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

在过去的一年里，我们已经能够用我们的Femtolaser V系统(800 nm，5 MJ，1 kHz，CEP稳定)在差动泵浦气室产生的灯丝中产生高达130 eV的高次谐波脉冲。我们计划用我们的TOPAS系统扩展这一地区。我们已经测量了2400 nm高达1mJ的CEP(载波包络相位)稳定激光脉冲。该装置将允许我们将HHG截止区域扩展到400 eV。*科学目标分为三个不同的轴。*在第一个项目中，我们想要研究分子和原子的激发过程和内层电离以及内层激发过程。*因此，我们将使用(飞秒)泵浦(亚飞秒)探测实验，同时测量两个激光脉冲的电子光谱。我们将在电子光谱的低能部分获得飞秒泵浦激光的阈值以上电离(ATI)谱，而在高能部分将测量探测脉冲。我们仅限于来自ATI光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。当抽运脉冲和探测脉冲的电子的动量分布不同时，这种限制可以克服。我们想升级我们的速度图成像(VMI)光谱仪，以测量这两种贡献。利用这一技术，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。这为我们提供了HHG实验的补充信息。*在第二个项目中，我们想要可视化发生在激光亚周期中的过程。*我们已经开始了偏振门控激光脉冲的原子电离/激发实验，并首次发表了VMI电子光谱。这些激光脉冲由脉冲前沿和后缘的圆偏振部分和实验定义的线偏振中心部分组成。中心部分可以限制到一个光学周期的一小部分。仅在线性极化下发生的过程可以在时间上限制到半个周期。我们想要将这种方法用于不同波长以及具有亚周期线偏振的激光脉冲的电离过程。*我们想要更好地了解灯丝中HHG的产生，并继续我们最近开始的800 nm的实验。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高次谐波信号的可能性。同时获得与强度和波长相关的高次谐波光谱的测量将是可视化原子/分子强激光场物理的理想工具。

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.*****