Vibrational state mixing of individual rotational levels in 1-butyne near 3333 cm−1

Abstract

The high resolution infrared spectrum of jet-cooled 1-butyne is reported for the acetylenic C–H stretch. The absorbance of an F-center laser beam is enhanced by a multiple reflection cell surrounding the free jet. Although the spectrum in the gas phase (300 K) is a featureless rotational contour, at 3 K sharp features are observed. For J′=0, 1, 2, the K′a =1 lines are spectroscopically well-behaved, but the Ka =0 lines are split into multiplets. Analysis of the K′a =1 lines gives the constants (B′+C′)/2=0.143 82(11) cm−1, (B′−C′)/2=0.007 20(11) cm−1, and ν0+A′−A″=3333.0182(9) cm−1. The quantities in parentheses are two standard deviations in units of the last digit. It is concluded that the Ka =0 levels are mixed with the bath of background vibrations by anharmonic matrix elements the order of 0.01 cm−1.