Hot Band Spectroscopy through Intermolecular Vibrational Energy Transfer

Abstract

In a seeded bimolecular jet, vibrational energy transfer between two different molecular species in combination with an infrared double resonance, technique permits to circumvent symmetry restrictions in high resolution hot band spectroscopy. One of the two species is resonantly excited by a cw $CO_{2}$ waveguide laser to a single rovibrational level around $1000 cm^{-1}$. In the collisional environment of the jet, vibrational energy is transferred to a second molecular species through V-V exchange processes. A single mode scanning F-center laser (FCL) is then applied to probe the transferred population. By modulating the $CO_{2}$ laser intensity and phase sensitively detecting the induced changes in FCL absorption, a background-free rotationally cold hot band spectrum (FWHM 100 MHz) is obtained. In the excitation step, the coincidences between $CO_{2}$ laser frequencies and molecular transitions in $SF_{6}$ and $C_{2}H_{4}$ have been used. Lower levels for hot band spectroscopy of $C_{2}H_{2}$ and $C_{2}H_{4}$ have been populated by collisions with the excited $SF_{6}$ and $C_{2}H_{4}$ molecules in the jet. Both the spectroscopic features and the relaxation processes have been studied. The low lying bending modes $\nu_{4}$ and $\nu_{5}$ in $C_{2}H_{2}$ become effectively excited in collisions with $C_{2}H_{4}$ molecules in the $\nu_{10}$ state. Even overtones of the bending modes are excited, due to energy pooling in collisions between two singly excited acetylene molecules. The weak Q-branches of several $\Delta l = 0$ hot band transitions have been resolved and assigned. Rotational constants and l-type doubling constants have been determined from these spectra and compared to the values of Ref. 1. In previous hot band studies of $C_{2}H_{4}$, a number of transitions escaped from observation due to symmetry restrictions implied by the pumped $transition^{2,3}$. Starting with vibrationally excited $SF_{6}$, all symmetries are equally accessed allowing to observe a more complete hot band spectrum.