Ro-vibrational spectroscopy of the formic acid-d1 monomer embedded in helium nanodroplets

Abstract

Rotationally resolved infrared spectra are reported for the fundamental CH and OD stretching bands of trans-formic acid-d1 (trans-HCOOD) embedded in helium nanodroplets. Altogether five and six ro-vibrational transitions were identified for the ν1(CH stretch) and ν2(OD stretch) bands of trans-HCOOD, respectively. Molecular constants were determined by using the measured transition frequencies as input in a fitting procedure that utilizes a standard gas phase Hamiltonian. The origins of the ν1 and ν2 bands were found to be at 2939.5827(2) and 2631.3713(2)cm−1, respectively. These band origins are redshifted by about 0.5 and 0.03cm−1, respectively, compared to the gas phase spectrum. The excited state rotational constants A′ [1.2725(3)cm−1], B′ [0.2816(2)cm−1], and C′ [0.19875(4)cm−1] for the ν1 mode and A′ [1.2918(3)cm−1], B′ [0.2837(2)cm−1], and C′ [0.20400(4)cm−1] for the ν2 mode of trans-HCOOD embedded in helium droplets are reduced by a factor of about 1.6 relative to the gas phase. The Lorentzian line widths of the observed transitions for the CH and OD stretches are in the range of ∼0.06cm−1 and ∼0.03cm−1, respectively. The broader lines of the CH stretch, as compared to the corresponding lines in HCOOH, may be a result of vibrational relaxation through a nearby combination band or through the OD vibration, which lies below the CH stretch in HCOOD.