GAs in Scattering Media Absorption Spectroscopy (GASMAS) is used to correlate the average pore size within mesoporous alumina samples to the broadening of the absorption lines of oxygen gas and water vapor entrapped within the pores. Collisions of gas molecules cause extra broadening to the absorption linewidths if the average time between collisions is smaller than the inverse of the linewidth of the absorption line. A gas molecule can collide either with another molecule or with the walls of its container. Hence, for a gas entrapped within a porous medium that has an average pore size comparable to the mean free path of intermolecular collisions, collisions of the gas molecules with the walls of the pores can cause extra broadening. This extra broadening is used to estimate the average size of the pores. At atmospheric pressure, the mean free path of intermolecular collision is about 100 nm and thus broadening due to collision with the walls of the pores should be noticeable for pore sizes of order of 100 nm or less. In this work, high resolution tunable singlemode diode lasers at 761 nm and 936 nm are employed to study the absorption from oxygen gas and water vapor, respectively. The samples used are made from porous pure 𝛼-alumina with average pore sizes ranging from 50 to 150 nm.
The broadening of the P9P9 absorption line of oxygen molecules entrapped in the pores of nanoporous alumina is studied using Gas in Scattering Media Absorption Spectroscopy (GASMAS). A narrow band tunable vertical-cavity surface-emitting laser (VCSEL), with emission peaking around 763 nm, is used to scan over the absorption line of the oxygen A-band. The oxygen line broadening in alpha alumina discs of pore sizes 150 nm, 80 nm and 50 nm, are measured to be 3.8 GHz, 4.2 GHz, and 4.8 GHz, respectively, and compared with the measured open-air oxygen line broadening of 3.3 GHz. The oxygen line broadenings are correlated with studied samples pore sizes and are found to agree well with the line broadenings evaluated using a model based on collisions of confined oxygen molecules with the bulk sample pore walls.
Collisions of gas molecules with the walls of small pores in nanoporous materials can cause the width of gas absorption lines to become wider. Also, the effective absorption path lengths through the gas become longer due to multi-scattering within the nanoporous materials. GAs in Scattering Media Absorption Spectroscopy (GASMAS) is an effective technique that can differentiate between the absorption from the gas within pores and the absorption from the bulk of scattering materials. A Vertical-Cavity Surface-Emitting Laser (VCSEL) emitting around 760 nm is used to investigate the absorption from molecular oxygen gas trapped within mesoporous alumina Al2O3. The pore size of the alumina samples are also characterized based on Barrett, Joyner and Halenda (BJH) method. In this work, we will present the correlation between the pore sizes of the mesoporous alumina samples the width of the absorption line of molecular oxygen gas under different experimental conditions.