氫分子離子再強雷射場下的中性解離之理論研究

Abstract

在各個雷射光束線叢(laser filamentation)的應用中，分子的強場激發是關鍵的機制。先前的研究已經發現激發的機率(excitation probabilities)和強場冪次(power)有關係。在800 nm的例子中，氧分子和氫原子有兩群(group)分別對應到不同的冪次，但是甲烷分子只有一群。我們無法清楚地知道造成不同的原因。利用時間－頻率的分析已經得知為什麼角動量量子數的宇稱決定了激發的行為(behavior)。

此研究以H_2^+為例，檢驗造成一群或兩群表現差異的原因。此例中解離位能(ionization potential)大約是氫原子的三倍大。這樣的情況下，躍遷機制很複雜且許多弗洛凱態(Floquet states)會耦合(couple)，導致激發行為中不明的對稱關連(symmetry dependence)。此外由於較大的解離位能，激發到高激發態所需的光子數很大。因此激發需要較強的雷射場,會造成較大的非絕熱過渡機率(nonadiabatic transition)。一個絕熱弗洛凱態(Floquet states)下創建的電子填充在廣泛再被分配到其他能量。這就是為什麼我們在800 nm對H_2^+的情況中只會找到一群。另一方面使用600 nm雷射所需的光子數小於800 nm，此情況中在雷射強度峰值附近，電子填充的發生在能量重新分配上能量的範圍。這是為什麼我們發現兩種群，依據不在於最終態的能階是否接近基態加上 n 個光子。 我們的分析預期能揭示甲烷分子只有一個群而氧分子有兩群的原因，以及在氧分子的案例中是什麼決定了群。

Intense field excitation of molecule is a key mechanism in various applications of laser fillamentation. In the previous studies, it has been found that the excitation probabilities have power dependences with large powers. In the case of 800nm, oxygen molecule and hydrogen atom have two groups with different powers, whereas the methane has one group with a common power. It was not clearly understood the reson for the difference. Using the time-frequency analysis, it has been understood the reason why the parity of the angular momentum quantum number determines the excitation behaviors.

In this study taking the H2+ molecule as as example, we examine the reson for the difference of one group and two group behaviors. In this case the ionization potential is about 3 times larger than hydrogen atom. In such a case transition mechanism is complicated and many Floquet states couples complicatedly. This leads to the unclear symmetry dependence in the excitation behavior. Futhermore the number of photons needed to excite to highly lying states is large because of the large ionization potential. Therefore excitation needs a large laser intensity, leading to a large nonadiabatic transition probabilities. The population created on an adiabatic Floquet state is redistributed widely in energy. This is why we find only one group in the case of 800nm with H2+ molecule. On the other hand with the 600nm laser, the number of photons needed is smaller than 800nm case. In this case the population created around the laser peak is not redistributed widely in energy. This is why we find two groups depending on whether the energy level of the final state is close to the photon dressed ground state. Our analysis is expected to reveal the reason why methane has one group, and oxygen molecule has two groups, or what determines the groups in the case of oxygen molecule.