The microwave-infrared double-resonance capabilities of an electric-resonance optothermal spectrometer have been used to assign the high resolution (5 MHz) infrared spectrum of the asymmetric =CH2 stretch of the cis conformer of methyl vinyl ether near 3130 cm−1. This vibrational state is anharmonically coupled to a near-resonant bath state by a 0.69 cm−1 matrix element resulting in two vibrational bands separated by about 1.44 cm−1. The two mixed states resulting from this interaction are further coupled to other near-resonant bath states with an average matrix element of about 0.01 cm−1. The coupled state density increases weakly with the total angular momentum, J, however, the intramolecular vibrational energy distribution (IVR) rate is approximately independent of the total angular momentum quantum number. Therefore, the rotationally mediated coupling mechanisms are weaker than the anharmonic terms in the redistribution process. A two-state analysis of the strong coupling, which includes a phenomenological IVR rate constant, suggests that the IVR rate in the two mixed states is dominated by the contribution from the coupled dark state. From the deconvolution of the IVR rates to remove the contribution from the dark state, the IVR lifetime of the asymmetric =CH2 stretch is determined to be 660 ps.
High-sensitivity, microwave–infrared double-resonance measurements can be made in molecular-beam spectrometers employing a single state-focusing device. The key feature of the double-resonance technique is the achievement of large signal modulations of infrared signals using microwave transitions, even in cases where the infrared transition cannot be saturated. A series of measurements is presented that shows that the technique is based on the transition moment and state-focusing properties of dressed molecular states in the presence of a strong microwave field. Using a state-focusing device, the spectroscopic measurements are doubly sensitive to the composition of the dressed states. The technique can be extended to other types of spectroscopy, such as electronic spectroscopy and the spectroscopy of weakly bound complexes.
The rotational spectra of molecular eigenstates of propynol in the region of the acetylenic C–H stretch (3330 cm−1) have been measured using infrared-microwave saturation spectroscopy. These spectra illustrate the basic properties of the rotational spectra of highly vibrationally mixed quantum states. From the measurements we are able to measure the average value of the rotational constant and the width of the rotational constant distribution. We determine that the average value of the quantity 12 (B+C) is 17 MHz smaller than the ground state value (a decrease of 0.4%). The width of the distribution (FHWM) is 90 MHz (1% of the ground state value). The distribution is approximately Gaussian. Narrowing of the rotational spectrum of single eigenstates by intramolecular vibrational energy redistribution (IVR) exchange processes is observed for the Ka=2 eigenstates. From the spectral narrowing we determine that the average IVR lifetime for vibrational states with Ka=2 near 3330 cm−1 is approximately 75 ps, about five times faster than the IVR lifetime of the Ka=2 states following coherent vibrational excitation of the acetylenic C–H stretch bright state (400 ps). Weak narrowing of the Ka=0 and Ka=1 eigenstates is observed below J=2. We estimate the IVR lifetime for Ka=0 and Ka=1 states as approximately 600 ps. The strong Ka dependence of the IVR rates of the bath states indicates that strong parallel Coriolis interactions play a primary role in the energy redistribution process.
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