Photoacoustic spectroscopy: a measurement technique for low absorption coefficients

Area of Science:

• Acoustic physics
• Laser-matter interactions
• Gas dynamics

Background:

• Laser beams interacting with gas cells can induce acoustic phenomena.
• Understanding absorption mechanisms (surface vs. bulk) is crucial for predicting these effects.

Purpose of the Study:

• To investigate the frequency dependence of acoustic stresses in a nonabsorbing gas.
• To differentiate and quantify acoustic stresses arising from surface and bulk absorption of a modulated laser beam.

Main Methods:

• Theoretical derivation of frequency-dependent acoustic stresses.
• Analysis of surface and bulk absorption contributions.
• Numerical simulations for specific materials (laser glass, nitrogen).

Main Results:

• Identified an intermediate frequency range for distinct acoustic stress behaviors.
• Surface absorption stress dependency: approximately omega(-1) with a ~90° phase shift.
• Bulk absorption stress dependency: approximately omega(-3/2) with a ~45° phase shift.

Conclusions:

• The frequency and phase shift of acoustic stress provide insights into absorption mechanisms.
• Distinct acoustic signatures differentiate surface and bulk absorption in gas cells.
• Provides a framework for analyzing laser-induced acoustics in gases.