摘要： Selective reflection spectroscopy at normal incidence provides signals with sub-Doppler resolution, linear with optical intensity. Frequency modulated (FM) selective reflection probes atomic vapours at distances comparable to the excitation wavelength (~λ/2π) and is used extensively to probe the Casimir-Polder interactions between an excited state atom and a macroscopic surface. Extending selective reflection spectroscopy to molecular gases allows probing a thin layer of molecular gas next to a surface. This is a very attractive prospect that allows envisaging high-resolution molecular spectroscopy and molecular frequency references in a compact and miniaturised apparatus, such as a thin cell [1] or a photonic crystal [2]. Additionally, it paves the way for spectroscopic probing of the Casimir-Polder interaction with molecules. The molecule-surface interaction has been the object of extensive theoretical investigations, focusing on the effects of molecular orientation and chirality. However, experimental tests are few and comparison with theoretical predictions has been challenging [3]. Here, we present selective reflection measurements on polyatomic molecules in gaseous form. We probe rovibrational molecular transitions of NH3 and SF6 using a Quantum Cascade Laser (QCL) at ~10.6μm. Reflection spectroscopy is performed in a vacuum chamber with transparent ZnSe windows. In a separate chamber, we perform simultaneous saturated absorption measurements, to get molecular frequency references in the volume. We also use an auxiliary set-up to lock the QCL laser, either on the derivative (after FM demodulation) of a Doppler linear absorption, or on the wings of the NH3 linear absorption (direct signal). This allows us to eliminate a frequency drift of the QCL source due to temperature fluctuations. By tuning the molecular pressure and therefore the absorption profile, the latter method (lock on the direct signal) allows stable frequency scanning for hundreds of MHz. A system of electronic valves allows us to empty and refill the chamber with molecules within tens of seconds. Detecting the difference between signals, as well as using multiple vibrating mirrors in our set-up, eliminates to about 0.1ppm an interferometric parasitic background, typical in infrared spectroscopy. Fig.1 shows our experimental results obtained for the isolated saP(1) transition of NH3 (Fig. 1a) and a multitude of transitions of SF6, mostly unidentified in molecular databases (Fig. 1b). Linear selective reflection allows us to pinpoint these transitions and easily determine their relative amplitude. At sufficiently low molecular pressure, the frequency resolution of our measurements is limited to ~0.5MHz essentially by laser linewidth. This allows partially resolving the hyperfine structure of NH3. The dotted curves represent theoretical predictions of selective reflection spectra, with transition amplitude adjustments, accounting for FM and laser linewidth. We are working on the fabrication of thin cells using ZnSe windows for rovibrational transmission spectroscopy in the mid-infrared, as well as glass windows, for probing C2H2 at telecommunication wavelengths. Due to the Dicke narrowing effect, thin cells are a step towards compact high-resolution frequency references. Furthermore, achieving, nanometric molecular confinement, defined by cell thickness, instead of wavelength (λ/2π for selective reflection, here ~1.5μm) will allow us to measure the Casimir-Polder interaction with molecules and to study the thermal coupling and energy transfer between rovibrational molecular transitions and surface polaritons [4].