Suppression of the nonresonant background in vibrational sum-frequency generation (SFG) in the broadband multiplex configuration is achieved using a time-asymmetric pulse, created by passing a femtosecond pulse through a Fabry-Perot e ´talon, to temporally discriminate between the faster nonresonant and slower resonant contributions. A mixed time and frequency domain explanation of the SFG process is presented, and spectra with high time resolutions and high degrees of nonresonant background suppression are obtained using selfassembled alkanethiolate monolayers on Au.

A method is described for obtaining ultrahigh time-resolution vibrational spectra of shocked polycrystalline materials. A microfabricated shock target array assembly is used, consisting of a polymer shock generation layer, a polymer buffer layer, and a thin sample layer. A near-IR pump pulse launches the shock. A pair of delayed visible probe pulses generate a coherent anti-Stokes Raman (CARS) spectrum of the sample. High-resolution Raman spectra of shocked crystalline anthracene are obtained. From the Raman shock shift, the shock pressure is determined to be 2.6 GPa. The rise time of shock loading is 400 ps. This rise time is limited by hydrodynamics of the shock generation layer. The shock velocity in the buffer layer is found to be 3.7 (±0.5) km/s, consistent with the observed shock pressure. As the shock propagates through a few μm of buffer material, the rise time and pressure can be monitored. The rise time decreases from ∼800 to ∼400 ps over the first 6 μm of travel, and the pressure begins to decline after about 12 μm of travel. The high-resolution CARS method permits detailed analysis of the vibrational line shape. Simulations of the CARS spectra show that when the shock front is in the crystal layer the spectral linewidths are inhomogeneously broadened by the distribution of pressures in the layers. When the crystal layer is behind the front, the spectral linewidth can be used to estimate the temperature. The increase of the spectral width from the ambient 4 to ∼6.5 cm−1 is consistent with the expected temperature increase of ∼200°.

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A new technique is described, where picosecond laser pulses generate and probe 4.2 GPa nanoshocks in polymeric and polycrystalline solids at a high repetition rate of ∼100/s. The term nanoshock refers to the short duration (a few ns) of the shock pulse and the very small shocked volume (a few ng). The nanoshock wave form is characterized by the shock front risetime, shock falltime, peak pressure, and velocity. Coherent Raman spectroscopy during nanoshock propagation in a 700-nm-thick layer of polycrystalline anthracene, called an optical nanogauge, is used to determine these quantities. A powerful method of analysis, singular value decomposition (SVD), is applied to Raman spectroscopy of shock waves for the first time. Using SVD analysis, the risetime of the nanoshock pulses is found to be less than 25 ps, and the velocity of the shock front in the nanogauge is monitored in real time. Some possible applications of nanoshock technology in the areas of shock-induced material transformation and shock-induced mechanical deformation processes, are discussed briefly.

Laser-driven shock waves (0−5 GPa) can be generated at high repetition rates (100/s) using a moderate-energy tabletop picosecond laser system and a multilayered microfabricated shock target array. High spatial resolution is needed to obtain high temporal resolution of the effects of a steeply rising shock front on molecular materials. The needed spatial resolution is obtained using a sandwich arrangement with a thin layer of sample material termed an “optical nanogauge”. Experiments with an anthracene nanogauge show that ultrafast vibrational spectroscopy can be used to determine the shock temperature, pressure, velocity, and shock front rise time. Shock pulses can be generated with rise times <25 ps, which generate irreversible shock compression, and with rise times of a few hundred picoseconds, which generate reversible compression. These pulses, which have a duration of a few nanoseconds, are termed “nanoshock” pulses. Nanoshock pulses produce large-amplitude mechanical perturbations and can initiate and turn off thermochemical reactions, produce highly excited vibrational populations, and heat and cool condensed matter systems at tremendous rates. These applications are illustrated briefly in nanoshock experiments on an energetic material and a heme protein. Using high repetition rate nanoshocks to study large-amplitude molecular dynamics in molecular materials important in chemistry and biology is the new wave in shock waves.

