1.College of Physics and Electronic Engineering, Xinjiang Normal University, Urumqi 830054, China 2.State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China
Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 21763027), the Autonomous Regional Collaborative Innovation Project of Xinjiang, China (Grant No. 2019E0223), the Tianshan Talent Program of Xinjiang, China (Grant No. 2018Q072), the Scientific Research in Higher Education of Xinjiang, China (Grant No. XJEDU2020Y029), the “13th Five-Year” Plan for KeyDiscipline Physics Bidding Project of Xinjiang Normal University, China (Grant No. 17SDKD0602), and the Undergraduate Teaching Research and Reform Project of Xinjiang Normal University, China (Grant No. SDJG2019-27)
Received Date:04 September 2020
Accepted Date:19 September 2020
Available Online:21 February 2021
Published Online:05 March 2021
Abstract:Interaction of light with matter has always been important in the field of natural science. Particularly, the ultrafast radiationless relaxation induced by UV light of molecular electronic excited states accompanied by ultrafast energy transfer plays an important role in the natural photophysical, photochemical and biological reactions. Generally, the molecular electronic excited state can be deactivated through a variety of decay channels, including dissociation, isomerization, internal conversion, intersysterm crossing, vibrational energy redistribution, and autoionization. This complexity of relaxation channels brings about a wide variety of deactivation mechanisms. The ultrafast nonadibatic relaxation dynamics of the excited state of phenylacetylene is studied by using femtosecond time-resolved photoelectron imaging and femtosecond time-resolved mass spectrometry. The first excited state S2 of phenylacetylene is excited by 235 nm pump light, and the excited state deactivation process is detected by 400 nm probe light. The time-dependent curves of parent ions include two exponential curves. One is the fast component with a time constant of 116 fs, and the other is the slow component with a time constant of 106 ps. The time-resolved photoelectron kinetic energy distribution is obtained from the time-resolved photoelectron images. Combined with the time-resolved photoelectron spectroscopy data, the fast component with a time constant of 116 fs is found to reflect the internal conversion process from S2 state to S1 state. The experimental results also show that S1 state is arranged by internal conversion, and the inter system jump process to T1 state is an important attenuation channel. This work provides a clearer physical picture for S1 state nonadibatic relaxation dynamics of phenylacetylene. Keywords:phenylacetylene/ photoelectron imaging/ intersystem crossing/ time-resolved spectroscopy
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3.1.S2态的衰减动力学
苯乙炔S2态的带源位于5.2 eV附近[25], 因此235 nm泵浦光(5.28 eV)可以将苯乙炔分子从电子基态激发到 S2态, 苯乙炔的电离能为8.8 eV[25], 要将被激发的S2态分子电离至少还需要2个400 nm的探测光子. 实验中调节探测光和泵浦光的光强, 保证在双光作用下的飞行时间质谱中母体离子${\rm{C}}_8{\rm{H}}_6^+$双光增益较强, 并且几乎无碎片离子出现. 因此在本实验中光电子信号都来自于母体离子. 图1显示了在235 nm泵浦、400 nm探测条件下的母体离子时间衰减曲线. 本实验中泵浦光是235 nm, 泵浦光激发苯乙炔的S2态, S2态被激发后很有可能内转换到S1态或S0态, 考虑到这样的一个动力学过程, 用双指数函数与高斯函数将母体离子的时间分辨质谱信号进行卷积拟合, 拟合得到两个明显不同的时间组分, 分别为τ1 = 116 fs和τ2 = 106 ps. 前人的研究中指出, 苯乙炔S2态很有可能快速内转换到S1态[15], 时间尺度为几十飞秒, 因此得出快速组分116 fs很有可能是S2态到S1态内转换的时间, 较慢的组分为S1态的衰减时间. 图 1 235 nm泵浦、400 nm探测条件下获得的母体离子时间衰减曲线, 圆圈代表实验结果, 实线代表拟合结果 Figure1. Time-resolved total ion signals of parent ion as a function of delay time between the pump pulse at 235 nm and the probe pulse at 400 nm. The circles are the experimental results, and solid lines are the fitting results.
