Intensities in local mode overtone spectra of dimethyl ether and acetone

Gas phase overtone spectra of dimethyl ether (ΔvCH=2–5) and acetone (ΔvCH=3) are measured using conventional near infrared (NIR) spectroscopy. Intracavity dye laser photoacoustic spectroscopy (IDL-PAS) has been used to measure the ΔvCH=5–7 gas phase spectra of both dimethyl ether and acetone. Oscillator strengths are calculated using a harmonically coupled anharmonic oscillator local mode description to obtain the vibrational wave functions and ab initio molecular orbital (MO) calculations to obtain the dipole moment function. The calculations, which use no adjustable parameters, can account for the magnitude of the intensities and for the falloff in intensity with increasing v, for both molecules. It can also account reasonably well for the relative intensity of various peaks within a given vibrational manifold and for the relative intensity in the spectra of these two molecules and of propane.