The passage of a 4 GPa shock front through an embedded optical nanogauge, a thin ͑ϳ700 nm͒ layer of polycrystalline molecular material (anthracene), is monitored in real time by picosecond coherent Raman scattering. Analysis of high resolution Raman spectra shows the shock rise time is less than 25 ps, and the front is less than 100 molecules wide. The rise time is faster than relaxation of nonequilibrium populations of molecular vibrations, which shows a shock front in a molecular material can leave highly nonequilibrium vibrational states in its wake. The implications for shock initiation of energetic materials, typically polycrystalline molecular solids, are briefly discussed.[S0031-9007(97)03191-8] PACS numbers: 62.50. + p, 31.70.Ks, 42.65.Dr, 78.47. + p We present a new spectroscopic method used to measure ultrafast shock-front rise times in molecular materials. The rise time of a shock front (4.2 GPa) in a polycrystalline layer of anthracene ͑C 14 H 10 ͒ is shown to be #25 ps. Our motivation is the development of a molecular level picture of energetic material initiation [1] by relatively low pressure (say 1-5 GPa) shocks. Most high performance energetic materials are formulations of polyatomic molecular solids. In contrast to simpler atomic solids such as metals, discussed briefly below, the basic unit of these energetic materials is a large molecule with a complicated vibrational structure. Interactions between a shock front and the internal molecular vibrations can transform some of the directed energy of the shock into internal energy in the form of molecular vibrational excitation [2][3][4]. This transformation, termed multiphonon up-pumping [2], is a dissipative process, which does not exist in atomic solids, that can broaden out the shock front [2,5,6]. If the shockfront rise time t r in the presence of up-pumping is faster than the characteristic time scale for thermal equilibration among internal molecular vibrations, typically a few tens of ps [2,3,7,8], the internal vibrations of the molecules can be pumped into highly nonequilibrium states. Several authors have discussed how such nonequilibrium populations might affect chemical reactivity and possibly affect the sensitivity of energetic materials [2-4], but until the present work there existed no direct evidence that shockfront rise times in molecular materials were fast enough to produce nonequilibrium vibrational excitations.For shocks in the 1-5 GPa range, conventional impact measurements ordinarily see steady-state shock fronts with rise times in the 10 28 10 26 s range [5]. These rise times were attributed to viscosity and other dissipative processes, and rise time decreases as shock pressure increases [5]. Recently, ultrashort laser shock generation and probing techniques have been used to investigate shocks which have propagated a very short distance and are not yet in steady state, in opaque atomic solids such as aluminum [9], gold [10], or silicon [11]. Several groups have observed fast rise times (a few tens of ps) for these extreme...

A 4.5 GPa shock pulse producing a cycle of compression heating ͑,25 ps͒ and expansion cooling ͑ϳ1.5 ns͒ is used to study fast mechanical dynamics of solid organic polymers and proteins. Coherent Raman spectroscopy of a dye in the sample shows that compression occurs by an instantaneous part followed by a second, ϳ300 ps, structural relaxation process. After expansion, a mechanically distorted structure is produced which does not relax on the ,15 ns time scale. The results are interpreted with an energy landscape model. PACS numbers: 61.41. + e, 62.50. + p, 78.47. + p, 87.15.He In this paper we use the nanoshock spectroscopy technique [1] to investigate the microscopic response of organic polymers and proteins, subjected to a cycle of shock compression heating followed by expansion cooling, which produces ultrafast large amplitude structural dynamics. The results are interpreted with an energy landscape model.The nanoshock [1] is a shock pulse with a duration of about 1 ns. The peak pressure is 4.5͑60.5͒ GPa. The rise time is ,25 ps. The pressure relaxes in a few ns. In a typical organic polymer [e.g., polymethylmethacrylate (PMMA)], shock volume compression is ϳ20%, the shock velocity is ϳ4 km͞s, and the temperature rise is ϳ125 K [2,3]. A microscopic understanding of fast large amplitude dynamics of materials helps extend current models beyond linear response, and might be relevant to practical problems such as impact resistance of polymers or biological effects of shock waves generated in pulsed laser surgery or lithotripsy.Structural relaxation of amorphous materials is usually treated in the context of potential energy landscape theories [4]. Relaxation response functions have been measured for weak external mechanical, acoustic, electrical, thermal, and optical perturbations which result from transitions between local energy minima [4,5]. Shock waves allow us to study fast structural evolution processes in the higher energy regions of the landscape difficult or impossible to access by conventional techniques. Figure 1 diagrams the energy landscape model for shock waves in amorphous materials. First consider the effects of slow, reversible isothermal compression or expansion [ Figs. 1(a) and 1(b)]. This problem has been treated recently in the context of molecular dynamics simulations of glasses [6,7]. With reversible compression, the ambient landscape's total potential energy is increased by an amount equal to the work done on the system. The additional potential energy is distributed among atom pairs in a complicated way which changes the entire topography [6,7], created a new "compressed landscape," whose local minima represent quite different structures from the corresponding ambient landscape. The global minima of compressed landscapes are displaced from the ambient landscapes, because compression favors configurational coordinates which increase the density. Slow reversible compression or expansion is a gradual evolution from the lower energy region of one landscape to another.Instantaneous shock com...

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