thermal diffusion depth

symbols: $L$, $\mu_{\rm{s}}$, units: $\pu{m}$, $\pu{\upmu\!m}$
synonym: thermal wave decay length
https://doi.org/10.1351/goldbook.08696
In photoacoustic spectroscopy, \(L = \sqrt{(D/\pi f)}\), where \(D\) is thermal diffusivity and \(f\) is modulation frequency of the electromagnetic radiation.
Notes:
  1. SI unit: \(\pu{m}\). Common unit: \(\pu{\upmu\!m}\).
  2. \(\pu{63 \%}\) of the PAS signal originates within the thermal diffusion depth.
Source:
PAC, 2021, 93, 647. 'Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)' on page 766 (https://doi.org/10.1515/pac-2019-0203)