We present absolute line center frequencies for 24 fundamental ν3 ro-vibrational P-branch transitions near 4.5 μm in N2O with an absolute expanded (multiplied by 2) frequency uncertainty of 800 kHz. The spectra are acquired with a swept laser spectrometer consisting of an external-cavity quantum cascade laser whose instantaneous frequency is continuously tracked against a near-infrared frequency comb. The measured absorbance profiles have a well-calibrated frequency axis, and are fitted to determine absolute line center values. We discuss the main sources of uncertainty.

The instantaneous optical frequency of an external-cavity quantum cascade laser (QCL) is characterized by comparison to a near-infrared frequency comb. Fluctuations in the instantaneous optical frequency are analyzed to determine the frequency-noise power spectral density for the external-cavity QCL both during fixed-wavelength and swept-wavelength operation. The noise performance of a near-infrared external-cavity diode laser is measured for comparison. In addition to providing basic frequency metrology of external-cavity QCLs, this comb-calibrated swept QCL system can be applied to rapid, precise broadband spectroscopy in the mid-infrared spectral region.

A computer controlled velocity modulation dye laser spectrometer is used to measure the A 2Πu←X 2Σ+g (7,3) band of N+2. The Doppler-limited absorption spectra are fit to a model Hamiltonian to obtain precise parameters for the states. The ion rotational temperature was determined to be 461±11, and the translational temperature was estimated to be near 1200 K.

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High resolution (Doppler-resolved) measurement of Doppler-shifted ion absorption frequencies allows the determination of the electric field responsible for the shift. A new method based on velocity modulation laser spectroscopy is described for the nonperturbative measurement of the axial electric fields in glow discharges. The A 'n, + X I&+ (4.0 ) band of NT has heen used as a probe of a He/N, discharge. Half-wave rectification of a sine-wave-driven discharge allows observation of Doppler shifts in the Ion transition frequencies since the ions are traveling in only one direction with respect to the laser wavevector. Electric fields and E/N values have been measured at pressures from 3 to I 1 Torr. Fields in the range of lo-I5 V/cm were observed.

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