We report a new kind of multiphoton dissociation in intense laser fields. H2 + molecular ions were formed in above-threshold ionization of H2 gas by intense 532-nm, 100-ps laser pulses. Observations suggest that multiphoton couplings soften the molecular bond, resulting in dissociation at laser intensities over 50 TW/cm 2 . The ion-dissociation spectra have multiple peaks caused by multiphoton transitions during dissociation of Hh + . PACS numbers: 33.80.WzInterest in small diatomic molecules irradiated by intense laser light has been stimulated by high-intensity photoionization experiments on atoms. At intensities above 10 TW/cm 2 atoms exhibit "above-threshold" ionization (ATI), where they absorb more photons than the minimum needed to ionize. 1,2 Molecules also exhibit ATI, but have additional degrees of freedom that lead to other new phenomena as well, such as multiphoton dissociation and dissociative ionization. 3 " 6 Intense laser fields can alter the kinetic energies of molecular fragments, through Coulomb repulsion following rapid multiple ionization. 7 " 9In this Letter, we report a new phenomenon in intense fields: the softening of molecular bonds. For this study we have chosen nature's simplest molecule, the H2 + molecular ion. We find that in laser fields comparable to the internuclear binding fields (« 3 V/A, or peak intensities of ~ 100 TW/cm 2 ) molecules can become unstable. In effect, the internuclear potentials become repulsive. However, in contrast to the "Coulomb explosions" described above, where dissociation is caused by sudden removal of electrons, the strength of the repulsion due to bond softening is quite gentle, and does not involve removal of any electrons at all: Molecular fragments usually emerge with kinetic energy equivalent to less than one photon.This new phenomenon is similar to ordinary photodissociation, where a molecule falls apart following excitation to a predissociating electronic state. The highintensity version of the process differs, however, in that it involves both multiphoton absorption and stimulated emission. Many photons are absorbed, but most of their energy is returned to the photon field via stimulated emission during the dissociation, resulting in slow ion fragments. The suggestion that molecules might deform in this manner was proposed previously by others. 10 " 12 This is the first experimental verification of this phenomenon.These experiments employed an amplified and frequency-doubled (A, =532 nm, ft

A quantum structuredynamic model of quarks, leptons, weak vector bosons, and Higgs mesons AIP Conf.

A generalized strong-field approximation is formulated to describe atoms interacting with intense laser fields. We apply it to determine angular distributions of electrons in above-threshold ionization (ATI). The theory treats the effects of an electron rescattering from its parent ion core in a systematic perturbation series. Probability amplitudes for ionization are interpreted in terms of quasiclassical electron trajectories. We demonstrate that contributions from the direct tunneling processes in the absence of rescattering are not sufBcient to describe the observed ATI spectra. We show that the high-energy portion of the spectrum, including recently discovered rings (i.e., complex features in the angular distributions of outgoing electrons) are due to rescattering processes. We compare our quasiclassical results with exact numerical solutions.PACS number(s): 32.80.Rm, 42.65.Ky

Abstract:The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-rayinduced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs. ©2010 Optical Society of America

Time-resolved x-ray diffraction with picosecond temporal resolution is used to observe scattering from impulsively generated coherent acoustic phonons in laser-excited InSb crystals. The observed frequencies and damping rates are in agreement with a model based on dynamical diffraction theory coupled to analytic solutions for the laser-induced strain profile. The results are consistent with a 12 ps thermal electron-acoustic phonon coupling time together with an instantaneous component from the deformation-potential interaction. Above a critical laser fluence, we show that the first step in the transition to a disordered state is the excitation of large amplitude, coherent atomic motion.

No abstract

At very high light intensities. the electron energy spectrum in multiphoton iozizatioii ( ~~i j spectroscopy of eve" ;he simpiesi aioms changes irom a singie, we:: defined threshold peak into multiple peaks, separated from one another by the photon energy. This phenomenon is generally referred to as 'above-threshold ionization' (ATI).The original experiments investigating ATI used relatively long laser pulses, with the result that amplitudes, energy widths and angular distributions of the individual photoelectron peaks depended on the laser intensity. I n addition, the widths of the peaks, as well as their absolute energy positions, changed according to the temporal width of the laser pulse. These dependencies were not intrinsic to the ionization process, but rather were all eventually ascribed to ponderomotive forces exerted on free photoelectrons by the laser focus. The ponderomotive effects frustrated comparisons between theoretical calculations and experimental data. More recent studies have shown that a dramatic simplification occurs when M P I is studied with extremely shon laser pulses: both the energies and the momenta of the AT1 electrons become independent of either the laser energy or the pulse duration. Under these cirmmstances, comparisons between theory and experiment can be made in sufficient detail 10 discriminate between competinz models of the high-intensity AT1 process.

Pump-probe time-resolved x-ray diffraction of allowed and nearly forbidden reflections in InSb is used to follow the propagation of a coherent acoustic pulse generated by ultrafast laser-excitation. The surface and bulk components of the strain could be simultaneously measured due to the large x-ray penetration depth. Comparison of the experimental data with dynamical diffraction simulations suggests that the conventional model for impulsively generated strain underestimates the partitioning of energy into coherent modes. 78.47.+p 61.10.-i 63.20.-e The absorption of ultrafast laser pulses in opaque materials generates coherent stress when the pulse length is short compared with time for sound to propagate across an optical penetration depth [1]. The resulting strain field consists of both a surface component, static on time scales where thermal diffusion can be ignored, and a bulk component that propagates at the speed of sound (coherent acoustic phonons). This strain is typically probed by optical methods that are sensitive primarily to the phonon component within the penetration depth of the light [1,2]. However, such methods give little information about the surface component of the strain, and, moreover, they are unable to give a quantitative measure of the strain amplitude.Due to their short wavelengths, long penetration depths, and significant interaction with core electrons, x-rays are a sensitive probe of strain. We note that coherent lattice motion adds sidebands to ordinary Bragg reflection peaks due to x-ray Brillouin scattering if the momentum transfer is large compared to the Darwin width, equivalent to phonons of GHz frequency for strong reflections from perfect crystals. This effect was demonstrated many years ago with acoustoelectrically amplified phonons using a conventional x-ray tube [3].With the recent availability of high brightness short-pulse hard x-ray sources, including third generation synchrotron sources and optical laser based sources [4-6], coherent strain generation and propagation can now be probed by x-ray methods in both the frequency and time domains. Recently, time-resolved diffraction patterns of cw ultrasonically excited crystals were obtained with a synchrotron source [7]. Other experiments have employed picosecond time-resolved x-ray diffraction to study transient lattice dynamics in metals [8], organic films [9], and impulsive strain generation and melting in semiconductors [10][11][12][13][14]. In particular Rose-Petruck et al.[10] demonstrated transient ultrafast strain propagation in GaAs by laser-pump x-ray-probe diffraction. In that experiment, x-rays were diffracted far outside the Bragg peak; however, no oscillations in the diffraction efficiency were detected, and the data were consistent with a unipolar strain pulse. In a similar experiment, Lindenberg et al. [13] detected oscillations in the sidebands for an asymmetrically cut InSb crystal using a streak camera. These oscillations were due to lattice compression and were probed for discrete phonon frequencies i...