Laser Centre and Department of Chemistry, Vrije Universiteit, de Boelelaan 1083, 1081 HV Amsterdam, The Netherlands.
J Phys Chem A. 2010 Mar 11;114(9):3157-66. doi: 10.1021/jp909031p.
The femtosecond multiphoton photoionization and dissociation dynamics of NO(2) have been studied in a two-color pump-probe experiment at 400 and 266 nm using velocity map ion imaging in conjunction with photoelectron imaging. We report here a series of experiments focusing on the oscillatory patterns in pump-probe transients of the photoelectron signal. By using the technique of spatially masked imaging detection, we can select different photoelectron channels enabling the rapid measurement of energy selected transients with good signal-to-noise ratio. At short delay times (<300 fs) the dominant process is dissociative multiphoton ionization by 3 x 400 nm + 1 x 266 nm excitation to a repulsive potential energy surface of the NO(2)(+) cation correlating to NO(+)((1)Sigma(+)) + O((3) P) and the ejection of a 0.37 eV electron. At longer delay times (>400 fs), the release of high-energy electrons (0.88 eV) is observed attributed to a three-photon absorption at 400 nm to Rydberg and valence type excited states of neutral NO(2) leading to predissociation and the production of NO(+) + O((3)P) from a one-photon ionization at 266 nm. At longer delay times (>400 fs) a second slow (near 0 eV) photoelectron channel is observed that is associated with one photon excitation at 400 nm to the first excited A(2)B(2) state of NO(2) followed by two-photon excitation at 266 nm leading to near threshold ionization and dissociation to NO(+) + O((3)P). Distinctive oscillatory patterns were found in the pump-probe transients of the photoelectron yield for both the slow and the fast photoelectron channels but with different periods of about 750 fs (slow) or 590 fs (fast). Extensive polarization experiments are reported for both linear and circular polarized pump and probe laser geometries. We discuss the oscillatory mechanism in relation to ab initio calculations of relevant Rydberg and valence type excited states of NO(2) near 9.3 eV. We propose that an oscillating wavepacket of mixed Rydberg and valence character that predissociates is responsible for the observed oscillations in the transients of the fast (0.88 eV) photoelectron channel.
飞秒双光子光电离和解离动力学的二氧化氮（2）已经在双色泵浦探测实验中研究了 400nm 和 266nm 用速度映射离子成像与光电子成像。我们在这里报告了一系列的实验，侧重于泵浦探测光电子信号的振荡模式。通过使用空间掩模成像检测技术，我们可以选择不同的光电子通道，从而快速测量具有良好信噪比的能量选择瞬态。在短的延迟时间（<300fs）下，主要的过程是通过 3x400nm+1x266nm 激发到一个排斥势能面的解离多光子电离，该势能面与二氧化氮（2）阳离子相关联，其关联到 NO(+)((1)Sigma(+))+O((3)P)和 0.37eV 电子的发射。在较长的延迟时间（>400fs），观察到高能电子（0.88eV）的释放归因于 400nm 的三光子吸收到中性二氧化氮的里德堡和价态激发态，导致预解离和通过 266nm 的单光子电离产生 NO(+) + O((3)P)。在较长的延迟时间（>400fs），观察到第二个较慢（接近 0eV）的光电子通道，其与 400nm 的单光子激发到二氧化氮的第一激发 A(2)B(2)态相关联，然后在 266nm 处进行双光子激发，导致近阈值电离和到 NO(+) + O((3)P)的解离。在光电子产额的泵浦探测瞬态中发现了两个慢（快）光电子通道的独特的振荡模式，但具有不同的周期，约为 750fs（慢）或 590fs（快）。报告了广泛的极化实验，包括线性和圆偏振泵浦和探测激光几何。我们讨论了与相关的里德堡和价态激发态的从头算计算有关的振荡机制在 9.3eV 附近的二氧化氮。我们提出，一个混合的里德堡和价态特征的振荡波包，它是预解离的，负责观察到的快速（0.88eV）光电子通道的瞬态中的振荡。
