發展紅外線/可見光合頻波成像顯微術以研究表面催化反應

Abstract

紅外/可見光合頻波產生（SFG）光譜術為二階非線性技術，其能取得介面的特徵振動光譜。在 等向介質中，例如：液體、溶液等具有反轉對稱中心者，無法產生合頻波，然而在不具有反轉對稱的 介面中可則產生合頻波。因此，合頻波產生光譜術在介面特徵振動光譜上提供了其他研究中難以得到 的資訊。自從沈元壤教授的團隊在1987 年首次在介面產生合頻波以來，合頻波產生術已廣泛運用於 研究在超高真空下的乾淨表面、自組裝單層分子，吸附於固/液介面的分子等系統。除此之外，此技 術亦能以皮秒之時間解析度監測超快動態學。 最近，已有人嘗試結合合頻波產生術及顯微鏡光學術以達到空間解析的測量。人們雖然在以奈米 探針進行的近場合頻波顯微技術上已取得一些成就，然而在遠場合頻波顯微技術之發展仍處於非常早 期的階段。此計畫的目標即是建構一遠場合頻波影像顯微鏡，其資料擷取時間可短於一分鐘，以利未 來應用於表面催化反應研究，例如：甲酸在二氧化鈦（110）表面之脫水反應。 合頻波影像顯微鏡的優點總結如下：首先，既然空間解析度是由合頻波產生的可見光訊號之波長 決定，合頻波顯微技術有其潛力以次微米空間解析度來提供分子振動（中紅外線範圍）的資訊。其次， 由於使用飛秒摻鈦藍寶石雷射，可得到高度時間解析度。最後，此計畫的合頻波顯微鏡是基於使用繞 射光柵的遠場影像技術，所以不需要掃瞄。理論上可減少資料擷取時間，俾使得到較佳的表面反應的 時間解析，並減少雷射輻射對表面損害的可能性。

IR–visible sum-frequency generation (SFG) spectroscopy is a second-order nonlinear technique that is capable of obtaining interface-specific vibrational spectra. An SFG process is not allowed in isotropic media such as liquids and solutions having a center of inversion symmetry, but it is allowed at interfaces that does not have inversion symmetry. Thus SFG spectroscopy provides otherwise inaccessible information on interface-specific vibrational spectra. Since the first demonstration by Shen’s group in 1987, SFG spectroscopy has been widely used to study clean surfaces under ultra-high vacuum, self-assembled monolayers, molecular species adsorbed on solid–liquid interfaces, and so on. In addition, ultrafast dynamics can be monitored typically with picosecond time resolution. Attempts to combine SFG spectroscopy with a microscope (SFG microscopy) have recently been made to achieve space-resolved measurements. Although there are a few successful results in near-field SFG microscope with nanoscale tips, far-field SFG microscope is still in the very early stage of development. Our objective in this proposal is to construct a far-field SFG imaging microscope with data-acquisition time short enough (<1 min) for future applications to surface catalytic reactions, such as the dehydration reaction of formic acid on TiO2(110) surface. The advantages of our SFG imaging microscope are summarized as follows: First, since the spatial resolution is determined by the wavelength of SFG signal (visible), SFG microscopy provides information on molecular vibrations (mid-IR range in energy) with potential sub-μm spatial resolution. Second, it has high time resolution due to the femtosecond Ti:sapphire laser used in our measurements. Finally our SFG microscope is based on far-field imaging technique using a diffraction grating, so it does not need scanning. This in principle leads to shorter data-acquisition time, which is desired for time-resolved measurements of surface reactions as well as for reducing possible surface damages due to the laser irradiation.