Dicke Narrowing Research Articles

Pressure effects on water vapour lines: beyond the Voigt profile

A short overview of recent results on the effects of pressure (collisions) regarding the shape of isolated infrared lines of water vapour is presented. The first part of this study considers the basic collisional quantities, which are the pressure-broadening and -shifting coefficients, central parameters of the Lorentzian (and Voigt) profile and thus of any sophisticated line-shape model. Through comparisons of measured values with semi-classical calculations, the influences of the molecular states (both rotational and vibrational) involved and of the temperature are analysed. This shows the relatively unusual behaviour of H(2)O broadening, with evidence of a significant vibrational dependence and the fact that the broadening coefficient (in cm(-1) atm(-1)) of some lines increases with temperature. In the second part of this study, line shapes beyond the Voigt model are considered, thus now taking 'velocity effects' into account. These include both the influence of collisionally induced velocity changes that lead to the so-called Dicke narrowing and the influence of the dependence of collisional parameters on the speed of the radiating molecule. Experimental evidence of deviations from the Voigt shape is presented and analysed. The interest of classical molecular dynamics simulations, to model velocity changes, together with semi-classical calculations of the speed-dependent collisional parameters for line-shape predictions from 'first principles', are discussed.

Speed-dependent effects in NH3self-broadened spectra: Towards the determination of the Boltzmann constant

In this paper we present an accurate analysis of the shape of an isolated rovibrational ammonia line from the strong nu2 band around 10 $\mu$m, recorded by laser absorption spectroscopy. Experimental spectra obtained under controlled temperature and pressure, are confronted to various models that take into account Dicke narrowing or speed-dependent effects. Our results show clear evidence for speed-dependent broadening and shifting, which had never been demonstrated so far in NH3. Accurate lineshape parameters of the nu2 saQ(6,3) line are obtained. Our current project aiming at measuring the Boltzmann constant, kB, by laser spectroscopy will straight away benefit from such knowledge. We anticipate that a first optical determination of kB with a competitive uncertainty of a few ppm is now reachable.

Absolute molecular density determinations by direct referencing of a quantum cascade laser to an optical frequency comb

We propose a new approach to chemical gas analysis in the gas phase by direct referencing of a quantum cascade laser (QCL) to a near-infrared optical frequency comb. The concept was demonstrated through measurements of CO2 molecular densities, reaching a precision of 0.2 %. Thanks to the robust phase lock of the QCL to the comb, absorption profiles could be recorded with high accuracy and repeatability by tuning the repetition rate of the comb. Such a scheme gave us the opportunity to test a variety of semiclassical line-shape models, accounting for both Dicke narrowing and speed-dependent effects. The success of the speed-dependent Nelkin–Ghatak model, with a hypergeometric dependence of the pressure broadening coefficient on the absorber speed, was demonstrated. Finally, the dependence of molecular density determinations on the choice of the line-shape model was investigated, thus demonstrating the inadequacy of the Voigt profile.

Study of the effect of perturber mass on collisional broadening coefficients of lines in the ν3 band of CS2

Using a tunable diode-laser spectrometer, we have recorded 28 lines in the fundamental ν3 band of carbon disulfide diluted in several rare gases: helium, neon and krypton. These lines are ranged from P(78) to R(72) and located in the spectral range 1514–1548cm−1.The collisional half-widths of each line have been obtained by fitting to the experimental lineshape, a Voigt profile and also the models developed by Rautian and Sobel’man and by Galatry. These two more elaborate profiles take into account the narrowing due to the molecular confinement (Dicke effect). From these results and those previously obtained for CS2+Ar, we have compared the collisional broadening coefficients of CS2 lines to put in evidence the effect of the perturber mass.

Improvement of the frequency stability below the dick limit with a continuous atomic fountain clock

The frequency instability of a shot-noise limited atomic fountain clock is inversely proportional to its signal-tonoise ratio. Therefore, increasing the atomic flux is a direct way to improve the stability. Nevertheless, in pulsed operation, the local oscillator noise limits the performance via the Dick effect. We experimentally demonstrate here that a continuous atomic fountain allows one to overcome this limitation. In this work, we take advantage of two-laser optical pumping on a cold cesium beam to increase the useful fountain flux and, thus, to reduce the frequency instability below the Dick limit. A stability of 6 × 10(-14)τ(-1/2) has been measured with the continuous cesium fountain FOCS-2.

