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SFA: Strong Field AMO Science

P. Bucksbaum; Adi Natan; Ryan Coffee; Matthew Ware, Lucas Zipp, Andrei Kamalov

Program Scope:  The SFA program studies short wavelength strong field interactions in small molecules and atoms. We explore the transient impulsive regime of bound electron dynamics in strongly driven molecules.  On a sub-picosecond time scale we have studied transient impulsive rotational Raman scattering, which is important for molecular alignment and is also displays quantum localization behavior as an archetype for a quantum kicked rotor.  The 10-100 femtosecond time scale is the regime of strongly driven Born-Oppenheimer dynamics, which includes wave packets interacting with and around conical interesections in molecules, as well as light-induced conical intersections that can control intramolecular motion, as well as Berry’s phase or other phenomena associated with separation of time scales among the protons, electrons, and heavier nuclei.  We also study X-ray-molecule interactions, which involve dynamics on the 0.1-10 femtosecond scale of electron-electron interactions.  Here we have studied the competition between proton migration and Auger relaxation in x-ray-ionized molecules at LCLS.  Finally on the shortest time scale studied, 10-100 attoseconds, we observe the temporal dynamics of above-threshold ionization.  This could be an important area of study at future high repetition rate soft x-ray FELs such as LCLS-II.  Progress has been made in all of these areas.

 

Attosecond dynamics in above-threshold ionization:  Attosecond delays in photoemission from atoms and molecules have been observed in several recent studies through the interference of short ionizing pulses of extreme ultraviolet (XUV) radiation with an infrared (IR) reference. Such phase measurement techniques allow the observation of ultrafast dynamics during photoionization. We have now extended these methods to ATI in an intense laser field by measuring the relative phase of different ATI peaks in a manner that is analogous to the XUV single photon phase measurements that employ RABBITT or the attosecond streak camera.

This is multi-path interference of adjacent ATI peaks by a weak probe field at half the frequency. Sideband peaks appear due to absorption or emission of a single probe photon from adjacent ATI levels (Fig. 2a). The outgoing photoelectron has no parity due to mixing of quantum pathways with even and odd ATI orders.  Therefore each sideband asymmetry oscillates with the relative optical phase, similar to the modulation of sideband peaks in RABBITT and the oscillations from interference of even and odd harmonics. Phase shifts in the modulation probe the phase delays in ATI, which are found to be greatly affected by non-perturbative processes.

We are expanding these observations to incorporate more detailed angular information, and also to study different regimes of strong-field ionization.

Fig. 1 (a-c): Electron VMI at relative phase delay phi = 0, pi/2 and pi show the phase- dependent shifts in the ATI distribution.

 

Dynamical localization of angular momentum in non-periodically kicked quantum rotors: Molecular alignment is a useful tool for studies of molecules in the body-centered frame.  Studies of field-free alignment following multiple nonresonant impulses have been particularly useful because they can lead to high degree of alignment while minimizing undesirable background processes such as ionization. Substantial literature exists on the alignment of linear molecules after the application of a multi-pulse trains with uniform pulse separation, but there have not been investigations using multi-pulse sequences with non-uniform time separation.

We have found evidence supporting Dynamic localization in the aperiodically kicked quantum linear rotor by studying the population alignment of 14N caused by pulse trains with non-uniform spacings between individual pulses. We find that the population alignment, a measure of the rotor’s rotational energy, exhibits resistance to change when the excitation train is periodic. Simulations and experiments show the population alignment experiences bounded growth for periodic pulse trains but experiences unbounded growth for disordered, non-periodic pulse trains.  This could point to methods to control molecular alignment in ultrafast experiments utilizing lasers or x-ray lasers in the future.

 

Light-induced conical intersections are laser-induced topological singularities in dressed molecular potential energy surfaces, similar to the natural conical intersections (CIs) responsible for non-radiative energy transfer between electronic states in polyatomic molecules. Like CIs, LICIs feature strong non-adiabatic couplings between electronic and nuclear degrees of freedom.  LICIs can be fully controlled through laser polarization, wavelength, and intensity.

Key Results:

  • First direct observation of LICIs, via LICI-induced interference.
  • Quantum interferences are a universal signature for LICIs.
  • The interferences depend strongly on the energy difference between the initial state and the LICI and are consistent with coherent diffraction around the LICI.

(a) The dressed potential energy surfaces of H2+ featuring LICIs.  (b) The calculated  instantaneous probability of dissociation P(θ,t)diss from a given vibrational state (for example, v=9)  during the interaction with the laser pulse reveals the interferences in the angular distribution of the outgoing fragments. (c) Experimental (top) and calculated (bottom) angular distributions of H2+ dissociation at kinetic energy releases that correspond to specific initial vibrations states. (Natan et al., 2016)

 

Ultrafast x-ray driven proton migration: We have collaborated with another program in the PULSE Institute to study fast proton migration dynamics in x-ray-ionized acetylene at LCLS.  A carbon atom in acetylene HCCH undergoes K-shell ionization, which induces proton motion to form vinylidete CCHH, and at the same time leads to dication formation via Auger decay.  These processes are detected by a second ionization event after a variable time delay.  This allows us to study the interplay between proton motion and Auger relaxation in the molecule.

