Burin Alexander L, Rubtsov Igor V
Department of Chemistry, Tulane University, New Orleans, Louisiana 70118, USA.
J Chem Phys. 2024 Sep 7;161(9). doi: 10.1063/5.0222580.
Molecular vibrations are generally responsible for chemical energy transport and dissipation in molecular systems. This transport is fast and efficient if energy is transferred by optical phonons in periodic oligomers, but its efficiency is limited by decoherence emerging due to anharmonic interactions with acoustic phonons. Using a general theoretical model, we show that in the most common case of the optical phonon band being narrower than the acoustic bands, decoherence takes place in two stages. The faster stage involves optical phonon multiple forward scattering due to absorption and emission of transverse acoustic phonons, i.e., collective bending modes with a quadratic spectrum; the transport remains ballistic and the speed can be altered. The subsequent slower stage involves phonon backscattering in multiphonon processes involving two or more acoustic phonons resulting in a switch to diffusive transport. If the initially excited optical phonon possesses a relatively small group velocity, then it is accelerated in the first stage due to its transitions to states propagating faster. This theoretical expectation is consistent with the recent measurements of optical phonon transport velocity in alkane chains, increasing with increasing the chain length.
分子振动通常负责分子系统中的化学能量传输和耗散。如果能量通过周期性低聚物中的光学声子转移，这种传输快速且高效，但由于与声学声子的非谐相互作用而出现的退相干会限制其效率。使用一个通用理论模型，我们表明，在光学声子能带比声学能带窄的最常见情况下，退相干分两个阶段发生。较快的阶段涉及由于横向声学声子的吸收和发射导致的光学声子多次向前散射，即具有二次谱的集体弯曲模式；传输保持弹道式，速度可以改变。随后较慢的阶段涉及在涉及两个或更多声学声子的多声子过程中的声子背散射，导致转变为扩散传输。如果最初激发的光学声子具有相对较小的群速度，那么它在第一阶段会由于向传播更快的状态跃迁而加速。这一理论预期与最近对烷烃链中光学声子传输速度的测量结果一致，该速度随链长增加而增加。
