Skip to content Skip to navigation

ATO: Attosecond Science

ATO – Attosecond Science @ Stanford PULSE Institute

PIs: James Cryan and Philip Bucksbaum
Post Doctoral Researchers: Elio Champenois, Taran Driver
Graduate Students: Andrei Kamalov, Jordan O’Neal, and Anna Wang

The Attosecond Science Group is currently looking for Graduate Students. Contact James Cryan for opportunities.

Electrons provide the screening to stabilize molecules, and thus electronic redistribution initiates all chemical changes, including isomerization and dissociation. This means that electron motion is the means by which light energy is harnessed in photochemistry. In order to understand the earliest processes involved in chemical change, it is only natural to attempt to track the evolution of electrons on their intrinsic timescales. The natural timescale for electron motion in atoms and molecules is about 100 times faster than the nuclei: electrons move across a molecular bond in 0.1 to 1 femtosecond. The attosecond science group at the Stanford PULSE Institute focuses on dynamics on this timescale, which is a science frontier both for experiments and for quantum many-body theory.

Ultrafast optical techniques are routinely used to interrogate chemical reactions on the timescale intrinsic to chemical change (i.e. the time scale for nuclear dynamics). These observations have given birth to the field of “femtochemistry.” But nuclear motion is rather slow compared to the rapid motion of electrons, which occurs on the attosecond timescale. The attosecond science group focuses on understanding electronic motion on its natural timescale. 

Recent Publications by the Attosecond Group:

Current Research Areas:

Charge Dynamics on the Attosecond Timescale 

What is particularly important for understanding the problem of electron motion is a comprehension of how a coherent superposition of electronic states evolves in time. More specifically, how do these superpositions couple to other degrees of freedom (e.g. nuclear motion) in a molecular system? It is not surprising that a superposition of excited electronic states with energy bandwidth of several electron volts will produce transient charge dynamics on sub-femtosecond timescales, but the more interesting question is what effect this coherence will have on nuclear motion and thus the chemistry of the molecule? This is the key question behind what has been dubbed attosecond chemistry. As electronic charge migrates across the molecular backbone, the nuclei respond. This charge-directed reactivity has been proposed as a mechanism for the fragmentation of large molecularsystems. Furthermore, coherent preparation of a superposition of electronic states could exert some control over chemical reactivity by effectively bypassing IVR (intramolecular vibrational energy redistribution). Nuclear motion (in all but the most extreme cases) will lead to a change in the electronic energies and thus the phase accumulation of the various components of the wavepacket, leading to dephasing of the initially coherent electronic wavepacket and eventual localization (decoherence) of the electronic charge. Attosecond chemistry is still very much in its infancy, and we are just now developing the requisite tools for probing electron motion.

References on this topic:

Attosecond Photoemission

Photoelectron spectrogram from two-color ionization of CO2 (bottom panel). The vertical lines show the energy values assigned to sidebands originating from the X,A, and B states of the CO2 cation. The top panel shows the normalized XUV only photoelectron spectrum, snd the middle plot shows the normalized amplitude of the 2ω Fourier component. All features in the middle plot can be assigned to either a HH or SB of a CO2 cationic system.

Time domain measurements of quantum mechanical phenomena often lead to interesting conceptual questions. As an example of a quantum mechanical phenomenon, we consider the photoelectric effect, which was originally discussed by Einstein. In molecular systems, photoelectron spectroscopy provides unique access to the transient dynamics of excited state species on ultrafast timescales. But despite the power of this technique,  the direct observation of the time evolution of the photoelectric effect has only recently become possible.  Such measurements require sub-femtosecond temporal resolution to capture the motion of photoelectrons as they escape the parent species.

A train of of extreme ultraviolet (XUV) light bursts produced by high harmonic generation with a multicycle laser pulse is capable driving of single-photon ionization to produce well-characterized electronic wavepackets (EWPs). The interaction of the outgoing EWP with the residual ionic potential leads to dispersion of the EWP. The main result of the dispersive potential is an overall time delay of the photoemission process. We probe these delays using two-color photoionization with synchronized attosecond pulse trains (APTs) and infrared (IR) dressing pulses. Recording the sideband modulations in the resultant measured photoelectron spectrum as a function of APT/IR delay allows us to reconstruct the EWP, and determine the attosecond delays in photoemission.