We have seen that proton migration begins with a few femtoseconds following K-shell ionization. In the future we believe it is possible to map out the electronic stated responsible for the isomerization.

 

References

Berrah, N. and P. H. Bucksbaum (2013). "The ultimate x-ray machine." Scientific American 310(1): 64-71.

Bostedt, C., J. Bozek, P. Bucksbaum, R. Coffee, J. Hastings, Z. Huang, R. Lee, S. Schorb, J. Corlett and P. Denes (2013). "Ultra-fast and ultra-intense x-ray sciences: First results from the linac coherent light source free-electron laser." Journal of Physics B: Atomic, Molecular and Optical Physics 46(16): 164003.

Bucksbaum, P. (2012). "Atomic and molecular physics at the lcls x-ray laser." Mexican Optics and Photonics Meeting.

Bucksbaum, P. (2014). "Molecular dissociation dynamics driven by strong-field multiple ionization." Bulletin of the American Physical Society 59.

Bucksbaum, P. and J. Glownia (2013). "Ultrafast amo physics at the lcls x-ray fel." EPJ Web of Conferences 57: 04001.

Bucksbaum, P., T. Möller and K. Ueda (2012). "Special issue on frontiers of free electron laser science." Journal of Physics B: Atomic, Molecular and Optical Physics 45(16): 160101.

Bucksbaum, P. H. (2011). "Observing structure and motion in molecules with ultrafast strong field and short wavelength laser radiation." ANL (Argonne National Laboratory (ANL), Argonne, IL (United States)).

Bucksbaum, P. H. (2013). "Ultrafast quantum control in atoms and molecules." Ultrafast nonlinear optics, Springer: 105-128.

Bucksbaum, P. H., R. Coffee and N. Berrah (2011). "The first atomic and molecular experiments at the linac coherent light source x-ray free electron laser." Advances in Atomic Molecular and Optical Physics 60: 239-289.

Cryan, J., J. Glownia, J. Andreasson, A. Belkacem, N. Berrah, C. Blaga, C. Bostedt, J. Bozek, N. Cherepkov and L. DiMauro (2012). "Molecular frame auger electron energy spectrum from n2." Journal of Physics B: Atomic, Molecular and Optical Physics 45(5): 055601.

Cryan, J., J. Glownia, N. Cherepkov, P. Bucksbaum and R. Coffee (2011). "Angular dependance of auger electrons from n $ _ {2} $." Bulletin of the American Physical Society 56.

Cryan, J. P., J. M. Glownia, D. W. Broege, Y. Ma and P. H. Bucksbaum (2011). "Ensemble of linear molecules in nondispersing rotational quantum states: A molecular stopwatch." Physical Review X 1(1): 011002.

Doumy, G., C. Roedig, S.-K. Son, C. I. Blaga, A. DiChiara, R. Santra, N. Berrah, C. Bostedt, J. Bozek and P. Bucksbaum (2011). "Nonlinear atomic response to intense ultrashort x rays." Physical Review Letters 106(8): 083002.

Fang, L., T. Osipov, B. Murphy, A. Rudenko, D. Rolles, V. Petrovic, C. Bostedt, J. Bozek, P. Bucksbaum and N. Berrah (2014). "Probing ultrafast electronic and molecular dynamics with free-electron lasers." Journal of Physics B: Atomic, Molecular and Optical Physics 47(12): 124006.

Fang, L., T. Osipov, B. Murphy, F. Tarantelli, E. Kukk, J. Cryan, M. Glownia, P. Bucksbaum, R. Coffee and M. Chen (2012). "Multiphoton ionization as a clock to reveal molecular dynamics with intense short x-ray free electron laser pulses." Physical Review Letters 109(26): 263001.

Farrell, J., B. McFarland, N. Berrah, C. Bostedt, J. Bozek, P. Bucksbaum, R. Coffee, J. Cryan, L. Fang and R. Feifel (2012). "Ultrafast x-ray probe of nucleobase photoprotection." Quantum Electronics and Laser Science Conference, Optical Society of America.

Fung, R., J. Glownia, J. Cryan, A. Natan, P. Bucksbaum and A. Ourmazd (2012). "Towards extracting dynamics below timing jiwer by manifold-‐based machine learning." Signal 1: i2.

Glownia, J. M., A. Ourmazd, R. Fung, J. Cryan, A. Natan, R. Coffee and P. Bucksbaum (2012). "Sub-cycle strong-field influences in x-ray photoionization." Quantum Electronics and Laser Science Conference, Optical Society of America.

