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Collisional quenching of electronically excited hydroxyl A2sigma+ radicals by molecular partners: Product branching and quantum state distributions.

机译：分子伴侣对电子激发的羟基A2sigma +自由基的碰撞猝灭：产物分支和量子态分布。

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摘要

Hydroxyl (OH) radicals are important in atmospheric and combustion environments where they are often detected by laser-induced fluorescence (LIF) on the A2Sigma+ - X2pi band system. Previous kinetic studies have shown that collisional quenching of electronically excited OH A2Sigma+ radicals impacts on LIF measurements of ground state populations in these environments. This thesis examines the dynamical outcomes of collisional quenching of OH A2Sigma+ radicals by several molecular collision partners, namely H2, D2, N2, O2, and CO2. The experiments utilize a pump-probe scheme to determine the OH X2pi product state distribution (PSD) following non-reactive quenching. A pump laser prepares OH A2Sigma+ (nu'=0, N'=0) in the collisional region of the pulsed supersonic region, which results in a significantly reduced fluorescence quantum yield due to quenching by collisions with the molecular partner, as evidenced by the shortened fluorescence lifetime. After a short time delay, a probe laser monitors the collision-induced population in OH X2pi (nu", N") levels under single collision conditions via LIF. The population in specific OH X2pi (nu", N") product states is also compared to that in the initially prepared OH A2Sigma + (nu'=0, N'=0) state to determine the branching fraction for OH X2pi.;For these quenchers, the OH X2pi products are released with little vibrational excitation, predominantly in nu"=0, with monotonically decreasing population in higher vibrational levels. The OH X2pi products from quenching with H2, D2, N2 and O 2 are found to be highly rotationally excited. In contrast, the OH X 2pi products from quenching with CO2 show a modest degree of rotational excitation indicative of a long-lived complex. Branching fraction measurements reveal that the branching to OH X2pi products varies significantly with collision partner, accounting for 12(5)% of the total quenching with H2, 15(8)% with D2, >88(3)% with N2, 40(1)% with O2, and 64(5)% with CO2. These observations provide new insight on the fundamental mechanism (or mechanisms) by which quenching occurs for OH A2Sigma+ with molecular partners.;For H2, D2 and N2, collisional quenching occurs through conical intersections (CI) that couple the excited- and ground-state potential energy surfaces (PES). The vibrational, rotational, and fine structure distributions of the OH X2pi products as well as the branching fractions are interpreted as dynamical signatures of nonadiabatic passage through the CI region. For these systems, the experimental measurements are supplemented by theoretical calculations examining the topography of the PESs in the vicinity of the CI. As a result, they are emerging as benchmark systems for understanding the nonadiabatic dynamics of collisional quenching.
机译：羟基（OH）自由基在大气和燃烧环境中很重要，在这些环境中，经常通过A2Sigma +-X2pi波段系统上的激光诱导荧光（LIF）来检测它们。先前的动力学研究表明，在这些环境中，电子激发的OH A2Sigma +自由基的碰撞猝灭会影响基态种群的LIF测量。本文研究了几种分子碰撞伴侣，即H2，D2，N2，O2和CO2对OH A2Sigma +自由基的碰撞猝灭的动力学结果。实验利用泵探针方案确定非反应性淬灭后的OH X2pi产物状态分布（PSD）。泵浦激光器在脉冲超声波区域的碰撞区域中制备OH A2Sigma +（nu'= 0，N'= 0），由于与分子配偶体的碰撞而猝灭，导致荧光量子产率显着降低，这证明了缩短荧光寿命。在短暂的时间延迟后，探测激光通过LIF在单个碰撞条件下监视碰撞诱导的OH X2pi（nu“，N”）含量。还将特定OH X2pi（nu“，N”）产物状态下的种群与最初制备的OH A2Sigma +（nu'= 0，N'= 0）状态下的种群进行比较，以确定OH X2pi的支化分数。在这些淬灭剂中，OH X2pi产物几乎没有振动激发，主要在nu“ = 0时释放，并且在较高的振动水平下单调减少了种群。发现用H2，D2，N2和O 2淬灭的OH X2pi产物非常高。相比之下，用CO2淬火得到的OH X 2pi产物显示出适度的旋转激发度，表明复合物是长寿命的，支化分数测量表明，向OH X2pi产物的支化程度随碰撞伴侣而变化很大，占12 H5占总淬灭的（5）％，D2占总淬灭的15％（8）％，N2> 88（3）％，O2占40（1）％，CO2占64（5）％，这些发现提供了新的见解OH A2Sigma + wi猝灭发生的基本机理对于H2，D2和N2，碰撞猝灭是通过将激发态和基态势能面（PES）耦合的圆锥形交叉点（CI）发生的。 OH X2pi产物的振动，旋转和精细结构分布以及分支部分被解释为非绝热通过CI区域的动力学特征。对于这些系统，通过理论计算来补充实验测量，以检查CI附近PES的地形。结果，它们正在成为理解碰撞猝灭的非绝热动力学的基准系统。

著录项

  • 作者

    Dempsey, Logan P.;

  • 作者单位

    University of Pennsylvania.;

  • 授予单位 University of Pennsylvania.;
  • 学科 Chemistry Physical.
  • 学位 Ph.D.
  • 年度 2009
  • 页码 268 p.
  • 总页数 268
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类 物理化学（理论化学）、化学物理学;
  • 关键词

  • 入库时间 2022-08-17 11:38:24

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