James B. Michael

Single-shot ultrafast coherent anti-Stokes Raman scattering of vibrational/rotational nonequilibrium

The study of internal molecular energy transfer is important for a variety of nonequilibrium and nonthermal environments, including plasma-based manufacturing and materials treatment, medical device treatment applications, and plasma-assisted combustion. In the current work, hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering spectroscopy is demonstrated for simultaneous, single-shot measurement of pure-rotational and rovibrational energy distributions in the highly nonequilibrium environment of a dielectric barrier discharge plasma. Detailed spatial distributions and shot-to-shot fluctuations of rotational temperatures spanning 325–450 K and corresponding vibrational temperatures of 1200–5000 K are recorded across the plasma and surrounding flow with high precision and accuracy. This approach allows concise measurements of vibrational/rotational energy distributions in nonequilibrium environments at kilohertz rates that are free of nonresonant background and minimize interference from molecular collisions.

Hybrid fs/ps Coherent anti-Stokes Raman Scattering for non-equilibrium Environments

N2 vibrational and rotational distributions are probed using hybrid CARS to investigate a ns-laser spark environment. Experiments and simulations explore measurement feasibility and accuracy when rotational and vibrational temperature and number density are unknown.

Evaluation of Hybrid fs/ps coherent anti-Stokes Raman scattering temperature and pressure sensitivity at combustor relevant conditions

Vibrational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (hybrid CARS) spectroscopy is evaluated for the simultaneous measurement of temperature and pressure through the use of a single probe pulse and multiple probe pulses at various time delays. The relevant temperature and pressure parameter space is explored with the goal of determining feasibility of simultaneous pressure and temperature measurements with simplified CARS probe schemes. The sensitivity of spectral features to pressure and temperature changes is evaluated quantitatively from 1-60 atm and 300-3000 K. This study establishes a framework for experimental design and testing under relevant engine conditions.