Molecular Attoscope: Pulse Shape Spectroscopy of Electronic Coherence
Loc Thi-Hoang Ngo, Julia Codere, Javin Ohara, Brian Kaufman, Martin G Cohen, Tamas Rozgonyi, Philipp Marquetand, Matthew Bain, Brett J Pearson, Ruaridh Forbes, Thomas Weinacht
quant-phSubmitted May 5, 2026• Announced May 6, 2026
Tracking the coupled motion of electrons and nuclei on their intrinsic timescales is essential to understanding and controlling photochemical transformations. While attosecond techniques have provided unprecedented insight into electronic dynamics, they have largely been restricted to ionic systems, with nuclear motion often neglected or indirectly inferred. Here, we demonstrate a ``molecular attoscope", which uses shaped laser pulses in the deep ultraviolet to perform a coherent measurement of electronic and nuclear dynamics in an entangled wave packet in neutral benzene. This enables us to trace both the 856-attosecond-period electronic motion and the 36-femtosecond-period nuclear motion. We observe electronic coherence persisting over hundreds of optical cycles, modulated by nuclear dynamics. Our holographic approach is general, and lays the groundwork for coherent measurements capable of visualizing the evolution of the coupled electronic-nuclear wave function in real time.
Tracking the coupled motion of electrons and nuclei on their intrinsic timescales is essential to understanding and controlling photochemical transformations. While attosecond techniques have provided unprecedented insight into electronic dynamics, they have largely been restricted to ionic systems, with nuclear motion often neglected or indirectly inferred. Here, we demonstrate a ``molecular attoscope", which uses shaped laser pulses in the deep ultraviolet to perform a coherent measurement of electronic and nuclear dynamics in an entangled wave packet in neutral benzene. This enables us to trace both the 856-attosecond-period electronic motion and the 36-femtosecond-period nuclear motion. We observe electronic coherence persisting over hundreds of optical cycles, modulated by nuclear dynamics. Our holographic approach is general, and lays the groundwork for coherent measurements capable of visualizing the evolution of the coupled electronic-nuclear wave function in real time.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.
Within this work, electron dynamics in atoms are experimentally and numerically investigated on their natural attosecond time scales. To access these dynamics, ultrashort and moderately-intense few-cycle laser pulses are superposed with attosecond pulsed radiation in the extreme-ultraviolet spectral region. Both these sources are combined in a new experimental vacuum setup which was designed and built up from scratch, also in order to generate the attosecond pulses. Novel experimental schemes are developed which involve the temporal and spectral interferometric utilization of both ultrashort light sources and include the multidimensional spectroscopy employing different dynamical parameters. These are the intensity and the carrier-envelope phase of the laser pulses, as well as their temporal delay with respect to the attosecond pulses. Scientifically, the observation and control of the quantum-motion of bound electrons, as well as the laser-driven quasi-classical motion of free electrons was a key goal of this work. In particular, a bound two-electron wave packet in helium on attosecond time scales was experimentally observed for the first time. This was realized by combining unprecedented high temporal and spectral resolution. In addition, a new coupling mechanism of several doubly-excited states to an effective single-electron continuum is identified and analyzed. Both these two-electron states as well as single-electron states are interferometrically investigated with access to the phase of their quantum-mechanical wave function. This allows the observation and control of the quantum dynamics of two correlated electrons in atoms on the attosecond time scale which was demonstrated within the scope of this work.
Attosecond chronoscopy enables the exploration of correlated electron dynamics in real time. One key observable of attosecond physics is the determination of "time zero" of photoionization, the time delay with which the wave packet of the ionized electron departs from the ionic core. This observable has become accessible by experimental advances in attosecond streaking and reconstruction of attosecond beating by interference of two-photon transitions (RABBIT) techniques. In this Letter, we explore photoionization time delays by strong extreme ultraviolet fields beyond the linear-response limit. We identify novel signatures in time delays signifying strong coupling between atoms and light fields and the light-field dressing of the ion. As a prototypical case, we study the interelectronic coherence and entanglement in helium driven by a strong extreme ultraviolet field. By the numerical solution of the time-dependent Schrödinger equation in its full dimensionality, we show that the time delay of the photoionized electron allows one to monitor the ultrafast variations of coherence dynamics and entanglement in real time.
Laser-dressed photoemission spectroscopy has established itself as the gold standard of attosecond temporal metrology.In this technique, the attosecond structure of an extreme-ultraviolet pulse is retrieved from the wave function of the electron wave packet released during photoionization.Here, we show that this electron wave packet should rather be described using the density matrix formalism, thus allowing one to account for all processes that can affect its coherence, from the attosecond pulse generation to the photoemission and the measurement processes.Using this approach, we reconstruct experimentally a partially coherent electron wave packet with a purity of 0.11 (1 for full coherence).Comparison with theoretical models then allows us to identify the origins of this decoherence and to overcome several limitations such as beam-line instabilities or spectrometer resolution.Furthermore, we show numerically how this method gives access to the coherence and eigencomponents of complex photoelectron wave packets.It thus goes beyond the current measurement of photoionization time delays and provides a general framework for the analysis and understanding of complex photoemission processes.
The quantum interference of two molecular wave packets has been precisely controlled in the B electronic state of the I2 molecule by using a pair of fs laser pulses whose relative phase is locked within the attosecond time scale and its real-time evolution has been observed by another fs laser pulse. It is clearly observed that the temporal evolution changes drastically as a function of the relative phase between the locked pulses, allowing us to read both amplitude and phase information stored in the wave functions of the molecular ensemble.