We report on the measurement of electron emission after the interaction of strong laser pulses with atoms and molecules. These electrons originate from high-lying Rydberg states with quantum numbers up to n 120 formed by frustrated field ionization. Simulations show that both tunneling ionization by a weak dc field and photoionization by the black-body radiation contribute to delayed electron emission on the nano-to microsecond scale. We measured ionization rates from these Rydberg states by coincidence spectroscopy. Further, the dependence of the Rydberg-state production on the ellipticity of the driving laser field proves that such high-lying Rydberg states are populated through electron recapture. The present experiment provides detailed quantitative information on Rydberg production by frustrated field ionization.PACS numbers: 32.80. Rm, 32.80.Fb, 42.50.Hz Ionization of atoms and molecules by strong laser fields is the starting point for a multitude of interesting phenomena, e.g., high harmonic generation or molecular fragmentation [1]. For sufficiently strong laser fields corresponding to intensities of the order of I ≈ 10 14 W/cm 2 , atoms and molecules are ionized via tunneling ionization, i.e., an electron passes through the potential barrier of the combined Coulomb and laser fields. After tunneling, electrons are steered by the laser field and most of them will eventually escape the Coulomb field of the remaining ion core. However, a fraction of them are recaptured into highly excited states by the ionic Coulomb field. This process frequently referred to frustrated field ionization (FFI) [2,3] leads to the formation of high-lying Rydberg states with binding energies extending from a fraction of an eV to values of µeV near threshold.Very high-lying Rydberg states with principal quantum numbers n ≈ 100 are quantum objects of macroscopic size allowing for studies of the border between the quantum and the classical worlds [4]. Formation and destruction of such mesoscopic objects can be described by semiclassical and classical methods [5]. Recent experiments on high harmonic generation and electron wave packet interferometry indicate the important contribution of such excited states to different processes [6][7][8][9][10] including ionization and molecular dissociation processes [11][12][13][14]. However, detailed and quantitative information on the FFI following the interaction of femtosecond laser pulses with atoms and molecules appears to be scarce.To explore the production process and the properties of high-lying Rydberg states formed in the strong field interaction with atoms and molecules, direct observation of such states is required. Traditionally, zero kinetic energy photoelectron spectroscopy is applied to study weakly bound states in atoms and molecules [15]. In case of strong field interaction, however, the ionization signal from Rydberg states is completely overshadowed by the dominant laser field-induced ionization signal from the target and the residual gas in the interaction chamber. There...

We perform a proof-of-principle demonstration of chemically specific standoff gas sensing, in which a coherent stimulated Raman signal is detected in the direction anticollinear to a two-color laser excitation beam traversing the target volume. The proposed geometry is intrinsically free space as it does not involve back-scattering (reflection) of the signal or excitation beams at or behind the target. A beam carrying an intense mid-IR femtosecond (fs) pulse and a parametrically generated picosecond (ps) UV Stokes pulse is fired in the forward direction. A fs filament, produced by the intense mid-IR pulse, emits a backward-propagating narrowband ps laser pulse at the 337 and 357 nm transitions of excited molecular nitrogen, thus supplying a counter-propagating Raman pump pulse. The scheme is linearly sensitive to species concentration and provides both transverse and longitudinal spatial resolution.

No abstract

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the lifetime of the long-lived ethylene dication leading to the delayed deprotonation and the relative channel strength of the delayed deprotonation compared to the prompt one have no evident dependence on the laser pulse duration and the laser peak intensity. Quantum chemical simulations suggest that such delayed fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on a dication electronic state along C-H stretching. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem crossing from electronically excited states to the electronic ground state in the case of acetylene.

No abstract