Pressure broadening of single vibrational-rotational transitions of acetylene at ν = 5

Abstract

To understand the mechanism of pressure broadening one must have accurate values of the pressure broadening coefficients as a function of vibrational quantum number. Unfortunately, such data for polyatomic molecules are scarce. The coefficient for self-broadening of methane has been found to be the same for the R(0) and R(1) lines of the 2{nu}{sub 3} and 3{nu}{sub 3} bands and for an unidentified line of the 5{nu}{sub 1} + {nu}{sub 3} band at 6190 {angstrom} (1). Measurements have also been performed on the {nu}{sub 2} (2) and {nu}{sub 1} + {nu}{sub 3} (3) bands of acetylene. In the work described here, high resolution spectra of single vibrational-rotational lines of the 5{nu}{sub 3} band of acetylene at 15,600 cm{sup -1} have been taken to determine the coefficients for self-broadening at a much higher level of vibrational excitation. A single frequency cw dye laser (Spectra-Physics 580A) with Rhodamine B as the lasing medium is used as a narrow bandwidth light source. The laser is continuously scannable over a 10 GHz region with 30 MHz linewidth. The unfocused beam is chopped and directed through a small, nonresonant optoacoustic cell. The pressure of the acetylene (> 99.99% purity) in the cell is measured withmore » a capacitance manometer. The optoacoustic signal is detected by a miniature electret microphone placed within the cell and is processed by a lock-in amplifier. The high resolution scans are calibrated ({+-} 5%) by monitoring the dye laser output with a spectrum analyzer equipped with 2 GHz FSR mirrors. All spectra were taken at room temperature (293 {+-} 2 K). At the relatively low pressures used in these experiments, the experimental linewidths are not more than three times the Doppler width (1.1 GHz FWHM). The method of Gronwall was used to calculate values of the Voigt lineshape. With these data, the pressure broadened widths, {Delta}{nu}{sub p}, were extracted from the experimental lineshapes and are plotted as a function of pressure for the R(3), R(9), and R(15) lines in Figure 1. The lines drawn through the data points were determined by the linear least squares method. The self-broadening coefficients, {gamma}, and their standard errors are summarized in Table 1 along with the results of other workers.« less