Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
J Chem Phys. 2017 Jul 7;147(1):013916. doi: 10.1063/1.4979952.
The 205-230 nm photodissociation of vibrationally excited CO at temperatures up to 1800 K was studied using Resonance Enhanced Multiphoton Ionization (REMPI) and time-sliced Velocity Map Imaging (VMI). CO molecules seeded in He were heated in an SiC tube attached to a pulsed valve and supersonically expanded to create a molecular beam of rotationally cooled but vibrationally hot CO. Photodissociation was observed from vibrationally excited CO with internal energies up to about 20 000 cm, and CO(XΣ), O(P), and O(D) products were detected by REMPI. The large enhancement in the absorption cross section with increasing CO vibrational excitation made this investigation feasible. The internal energies of heated CO molecules that absorbed 230 nm radiation were estimated from the kinetic energy release (KER) distributions of CO(XΣ) products in v″ = 0. At 230 nm, CO needs to have at least 4000 cm of rovibrational energy to absorb the UV radiation and produce CO(XΣ) + O(P). CO internal energies in excess of 16 000 cm were confirmed by observing O(D) products. It is likely that initial absorption from levels with high bending excitation accesses both the AB and BA states, explaining the nearly isotropic angular distributions of the products. CO(XΣ) product internal energies were estimated from REMPI spectroscopy, and the KER distributions of the CO(XΣ), O(P), and O(D) products were obtained by VMI. The CO product internal energy distributions change with increasing CO temperature, suggesting that more than one dynamical pathway is involved when the internal energy of CO (and the corresponding available energy) increases. The KER distributions of O(D) and O(P) show broad internal energy distributions in the CO(XΣ) cofragment, extending up to the maximum allowed by energy but peaking at low KER values. Although not all the observations can be explained at this time, with the aid of available theoretical studies of CO VUV photodissociation and O + CO recombination, it is proposed that following UV absorption, the two lowest lying triplet states, aB and bA, and the ground electronic state are involved in the dynamical pathways that lead to product formation.
在高达 1800 K 的温度下，使用共振增强多光子电离 (REMPI) 和时间切片速度映射成像 (VMI) 研究了振动激发的 CO 在 205-230nm 光解。在附着于脉冲阀的 SiC 管中加热 He 中掺入的 CO 分子，并使它们超音速膨胀，从而产生旋转冷却但振动加热的 CO 分子束。通过 REMPI 检测到振动激发的 CO 产生的光解，其内部能量高达约 20000cm，检测到 CO(XΣ)、O(P)和 O(D)产物。随着 CO 振动激发的增加，吸收截面的大幅增加使得这项研究成为可能。吸收 230nm 辐射的加热 CO 分子的内部能量是根据 v″=0 时 CO(XΣ)产物的动能释放 (KER) 分布来估计的。在 230nm 时，CO 需要至少 4000cm 的转动振动能量才能吸收 UV 辐射并产生 CO(XΣ)+O(P)。通过观察 O(D)产物，证实 CO 内部能量超过 16000cm。最初从具有高弯曲激发的能级吸收可能同时进入 AB 和 BA 态，这解释了产物几乎各向同性的角分布。通过 REMPI 光谱学估计 CO(XΣ)产物的内部能量，并通过 VMI 获得 CO(XΣ)、O(P)和 O(D)产物的 KER 分布。CO 产物的内部能量分布随 CO 温度的升高而变化，这表明当 CO 的内部能量（和相应的可用能量）增加时，涉及到不止一种动力学途径。O(D)和 O(P)的 KER 分布在 CO(XΣ)共碎片中显示出广泛的内部能量分布，扩展到允许的最大能量，但在低 KER 值处达到峰值。尽管目前还不能解释所有的观察结果，但借助于现有的 CO VUV 光解和 O+CO 复合的理论研究，提出在 UV 吸收后，两个最低的三重态 aB 和 bA 以及基态电子态参与了导致产物形成的动力学途径。
