皮秒脈衝與連續光在有機溶液中引發之質量傳輸效應研究

Study of Mass Transport in Organic Solutions Induced by Picosecond Laser Pulses and CW Laser Light

李麗淑(Li-Shu Lee)

指導教授 ： 魏台輝

國立中正大學/理學院/物理學系/博士(2013年)

摘要

在本論文中，我們以溶在乙醇的有機染料氯鋁酞青素(chloroaluminum phthalocyanine molecules，化學式為C32H16AlClN8)溶液(簡稱ClAlPc-ethanol)為樣品，分別利用波長為632.8 nm的CW laser light (連續光)及波長為532 nm的19 picosecond(ps) laser pulses(19 ps 脈衝)探討溶質分子質量傳輸效應之機制。
在CW laser light的研究中，我們分別量測平行及垂直於桌面的光束入射到兩個濃度樣品(4.21017 cm3和1.21017 cm3)的穿透光功率及光束之偏折現象。對於前者光路，我們發現穿透光功率隨時間單調遞增，而經過樣品的光束沒有發生偏折現象。因為高濃度溶液可產生較高之溫度梯度，而實驗結果又顯現出較較高的光穿透率變化，定性上符合熱擴散效應(一種準靜過程)的預期；另方面，透過理論擬合，我們的確以熱擴散效應定量地解釋了實驗結果。對於後者光路，我們一方面在實驗上發現光穿透率先隨時間之增加而增加，而後又隨時間之繼續增加而減少，另方面也發先穿透光向下偏折的現象，目前我們只能結合熱擴散與對流效應以定性方式解釋實驗結果。定量的解釋有賴日後進一步的努力。
在19 ps脈衝研究中，我們透過Z-scan技術量測兩個濃度溶液(4.21017 cm3 和 1.21017 cm3)的溶質分子質量傳輸效應，結果發現高濃度溶液有較強之吸收而產生較大之溫度梯度但卻較不容易產生質量傳輸效應，因而排除熱擴散之可能性。再則，透過理論模擬，我們發現在受激發溶液尚未回復區域熱平衡之前，低濃度溶液中之個別溶質分子透過非線性之激發可得到較多之平移過剩能量(translational excess energy，t)，而此t即為質量傳輸之驅動力，因此我們認為19 ps脈衝所造成的溶質分子質量傳輸效應是非準靜過程。
藉比較光能傳予溶質分子的速率及溶質分子將intra-molecular excess energy t傳予鄰近溶劑分子的速率，我們解釋為何CW laser light所引起之質量傳輸效應是準靜過程(quasistatic)，而19 ps雷射脈衝所引起之質量傳輸效應是非準靜過程(non-quasistatic)。因為CW laser light將光能傳予溶質分子的速率遠低於溶質分子將intra-molecular excess energyt透過inter-molecular relaxation方式傳予鄰近溶劑分子的速率，因此在溶液保持區域熱平衡的狀態下，被吸收CW laser light能量很均勻地儲存於溶質與溶劑分子的各個自由度內，使溶液始終非常接近熱力學平衡，致CW laser light引起的溶質分子質量傳輸可被視為準靜過程。這裡所說的區域熱平衡，是指在特定的dV內的，每個溶質和每個溶劑分子平移動能t ( =t0+t ，其中t0 表示樣品在完全熱力學平衡時之個別溶質分子的translational energy)幾乎是完全相等的，因此溶液中的特定dV內溫度 是可定義的，但和鄰近的dV有些許的差異。這裡所指的dV大小量級和光源波長相當，所以在dV內的光強大小可視為均勻的。根據能量均分原理(equipartition theorem)，在溶液為完全（區域）熱平衡時，  t，因此（t）為溶質分子質量傳輸的驅動力，即是所謂的熱擴散。當樣品在所有時間都處於接近區域熱平衡狀態時，二元系統(溶質與溶劑)中兩種成分的t是一樣的。
另方面，因為19 ps脈衝將光能傳予溶質分子的速率遠高於溶質分子將intra-molecular excess energy t傳予鄰近溶劑分子的速率，故可能將大量的脈衝能量儲存於溶質分子內，因此我們首先模擬19 ps脈衝通過樣品之瞬時，個別溶質分子之translational excess energy (t)，然後再模擬intra- 及 inter-molecular relaxation使溶液趨於區域熱平衡後個別溶質分子的t，結果我們發現，當溶液趨於區域熱平時，高濃度溶液可產生較高之溫度梯度，而低濃度溶液之個別溶質分子在溶液未達區域熱平衡前，可獲得較大之translational excess energy (t)。綜合實驗結果和理論模擬，我們認為19 ps 脈衝所造成的溶質分子質量傳輸發生於受激溶液回復區域熱平衡之前，屬非準靜過程。

