What Is Collisional Broadening?
When gas molecules absorb or emit radiation, they exhibit discrete energy transitions, which show up as sharp spectral lines. However, in real systems, these lines broaden due to various effects—one of them being collisions between particles.
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Mechanism: Collisions disturb the energy levels of molecules or atoms, causing uncertainty in the energy of transitions (Heisenberg uncertainty principle). The result is a spread in the frequency of the absorbed/emitted radiation, i.e., broader spectral lines.
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Line shape: Typically Lorentzian in nature.
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When Does It Happen?
Collisional broadening is significant under the following conditions:
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High pressure: More molecules per unit volume → more collisions.
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High gas density: Denser gases increase the frequency of collisions.
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Elevated temperatures: Increase kinetic energy, leading to more frequent and energetic collisions.
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Low resolution: In low-resolution instruments, the effects of broadening become harder to distinguish from instrumental limitations.
1. UV (Ultraviolet) Spectroscopy
UV analyzers generally operate in low-pressure environments to reduce collisional broadening. Strategies include:Operate at reduced pressure: Use vacuum systems or pressure-controlled sample chambers.Use lower concentration samples: Reduces the number of collisions.Calibrate carefully: Use reference cells and account for line broadening in calibration curves.
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2. IR (Infrared) Spectroscopy
Infrared analyzers are more affected by collisional broadening in gas-phase samples.
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Dilution with inert gases (e.g., Nâ‚‚, Ar): Reduces partial pressure of the absorbing species.
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Pressure control: Use pressure regulators or vacuum cells to manage sample pressure.
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High-resolution spectrometers (e.g., FTIR): Help resolve broadened lines.
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​ Separating the molecules, under low pressure sampling minimizes the broadening of the spectral lines
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3. TDLAS (Tunable Diode Laser Absorption Spectroscopy)
TDLAS is highly sensitive to line shape and is often used to quantify gas species in trace amounts.
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Low-pressure sampling: Sampling at low pressure is standard to reduce broadening and improve resolution.
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Temperature stabilization: Helps reduce thermal effects on collision rates.
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Line shape fitting models: Use Voigt or Lorentzian profiles in software to fit and correct for pressure broadening.
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Use of reference spectra: Use HITRAN or similar databases with pressure-dependent line shape parameters.
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Need help choosing or optimizing a spectroscopic analyzer?
Whether you're working with UV, IR, or TDLAS systems, minimizing collisional broadening is key to achieving accurate results—especially in gas-phase applications. If you're unsure how to manage this in your specific setup, reach out today. Let’s talk instrumentation, sample conditioning, or even custom calibration strategies tailored to your needs!
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