Physikalische und Theoretische Chemie, Institut für Chemie und Biochemie, Freie Universität Berlin, Takustrasse 3, 14195 Berlin, Germany.
J Phys Chem A. 2012 Nov 26;116(46):11398-405. doi: 10.1021/jp3063977. Epub 2012 Sep 18.
We study pump-probe schemes for the real time observation of electronic motion on attosecond time scale in the molecular ion H(2)(+) and its heavier isotope T(2)(+) while these molecules dissociate on femtosecond time scale by solving numerically the non-Born-Oppenheimer time-dependent Schrödinger equation. The UV pump laser pulse prepares a coherent superposition of the three lowest lying quantum states and the time-delayed mid-infrared, intense few-femtosecond probe pulse subsequently generates molecular high-order harmonics (MHOHG) from this coherent electron-nuclear wavepacket (CENWP). Varying the pump-probe time delay by a few hundreds of attoseconds, the MHOHG signal intensity is shown to vary by orders of magnitude. Due to nuclear movement, the coherence of these two upper states and the ground state is lost after a few femtoseconds and the MHOHG intensity variations as function of pump-probe delay time are shown to be equal to the period of electron oscillation in the coherent superposition of the two upper dissociative quantum states. Although this electron oscillation period and hence the periodicity of the harmonic spectra are quite constant over a wide range of internuclear distances, a strong signature of nuclear motion is seen in the actual shapes and ways in which these spectra change as a function of pump-probe delay time, which is illustrated by comparison of the MHOHG spectra generated by the two isotopes H(2)(+) and T(2)(+). Two different regimes corresponding roughly to internuclear distances R < 4a(0) and R > 4a(0) are identified: For R < 4a(0), the intensity of a whole range of frequencies in the plateau region is decreased by orders of magnitude when the delay time is changed by a few hundred attoseconds whereas in the cutoff region the peaks in the MHOHG spectra are red-shifted with increasing pump-probe time delay. For R > 4a(0), on the other hand, the peaks both in the cutoff and plateau region are red-shifted with increasing delay times with only slight variations in the peak intensities. A time-frequency analysis shows that in the case of a two-cycle probe pulse the sole contribution of one long and associated short trajectory correlates with the attenuation of a whole range of frequencies in the plateau region for R < 4a(0) whereas the observed red shift for R > 4a(0), even in the plateau region, correlates with a single electron return within one-half laser cycle.
我们研究了泵浦探测方案，以便在分子离子 H(2)(+)及其较重的同位素 T(2)(+)上实时观察飞秒时间尺度上的电子运动，同时这些分子在飞秒时间尺度上通过数值求解非玻恩-奥本海默含时薛定谔方程而离解。紫外泵浦激光脉冲将三个最低能量子态的相干叠加态准备好，随后通过延迟的中红外强飞秒探测脉冲从这个相干电子-核波包（CENWP）中产生分子高次谐波（MHOHG）。通过改变几百个阿秒的泵浦探测延迟时间，可以看出 MHOHG 信号强度的数量级发生了变化。由于核运动，这两个上态和基态的相干性在几飞秒后就会丧失，MHOHG 强度随泵浦探测延迟时间的变化被证明与两个上离解量子态相干叠加中的电子振荡周期相等。虽然这个电子振荡周期，以及因此谐波谱的周期性在相当宽的核间距范围内相当稳定，但在实际形状和方式上，我们可以看到核运动的强烈特征，这种特征体现在这些谱随泵浦探测延迟时间的变化方式上，通过比较 H(2)(+)和 T(2)(+)两种同位素产生的 MHOHG 谱可以说明这一点。可以识别出大致对应于核间距 R < 4a(0)和 R > 4a(0)的两个不同区域：对于 R < 4a(0)，当延迟时间改变几百个阿秒时，在平台区域的整个频率范围内的强度会降低几个数量级，而在截止区域，MHOHG 谱中的峰随着泵浦探测时间延迟的增加而红移。另一方面，对于 R > 4a(0)，截止区和平台区的峰都随着延迟时间的增加而红移，只有峰强度有轻微变化。时频分析表明，在双周期探测脉冲的情况下，唯一的长轨迹和相关短轨迹的贡献与 R < 4a(0)时平台区整个频率范围的衰减相关，而对于 R > 4a(0)，即使在平台区，观察到的红移也与单个电子在半个激光周期内的返回相关。
