A. Vitcu

High-resolution tunable mid-infrared spectrometer based on difference-frequency generation in AgGaS 2

We have built a high-resolution and high-signal-to-noise ratio spectrometer for line shape studies of greenhouse gases in the mid infrared. The infrared radiation is generated in a AgGaS2 nonlinear crystal by the well-known difference-frequency method. The choice of crystal is explained, and a brief literature review is presented. With two tunable dye lasers and a type I, 90 degrees phase-matching geometry, the infrared is continuously tunable from 7 to 9 microm when Rhodamine 6G and Sulforhodamine 640 dyes are used. The total infrared power exceeds 30 nW and is limited by both the damage threshold and thermal loading of the crystal. Phase-sensitive detection allows us to reach signal-to-noise ratios in excess of 3500:1 while maintaining an instrumental linewidth of 1.5 MHz. However, we show that the spectrometer may be used to measure the positions of spectral lines within +/-400 kHz.

Dynamic spectroscopic measurements of the temperature and pressure cycles in a MOPITT pressure modulator cell

The temperature and pressure cycles inside a pressure modulator cell (PMC) of the type used for gas-correlation radiometry aboard the Measurements of Pollution in the Troposphere (MOPITT) satellite instrument have been determined from dynamic measurements of the spectral line shapes of the R(0) and R(18) transitions in the fundamental vibrational-rotational band of carbon monoxide. The line strengths and linewidths were used to calculate the temperature and pressure, respectively, with a temporal resolution of approximately 200 micros, or 1/100 of a PMC cycle. The results are compared with a thermodynamic box model.

Dicke-narrowed line shapes in CO-Ar: Measurements, calculations, and a revised interpretation

New line shape calculations for CO buffered by Ar are compared to high‐resolution measurements from a difference‐frequency laser spectrometer, over a range of thermodynamic conditions relevant to the atmosphere. The calculations are based on solving the quantum kinetic (i.e. transport/relaxation) equation for the molecules within the impact approximation, and rely on the commonly used MOLSCAT and MOLCOL codes to determine the speed‐dependent collisional relaxation rate. Velocity‐changing effects are treated classically using a rigid sphere potential. The comparison initially reveals that the experimental profiles exhibit only 10% to 30% of the expected Dicke narrowing, which leads us to reevaluate our understanding of the narrowing process. A more subtle aspect of the disagreement between theory and experiment draws our attention to an assumption implicit in the calculation of the collisional relaxation rate: the assumption of a Maxwellian form for the velocity dependence of the off‐diagonal elements of the density matrix (i.e. the optical coherences). This assumption allows for an analytical simplification of the problem, but eliminates velocity‐changing effects (so that they must be added back in using a supplementary classical calculation, which is based here on a rigid sphere interaction). We find that the removal of the above‐mentioned assumption should allow for accurate and fully quantum mechanical (but numerical) line shape calculations for systems like CO‐Ar on existing computers.

High Resolution and High Signal‐to‐Noise Measurements in the 0310 ← 0110 Q‐Branch of N2O at 1160 cm−1

High‐resolution measurements of the Π ← Π Q‐branch of pure N2O near 1160 cm−1 were made using a difference‐frequency spectrometer with resolution of 5 × 10−5 cm−1 and a signal‐to‐noise ratio of 2000:1. Lines Q18F through Q12E have been recorded in a single scan, at room temperature and at pressures ranging from 1 to 130 torr. The spectra are analyzed up to 23 torr on a line‐by‐line basis using a hard collision profile including Dicke narrowing and line mixing. Since the separation of the central lines of this double‐sided Q‐branch is of the same order of magnitude with the collisional broadening, line mixing is considered in the analysis even at 1 torr and its dependence with pressure is studied.

Comparison of an ab initio calculation of the CO-Ar P(2) line shape with high-resolution measurements

A practical matrix‐based formalism for solving the master equation for a spectral line is applied to the P(2) transition in the fundamental band of carbon monoxide perturbed by argon. The method assumes that the effect of intermolecular collisions on the internal relaxation of the molecules is uncorrelated with the effect of those collisions on the translational motion of the molecules. Comparison with high‐resolution line shape measurements reveals that at low pressures, the omission of statistical correlation leads to a miscalculation of the shape of the line.