關鍵字

質量傳輸 ； 熱擴散 ； 準靜過程 ； 非準靜過程

並列摘要

Using chloroaluminum phthalocyanine molecules (C32H16AlClN8) dissolved in ethanol, dubbed as ClAlPc-ethanol, as an example, we investigate the mechanism of solute migration induced by 632.8 nm CW laser light and 532 nm 19 picosecond (ps) laser pulses.
In the study with CW laser light, we measure the transmittance and deflection of the beam normally incident on the sample, prepared to have two concentrations (4.21017 cm3 and 1.21017 cm3), with the entrance surface parallel and normal to the optical table respectively. In the former geometry, we find a monotonous increase of transmittance with time and no transmitted beam deflection. When the higher-concentrated solution shows a temperature gradient larger than the lower-concentrated one, we find that it shows a stronger solute migration. Furthermore, we quantitatively simulate the observed transmittance by involving thermal diffusion alone. Here by thermal diffusion, we mean temperature gradient driven mass transport of a component in a binary system. It is a quasistatic process. In the latter geometry, we find a transition of transmittance from ascent to descent with time and a downward transmitted beam deflection. We additionally invoke convection to explain the results.
In the study with 19 ps laser pulses, we conduct Z-scan measurements on the samples at two concentrations (4.21017 cm3 and 1.21017 cm3). With stronger absorption and larger temperature rise, the higher-concentrated solution shows less migration behavior. Thermal diffusion is thus disregarded. On the other hand, simulation shows that before the excited solution restores local thermal equilibrium, individual solute molecules in the lower concentrated solution gain, due to nonlinear excitation, more translational excess energy t which drives the solute migration. We, therefore, conclude that this migration is non-quasistatic.
To explain how the CW laser light induced-solute migration is quasistatic and the 19 ps laser pulse-induced migration is non-quasistatic, we compare the rate of photo energy deposition into the solute molecules and that of excess energy dissipation throughout the neighboring solvent molecules. Since a CW laser converts its photo energy into the (solute and solvent) molecules translational excess energy, by photo absorption and the subsequent intra- and inter-molecular relaxation, at a rate lower than that the excited samples restore local thermal equilibrium, the samples deviate from local thermal equilibrium infinitesimally in the middle of solute migration. Therefore, this migration tends to be quasistatic. Here, by local thermal equilibrium, we mean the translational energy t (= t0+t with t0 denoting the translational energy of individual solute molecules given the sample in full thermodynamic equilibrium) of each individual molecule contained within the same macroscopic volume element dV is nearly equal and, hence, the solution temperature  pertaining to this dV becomes definable but appears minutely different from that pertaining to a neighboring dV. Here a dV has dimensions in the order of a wavelength and thus light intensity can be considered uniform therein. Since, according to the equipartition theorem,  is proportional to t given the sample in (full or local) thermal equilibrium,  ( t) is taken as the driving force of the solute migration, namely thermal diffusion which is a quasistatic process. Note that when the samples are nearly in local thermal equilibrium all the time, t pertains to both species of molecules composing the binary systems.
On the other hand, since a 19 ps pulse converts its photo energy into the solute molecules translational excess energy at a rate higher than that the excited solution restores local thermal equilibrium, we first simulate translational excess energy (t) gained by individual solute molecules and then calculate t retained in individual solute molecules after the solution restores local thermal equilibrium by intra- and inter-molecular relaxation. As a result, we discover that a larger temperature gradient is formed in the higher concentrated solution after the excited solution restores local thermal equilibrium; however, individual solute molecules in the lower concentrated solution possess more translational excess energy t before the solution restores local thermal equilibrium.Combing the experimental results and theoretical simulation, we determine that 19 ps pulse-induced solute migration is non-quasistatic, initiated before the excited solution restores local thermal equilibrium.

並列關鍵字

mass transport ； thermal diffusion ； quasistatic ； non-quasistatic

參考文獻

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