Shock compression and flash-heating of molecular adsorbates on the picosecond time scale

sum-frequency generation spectroscopy;ultrafast;vibrational spectroscopy;shock compression;flash-heating;molecular adsorbates;self-assembled monolayers;temperature jumps;thermoreflectance

Berg, Christopher

关键词: sum-frequency generation spectroscopy;    ultrafast;    vibrational spectroscopy;    shock compression;    flash-heating;    molecular adsorbates;    self-assembled monolayers;    temperature jumps;    thermoreflectance;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/49566/Christopher_Berg.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

An ultrafast nonlinear coherent laser spectroscopy termed broadband multiplex vibrational sum-frequency generation (SFG) with nonresonant suppression was employed to monitor vibrational transitions of molecular adsorbates on metallic substrates during laser-driven shock compression and flash-heating. Adsorbates were in the form of well-ordered self-assembled monolayers (SAMs) and included molecular explosive simulants, such as nitroaromatics, and long chain-length alkanethiols.Based on reflectance measurements of the metallic substrates, femtosecond flash-heating pulses were capable of producing large-amplitude temperature jumps with ΔT = 500 K.Laser-driven shock compression of SAMs produced pressures up to 2 GPa, where 1 GPa ≈ 10000 atm.Shock pressures were estimated via comparison with frequency shifts observed in the monolayer vibrational transitions during hydrostatic pressure measurements in a SiC anvil cell.Molecular dynamics during flash-heating and shock loading were probed with vibrational SFG spectroscopy with picosecond temporal resolution and sub-nanometer spatial resolution. Flash-heating studies of 4-nitrobenzenethiolate (NBT) on Au provided insight into effects from hot-electron excitation of the molecular adsorbates at early pump-probe delay times.At longer delay times, effects from the excitation of SAM lattice modes and lower-energy NBT vibrations were shown.In addition, flash-heating studies of alkanethiolates demonstrated chain disordering behaviors as well as interface thermal conductances across the Au-SAM junction, which was of specific interest within the context of molecular electronics. Shock compression studies of molecular explosive simulants, such as 4-nitrobenzoate (NBA), demonstrated the proficiency of this technique to observe shock-induced molecular dynamics, in this case orientational dynamics, on the picosecond time scale.Results validated the utilization of these refined shock loading techniques to probe the shock initiation or first bond-breaking reactions in molecular explosives such as δ-HMX:a necessary study for the development of safer and more effective energetic materials.