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

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 (see article)
James M. Glownia, J. Cryan, M. Gühr, B. K. McFarland, H. Merdji, V. S. Petrovic, D. A. Reis, M. Trigo, J. L. White, R. N. Coffee, and P. H. Bucksbaum from PULSE, plus collaborators from ANL, Imperial College, Western Michigan University, Georgia Tech, SLAC, Ohio State, LBNL,  Opt. Express 18, 17620-17630 (2010)

  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 (see article)
J. P. Cryan, P. H. Bucksbaum, and R. N. Coffee, "," Physical Review A (Atomic, Molecular, and Optical Physics), 80:6 (2009), 063412-063415.

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.

Double core hole production in N2: Beating the Auger clock (see article)
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:

 

Auger electron angular distribution of double core hole states in the molecular reference frame  (see article)
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.

 

 

 

Molecular Alignment
Doug Broege, James Cryan, Ryan N. Coffee, and Phil H. Bucksbaum

Figure 1 Transient birefringence I2 following impulsive excitation with 800 nm, 40 fs pulses. Data are represented by gray circles and the semi-classical simulation is represented by the solid line.

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 [1]. 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. [1] 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.

 

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