Detection of electric-quadrupole transitions in water vapour near 5.4 and 2.5 μm

Abstract

Electric-quadrupole transitions, presently detected by Fourier transform spectroscopy, contribute to the infrared absorption spectrum of water vapor.

The 2020 edition of the GEISA spectroscopic database

“…largely below intensity values of the strongest electric-quadrupole lines (e.g. up to 10 -26 cm/molecule for water vapor [17]). The laboratory measurements of Refs [16,17] (in total, about twenty quadrupole lines near 5.4, 2.5 and 1.3 µm) have validated the ab initio intensities computed in Ref.…”

“…Electric-quadrupole transitions do also exist, but they are typically a million times weaker than electric-dipole transitions, making their observation extremely challenging. The recent detection of weak quadrupole transitions in the spectrum of water vapor [16,17] has shown that in their present status the spectroscopic databases are not complete for polyatomic molecules. Actually, databases provide lists of electric-dipole transitions above an intensity cut-off (e.g.…”

“…up to 10 -26 cm/molecule for water vapor [17]). The laboratory measurements of Refs [16,17] (in total, about twenty quadrupole lines near 5.4, 2.5 and 1.3 µm) have validated the ab initio intensities computed in Ref. [16] within the experimental error bars.…”

“…As line shape parameters are critical for atmospheric applications, an updated calculation for H2O-H2O, H2O-N2 and H2O-O2 broadenings and shifts, along with the temperature dependence of these parameters was provided by Gamache and Vispoel (unpublished data, 2020). These parameters were calculated for H2 16 O, H2 18 O, H2 17 O (but also HD 17 O, HD 18 O, D2 16 O, D2 18 O, and D2 17 O species), and include the H2 16 O quadrupole transitions. From values calculated using the Modified Complex Robert Bonamy (MCRB) model by Vispoel et al [14] for H2O-N2 and H2O-O2 (Vispoel and Gamache, unpublished data, 2020), the pressure broadening information for H2O-air was produced considering (air) = 0.79(N2) + 0.21(O2), with a similar formula for the line shift.…”

Artificial Symmetries for Calculating Vibrational Energies of Linear Molecules

“…The theory and programming work reported in Ref. [19] and in the present work have already been employed for generating hot molecular line lists for SiO 2 [45] and CO 2 [46] as well as to produce a quadrupole spectrum of H 2 O [47]. TROVE uses an automatic approach for constructing a symmetry-adapted basis set to be used in setting up a matrix representation of the molecular rotation-vibration Hamiltonian [48] In order to compute the vibrational energies for a linear molecule, we solve the Schrödinger equation for the HamiltonianĤ (0) vib,lin , which involves the bending and stretching motions and, as explained in Introduction, the rotational motion about the a axis.…”