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...