Signatures of Reaction Mechanisms Encoded in the Vibrational Population Distribution of Small-Molecule Products: Photodissociation of Symmetric-Triazine Using 266 nm Radiation

We utilize rotationally resolved Chirped-Pulse Fourier Transform millimeter-wave spectroscopy to study photodissociation dynamics of 1,3,5-Triazine (symmetric-Triazine) to form 3 HCN molecules. The state-specific vibrational population distribution (VPD) of the photofragments contains mechanistic details of the reaction. This photodissociation is performed using 266 nm radiation transverse to a seeded supersonic jet. The vibrational cooling inefficiency in the jet preserves the VPD of the photofragments, while rotational cooling enhances the signal of low-J pure-rotational transitions. The multiplexed nature of the spectrometer enables simultaneous sampling of several “vibrational satellites” of the J = 1 ← 0 transition of HCN. Excited state populations along the HCN bend (v2) and CN stretch (v3) modes are observed, which show ≥3.2% vibrational excitation of the photofragments. Observation of an at least bimodal VPD, along the even-v states of v2, implies an asymmetric partitioning of vibrational energy among the HCN photofragments. This suggests a sequential dissociation mechanism of symmetric-Triazine initiated by 266 nm radiation.本研究采用转动分辨啁啾脉冲傅里叶变换毫米波光谱学（rotationally resolved Chirped-Pulse Fourier Transform millimeter-wave spectroscopy），探究1,3,5-三嗪（symmetric-Triazine，均三嗪）光解为3分子氰化氢（HCN）的动力学过程。光解碎片的态分辨振动布居分布（Vibrational Population Distribution, VPD）蕴含了该反应的微观机理细节。该光解过程采用垂直于掺杂型超声超音速射流的266纳米辐射完成。射流中振动冷却效率不足得以保留光解碎片的VPD，而转动冷却则强化了低J纯转动跃迁的信号强度。该光谱仪的多路复用特性可同时采集HCN的J=1←0跃迁的多条“振动伴峰”信号。研究观测到HCN的弯曲振动（v2）与CN伸缩振动（v3）模式的激发态布居，结果显示光解碎片的振动激发占比不低于3.2%。针对v2的偶宇称振动能级观测到至少呈双峰分布的VPD，这表明HCN光解碎片间的振动能量分配存在不对称性。这表明均三嗪在266纳米辐射引发下遵循分步解离机理。