CREOL: College of Optics and Photonics, University of Central Florida, Orlando, Florida 32816, USA.
J Phys Chem A. 2012 May 24;116(20):4833-41. doi: 10.1021/jp301051u. Epub 2012 May 10.
The double pump-probe technique (DPP), first introduced by Swatton et al. [Appl. Phys. Lett. 1997, 71, 10], is a variant of the standard pump-probe method but uses two pumps instead of one to create two sets of initial conditions for solving the rate equations, allowing a unique determination of singlet- and triplet-state absorption parameters and transition rates. We investigate the advantages and limitations of the DPP theoretically and experimentally and determine the influence of several experimental parameters on its accuracy. The accuracy with which the DPP determines the triplet-state parameters improves when the fraction of the population in the triplet state relative to the ground state is increased. To simplify the analysis of the DPP, an analytical model is presented, which is applicable to both the reverse saturable and the saturable absorption regimes. We show that the DPP is optimized by working in the saturable absorption regime. Although increased accuracy is in principle achievable by increasing the pump fluence in the reverse saturable absorption range, this can cause photoinduced decomposition in photochemically unstable molecules. Alternatively, we can tune the excitation wavelength to the spectral region of larger ground-state absorption, to achieve similar accuracy. This results in an accurate separation of triplet yield and excited-state absorption cross section. If the cross section at another wavelength is then desired, a second pump-probe experiment at that wavelength can be utilized given the previously measured triplet yield under the usually valid assumption that the triplet yield is independent of excitation wavelength.
双泵探测技术（DPP），由 Swatton 等人首次提出 [Appl. Phys. Lett. 1997, 71, 10]，是标准泵探测方法的一种变体，但使用两个泵而不是一个来为求解速率方程创建两组初始条件，从而可以唯一确定单重态和三重态吸收参数和跃迁速率。我们从理论和实验两方面研究了 DPP 的优势和局限性，并确定了几个实验参数对其准确性的影响。当三重态相对于基态的粒子数分数增加时，DPP 确定三重态参数的准确性会提高。为了简化 DPP 的分析，提出了一个适用于反饱和和饱和吸收两种情况的解析模型。我们表明，DPP 在饱和吸收区工作时得到了优化。虽然通过增加反饱和吸收区的泵浦强度可以在理论上提高准确性，但这可能会导致光化学不稳定分子的光诱导分解。或者，我们可以调整激发波长到基态吸收较大的光谱区域，以获得类似的准确性。这可以实现三重态产率和激发态吸收截面的准确分离。如果需要另一个波长的截面，则可以在该波长下进行第二次泵探测实验，前提是在通常有效的假设下，三重态产率与激发波长无关，即根据之前测量的三重态产率。
