Fig. 4: QEMR-PAS spectra of ammonia measured at different spectral resolutions and pressures.
From: Quartz-enhanced multiheterodyne resonant photoacoustic spectroscopy

a Spectral resolution of 1 GHz at the pressure of 670 Torr. Three spectral features overlap each other. b Spectral resolution of 700 MHz at the pressure of 200 Torr. The third feature completely separates from the first two. c Spectral resolution of 43 MHz at the pressure of 100 Torr. The three features are well separated. The horizontal axis represents the offset frequency from 195.7368 THz (1531.68 nm)