Chloe E. Dedic

Interference-free gas-phase thermometry at elevated pressure using hybrid femtosecond/picosecond rotational coherent anti- Stokes Raman scattering

Rotational-level-dependent dephasing rates and nonresonant background can lead to significant uncertainties in coherent anti-Stokes Raman scattering (CARS) thermometry under high-pressure, low-temperature conditions if the gas composition is unknown. Hybrid femtosecond/picosecond rotational CARS is employed to minimize or eliminate the influence of collisions and nonresonant background for accurate, frequency-domain thermometry at elevated pressure. The ability to ignore these interferences and achieve thermometric errors of <5% is demonstrated for N2 and O2 at pressures up to 15 atm. Beyond 15 atm, the effects of collisions cannot be ignored but can be minimized using a short probe delay (~6.5 ps) after Raman excitation, thereby improving thermometric accuracy with a time- and frequency-resolved theoretical model.

Dual-pump vibrational/rotational femtosecond/ picosecond coherent anti-Stokes Raman scattering temperature and species measurements

A method for simultaneous ro-vibrational and pure-rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) is presented for multi-species detection and improved temperature sensitivity from room temperature to flame conditions. N₂/CH₄ vibrational and N₂/O₂/H₂ rotational Raman coherences are excited simultaneously using fs pump pulses at 660 and 798 nm, respectively, and a common fs Stokes pulse at 798 nm. A fourth narrowband 798 nm ps pulse probes all coherence states at a time delay that minimizes nonresonant background and the effects of collisions. The transition strength is concentration dependent, while the distribution among observed transitions is related to temperature through the Boltzmann distribution. The broadband excitation pulses and multiplexed signal are demonstrated for accurate thermometry from 298 to 2400 K and concentration measurements of four key combustion species.

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.

Vibrational/Rotational Hybrid fs/ps Coherent Anti-Stokes Raman Scattering for Combustion Analysis

A method for simultaneous vibrational and rotational hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) spectroscopy is presented for multispecies detection and improved temperature sensitivity over a wide range of flame conditions. N 2 /CH 4 vibrational and N 2 /O 2 /H 2 rotational Raman coherences are excited simultaneously using fs pump/Stokes pulses at 675/799 nm and 799/799 nm, respectively, and a fourth narrowband ps pulse at 799 nm is used to probe all coherence states at a time delay that minimizes non-resonant background and the effects of collisions. The strength of these transitions is concentration dependent, while the distribution among observed transitions can be related to temperature through the Boltzmann distribution of energy states. The multiplexed signal covers the entire range of temperatures observed in combustion reactants and products (298‐2300 K) with one measurement and can detect four species (N 2 , O 2 , CH 4 , H 2 ) using broadband excitation. This approach is discussed in the context of the feasibility for simultaneous, high-speed, interference-free measurements of key combustion species and temperature in flames with widely varying conditions.

Hybrid fs/ps CARS Spectroscopy for Single-Shot kHz-Rate Thermometry in High-Temperature Flames

This work expands on previous studies to utilize hybrid femtosecond/picosecond coherent anti-Stokes Raman scattering (fs/ps CARS) in high-temperature flames for background-free thermometry. The goal of the current work is to quantify the precision and accuracy of the temperature measurements over an expanded temperature range from 1250 – 2400 K, while also quantifying the influence of nonresonant background on the measurements. Temporal suppression of the nonresonant background by three orders of magnitude while retaining up to 20% of the time-zero signal is demonstrated while the background-free single-shot spectra show a signal-to-noise ratio of ~ 200:1 at ~2400 K. The accuracy and precision of the singleshot temperature measurements are investigated over an expanded range of 1250 – 2400 K to address hybrid fs/ps CARS applicability to lower temperature flames. The resulting accuracy and precision of the measurements is better than 10% and 3.5%, respectively, at all conditions. Finally, the effect of the nonresonant background on the accuracy and precision of the temperature measurement is quantified at ~2400 K, and the need for nonresonant-free detection is discussed.