Coherent phase lock of a 9 µm quantum cascade laser to a 2 µm thulium optical frequency comb.

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1 µm to a Tm-fiber laser frequency comb centered at 2 µm, with an integrated residual phase error of 0.9 rad (30 mHz to 1.5 MHz). This resulted in a QCL linewidth reduction from 525 to 25 kHz at 1 ms observation time, limited by the linewidth of the free-running frequency comb.

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.

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.

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.

Communications: Development and characterization of a source of rotationally cold, enriched para-H3+.

In an effort to develop a source of H(3)(+) that is almost entirely in a single quantum state (J=K=1), we have successfully generated a plasma that is enriched to approximately 83% in para-H(3)(+) at a rotational temperature of 80 K. This enrichment is a result of the nuclear spin selection rules at work in hydrogenic plasmas, which dictate that only para-H(3)(+) will form from para-H(2), and that para-H(3)(+) can be converted to ortho-H(3)(+) by subsequent reaction with H(2). This is the first experimental study in which the H(2) and H(3) (+) nuclear spin selection rules have been observed at cold temperatures. The ions were produced from a pulsed solenoid valve source, cooled by supersonic expansion, and interrogated via continuous-wave cavity ringdown spectroscopy.

Dissociative recombination of highly enriched para-H3+.

The determination of the dissociative recombination rate coefficient of H(3) (+) has had a turbulent history, but both experiment and theory have recently converged to a common value. Despite this convergence, it has not been clear if there should be a difference between the rate coefficients for ortho-H(3) (+) and para-H(3) (+). A difference has been predicted theoretically and could conceivably impact the ortho:para ratio of H(3) (+) in the diffuse interstellar medium, where H(3) (+) has been widely observed. We present the results of an experiment at the CRYRING ion storage ring in which we investigated the dissociative recombination of highly enriched ( approximately 83.6%) para-H(3) (+) using a supersonic expansion source that produced ions with T(rot) approximately 60-100 K. We observed an increase in the low energy recombination rate coefficient of the enriched para-H(3) (+) by a factor of approximately 1.25 in comparison to H(3) (+) produced from normal H(2) (ortho:para=3:1). The ratio of the rate coefficients of pure para-H(3) (+) to that of pure ortho-H(3) (+) is inferred to be approximately 2 at low collision energies; the corresponding ratio of the thermal rate coefficients is approximately 1.5 at electron temperatures from 60 to 1000 K. We conclude that this difference is unlikely to have an impact on the interstellar ortho:para ratio of H(3) (+).

Optical measurements of ion trajectories through the vacuum interface of an inductively coupled plasma mass spectrometer.

A flexible laser excited fluorescence system has been used to trace ion trajectories in the first vacuum stage of an inductively coupled plasma mass spectrometer. The system has been used to record the spatial distributions of ions immediately upstream from the sampling cone and in the 10 mm downstream from the cone for a variety of positions of the plasma with respect to the sampling cone. The data were used in turn to test the efficacy of scanning the plasma across the sampling cone to generate maps of ion distributions in the plasma. The maps generated by scanning the plasma across the cone are close approximations of the ion distribution immediately upstream from the sampling cone, but are not representative of distributions in an unperturbed plasma.