摘要
随着飞秒激光脉冲技术的发展及其应用,人们已经观测到许多有趣的强场现象,包括超阈值电离和解离、高次谐波等。这激发了很多理论研究者的兴趣,人们期望从理论上解释、阐述乃至探索这些强场现象。由于含时量子波包方法具有物理图像清晰和计算简便快捷等优点,因此它被广泛应用于研究强场现象。
本文采用考虑分子振动和转动自由度的含时量子波包方法从理论上研究了线偏振飞秒激光场中HD+的超阈值解离动力学。在Born-Oppenheimer近似下,我们采用了HD+的双态模型(基电子态1sσg和激发电子态2pσu),利用动量空间中渐近流表达式计算了由超阂值解离产生的解离碎片的能量分布。
我们选择激光脉冲的包络为高斯函数包络,其波长λ=800nm,半高全宽τ=30fs。当激光脉冲峰强度为I=5.0×1012W／cm2,我们仅能观测到双光子解离。但是当激光脉冲峰强度增大到I=1.5×1015W／cm2时,四光子解离主导了超阈值解离过程。这个结果与Orr等人的实验结果基本一致。同时,我们还发现强激光场引起的ac-Stark能级移动能够改变解离碎片的能谱,分子的转动和取向对解离碎片的能谱有一定影响。分子的转动能够减弱ac-Stark能级移动并加宽解离碎片的能谱峰。而强激光场能够有效地影响分子取向并增加超阈值解离的几率。我们通过改变调制激光脉冲的载波相位可以有效地控制HD+的超阈值解离。强激光场引起的ac-Stark能级移动对解离碎片的能谱有明显的影响。我们采用光缀饰势或者光诱导势的概念来解释HD+的超阈值解离动力学过程。
With the development and application of femtosecond laser pulse technology, a lot of interesting strong field phenomena was observed, including above threshold ionization and dissociation, high order harmonic and so on. Many researchers expect to interpret and describe these strong field phenomenain theory. The time-dependent quantum wavepacket method has been used to study the strong field phenomena because of its clear physical picture and rapid computation.
The above threshold dissociation dynamics of HD+ in linearly polarized femtosecond laser fields is theoretically studied using the time-dependent wavepacket method including the molecular vibrational and rotational degrees of freedom. Based on the Born-Oppenheimer approximation, the calculations are performed on two electronic states, the ground state 1sσg and the excited state 2pσu, and the energy-dependent distributions of the dissociated fragments resulting from the above threshold dissociation are calculated by using an asymptotic-flow expression in the momentum space.
A laser field with a Gaussian pulse envelope, wavelengthλ=800nm and full-width at half-maximumτ=30fs is employed in our calculations. Only two-photon dissociation is observable when the laser intensity is 5.O×1012W/cm2. While the laser intensity is increased to 1.5×1015W/cm2, the dissociated fragments resulting from four-photon absorption dominate the photodissociation process. These results are consistent with the experimental observation of Orr et al. Moreover, the ac-Stark shift caused by the intense laser field can change the kinetic energy spectra of the dissociated fragments, and the molecular rotation and alignment also has some effects on the kinetic energy spectra of the dissociated fragments. The molecular rotation reduces the ac-Stark shift and broadens the peaks of kinetic energy spectra of the dissociated fragments. The intense laser field can effectively align the molecule and increase the probability of the above threshold dissociation. We also control the above threshold dissociation of HD+ by changing the carrier-envelope phase of the modulated laser pulse. The intense laser field also causes the ac-Stark shift which changes the kinetic energy spectra of the dissociated fragments. The above threshold dissociation dynamics process is interpreted in terms of the light-dressed potential or the light-induced potential.
引文
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