No abstract

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.

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).

In this work, colloidal zinc sulfide nanocrystals, prepared with selected concentrations of manganese(II) ions in the range of 0-2% and stabilized with a polymeric capping agent, were used in the photocatalytic degradation of 2-chlorophenol in aqueous solutions. Particle sizes for the synthesized nanocrystals were determined using UV-vis absorbance spectroscopy and X-ray diffraction (XRD). UV-vis spectra for all of the synthesized nanocrystals display an excitonic peak at around 290 nm, which corresponds to a particle size of approximately 3 nm, in agreement with the XRD analysis. Photoluminescence emission spectra recorded for undoped ZnS nanocrystals exhibit an emission peak at 460 nm in the spectrum, whereas peaks corresponding to the dopant ion are red-shifted to 590 nm. With increasing addition of the dopant ion, the area of the peak at 460 nm first increases and then decreases for higher dopant concentrations, whereas the area of the peak at 590 nm increases. This finding clearly indicates that the presence of the dopant ion alters the electronic states of the nanocrystals, which, in turn, affects the photocatalytic properties of the colloidal nanocrystals. Apparent rate constants for the photodegradation of 2-chlorophenol in aqueous solution utilizing undoped and doped colloidal ZnS nanocrystals were calculated using pseudo-first-order kinetic analysis. As the dopant concentration used during synthesis was increased, the apparent rate constants increased initially and then decreased. Results from kinetic experiments performed after nitrogen purging showed a decrease in the apparent rate constant, suggesting that the reaction mechanism involves a reactive oxygen species such as superoxide. Taken together, these results provide further insight into the effects of dopants on the activity of colloidal semiconductor nanocrystals employed as photocatalysts in environmental remediation applications.

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.