为了得到S2态向S1态内转换的更多证据, 实验中采集了不同泵浦探测时间延迟下的光电子影像, 用BASEX程序[26]对光电子影像进行变换得到电子影像的三维空间分布. 为了更清楚地探究不同能态之间的能量转移过程, 我们通过光电子影像得到了光电子能谱. 图2给出了0时刻和163 fs时的光电子能谱, 从图中我们可以看到有明显的3个带, 第1个带为0.4—0.7 eV左右的比较宽的带, 第2个带为比较强比较尖的0.7—1.3 eV带, 第3个带为1.3—2.5 eV的比较宽的带, 依次被标记为 1, 2, 3, 从能谱中可以看到随着泵浦-探测延迟时间不同, 光电子能谱有明显的变化. 随着时间的演化, 第2和第3光电子能带在衰减的同时, 第1光电子能带在上升, 这个很有可能表明存在两个态之间的耦合. 根据文献[15]苯乙炔的电离能IP为8.83 eV, S2态的0振动态的能量为5.2 eV, S1 态0振动态的能量为4.45 eV, S2态和S1态能量差为0.75 eV, 235 nmm双光探测光电子最大动能Epump+Eprobe – IP为2.65 eV. 我们泵浦光泵浦S2态带源附近, 因此第3光电子能带来自于S2态电离. 图中第2个光电子能带的衰减趋势和第3个光电子能带相似, 随着时间延迟, 第2个和第3个光电子能带都在衰减, 因此我们认为第2光电子能带来自于S1态的电离. 第2个和第3个光电子能带衰减的同时, 第1个光电子能带在增加, 而且第1个光电子带和第2个光电子带的能量差为0.7 eV, 与S2态和S1态能量差为0.75 eV吻合, 因此, 第1个光电子能带很有可能来自与S1态的电离. 第2和第3光电子能带衰减的同时第1光电子能带增加, 很有可能反映的是S2到S1内转换过程. 从母体离子时间分辨质谱中得到的快速衰减时间116 fs很有可能是S2态内转换时间. 图 2 从Δt = 0 fs和Δt = 163 fs的影像中提取得到的光电子动能分布, 位于D0处的箭头表示(1+2')电离机制下最大的可资用能 Figure2. Photoelectron kinetic energy distributions at Δt = 0 ps and Δt = 92 ps. The arrow at D0 indicates the maximum electron energy by two-photon absorption of probe beam at 400 nm after one-photon excitation of pump at 235 nm.
23.2.S1态的衰减动力学 -->
3.2.S1态的衰减动力学
上面的研究中我们观察到了S2态被泵浦光布局后内转换到S1态, 为了研究S1态的衰减过程, 在实验中我们采集了长时间延迟下的时间分辨离子信号, 同时也采集了长时间延迟下的光电子影像, 图3为长时间延迟下的时间分辨离子信号. 考虑到S1态有可能发生的动力学过程, 我们把离子信号用一个衰减函数、一个上升函数和高斯函数(半高宽为200 fs)的卷积来拟合. 与前面的讨论相结合, 我们认为衰减寿命106 ps (不确定度为 ±2 ps)应该是S1态的寿命, 上升时间60 ps (不确定度为 ±3 ps)很有可能是三重态T1态布局的时间. 图 3 长时间延迟的235 nm泵浦、400 nm探测条件下获得的母体离子时间衰减曲线, 圆圈代表实验结果, 实线代表拟合结果 Figure3. Time-resolved total ion signals of parent ion as a function of delay time between the pump pulse at 235 nm and the probe pulse at 400 nm. The circles are the experimental results, and solid lines are the fitting results.
实验中我们采集了长时间泵浦-探测延迟下的光电子影像, 图4为从光电子影像中得到的光电子能谱. 从能谱中可以看到随着泵浦探测延迟时间的增加第2和第3光电子能带快速衰减, 而第1个光电子能带向低的光电子动能方向移动, 在553 ps时我们观察到了明显的光电子信号. 在553 ps时S1态的衰减已经结束, 因此0—0.4 eV的这个光电子能带很有可能来自于三重态T1态的电离, 从母体离子时间分辨质谱中拟合得到的60 ps的上升时间就是三重态布局的时间. 实验中观察到的S1态的衰减和T1态的布局, 是内转换布局的S1态继续通过系间交叉衰减到T1态的过程. S1态衰减可能的另外一个衰减通道为内转换到S0态, 但由于我们的探测光没办法探测来自于S0态的信号, 因此不能排除S1向S0内转换的衰减通道, S1到S0的内转换很有可能是S1态衰减的另外一个很重要的通道. 图 4 在235 nm泵浦、400 nm探测不同泵浦-探测延迟下的光电子能谱 Figure4. Photoelectron kinetic energy distributions (PKE) at different time delay observed at 235 nm pump and 400 nm probe.