sub-Doppler Spectral Resolution Research Articles

Sub-Doppler spectral resolution in a resonantly driven four-level coherent medium

Sub-Doppler spectral resolution has been studied in Doppler-broadened multi-level 87Rb atoms coherently coupled by two strong laser fields. Narrow spectral features of absorption or gain are observed in the center or sides of the Doppler-broadened absorption profile. Analytical and numerical calculations based on a four-level N type model are presented to explain the experimental results.

Multiphoton laser wave-mixing absorption spectroscopy for samarium using a graphite furnace atomizer

Nonlinear laser wave-mixing optical technique is presented as a sensitive atomic spectroscopic method for the analysis of rare earth elements using an unmodified commercially available graphite furnace (GF) atomizer. A simple nonplanar backward-scattering degenerate four-wave mixing optical arrangement offers sub-picogram detection sensitivity with sub-Doppler Lorentzian-broadened resolution. Nonlinear wave mixing is an unusually sensitive absorption-based optical method that offers both excellent detection sensitivity and sub-Doppler spectral resolution. A mass detection limit of 0.7 pg and a concentration detection limit of 70 pg/ml are determined for a rare earth element, samarium, using the 429.7-nm excitation line.

Trace analysis of rubidium hyperfine structure in a flame atomizer using sub-Doppler laser wave-mixing spectroscopy.

Nonlinear laser wave mixing is a versatile spectroscopic method for trace elemental analysis at high spectral resolution. Sub-Doppler spectral resolution allows isotope and hyperfine structure measurements of some of the elements even when using a room-pressure analytical flame (i.e., sub-Doppler but Lorentzian broadened spectra). A non-planar wave-mixing optical setup offers some advantages as compared to the conventional planar wave-mixing setup including better signal collection efficiency and easier optical alignment. Using our absorption-based wave mixing, a detection limit of 0.05 ng/mL (i.e., 50 parts-per-trillion) is reported for Rb in an air/acetylene flame, while still maintaining sub-Doppler spectral resolution for the infrared 780.0-nm Rb transition line.

Absorption by water vapour in the 1 to [formula omitted] region

The near-IR (in the range 5000– 10 000 cm −1 , 1– 2 μm ) bands of water vapour have been measured in absorption in the laboratory at sub-Doppler spectral resolution (up to 0.0054 cm −1 after numerical apodisation) by Fourier transform spectroscopy. Measurements have been made at 296 K on pure water vapour (at pressures between 2 and 20 hPa ) and mixtures of water and air (at total pressures of 100 and 1000 hPa ), at optical path lengths in the range 0.26– 9.75 m . Measured absorption intensities have been compared with values calculated using the HITRAN 2000 molecular database. These comparisons indicate that the intensities of the 2ν(1.4 μm) and 2ν+δ(1.14 μm) bands are underestimated in HITRAN 2000 by approximately 15% and 20%, respectively, for pure water vapour measurements, and 12% for both bands in the case of water–air mixtures. The ν+δ (1.86 μm) band is in good agreement (0.4% for pure water vapour and less than 6% for mixtures with air) with HITRAN 2000. For typical atmospheric conditions, these absorption bands are sufficiently strong that radiation is fully absorbed at wavelengths in the region of the band centres. Hence the extra absorption that has been identified has only a modest impact ( 0.16 W m −2 or about 0.2%) on the global-mean clear-sky absorption of solar radiation. The impact in the upper troposphere is several times larger.

Moving object detection using a cesium resonance fluorescence monochromator

The measurement of Doppler-shifted radiation using a cesium resonance fluorescence monochromator (RFM) is described. The RFM is based on two-step excitation ( 6 2 S 1/2→6 2 P 0 3/2→6 2 D 5/2 ) followed by wavelength-shifted fluorescence and detection of resonance photons with sub-Doppler spectral resolution (200 MHz). We demonstrate the resolution of velocity differences by comparing excitation profiles of a reference beam and Doppler-shifted scatter. A rotating disc of variable speed was used to generate the Doppler-shifted photons. Velocity changes on the order of 25 m/s were resolved, and the upper velocity limit (1500 m/s) is limited only by the extent of the frequency scan of the first excitation laser.

