Long-lived nitric oxide molecular tagging velocimetry with 1 + 1 REMPI

Area of Science:

• Fluid dynamics
• Laser-based diagnostics
• Spectroscopy

Background:

• Molecular Tagging Velocimetry (MTV) is a technique used for measuring flow velocities.
• Nitric oxide (NO) fluorescence is commonly used for MTV, but its short lifetime can limit precision.
• Existing NO fluorescence lifetimes are typically in the nanosecond range at relevant pressures.

Purpose of the Study:

• To demonstrate a method for generating long-lived NO fluorescence for MTV.
• To investigate the physical mechanism behind the long-lived NO signal.
• To assess the potential of this technique for enhanced velocimetry accuracy and precision.

Main Methods:

• Utilized 1+1 resonance-enhanced multiphoton ionization (REMPI) of NO near 226 nm.
• Targeted overlapping Q1(7) and Q21(7) lines of the A-X (0,0) electronic system.
• Measured fluorescence lifetime in a 100-Torr cell with 2% NO in nitrogen.

Main Results:

• Achieved a long-lived NO fluorescence signal with a lifetime of approximately 8.6 µs.
• Observed a fluorescence lifetime significantly longer (by orders of magnitude) than traditional single-photon NO fluorescence (approx. 43 ns).
• Successfully employed the technique for precise flow velocity measurements in a high-speed jet flow.

Conclusions:

• The demonstrated long-lived NO fluorescence offers potential for an order of magnitude improvement in velocimetry accuracy and precision.
• This technique is particularly promising for lower-speed regions of hypersonic flows, such as wakes and boundary layers.
• The detailed understanding of the physical mechanism enables further optimization and application of this advanced MTV method.