Mode specific vibrational relaxation in the acetylene–hydrogen fluoride binary complex

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Abstract
The optothermal molecular beam technique has been used to record the near infrared spectrum of the acetylene–HF binary complex associated with the ν7 vibrational mode (asymmetric C–H stretch). A fit to this perpendicular band spectrum gives accurate rotational constants for both the ground and excited vibrational states, as well as the vibrational origin, namely A″=1.192 60(5) cm−1 , B″=0.157 34(1) cm−1 , C″=0.138 10(1) cm−1 , A′=1.186 90(5) cm−1 , B′=0.157 34(1) cm−1 , C′=0.138 08(1) cm−1, and ν0 =3276.2889(30) cm−1 . From the homogeneous linewidths of the observed transitions the lifetime of the excited vibrational state has been determined to be 3.6 ns. When compared with the lifetime determined previously for the ν1 mode (0.8 ns) and the data obtained previously for several other systems, it is clear that vibrational relaxation in this molecule is mode specific. A discussion is given concerning the nature of this relaxation process.