Sub-Doppler Spectral Resolution and Improved Sensitivity in a Cesium Resonance Fluorescence Imaging Monochromator

Improvements in spectral resolution and sensitivity of a cesium based resonance fluorescence imaging monochromator (RFIM) are described. Alignment of the two lasers used in the excitation scheme in a co-propagating geometry have resulted in a detector spectral bandpass (270 MHz) that is narrower than the Doppler broadened profile (380 MHz) of cesium vapor at room temperature. Improvements in the experimental design, namely the addition of an image intensifier and cooled charge-coupled device (CCD) camera, have resulted in increased sensitivity; we report a limit of detection of 108 photons per pixel limited by image intensifier noise. Images of the cesium D2 line emission (852.12 nm) from a cesium hollow cathode lamp were compared to an argon emission line (4Sº1/2 → 4P3/2) at 852.14 nm from a silver/argon hollow cathode lamp as a demonstration of the spectral resolution. The ability of the RFIM to discriminate between photons exciting the F = 4 vs. F = 3 ground state hyperfine level (a frequency difference of 9.2 GHz) of the cesium 62S1/2 state was also investigated to further demonstrate the spectral selectivity. The potential use of the described RFIM for chemical imaging applications is discussed.

Tunable resonance fluorescence monochromator with sub-Doppler spectral resolution

A cesium-based resonance fluorescence monochromator with a spectral resolution of 200 MHz and a tunable response over the D(2) absorption line of cesium (380 MHz) is described. The narrow spectral response is achieved through excitation of a monokinetic population of the 6(2)P(3/2)( degrees ) state by arrangement of the excitation lasers in either a copropagating or a counterpropagating orientation. The narrow spectral response of the detector allows for excitation of specific hyperfine components involved in the 6(2)P(3/2)( degrees ) (F=3-5) to 6(2)D(5/2) (F=2-6) transition (917.23 nm). The selectivity gained through resolving specific hyperfine transitions allows for a photon detector that is both spectrally tunable and narrow. We report the sub-Doppler linewidths achieved through various laser beam orientations. We also describe how these beam geometries can be applied to spectrally narrow and tunable image detection.

High Resolution and High Sensitivity Laser Spectroscopy ofSmall Molecules and Clusters

With sensitive techniques, such as phase-modulation diode laserspectroscopy, opto-acoustic and opto-thermal spectroscopy, overtonespectra of small molecules are recorded which allow the specificdetection of pollutant molecular gases and investigations ofintramolecular vibrational energy transfer. Examples of moleculesstudied so far are H2S, CS2,C2H2, OCS, CHCl3 andC2H4. With resonant two-photon two colorionization spectroscopy, isotope-selective detection of small metalclusters in cold molecular beams has been achieved with sub-Dopplerspectral resolution. Some of these techniques and spectroscopicresults are discussed in the present paper.

High Resolution and High Sensitivity Laser Spectroscopy ofSmall Molecules and Clusters

With sensitive techniques, such as phase-modulation diode laserspectroscopy, opto-acoustic and opto-thermal spectroscopy, overtonespectra of small molecules are recorded which allow the specificdetection of pollutant molecular gases and investigations ofintramolecular vibrational energy transfer. Examples of moleculesstudied so far are H2S, CS2,C2H2, OCS, CHCl3 andC2H4. With resonant two-photon two colorionization spectroscopy, isotope-selective detection of small metalclusters in cold molecular beams has been achieved with sub-Dopplerspectral resolution. Some of these techniques and spectroscopicresults are discussed in the present paper.

The TiOc1Φ–a1Δ (0–0) Band at Sub-Doppler Spectral Resolution: Spin–Orbit Interactions in thecState

Thec1Φ–a1Δ (0–0) band of titanium monoxide has been studied at sub-Doppler resolution by crossing a beam of TiO molecules with a cw tunable laser beam and collecting the laser-induced fluorescence. The rotational structures of theP,Q, andRbranches have been observed up to rotational quantum numbers equal to 52, 94, and 96, respectively. The spectroscopic data have been reduced to a set of 11 effective molecular constants with a root mean squared values of the residuals equal to 7 × 10−4cm−1. However, the necessity of including seventh order terms in the rotational development of the energy put into evidence a very strong spin–orbit interaction between thec1Φ (v= 0) andC3Δ3(v= 2 andv= 3) levels. The derived spin–orbit parameter allows for the reproduction of the measured wavenumbers without the introduction of higher order rotational terms into the energy development. The value of the spin–orbit parameter indicates the necessity of revisiting the configuration scheme of the involved electronic states.

