Carter Jeffrey A, Wang Zhaohui, Dlott Dana D
School of Chemical Sciences, University of Illinois at Urbana-Champaign, Chemical and Life Sciences Laboratory, 600 South Mathews Avenue, Urbana, Illinois 61801, USA.
J Phys Chem A. 2008 Apr 24;112(16):3523-9. doi: 10.1021/jp800278c. Epub 2008 Mar 18.
We have shown that it is possible to input heat to one location of a molecule and simultaneously measure its arrival in real time at two other locations, using an ultrafast flash-thermal conductance technique. A femtosecond laser pulse heats an Au layer to approximately 800 degrees C, while vibrational sum-frequency generation spectroscopy (SFG) monitors heat flow into self-assembled monolayers (SAMs) of organic thiolates. Heat flow into the SAM creates thermally induced disorder, which decreases the coherent SFG signal from the CH-stretching transitions. Recent improvements in the technique are described, including the use of nonresonant background-suppressed SFG. The improved apparatus was characterized using alkanethiolate and benzenethiolate SAMs. In the asymmetric 2-methyl benzenethiolate SAM, SFG can simultaneously monitor CH-stretching transitions of both phenyl and methyl groups. The phenyl response to flash-heating occurs at least as fast as the 1 ps time for the Au surface to heat. The methyl response has a faster portion similar to the phenyl response and a slower portion characterized by an 8 ps time constant. The faster portions are attributed to disordering of the methyl-substituted phenyl rings due to thermal excitation of the Au-S adbonds. The slower portion, seen only in the methyl SFG signal, is attributed to heat flow from the metal surface into the phenyl rings and then to the methyl groups.
我们已经表明，使用超快闪光热导技术，可以在分子的一个位置输入热量，并同时实时测量其在另外两个位置的热量到达情况。飞秒激光脉冲将金层加热到约800摄氏度，同时振动和频产生光谱（SFG）监测热量流入有机硫醇盐的自组装单分子层（SAMs）。流入SAM的热量会产生热诱导无序，这会降低来自CH伸缩跃迁的相干SFG信号。描述了该技术最近的改进，包括使用非共振背景抑制SFG。使用链烷硫醇盐和苯硫醇盐SAM对改进后的仪器进行了表征。在不对称的2-甲基苯硫醇盐SAM中，SFG可以同时监测苯基和甲基的CH伸缩跃迁。苯基对快速加热的响应至少与金表面加热的1皮秒时间一样快。甲基响应有一个与苯基响应相似的较快部分和一个以8皮秒时间常数为特征的较慢部分。较快部分归因于由于Au-S键的热激发导致甲基取代苯环的无序化。较慢部分仅在甲基SFG信号中出现，归因于热量从金属表面流入苯环，然后再流入甲基。
