Ultra-Sensitive High-Precision Spectroscopy of a Fast Molecular Ion Beam

Direct spectroscopy of a fast molecular ion beam offers many advantages over competing techniques, including the generality of the approach to any molecular ion, the complete elimination of spectral confusion due to neutral molecules, and the mass identification of individual spectral lines. The major challenge is the intrinsic weakness of absorption or dispersion signals resulting from the relatively low number density of ions in the beam. Direct spectroscopy of an ion beam was pioneered by Saykally and co-workers in the late 1980s, but has not been attempted since that time. Here, we present the design and construction of an ion beam spectrometer with several improvements over the Saykally design. The ion beam and its characterization have been improved by adopting recent advances in electrostatic optics, along with a time-of-flight mass spectrometer that can be used simultaneously with optical spectroscopy. As a proof of concept, a noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) setup with a noise equivalent absorption of ~2 × 10(-11) cm(-1) Hz(-1/2) has been used to observe several transitions of the Meinel 1-0 band of N(2) (+) with linewidths of ~120 MHz. An optical frequency comb has been used for absolute frequency calibration of transition frequencies to within ~8 MHz. This work represents the first direct spectroscopy of an electronic transition in an ion beam, and also represents a major step toward the development of routine infrared spectroscopy of rotationally cooled molecular ions

Refractive index measurements of solid parahydrogen

Solid para-H2 is a promising gain medium for stimulated Raman scattering, due to its high number density and narrow Raman linewidth. In preparation for the design of a cw solid hydrogen Raman laser, we have made the first measurements, to our knowledge, of the index of refraction of a solid para-H2 crystal, in the wavelength range of 430–1100 nm. For a crystal stabilized at 4:4K, this refractive index is measured to be np−H2 = 1.130 ±0.001 at 514 nm. A slight, but significant, dependence on the final crystal-growth temperature is observed, with higher np−H2 at higher crystal-growth temperatures. Once a crystal is grown, it can be heated up to 10 K with no change in np−H2. The refractive index varies only slightly over the observed wavelength range, and no significant birefringence was observed