Atomic and Molecular Dynamics
Strongly Driven Molecules at LCLS
We are currently investigating various coherent processes in both atoms and molecules that are of value as either LCLS experiments or ultrafast x-ray diagnostics. We have studied and developed methods for rovibrational control in small molecules, and have used them to perform several of the first experiments at LCLS. This LCLS experiment to date have focused on the properties of intense x-ray interactions on laser-aligned molecular gases.
We also have strong LCLS collaborations with other groups at SLAC, LBNL, ANL, Ohio State University and Western Michigan University. We have actively participated in LCLS commissioning this year.
In the VUV regime, we are pursuing selective nonlinear absorbers as a possible means for LCLS beam conditioning.
First lasing and operation of an angstrom-wavelength
PULSE participated in several strong field and short pulse experimentts in atoms, small molecules and clusters, and in the first X-ray imaging of nanoscale materials, in the soft x-ray (sub 2keV) region. The installed Kirkpatrick–Baez mirror producing a focused beam as small as 3 µm in diameter. Several new phenomena were explored, as summarized in the next sections of this summary.
First experiments verified the production of very high charge states through rapid sequential photoionization. The competition between ionizaion and competing Auger relaxation was studied. Indications are that the pulse lengths were as short as less than 10 fs.
LCLS Instrument Scientist John Bozek, and the AMO instrument where PULSE work on ultrafast x-ray science have been performed.
PULSE scientists led the commissioning of timing between the X-rays and an optical laser. and continue to work with LCLS to improve this difficult feature.
Femtosecond electronic response of atoms to ultra-intense
X-rays (see article
This was the first experiment performed at LCLS. The Young group at ANL took the lead, with strong participation from PULSE. The intense focused x-ray laser was capable of fully stripping Ne atoms through rapid sequential inner shell ionization and Auger relaxation. and how very short pulses were unable to fully strip the atoms because the pulse duration was shorter than the spontaneous relaxation process. This leads to the formation of double core vacancies, which are relatively transparent to x-rays.
Ultraintense X-Ray Induced Ionization, Dissociation, and Frustrated Absorption in Molecular Nitrogen. (see article)
M. Hoener, L. Fang, et al., Western Michigan; PULSE participants R. Coffee, J. Cryan, J.M. Glownia, M. Guehr, R. McFarland. Phys. Rev. Lett. 104(25): 253002.
A following experiment continued to study the process of frustrated absorption due to ultrashort and ultraintense x-ray pulses, using N2 as a target gas. This data for this work extended over two successive LCLS runs in Fall 2009: The first run was led by the group of Nora Berrah from Western Michigan University, and the second run, using shorter LCLS pulses, was led by the Bucksbaum group at PULSE, with Ryan Coffee as spokesperson. The lead for the paper was the Berrah group.
Sequential multiple photoionization of molecular nitrogen was suppressed due to frustrated absorption by high charge states at short pulse durations. The inverse scaling of the average target charge state with x-ray peak brightness has possible implications for single-pulse imaging applications.
Ultrafast pump-probe experiments on aligned molecules
Ultrafast pump-probe experiments on aligned molecules
We have commissioned the ability to align the molecules using the impulsive alignment methods we have developed over the past several years at PULSE.
Time-resolved pump-probe experiments at the LCLS
Time-resolved x-ray/optical pump-probe experiments at LCLS require a both feedback methods and post-analysis time binning in order to achieve the best synchronization between ultrafast optical laser pulses and the x-ray laser.
We used transient molecular alignment to test this, by measuring in time-dependent x-ray fragmentation spectra. In these first experiments, the relative arrival time had a standard deviation jitter of 120 fs, mostly due to locking the laser to the reference RF of the accelerator. Simple technical improvements could reduce the jitter to better than 50 fs.