Intensities and shapes of H2O lines in the near-infrared by tunable diode laser spectroscopy

The integrated intensities, self- and air-broadening coefficients of thirteen transitions of H216O in the 11980–12260cm−1 region, belonging to the 2ν1+ν2+ν3 band, were measured. Using a tunable diode laser system, spectra were recorded at room temperature for a wide range of pressure (2–15Torr for pure H2O and 50–760Torr for H2O in air). Line parameters were adjusted from experiments using three line-shape models: the Voigt profile (VP), the (hard collision) Rautian profile (RP) and the speed dependent Voigt profile (SDVP). The results show that the RP and SDVP are in better agreement with measurements than the VP and that they lead to larger values of the line parameters (about 5% for the line broadening, and 0.8% for the line intensity). Comparisons of the present results with HITRAN 2008 [Rothman et al., JQSRT 2009, 110:533-72] show that the HITRAN intensities of the studied lines are overestimated by about 9.4%, suggesting a more complete study of the H2O line parameters in the considered region. The Dicke narrowing and speed dependence parameters deduced from this work are also presented and discussed, demonstrating the need for a more refined line-shape model.

Dicke narrowing in the dispersion mode of detection and in noise-immune cavity-enhanced optical heterodyne molecular spectroscopy—theory and experimental verification

Dicke narrowing in both the absorption and dispersion modes of detection have been scrutinized by noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) using an isolate ...

The air-broadened, near-infrared CO2 line shape in the spectrally isolated regime: Evidence of simultaneous Dicke narrowing and speed dependence

Frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) was employed to measure air-broadened CO(2) line shape parameters for transitions near 1.6 μm over a pressure range of 6.7-33 kPa. The high sensitivity of FS-CRDS allowed for the first measurements in this wavelength range of air-broadened line shape parameters on samples with CO(2) mixing ratios near those of the atmosphere. The measured air-broadening parameters show several percent deviations (0.9%-2.7%) from values found in the HITRAN 2008 database. Spectra were fit with a variety of models including the Voigt, Galatry, Nelkin-Ghatak, and speed-dependent Nelkin-Ghatak line profiles. Clear evidence of line narrowing was observed, which if unaccounted for can lead to several percent biases. Furthermore, it was observed that only the speed-dependent Nelkin-Ghatak line profile was able to model the spectra to within the instrumental noise level because of the concurrent effects of collisional narrowing and speed dependence of collisional broadening and shifting.

Speed-dependent effects in the near-infrared spectrum of self-collidingH2O18molecules

The absorption line shape of a given ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ vibration-rotation line at 1.38 $\ensuremath{\mu}$m was deeply investigated using a spectroscopic approach based upon the use of a pair of offset-frequency locked extended-cavity diode lasers. This dual-laser apparatus ensures extreme levels of accuracy in controlling and measuring any variation of the laser frequency around a given absolute reference. As a result, high levels of precision and accuracy were reached in the observation of a molecular absorption line shape in the near infrared. A variety of semiclassical models, accounting for Dicke narrowing and speed-dependent effects, were implemented and tested in order to describe the physical situation of self-colliding ${\mathrm{H}}_{2}$$^{18}\mathrm{O}$ molecules. Our study demonstrates that the molecular confinement alone is unable to explain entirely the departures from the Voigt profile and that the speed dependence of pressure-induced broadening and shift cannot be ignored, even in the case of pure water samples at relatively small pressures. The absorption spectrum was successfully interpolated using the uncorrelated version of the speed-dependent Galatry profile, with a hypergeometric dependence on the absorber speed for both pressure broadening and pressure shift parameters, thus reaching an agreement between theory and experiment at the level of $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$.

Frequency comparison of optical lattice clocks beyond the Dick limit

Researchers cancel out the Dick effect through a synchronous frequency comparison between two optical lattice clocks based on 87Sr and 88Sr atoms. This scheme achieves an Allan standard deviation of around 10−17, which represents a significant advantage when using a large number (2,000) of atoms in an optical clock.

Robert dicke and atomic physics

In this article, we highlight the contributions of Dicke to atomic physics. The effects, called Dicke narrowing and Dicke superradiance, are contained in two seminal papers published in the 1950’s. The description below should be accessible to anyone who has had a brief introduction to quantum mechanics.

Modulation Transfer Spectroscopy of Ytterbium Atoms in a Hollow Cathode Lamp

We present the experimental study of modulation transfer spectroscopy of ytterbium atoms in a hollow cathode lamp. The dependences of its linewidth, slope and magnitude on the various experimental parameters are measured and fitted by the well-known theoretical expressions. The experimental results are in good agreement with the theoretical prediction. We have observed the Dicke narrowing effect by increasing the current of the hollow cathode lamp. It is also found that there are the optimal current and laser power to generate the better modulation transfer spectroscopy signal, which can be employed for locking the laser frequency to the atomic transition.

