecond timescale experiments, and a discussion of their potential to reach the zeptosecond regime are presented.« less

D₂ molecules, excited by linearly cross-polarized femtosecond extreme ultraviolet (XUV) and near-infrared (NIR) light pulses, reveal highly structured D⁺ ion fragment momenta and angular distributions that originate from two different four-step dissociative ionization pathways after four-photon absorption (one XUV + three NIR). We show that, even for very low dissociation kinetic energy release ≤ 240 meV, specific electronic excitation pathways can be identified and isolated in the final ion momentum distributions. With the aid of ab initio electronic structure and time-dependent Schrödinger equation calculations, angular momentum, energy, and parity conservation are used to identify the excited neutral molecular states andmore » molecular orientations relative to the polarization vectors in these different photoexcitation and dissociation sequences of the neutral D₂ molecule and its $$D_2^+$$ cation. In one sequential photodissociation pathway, molecules aligned along either of the two light polarization vectors are excluded, while another pathway selects molecules aligned parallel to the light propagation direction. The evolution of the nuclear wave packet on the intermediate $$B^1Σ_u^+$$ electronic state of the neutral D₂ molecule is also probed in real time.« less

Clusters in ³He and ⁴He nanodroplets reveal markedly different shapes showcasing the effect of superfluidity on aggregation.

In quantum systems, coherent superpositions of electronic states evolve on ultrafast time scales (few femtoseconds to attoseconds; 1 attosecond = 0.001 femtoseconds = 10^-18 seconds), leading to a time-dependent charge density. Here we performed time-resolved measurements using attosecond soft x-ray pulses produced by a free-electron laser, to track the evolution of a coherent core-hole excitation in nitric oxide. Using an additional circularly polarized infrared laser pulse, we created a clock to time-resolve the electron dynamics and demonstrated control of the coherent electron motion by tuning the photon energy of the x-ray pulse. Core-excited states offer a fundamental test bed formore » studying coherent electron dynamics in highly excited and strongly correlated matter.« less

Previous single-pulse extreme ultraviolet and X-ray coherent diffraction studies revealed that superfluid 4He droplets obtained in free jet expansion acquire sizable angular momentum, resulting in significant centrifugal distortion. Similar experiments with normal fluid 3He droplets may help elucidating the origin of the of the large degree of rotational excitation and highlight similarities and differences of dynamics in normal and superfluid droplets. Here, we present the first comparison of the shapes of isolated 3He and 4He droplets following expansion of the corresponding fluids in vacuum at temperatures as low as ~ 2 K. Large 3He and 4He droplets with average radiimore » of ~160 nm and ~350 nm, respectively, were produced. We find that the majority of the 3He droplets in the beam correspond to rotating oblate spheroids with reduced average angular momentum ($$\Lambda$$) and reduced angular velocities ($$\Omega$$) similar to that of 4He droplets. Given the different physical nature of 3He and 4He, this similarity in $$\Lambda$$ and $$\Omega$$ may be surprising and suggest that similar mechanisms induce rotation regardless of the isotope. We hypothesized that the observed distribution of droplet sizes and angular momenta stem from processes in the dense region close to the nozzle. In this region, the significant velocity spread and collisions between the droplets induce excessive rotation followed by droplet fission. The process may repeat itself several times before the droplets enter the collision-fee high vacuum region further downstream.« less

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using ultrafast transient polarization spectroscopy, extending the well-known technique of optical Kerr effect spectroscopy to excited electronic states. The third-order nonlinear response of the excited molecular ensemble is measured using a pair of femtosecond pulses following a third femtosecond pulse that populates the S₁ excited state. By measuring this response, which is highly sensitive to details of the excited state character and structure, as a function of time delays between the three pulses involved, we extract the dephasing time of the wave packet on the excitedmore » state. The dephasing time, measured as a function of time delay after pump excitation, shows oscillations indicating oscillatory wave packet dynamics on the excited state. Finally, from the experimental measurements and supporting theoretical calculations, we deduce that the wave packet completely leaves the S₁ state potential energy surface after three traversals of the intersystem crossing between the singlet S₁ and triplet T₂ states.« less

The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime. In this paper we propose and demonstrate a new technique to conduct transient absorption spectroscopy with broadmore » bandwidth attosecond pulses with the aid of ghost imaging, recovering sub-bandwidth resolution in photoproduct-based absorption measurements.« less

The electronic and nuclear dynamics in methanol, following 156 nm photoexcitation, are investigated by combining a detailed analysis of time-resolved photoelectron spectroscopy experiments with electronic structure calculations. The photoexcitation pump pulse is followed by a delayed 260 nm photoionization probe pulse to produce photoelectrons that are analyzed by velocity map imaging. The yields of mass-resolved ions, measured with similar experimental conditions, are found to exhibit the same time-dependence as specific photoelectron spectral features. Energy-resolved signal onset and decay times are extracted from the measured photoelectron spectra to achieve high temporal resolution, beyond the 20 fs pump and probe pulse durations.more » When combined with ab initio calculations of selected cuts through the excited state potential energy surfaces, this information allows the dynamics of the transient excited molecule, which exhibits multiple nuclear and electronic degrees of freedom, to be tracked on its intrinsic few-femtosecond time scale. Within 15 fs of photoexcitation, we observe nuclear motion on the initially bound photoexcited 2¹A" (S₂) electronic state, through a conical intersection with the 1¹A' (S3) state, which reveals paths to photodissociation following C–O stretch and C–O–H angle opening.« less

The quantum-mechanical motion of electrons in molecules and solids occurs on the sub-femtosecond timescale. Consequently, the study of ultrafast electronic phenomena requires the generation of laser pulses shorter than 1 fs and of sufficient intensity to interact with their target with high probability. Probing these dynamics with atomic-site specificity requires the extension of sub-femtosecond pulses to the soft X-ray spectral region. Here, we report the generation of isolated soft X-ray attosecond pulses with an X-ray free-electron laser. Our source has a pulse energy that is millions of times larger than any other source of isolated attosecond pulses in the softmore » X-ray spectral region, with a peak power exceeding 100 GW. This unique combination of high intensity, high photon energy and short pulse duration enables the investigation of electron dynamics with X-ray nonlinear spectroscopy and single-particle imaging, unlocking a path towards a new era of attosecond science.« less

We present a measurement of the time-resolved photoelectron kinetic energy spectrum of ethylene using 156 nm and 260 nm laser pulses. The 156 nm pulse first excites ethylene to the 1B1u (ππ∗) electronic state where 260 nm light photoionizes the system to probe the relaxation dynamics with sub-30 fs resolution. Recent ab initio calculations by Mori et al. [J. Phys. Chem. A 116, 2808-2818 (2012)] have predicted an ultrafast population transfer from the initially excited state to a low-lying Rydberg state during the relaxation of photoexcited ethylene. The measured photoelectron kinetic energy spectrum reveals wave packet motion on the valencemore » state and shows indications that the low-lying π3s Rydberg state is indeed transiently populated via internal conversion following excitation to the ππ∗ state, supporting the theoretical predictions.« less