Lavoine J P
Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 ULP-CNRS, Groupe d'Optique Nonlinéaire et d'Optoélectronique, 23, rue du Loess, B.P. 43, 67034 Strasbourg Cedex 2, France.
J Chem Phys. 2007 Sep 7;127(9):094107. doi: 10.1063/1.2753472.
A two-pulse degenerate four-wave mixing experiment is analyzed in the case where the medium under investigation can be modeled by two-level systems having unequal permanent dipole moments. By modeling the light pulses by double exponentials [exp(-Gamma/t/)], we give an analytical expression of the third-order nonlinear polarization of the medium. We apply this result to simulate the measured signal in such experiment. We show that in the case of a two-photon transition, a signal can be detected if the pump pulse interacts with the medium before the probe pulse contrary to what is observed for excitations in the resonance region. An attempt to explain this behavior is made and the detected signal is analyzed in terms of pure coherent processes. This effect appears as a signature of the presence of permanent dipole moments. To test this property on a more realistic system, we then have considered a one-dimensional frequency-selected infrared degenerate four-wave mixing experiment on a molecular anharmonic vibrational mode modeled by a Morse potential and coupled to a dissipative bath of harmonic oscillators. We show that the two-photon transitions allowed by the presence of permanent dipole moments enable to analyze the multilevel system dynamics as if they were the one of a two-level system. Our results can also be extended to the case of inhomogeneous broadening and are of interest to study the infrared photon-echo response of anharmonic vibrational modes.
在被研究介质可用具有不等永久偶极矩的两能级系统建模的情况下，对双脉冲简并四波混频实验进行了分析。通过用双指数函数[exp(-Gamma/t/)]对光脉冲进行建模，我们给出了介质三阶非线性极化的解析表达式。我们将此结果应用于模拟该实验中测量的信号。我们表明，在双光子跃迁的情况下，如果泵浦脉冲在探测脉冲之前与介质相互作用，就可以检测到信号，这与在共振区域激发时观察到的情况相反。我们尝试解释这种行为，并根据纯相干过程对检测到的信号进行分析。这种效应表现为永久偶极矩存在的一个特征。为了在更现实的系统上测试此特性，我们随后考虑了在由莫尔斯势建模并耦合到简谐振子耗散浴的分子非谐振动模式上进行的一维频率选择红外简并四波混频实验。我们表明，由永久偶极矩导致的双光子跃迁能够分析多能级系统的动力学，就好像它们是两能级系统的动力学一样。我们的结果还可以扩展到非均匀展宽的情况，并且对于研究非谐振动模式的红外光子回波响应具有重要意义。
