High-precision and high-accuracy rovibrational spectroscopy of molecular ions.

We present a versatile new instrument capable of measuring rovibrational transition frequencies of molecular ions with sub-MHz accuracy and precision. A liquid-nitrogen cooled positive column discharge cell, which can produce large column densities of a wide variety of molecular ions, is probed with sub-Doppler spectroscopy enabled by a high-power optical parametric oscillator locked to a moderate finesse external cavity. Frequency modulation (heterodyne) spectroscopy is employed to reduce intensity fluctuations due to the cavity lock, and velocity modulation spectroscopy permits ion-neutral discrimination. The relatively narrow Lamb dips are precisely and accurately calibrated using an optical frequency comb. This method is completely general as it relies on the direct measurement of absorption or dispersion of rovibrational transitions. We expect that this new approach will open up many new possibilities: from providing new benchmarks for state-of-the-art ab initio calculations to supporting astronomical observations to helping assign congested spectra by combination differences. Herein, we describe the instrument in detail and demonstrate its performance by measuring ten R-branch transitions in the ν2 band of H3(+), two transitions in the ν1 band of HCO(+), and the first sub-Doppler transition of CH5(+).

Indirect rotational spectroscopy of HCO+.

Spectroscopy of the ν1 band of the astrophysically relevant ion HCO(+) is performed with an optical parametric oscillator calibrated with an optical frequency comb. The sub-MHz accuracy of this technique was confirmed by performing a combination differences analysis with the acquired rovibrational data and comparing the results to known ground-state rotational transitions. A similar combination differences analysis was performed from the same data set to calculate the previously unobserved rotational spectrum of the ν1 vibrationally excited state with precision sufficient for astronomical detection. Initial results of cavity-enhanced sub-Doppler spectroscopy are also presented and hold promise for further improving the accuracy and precision in the near future.

Broadly tunable mid-infrared noise-immune cavity-enhanced optical heterodyne molecular spectrometer.

The sensitive spectroscopic technique noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) has been successfully used in a variety of systems; however, no broadly tunable setup has been developed for the mid-infrared. To this end, we have integrated a difference frequency generation system into a NICE-OHMS setup. Initial optimization and characterization was completed with ro-vibrational spectroscopy of methane. Doppler-broadened frequency-modulated NICE-OHMS spectra were recorded at a sensitivity of 2×10(-7) cm(-1) Hz(-1/2). Sub-Doppler saturation signals (Lamb dips) were also observed using wavelength-modulated NICE-OHMS, achieving a sensitivity of ~6×10(-9) cm(-1) Hz(-1/2).

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.

Noise immune cavity enhanced optical heterodyne velocity modulation spectroscopy.

The novel technique of cavity enhanced velocity modulation spectroscopy has recently been demonstrated as the first general absorption technique that allows for sub-Doppler spectroscopy of molecular ions while retaining ion-neutral discrimination. The previous experimental setup has been further improved with the addition of heterodyne detection in a NICE-OHMS setup. This improves the sensitivity by a factor of 50 while retaining sub-Doppler resolution and ion-neutral discrimination. Calibration was done with an optical frequency comb, and line centers for several N(2)(+) lines have been determined to within an accuracy of 300 kHz.

Cavity-enhanced velocity modulation spectroscopy.

The spectroscopic study of molecular ions is of great importance to a variety of fields, but is challenging as ions are typically produced in plasmas containing many orders of magnitude more neutral molecules than ions. The successful technique of velocity modulation permits discrimination between ion and neutral absorption signals and has allowed the study of scores of molecular ions in the past quarter century. However, this technique has long been considered to be inappropriate for use with cavity-enhanced techniques, owing to the directional nature of the velocity modulation. Here we report what we believe to be the first demonstration of cavity-enhanced velocity modulation spectroscopy, utilizing a 2f phase-sensitive demodulation scheme. This approach offers the promise of combining very high-sensitivity spectroscopic techniques with ion-neutral discrimination, which could extend the applicability of velocity modulation to intrinsically weak transitions and to ions that cannot be produced in high abundance. The use of a cavity also permits Lamb dip spectroscopy, which offers higher resolution and precision in frequency measurements and may be useful in measuring collisional rate coefficients.