High‐resolution stimulated Raman spectroscopy of low‐frequency rotational transitions: Obtention of collisional broadening coefficients in the rotational S branch of CO perturbed by H₂

Abstract

We demonstrate the application of high‐resolution quasi‐continuous (q‐cw) stimulated Raman spectroscopy (SRS) to the study of low‐frequency, pure rotational transitions of molecules in diluted samples in order to obtain collisional information. The main problems encountered in the few pioneering q‐cw SRS rotational works, namely weak signals because of the small population differences between the upper and lower states of the transitions and the presence of alternating current (AC) Stark effect, have been addressed: technical improvements in the optical elements that perform the beam mixing, separation, and filtering have allowed us to use a collinear beam geometry and a triple pass configuration to enhance the Raman signals. The distortions in the line profiles caused by AC Stark effect have been measured and included in the numerical treatment of the spectra. With these extended capabilities, we have been able to register stimulated Raman transitions with wavenumbers as low as ∼ 11.5 cm−1, corresponding to the S(0) pure rotational transition of the molecule of CO. We have applied the setup to the study of collisional broadening of CO rotational lines perturbed by H2 and have obtained broadening coefficients for lines from S(0) to S(20) at 77, 180, and 298 K. A comparison between these anisotropic coefficients and those obtained from IR rovibrational experiments shows excellent agreement. This is, to our knowledge, the first demonstration of the use of rotational SRS for the obtention of collisional information in diluted gas samples.