Glownia, J. M., A. Ourmazd, R. Fung, J. Cryan, A. Natan, R. Coffee and P. Bucksbaum (2012). "Sub-cycle strong-field influences in x-ray photoionization." Quantum Electronics and Laser Science Conference, Optical Society of America.

Kanter, E., B. Krässig, Y. Li, A. March, P. Ho, N. Rohringer, R. Santra, S. Southworth, L. DiMauro and G. Doumy (2011). "Unveiling and driving hidden resonances with high-fluence, high-intensity x-ray pulses." Physical Review Letters 107(23): 233001.

Li, H., L.-J. Chen, H. P. H. Cheng, J. May, S. Smith, K. Muehlig, J. C. Frisch, A. R. Fry, F. X. Kärtner and P. H. Bucksbaum (2013). "Femtosecond fiber timing distribution system for the linac coherent light source." CLEO: Applications and Technology, Optical Society of America.

McFarland, B., N. Berrah, C. Bostedt, J. Bozek, P. Bucksbaum, J. Castagna, R. Coffee, J. Cryan, L. Fang and J. Farrell (2014). "Experimental strategies for optical pump–soft x-ray probe experiments at the lcls." Journal of Physics: Conference Series 488: 012015.

McFarland, B., J. Farrell, N. Berrah, C. Bostedt, J. Bozek, P. Bucksbaum, R. Coffee, J. Cryan, L. Fang and R. Feifel (2013). "Probing nucleobase photoprotection with soft x-rays." EPJ Web of Conferences 41, 07004 (2013) 41: 07004.

McFarland, B., J. Farrell, N. Berrah, C. Bostedt, J. Bozek, P. Bucksbaum, R. Coffee, J. Cryan, L. Fang and R. Feifel (2013). "Probing nucleobase photoprotection with soft x-rays." EPJ Web of Conferences 41, 07004 (2013) 41: 07004.

McFarland, B., J. Farrell, N. Berrah, C. Bostedt, J. Bozek, P. H. Bucksbaum, R. Coffee, J. Cryan, L. Fang and R. Feifel (2012). "Ultrafast nucleobase photoprotection probed by soft x-rays." Laser Science, Optical Society of America.

Natan, A., M. R. Ware and P. H. Bucksbaum (2014). "Experimental evidence of light induced conical intersections in dissociation of diatomic molecules." International Conference on Ultrafast Phenomena, Optical Society of America.

Natan, A., M. R. Ware and P. H. Bucksbaum (2014). "Experimental observation of light induced conical intersections in a diatomic molecule." CLEO: QELS_Fundamental Science, Optical Society of America.

Natan, A., M. R. Ware, V. S. Prabhudesai, U. Lev, B. D. Bruner, O. Heber, P. H. Bucksbaum (2016). "Observation of Quantum Interferences via Light-Induced Conical Intersections in Diatomic Molecules" Phys Rev Lett 116(14): 143004.

Petrovic, V., S. Schorb, J. Kim, J. White, J. Cryan, J. M. Glownia, L. Zipp, D. Broege, S. Miyabe and H. Tao (2013). "Enhanced strong-field multiple ionization in the vicinity of s-1/s-0 conical intersection in 1, 3-cyclohexadiene." ABSTRACTS OF PAPERS OF THE AMERICAN CHEMICAL SOCIETY, AMER CHEMICAL SOC 1155 16TH ST, NW, WASHINGTON, DC 20036 USA.

Petrovic, V. S., S. Schorb, J. Kim, J. White, J. P. Cryan, J. M. Glownia, L. Zipp, D. Broege, S. Miyabe and H. Tao (2012). "High sensitivity of strong-field multiple ionization to conical intersections." arXiv preprint arXiv:1212.3728.

Petrovic, V. S., S. Schorb, J. Kim, J. White, J. P. Cryan, J. M. Glownia, L. Zipp, D. Broege, S. Miyabe and H. Tao (2013). "Enhancement of strong-field multiple ionization in the vicinity of the conical intersection in 1, 3-cyclohexadiene ring opening." The Journal of chemical physics 139(18): 184309.

Petrović, V. S., M. Siano, J. L. White, N. Berrah, C. Bostedt, J. D. Bozek, D. Broege, M. Chalfin, R. N. Coffee and J. Cryan (2012). "Transient x-ray fragmentation: Probing a prototypical photoinduced ring opening." Physical Review Letters 108(25): 253006.

Roedig, C., G. Doumy, S.-K. Son, C. Blaga, A. DiChiara, R. Santra, N. Berrah, C. Bosteadt, J. Bozek and P. H. Bucksbaum (2011). "Multi photon physics at the lcls." Laser Science, Optical Society of America.

Zipp, L., A. Natan and P. Bucksbaum (2014). "Quantum phase measurements in two color above threshold ionization." Bulletin of the American Physical Society 59.

Zipp, L., A. Natan and P. H. Bucksbaum (2014). "Two color interferences and quantum phase shifts in above threshold ionization." CLEO: QELS_Fundamental Science, Optical Society of America.