Measurement of the dependence of ultra diluted gas transmittance on the size of the detector

Abstract: We show that measured optical transmittance of an ultra thin gas depends on the detector size. To this end we conducted an experiment that compares transmittances measured in parallel with a pair of detectors with different diameters ranging from 2 ${\mu}$m to 200 ${\mu}$m. A Tunable Diode Laser Absorption Spectroscopy type system was used. Transmittance of $\sim$1e-2 mbar water vapor on NIR absorption line $\lambda$=1368.60 nm was measured using a 60 m long multi-pass cell placed inside the 300 l vacuum chamber. The result of the experiment shows higher transmittances when the measurement is performed using smaller detectors. The difference reaches as much as 1.23 $\pm$0.1 %, which is greater than 0 with >5$\sigma$ statistical significance. Qualitatively it is in agreement with the recently developed model of thin gas optical transmittance taking into account the quantum mechanical effects of spreading of the wave functions of individual gas particles.