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Chemical Sciences and Engineering Division |
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Atomic, Molecular and Optical PhysicsX-ray processes intrinsic to an atom or molecule can be controlled using intense optical fields. Optical fields in the range of 1012 W/cm2 to 1015 W/cm2 create changes in outer shell electronic structure which in turn modify x-ray absorption and scattering processes. In order to interrogate atoms and molecules in the extreme environment of a high-intensity optical pulse, four-dimensional overlap (three spatial, one temporal dimension) of an optical pulse with an x-ray pulse is necessary. Imagine two bullets (of few micron diameter) racing at the speed of light which must coincide with each other. Experimental methods to achieve this level of overlap have been developed and are combined with theoretical analysis of x-ray interactions in the presence of intense optical fields to demonstrate optical control of x-ray absorption and scattering. The control of x-ray processes is demonstrated in simple atoms and molecules to develop an understanding from first principles. This frontier area exploring the control of x-ray processes is made possible by combining advances in laser technology with tunable polarized ultrafast x-rays. Ultrashort x-ray pulses from a variety of sources with unique properties (Argonne's Advanced Photon Source (APS), Berkeley's femtosecond slicing soft x-ray beamline, Ohio State University's attosecond/femtosecond EUV source, eventually Stanford's Linac Coherent Light Source, world's first x-ray free electron laser) are used to probe electron and molecular dynamics with atomic-scale temporal, spatial, and spectral resolution. Four lines of research are currently being pursued:
Ultrafast Field Ionization: Polarization and Quantum State Properties of the Residual IonModern ultrafast optical lasers, when focused to intensities of 1015W/cm2, provide field strengths (~10 Volts/Ångstrom) comparable to the binding energies of electrons in outer orbits of atoms. When an atom is subjected to a linearly polarized electric field at this strength, the Coulomb barrier is suppressed and an outer electron tunnels into the continuum. This creates an ion with a hole orbital aligned with the laser polarization axis. Because the x-ray absorption process promotes a localized inner-shell electron to an empty orbital, i.e. a hole orbital, polarized x-rays can be used to probe the direction of the hole orbital. That is, if the hole orbital is aligned with the x-ray polarization axis (when the laser and x-ray polarizations are parallel) then there will be a strong x-ray absorption, whereas if the hole orbital is perpendicular to the x-ray polarization, there will be a weak x-ray absorption. To be concrete, we have observed that optical field ionization of krypton produces a hole orbital that is aligned with the polarization axis of the ionizing laser. However, we find that the degree of orbital alignment is not 100%, as might be expected for the simplest model, but rather is suppressed by internal atomic interactions; spin-orbit recoupling occurs on a timescale of 6.2 fs [1,2]. The quantum state distribution (i.e. the j,m-sublevel populations) in the field-ionized residual ion can also be obtained. Inspired by the successful extraction of the quantum state populations of laser-generated Xe ions [3], we applied a similar procedure to the krypton [4]. The situation is more challenging because of the large decay width of the K-shell excited states observed in the Kr experiment carried out at the APS. Therefore, more care is needed to separate the individual fine-structure components of the 1s → 4p transition. We have formulated the theoretical tools needed for extracting the j,m- sublevel populations from the experimental data [4]. It turns out that in the Kr experiment at the APS, the sensitivity to the spatial anisotropy of the laser-produced ions is higher by an order of magnitude than in the Berkeley experiment on Xe [3]. We further find in this gas phase system, with ion/electron density ~1015/cm3, that memory of the orbital alignment persists indefinitely until an electron-ion dealignment collision [5]. The collection of aligned ions, randomly distributed in space, but all pointing in the same direction can be induced to precess about an externally applied magnetic field, giving an in situ measure of magnetic field in a laser-produced plasma [5].
(1) L. Young et al., Phys. Rev. Lett. 97,
083601 (2006). X-ray microprobe of orbital alignment in strong-field ionized
atoms. Electromagnetically Induced Transparency (EIT) for X-RaysIf we turn down the intensity of the optical laser to 1013 W/cm2, the ultrafast field ionization described above does not occur in inert gas atoms. Nevertheless, this intensity is sufficient to drive resonant transitions between outer-shell electronic states (Rabi flopping) on the fs timescale, comparable to inner-shell decay lifetimes. Combining the rapid Rabi flopping between the outer shell electronic states with a resonant inner-shell transition creates a coupled three-level “lambda” system. With both laser and x-rays on resonance in the lambda system, the x-ray absorption on the inner-shell resonance can be suppressed – a phenomenon known as electromagnetically induced transparency (EIT) for x rays [1]. Because the laser-induced effect is reversible, EIT can shape x-rays pulses using ultrafast optical pulses. Shown is the creation of two short x-ray pulses from a single long pulse using two short laser control pulses. Electromagnetically induced transparency has been a topic of much interest in the visible region of the spectrum. Research has focused primarily on three-level lambda systems, where the upper level decays radiatively while the two lower levels are stable. A coupling laser is used to modify the absorptive and dispersive properties of the system to generate transparency and slow light, respectively. In the x-ray regime, the situation is considerably more complex. Theory demonstrates that the x-ray absorption spectrum of a neon atom can be rendered transparent at a selected wavelength by application of a strong optical field (1013 W/cm2 at 800 nm) [1,2]. The actual absorption spectrum is considerably more complex than the simple three-level model would predict. This work points the way toward producing ultrafast, shaped x-ray pulses by laser irradiation of a simple gaseous target. An experimental demonstration is being attempted at Berkeley’s Advanced Light Source at the femtosecond slicing beamline, 6.0.1. (1) C. Buth, R. Santra, and L. Young, Phys. Rev. Lett. 98,
