強短脈衝雷射產生的超激發態的研究

Abstract

當分子經由強雷射照射發光(波長在800 nm情況下), 會導致各種非線性動力學的過程。例如分子的校準、結構改變離子化高階簡諧的產生或庫倫爆炸。然而，經由強雷射所導致的電子激發，比較不被重視。近年來，Kong等人由強雷射照射各種氣體分子找到了電子激發態的中性碎片，且已知庫倫爆炸同時會導致各種化學鍵斷裂並產生帶電的碎片。即使Kong發現化學鍵斷裂的結果，也不會因為碎片是中性而歸因於庫倫爆炸，故這是一個新的機制。 我們假設碎片的產生是經由激發母體分子而來的，並且呈現寬廣的分佈。光子能量必須遠小於第一激發態，且大於或等於其他激發態之間的能量，就能呈現寬廣的分佈態。因為光子能量遠小於第一激發能，導致從基態到激發態的躍遷不易發生，而激發態之間的躍遷速度又比前者來的迅速。 無論最終態為何，都不會影響基態的躍遷趨勢，因為激發態之分布會重新且快速的平均分配至每個激發態(包含超激發態)上。其計算結果與近期實驗所發現的超激發態形成原因相符。 傳統的研究指出，可利用電子碰撞或同步輻射源來產生超激發態之現象，而強短脈衝亦具有開啟這方面研究的可能性，其原因為非線性激發過程能使強雷射製造出電子碰撞或同步輻射源很難達成的超激發態，Kong等人也發現某些氧氣分子確實具有超激發態之性質。另一因素為可利用時間解析光譜儀來研究超激發態。Chin’s研究團隊在pump-probe實驗中得知第一個脈衝能產生超激發態之現象，第二個脈衝則導致超激發態受到破壞，其結果能推測出超激發態的可能壽命。 在超激發態的研究中，雖然已經證明強短脈衝雷射是一個優良的方法，但仍有幾個基本問題依舊存在：為什麼超激發態可以存在於強雷射場中？母體分子在超激發態的碎片機制是什麼？pump-probe實驗中，第二道脈衝又是如何破壞超激發態？故本篇研究重點著重於(a)理論計算強短脈衝雷射到達超激發態或高階簡諧之過程(b)超激發態的飛秒與阿秒動力學。

When molecules are irradiated by an intense laser (which typical wavelength is about 800 nm), various nonlinear dynamical processes are induced. Examples are alignment, structural change, and ionization followed by high order harmonic generation or Coulomb explosion. Electronic excitation by an intense laser, however, has been paid less attention. Recently, Kong et al. found electronically excited neutral fragments after irradiating intense laser to various kind of gaseous molecule. It has been known that Coulomb explosion results in simultaneous breakings of various chemical bonds leading to charged fragments. Even though Kong’s finding also results in breaking almost all the chemical bonds, it is not attributed to the Coulomb explosion, but to a new mechanism because the fragments are neutral. We have proposed a theoretical model that explains this new phenomenon. We assume that fragmentation occurs from excited parent molecule created by a new excitation mechanism having a wide distribution of the final states. We assume that the first excitation energy is much larger than the photon energy, and that energy spacing between excited states are as same as or smaller than the photon energy. Accordingly the excitation hardly takes place from the ground state because of the large excitation energy, whereas the transitions between excited states are much faster than the transition from the ground state. Excitation probabilities from the ground state consequently do not depend on the final state because the population of an excited state tends to be redistributed rapidly among excited states. Our model indicates that even super excited states (SES’s) are created, which was confirmed by recent experiments. Conventional studies utilize electron scattering or synchrotron radiation to prepare SES’s. Intense short pulsed laser, on the other hand, may open new possibilities in this research field. Due to the non linearity of the pumping process, intense laser can produce SES’s that can hardly be created by electron scattering or synchrotron radiation. Kong et al. actually found several new SES’s of O2 molecule. Another consequence of intense short pulse laser is that time resolved spectroscopy is made possible. Chin’s group performed a kind of pump-probe experiment, in which pump pulse creates SES’s and probe pulse destroys them, obtaining a possible lifetime of SES. Even though it is evident that intense short pulse laser is a new promising tool to study super excited states, several basic problems are still open as follows. Why can SES’s survive in intense laser field? What is the fragmentation mechanism of parent molecule in SES? What is the mechanism to destroy SES in the pump probe experiment? The purpose of this research project is to give concrete answers to these problems. I am going to study on (a) pumping process to SESs and high harmonic generation by intense short pulse laser, and (b) femto- and atto- second dynamics of SES with/without laser iradiation.