Higher alignment through pulse stacking:
A multiple pulse "stacker" can be used to create much higher degrees of alignment for more precise studies at LCLS in the future. Our demonstration of this new idea was also published in the last year:
Field-free alignment in repetitively kicked nitrogen gas
This figure shows the increasing degree of alignment if eight laser pulses are spaced by the rotational revival time. (Cryan et al. 2009)
This creates a high level of laser-induced transient alignment in room temperature and density N2 but avoids laser field ionization. The alignment approaches the theoretical maximum value. We employ eight equally spaced ultrafast laser pulses with a separation that takes advantage of the periodic revivals for the ensemble of quantum rotors. Each successive pulse increases the transient alignment [<cos2θ(t)>] and also moves the rotational population away from thermal equilibrium. These measurements are combined with simulations to determine the value of <cos2θ>, the J-state distributions, and the functional dependencies of the alignment features. (Cryan et al. 2009)
Auger spectroscopy at LCLS
This year we completed our first studies of Auger electron spectroscopy of exotic multiple core-hole states that can be made in abundance at LCLS, but nowhere else. Emphasis was on spectroscopy of double core holes created in a single molecule.
L. Fang, M. Hoener, et al., Western Michigan Collaboration; PULSE participation by J. Cryan, J.M. Glownia, P.H. Bucksbaum, “,” Phys. Rev. Letters, in press 2010, We investigate the creation of double K-shell holes in N2 molecules via sequential absorption of two photons on a timescale shorter than the core hole lifetime by using intense x-ray pulses from the Linac Coherent Light Source free electron laser. The production and decay of these states is characterized by photoelectron spectroscopy and Auger electron spectroscopy. In molecules, two types of double core holes are expected, the first with two core holes on the same N atom, and the second with one core hole on each N atom. We report the first direct observations of the former type of core hole in a molecule, in good agreement with theory, and provide an experimental upper bound for the relative contribution of the latter type.
Following the initial work on double core holes, we continued looking at the details Auger electrons from double-core holes that provide information about correlated molecular electrons. Using the pump-probe timing protocol we established for LCLS, we then proceeded to repeat the nitrogen double-core hole Auger spectroscopy, but with angular resolution. This gave us the first look at correlation effects in this exotic system, which may be an important mode for transient chemical analysis at intense fourth generation x-ray sources:
James P. Cryan,J. M. Glownia, M. Guehr,B. McFarland, V. Petrovic, D. Reis, J.L. White, P. H. Bucksbaum, R. N. Coffee, PULSE Insitute; plus collaborators from SLAC, Uppsala, LBNL, Western Michigan, Ohio State, LSU, Imperial College, ANL, Saclay, Georgia Tech, Kansas State. Phys. Rev. Lett. 105(8): 083004 (2010)
We observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame. Core-level double vacancies have been observed in atomic systems via a single photon double ionization process. However, the sequential formation of double core vacancies relies on the extremely high peak intensity LCLS to induce photoionization rates that exceed Auger relaxation rates.
We investigated impulsive laser alignment of molecular iodine at a density of ~1018 molecules/cm3 and a temperature of 120 C using a 1.5 mJ 40 fs 800 nm laser system . In this experiment, we split the 1.5mJ beam into two arms. One arm is sent through a 250mm BBO crystal and produces ~20uJ of 400nm light. A polarizer ensures that this beam is horizontally polarized. The other arm serves as the laser alignment pulse. The alignment arm is polarization rotated to 45 degrees to the horizontal. This induces a transient birefringence in the gas cell. We observe this transient birefringence by measuring the amount of 400 nm light exiting the cell with vertical polarization. In Figure 1 we show the half- and full-revivals for iodine. The oscillatory signal is due to the strong effect of centrifugal distortion that exists for such a thermally hot ensemble of rotors.
We have also begun a program to produce and detect impulsive molecular alignment along the direction of laser propagation. We have constructed an interferometric technique that is capable of detecting molecular alignment in a gas cell that does not produce a birefringence signal. This technique is run in parallel to the previously developed birefringence technique of Ref.  and allows us to measure and distinguish molecular alignment along any of the lab-fixed directions with only one, co-propagating, probe beam. This experiment is nearing completion. Upon final development of this technique, it will be used to produce laser aligned molecules at the LCLS where the alignment axis can be switched in real time from X to Y to Z for scattering experiments.