Abstract
A microwave-sideband CO 2 laser is used together with an electric-resonance optothermal spectrometer to measure the infrared spectrum of the NH 3 umbrella vibration in NCH-NH 3 at a resolution of ≈ 3 MHz. The infrared radiation is produced by mixing Lamb-dip-stabilized CO 2 laser radiation with synthesizer-derived microwave radiation in a CdTe-buffered GaAs stripline electrooptic waveguide modulator. For NCH-NH 3 a symmetric top spectrum is observed with a band origin at 1041.7 cm −1, blue-shifted ≈ 91.8 cm −1 from the hypothetical inversion-free ν 2 band origin of free NH 3, which indicates a decrease in the van der Waals zero-point binding energy, D 0, for the excited state. The observed Δ B of −14.3 MHz, implying a hydrogen-bond extension, is consistent with this blue shift. The vibrationally excited complex does not predissociate within the ≈ 1 ms transit time between the laser excitation region and the bolometer detector, implying that D 0 is greater than the laser frequency, ≈ 1042 cm −1.
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