The absorption spectrum of H2: CRDS measurements of the (2-0) band, review of the literature data and accurate ab initio line list up to 35 000 cm−1

Abstract

Five very weak transitions—O(2), O(3), O(4), O(5) and Q(5)—of the first overtone band of H₂ are measured by very high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) between 6900 and 7920 cm⁻¹. The noise equivalent absorption of the recordings is on the order of α_min ≈ 5 × 10⁻¹¹ cm⁻¹ allowing for the detection of the O(5) transition with an intensity of 1.1 × 10⁻³⁰ cm per molecule, the smallest intensity value measured so far for an H₂ absorption line. A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift of the O(2) and O(3) lines was accurately determined from a series of recordings with pressure ranging between 10 and 700 Torr. From an exhaustive review of the literature data, the list of H₂ absorption lines detected so far has been constructed. It includes a total of 39 transitions ranging from the S(0) pure rotational line near 354 cm⁻¹ up to the S(1) transition of the (5-0) band near 18 908 cm⁻¹. These experimental values are compared to a highly accurate theoretical line list constructed for pure H₂ at 296 K (0–35 000 cm⁻¹, intensity cut off of 1 × 10⁻³⁴ cm per molecule). The energy levels and transition moments were computed from high level quantum mechanics calculations. The overall agreement between the theoretical and experimental values is found to be very good for the line positions. Some deviations for the intensities of the high overtone bands (V > 2) are discussed in relation with possible pressure effects affecting the retrieved intensity values. We conclude that the hydrogen molecule is probably a unique case in rovibrational spectroscopy for which first principles theory can provide accurate spectroscopic parameters at the level of the performances of the state of the art experimental techniques.