253001 (2007). Electromagnetically induced transparency for x-rays. X-Ray Probes of Laser-Aligned MoleculesSubjecting a molecule to a non-resonant, linearly-polarized laser field of intensity of 1012 W/cm2 causes it to align along its most polarizable axis. Three-dimensional alignment can be achieved for molecules with three distinct moments of inertia using elliptically polarized light. Such methods are expected to be useful for studying biomolecule structure with intense x-ray free-electron lasers. At Argonne, we are developing x-ray methods to understand quantitatively the structure of molecules aligned by non-resonant laser fields. At intensities of ~1012 W/cm2 molecules align due to the interaction between the polarized laser field and the anisotropic polarizability of the molecule. At Argonne, we align small molecules with laser fields and determine their structure in the presence of the fields using x-ray methods: such as extended x-ray absorption fine structure or x-ray diffraction. In the simplest picture, it is assumed that the molecular structure is unperturbed by the presence of the aligning field. However, the molecular framework can be distorted from the equilibrium structure by multiphoton mixing between ground and excited states. A major goal is to develop methods to predict such structural distortions through a combination of theory and experiment. Even in so-called “field-free” alignment studies, where a rotational wavepacket is created by an impulse from a non-resonant laser, the molecular structure will not be the ground state equilibrium structure. These distortions could also be measured with x-ray methods. We have developed an x-ray probe of laser-aligned molecules and a theoretical model that describes x-ray absorption in samples aligned by optical pulses of arbitrary shape [1, 2]. An ensemble of aligned CF3Br molecules was created with an 800 nm laser pulse of ~100 ps duration at an intensity of 1012 W/cm2. CF3Br is a symmetric top molecule where the C-Br axis is the most polarizable and the s* lowest unoccupied molecular orbital lies along this axis. As a simple initial step, we use resonant x-ray absorption in the near-edge region to probe the aligned molecules [1,2]. Absorption at the dipole transition, Br 1s®s*, probes the alignment of the C-Br axis relative to the x-ray polarization axis. We use the change in absorption of the Br 1s®s* resonance to measure the alignment of the ensemble of molecules [1]. For our jet-cooled sample (10% CF3Br/90% He) we find an alignment consistent with a rotational temperature of 20 K, as determined by a comparison with theory. Due to the duration (100 ps) of the x-ray pulses at the Advanced Photon Source we currently are using adiabatic (Trot<<Tlaser) alignment methods, where the laser field is present to align molecules. However, the short pulse x-ray project at the Advanced Photon Source promises 1-ps duration x-rays which will be sufficiently short for impulsive (field-free) alignment studies. In the larger picture, our scheme to produce an ensemble of ~107 aligned molecules in the gas phase and x-ray the ensemble with the Advanced Photon Source, can be an alternative to atomic-level structure determination of non-crystalline samples using x-ray free-electron lasers. [1] E. R. Peterson, C. Buth, D.
A. Arms, R. W. Dunford, E. P. Kanter, B. Krässig, E. C. Landahl, S. T. Pratt, R.
Santra, S. H. Southworth, and L. Young, Appl. Phys. Lett. 92, 094106
(2008). An x-ray probe of laser-aligned molecules. Nonlinear Interactions at LCLS: World’s First X-Ray Free Electron LaserThe world’s first x-ray free electron laser, the Linac Coherent Light Source (LCLS) at Stanford, is due to be commissioned in August 2009 and the AMO group is part of a team of scientists that will be the first to use this novel light source. Since the LCLS is based upon SASE (self-amplified spontaneous emission) the pulses are expected to be temporally and energetically chaotic. Thus, initial studies characterizing source properties are expected to be extremely valuable for all experimenters. The extreme per pulse brilliance of the LCLS also provides an opportunity to investigate nonlinear hard x-ray processes for the first time. Focussing to micron-size dimensions to obtain 1018 W/cm2 is critical for observing nonlinear processes. In contrast to the long-wavelength regime, x-ray nonlinear optical processes are characterized in general by sequential single-photon single-electron interactions. In principle, despite this fact, probabilities for these multiphoton processes involve higher-order correlation functions of the radiation field. We demonstrated that double-core-hole formation via x-ray two-photon absorption is enhanced by chaotic photon statistics. Numerical calculations using rate equations illustrated the impact of field chaoticity on x-ray nonlinear ionization of helium and neon for photon energies near 1 keV. In the case of neon, processes were discussed that involve up to seven photons. Assuming an x-ray coherence time of 2.6 fs, double-core-hole formation in neon was found to be statistically enhanced by about 30% at an x-ray intensity of 1016 W/cm2 [1]. It also became clear during the course of this investigation that for processes that do not require photon absorption to take place within the lifetime of an inner-shell vacancy, the detailed x-ray pulse properties are irrelevant, as long as one averages over a number of shots. This means that for most experiments using LCLS, there is no need for a detailed shot-by-shot characterization of the x-ray pulses. [1] N. Rohringer and R. Santra, Phys. Rev. A 76, 033416 (2007). X-ray nonlinear optical processes using a self-amplified spontaneous emission free-electron laser. ContactLinda Young, Group Leader |
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