References on this topic:
ATO Publications on this topic:

Impulsive Stimulated X-ray Raman Scattering

Another approach for initiating transient charge dynamics in molecular systems is impulsive stimulated X-ray Raman scattering (SXRS), where an inner-shell electron is excited by an incident X-ray pulse to an unoccupied valence or continuum orbital.  Before this core-excited state decays, a second interaction with the incident  X-ray pulse stimulates a transition to refill the inner-shell vacancy from an occupied valence orbital. In the impulsive limit of SXRS the pump and Stokes (stimulating) photons come from the same pulse, and for sufficient coherent X-ray bandwidth, the ground state undergoes a transition to a coherently phased combination of electronic states, which evolves as an electronic wavepacket.

SXRS is an attractive alternative to single photon impulsive ionization, because (1) it creates coherences in the neutral molecule, which is more relevant for chemical applications, (2) it creates spatially localized coherences, because the SXRS excitation process results from the oscillator strength of a particular atomic site, and (3) SXRS is a stepping stone to non-linear spectroscopy, such as four wave mixing schemes, which are indispensable tools for studying vibrational coherence with visible pulses. Numerous non-linear spectroscopies have been proposed for studying ultrafast charge dynamics, all of which are predicated on the ability to drive SXRS.

References on this topic:
ATO Publications on this topic:

In SXRS, a molecular system simultaneously absorbs and emits two different photons from an attosecond x-ray pulse, which has a bandwidth greater than the ground-to-valence excited state energy, i.e. δω>Ev-Eg. The intermediate state, Ec, is typically a core-excited state where an inner shell electron has been moved to a previously unoccupied valence orbital.

Attosecond Source Development

Until recently, the direct measurement of dynamics on the attosecond scale was impossible. A 500 as pulse has a minimum coherent spectral bandwidth of ~3.5 eV. Such large bandwidths are nearly impossible to produce with visible wavelengths. Such bandwidths can only be achieved with higher frequency electromagnetic radiation, e.g. extreme ultraviolet (XUV) light or X-rays. With innovations in strong-field driven high harmonic generation (HHGit has become possible to synthesize light pulses with sub-femtosecond duration and begin to probe dynamics on these extreme timescales. As a compliment to HHG-based X-ray sources, free electron laser facilities (FELs) routinely produce high pulse energy, soft X-ray (SXR) pulses. However, the pulse duration of FEL sources is limited by the FEL amplification bandwidth.

We have two different attosecond source development projects based on HHG. In our main laboratory space in the PULSE Institute, we have a laser source and beam line designed to produce high intensity XUV pulses with sub-femtosecond pulse durations. HHG from laser pulses containing multiple laser cycles produces attosecond bursts of VUV/XUV radiation every half cycle of the driving infrared laser pulse. In order to produce an IAP, one needs to filter out the attosecond burst from a single cycle of the driving laser pulse. In our laboratory, we have employed a spatio-temporal coupling (STC) scheme. In a spatio-temporal coupled laser field, spatial properties of the beam vary in time; for example, the direction of propagation can vary throughout the laser pulse duration, which is referred to as ultrafast wavefront rotation (WFR). An intense pulse with this particular STC will produce XUV bursts (or beamlets) every half laser cycle, but the beamlets will propagate in different angular directions. These beamlets can be filtered to isolate a single attosecond burst.

In addition to our high energy attosecond pulse source, the Stanford PULSE Institute has partnered with the LCLS laser group (Alan Fry and Franz Tavella) to build a 100 kHz, HHG based, soft X-ray beam line. Based on previous results, we anticipate nearly 1010 photons/sec from this source. The SLAC Laboratory Directed Research and Development (LDRD) program currently supports this project, with the end goal of enabling time-resolved X-ray absorption spectroscopy (trXAS) in gas phase molecular systems. The existing project is focused on chemical dynamics on the 10-100 fs scale. The ATO group is working to extend the capabilities to break below the femtosecond barrier. The HHG emission spectrum from a similar experimental setup has demonstrated that this extremely broad emission spectrum is coherently phased, such that the time-domain emission is below 100 as.

The ATO group is also involved in the development of attosecond pulses from FELs. Through an ongoing collaboration, led by Agostino Marinelli, we have developed an electron bunch shaping method based on the enhanced self-amplified spontaneous emission (ESASE) technique. This technique is being implemented at LCLS through the XLEAP project. Recently, we conducted experiments to measure the duration of soft X-ray pulses generated using the ESASE method. In these experiments, we recorded a 2-dimensional projection of the 3-dimensional photoelectron momentum distribution produced in two-color photoionization of gas-phase targets. Depending on the phase of the infrared streaking field at the time when the X-ray pulses photoionizes the target, the photoelectrons will experience a momentum shift proportional to the magnitude of the streaking laser vector potential. We use this phase dependent impulse can be used to reconstruct the temporal profile (and phase) of the incident X-ray pulse.

References on this topic:
ATO Publications on this topic:

Collaborations Within The Stanford PULSE Institute: