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The dynamics of photoexcited 2,4-dimethylpyrrole (DMP) were studied using time-resolved velocity map imaging spectroscopy over a range of photoexcitation wavelengths (276238 nm). Two dominant H atom elimination channels were inferred from the time-resolved total kinetic energy release spectra, one which occurs with a time constant of similar to 120 fs producing H atoms with high kinetic energies centered around 50007000 cm(1) and a second channel with a time constant of similar to 3.5 ps producing H atoms with low kinetic energies centered around 25003000 cm(1). The first of these channels is attributed to direct excitation from the ground electronic state (S-0) to the dissociative 1(1)pi sigma* state (S-1) and subsequent NH bond fission, moderated by a reaction barrier in the NH stretch coordinate. In contrast to analogous measurements in pyrrole (Roberts et al. Faraday Discuss. 2013, 163, 95116), the NH dissociation times are invariant with photoexcitation wavelength, implying a relatively flatter potential in the vertical FranckCondon region of the 1(1)pi sigma* state of DMP. The origins of the second channel are less clear-cut, but given the picosecond time constant for this process, we posit that this channel is indirect and is likely a consequence of populating higher-lying electronic states [e.g., 2(1)pi sigma*(S-2)] which, following vibronic coupling into lower-lying intermediary states (namely, S-1 or S-0), leads to prompt NH bond fission.

As part of a strategic, multi-facility test program, subscale testing of NASA s Crew Exploration Vehicle was conducted in both legs of NASA Langley s Unitary Plan Wind Tunnel. The objectives of these tests were to generate aerodynamic and surface pressure data over a range of supersonic Mach numbers and reentry angles of attack for experimental and computational validation and aerodynamic database development. To provide initial information on boundary layer transition at supersonic test conditions, transition studies were conducted using temperature sensitive paint and infrared thermography optical techniques. To support implementation of these optical diagnostics in the Unitary Wind Tunnel, the experiment was first modeled using the Virtual Diagnostics Interface software. For reentry orientations of 140 to 170 degrees (heat shield forward), windward surface flow was entirely laminar for freestream unit Reynolds numbers equal to or less than 3 million per foot. Optical techniques showed qualitative evidence of forced transition on the windward heat shield with application of both distributed grit and discreet trip dots. Longitudinal static force and moment data showed the largest differences with Mach number and angle of attack variations. Differences associated with Reynolds number variation and/or laminar versus turbulent flow on the heat shield were very small. Static surface pressure data supported the aforementioned trends with Mach number, Reynolds number, and angle of attack.