Visualization of atomic and molecule excitation/ionization processes with ultra-short (attosecond) and optical gated (femtosecond) laser pulses and study of high harmonic generation (HHG) in a filament

在过去的一年中，我们已经能够用我们的Femtolaser V系统（800 nm，5 mJ，1 kHz，CEP稳定）产生高达130 eV脉冲的高谐波光谱，在差分泵浦气体电池中产生的灯丝。我们计划通过TOPAS系统扩展该区域。我们已经测量了CEP（载波包络相位）稳定的激光脉冲的2400 nm至1 mJ。 该装置将允许我们将HHG截止区扩展到400 eV。*科学目标分为三个不同的轴。*在第一个项目中，我们想研究激发过程和内壳层电离以及分子和原子的内壳层激发过程。因此，我们将使用（飞秒）泵浦-（亚飞秒）探测实验，并测量两个激光脉冲的电子光谱在一起。在电子谱的低能部分得到飞秒泵浦激光的ATI（阈上电离）谱，在高能部分测量探测脉冲。我们仅限于来自ATI谱的电子的能量分布不与来自探测脉冲的电子能谱重叠的激光强度。当来自泵浦脉冲和探测脉冲的电子的动量分布不同时，可以克服这种限制。 我们希望升级我们的VMI（速度图成像）光谱仪来测量这两种贡献。利用这种技术，我们将能够研究分子和原子的内壳层电离、内壳层激发过程。这为HHG实验提供了补充信息。*在第二个项目中，我们希望可视化激光子周期中发生的过程。*我们已经开始使用偏振门控激光脉冲进行原子电离/激发实验，并率先发布了VMI电子光谱。这些激光脉冲由脉冲前沿和后沿的圆偏振部分和实验定义的线性偏振中心部分组成。中心部分可以被限制到一个光学周期的一小部分。 仅在线性偏振下发生的过程可以在时间上限制为半个周期。我们希望将这种方法用于不同的波长以及具有子周期线性偏振的激光脉冲的电离过程。我们希望更好地了解灯丝中HHG的产生，并继续我们最近开始的800 nm实验。稍后，我们将把这些实验扩展到TOPAS系统产生的其他波长。在那里，我们想要研究原子/分子共振的影响以及放大灯丝中HHG信号的可能性。测量，其中强度以及波长相关的HHG光谱同时进行将是一个理想的工具，以可视化原子/分子高激光场物理。

In the past year, we have been able to generate with our Femtolaser V system (800 nm, 5 mJ, 1 kHz, CEP stabilized) high harmonic spectra up to 130 eV pulses in a filament created in a differential pumped gas cell. We plan to extend this region with our TOPAS system. We have already measured CEP (carrier envelope phase) stabilized laser pulses of 2400 nm up to 1mJ. The setup will allow us to extend the HHG cut-off region up to 400 eV.****The scientific goals are divided into 3 different axes.****In a first project we want to study excitation processes and inner-shell ionization as well as inner shell excitation processes of molecules and atoms.****Therefore we will use (femtosecond) pump - (sub-femtosecond) probe experiments and measure the electron spectrum of both laser pulses together. We will obtain in the low energy part of the electron spectrum the ATI (above threshold ionization) spectrum of the femtosecond pump laser, the high energy part will measure the probe pulse. We are only limited to laser intensities where the energy distribution of electrons from the ATI-spectrum does not overlap the electron energy spectrum from the probe pulse. This restriction can be overcome when the momentum distribution from the electrons of the pump and probe pulse is different. We want to upgrade our VMI (velocity map imaging) spectrometer to measure both contributions. With this technique we will be able to study inner-shell ionization, inner shell excitation processes of molecules and atoms. This offers us complementary information to HHG experiments.****In a second project we want to visualize processes taking place in a sub cycle of a laser.****We have started atom ionization/excitation experiments with polarization gated laser pulses and have been the first to publish VMI electron spectra. These laser pulses are composed of circularly polarized sections at the leading and trailing edges of the pulse and of an experimentally defined linearly polarized central part. The central part can be restricted down to a fraction of an optical cycle. Processes taking place only at linear polarization can be restricted in time down to a half cycle. We want to use this method for different wavelengths as well as ionization processes with laser pulses with sub-cycle linear polarization.****We want to better understand HHG generation in a filament and continue the experiments we have started recently with 800 nm. Later we will extend these experiments to other wavelengths generated by the TOPAS system. There we want to study the effect of atomic/molecular resonances as well as the possibility to amplify a HHG signal in a filament. Measurements, where the intensity as well as the wavelength dependent HHG spectra are taken at the same time will be an ideal tool to visualize atom/molecular high laser field physics.*****