Stratospheric Ammonia on Jupiter after the SL9 Collision

We report results from measurements of NH3emission lines from the stratosphere of Jupiter following the impacts of fragments of Comet P/Shoemaker–Levy 9. Measurements were taken at the NASA Infrared Telescope Facility on 29 July 1994 on two impact regions, Q1 and the RSG complex, ∼8 days after the impacts. Emission line profiles were obtained near 11.2 μm at sub-Doppler spectral resolution (λ/Δλ ∼ 106). The measurements of the lineshapes permit the retrieval of NH3abundance and provide constraints on its altitude distribution in the stratosphere. If a uniformly mixed altitude distribution is assumed above the tropopause, the retrieved NH3mole fractions are 7.8 ± 1.6 × 10−9for the Q1 impact region and 4 ± 0.8 × 10−9for the RSG impact region. The total derived mass of NH3, ∼4 × 1012g within our field of view, is approximately 20 times greater than that possible from cometary origin. Near the Q1 impact site, ammonia emission is found to originate primarily from altitudes above the 10 mbar level, with a mole fraction of 18 ± 4 × 10−9corresponding to a column abundance of 2 ± 0.4 × 1016cm−2. Results are compared with other infrared and ultraviolet measurements and with recent photochemical models for jovian stratospheric NH3. These comparisons provide evidence for the presence of haze up to the 10 mbar level and provide an initial data set for study of the temporal behavior of NH3in the stratosphere of Jupiter.

Polar warming in the middle atmosphere of Mars

We report ground-based laser heterodyne spectroscopy of non-thermal emission in the cores of the 10.33- μm R(8) and 10.72- μm P(32) lines of 12C 16O 2, obtained at 23 locations on the disk of Mars during the 1984 opposition, at L s = 130°. The data were obtained at a sub-Doppler spectral resolution, and the temperature of the middle Martian atmosphere (50–85 km) is derived from the frequency width and intensity of the R(8) emission, and from the total intensity of the P(32) emission. We find that the temperature of the middle Martian atmosphere varies with latitude. Near the subsolar latitude, the average 50- to 85-km temperature is close to the radiative equilibrium value for a CO 2 atmosphere. However, at high latitudes in both the northern (summer) and southern (winter) hemispheres the 50- to 85-km temperature exceeds the CO 2 radiative equilibrium value; a meridional gradient in the range of 0.4 – 0.9°K per degree of latitude is indicated by our data. The highest temperatures are seen at high latitudes in the winter hemisphere, reminiscent of the seasonal effects seen at the Earth's mesopause. As in the terrestrial case, this winter polar warming in the Martian middle atmosphere necessitates departures from radiative equilibrium; dynamical heating of order 4 × 10 2 ergs g −1 sec −1 is required at the edge of the winter polar night. A comparison with 2-D circulation models shows that the presence of atmospheric dust may enhance this dynamical heating at high winter latitudes, and may also account for heating at high latitudes in the summer hemisphere.

High Resolution Atmospheric Spectroscopy Using A Diode Laser Heterodyne Spectrometer

Atmospheric'spectroscopy, employing the sun as a source, has been routinely used for a number of years as a means for identification and quantitative measurements of atmospheric species. Recent improvements in Pb-salt semiconductor laser technology when combined with infrared heterodyne technology provide the capability of obtaining solar spectra with sub-Doppler spectral resolution. In this paper we discuss ultrahigh resolution (0.007 cm-1) atmospheric solar absorption spectra that have been obtained from a tunable infrared heterodyne radiometer. The radiometer was developed for ground based observations in the 8 to 12µm region, and tunability is achieved through the use of Pb-salt semiconductor laser local oscillators (L0). Spectra have been obtained in a piece-wise fashion from 9.1 to 11.1 um using laser emission modes that exhibit characteristics suitable for LO operation. Spectra showing absorption features of HNO3, 03, CO2, and H2O are presented along with comparisons of experimental and synthetic spectra calculated using a line-by-line atmospheric transmission model.