Plasmonic Dicke effect

We study cooperative emission by an ensemble of emitters, such as fluorescing molecules or semiconductor quantum dots, near a metal nanoparticle. The primary mechanism of cooperative emission is resonant energy transfer between emitters and plasmons rather than Dicke radiative coupling between emitters. The emission is dominated by three superradiant states with the same quantum yield as a single emitter, leading to a drastic reduction of ensemble radiated energy down to just thrice of that by a single emitter, the remaining energy being dissipated in the metal through subradiant states. We perform numerical calculations of system eigenstates and find that the plasmonic Dicke effect interactions affect is not impacted by the interactions between emitters or non-radiative losses in the metal.

Self-collisional broadening and shift coefficients of lines in the [formula omitted] band of [formula omitted] from 173.2 to 298.2 K by diode-laser spectroscopy

The pressure-induced self-shift and broadening coefficients for seven lines of C 2 12 H 2 in the ν 4 + ν 5 band located around 1330 cm −1 have been measured using a tunable diode-laser spectrometer coupled with a low temperature absorption cell. The measurement of these coefficients was realized at five temperatures ranging from 173.2 to 273.2 K (and also 298.2 K for shift) in order to determine their temperature dependence. The broadening coefficients were obtained by fitting to the experimental profile the Voigt lineshape and the Rautian and the Galatry models which take into account the collisional narrowing due to molecular confinement (Dicke effect). The shift coefficients were determined using a procedure, where calibration lines were simultaneously recorded with transitions from the gas sample under study. The results have been compared with previous experimental data. Our coefficients of collisional broadening, γ 0 , as well as the temperature dependence parameters, n, are in satisfactory agreement with previous studies. The pressure broadening coefficients, γ 0 , are in good agreement with those reported for various vibrational bands, indicating that they are insensitive to vibrational excitation. The pressure shift coefficients, δ 0 , by contrast, are found to differ substantially among vibrational bands.

Suppression of Kondo screening by the Dicke effect in multiple quantum dots

The interplay between the coupling of an interacting quantum dot to a conduction band and its connection to localized levels has been studied in a triple quantum dot arrangement. The electronic Dicke effect, resulting from quasi-resonant states of two side-coupled non-interacting quantum dots, is found to produce important effects on the Kondo resonance of the interacting dot. We study in detail the Kondo regime of the system by applying a numerical renormalization group analysis to a finite-U multi-impurity Anderson Hamiltonian model. We find an extreme narrowing of the Kondo resonance, as the single-particle levels of the side dots are tuned towards the Fermi level and "squeeze" the Kondo resonance, accompanied by a strong drop in the Kondo temperature, due to the presence of a supertunneling state. Further, we show that the Kondo temperature vanishes in the limit of the Dicke effect of the structure. By analyzing the magnetic moment and entropy of the three-dot cluster versus temperature, we identify a different local singlet that competes with the Kondo state, resulting in the eventual suppression of the Kondo temperature and strongly affecting the spin correlations of the structure. We further show that system asymmetries in couplings, level structure or due to Coulomb interactions, result in interesting changes in the spectral function near the Fermi level. These strongly affect the Kondo temperature and the linear conductance of the system.

Influence of the line-shape model on the spectroscopic determination of the Boltzmann constant

Recent high-resolution spectroscopic experiments demonstrated the possibility of measuring the Boltzmann constant directly from the Doppler width of atomic or molecular lines; however, both experimental setups and data analysis methods still need to be improved to compete with the best acoustic methods. In this paper, the influence of choice of spectral line-shape model for data analysis on systematical error of the Doppler-width determination is analyzed on the example of the oxygen line near $\ensuremath{\lambda}=687$ nm. Profile simulations were performed for many different models at pressures ranging from 0.012 Pa to 120 kPa, taking into account such line-shape effects as the speed dependence of collisional broadening and Dicke narrowing. We discuss limitations of an applicability of the Voigt profile for purposes of the Boltzmann constant determination. The influence of the model line-shape profile and the gas pressure range on the Doppler width extrapolated to the zero-pressure value is investigated.

Plasmon-mediated superradiance near metal nanostructures

We develop a theory of cooperative emission of light by an ensemble of emitters, such as fluorescing molecules or semiconductor quantum dots, located near a metal nanostructure supporting surface plasmon. The primary mechanism of cooperative emission in such systems is resonant energy transfer between emitters and plasmons rather than the Dicke radiative coupling between emitters. We identify two types of plasmonic coupling between the emitters, (i) plasmon-enhanced radiative coupling and (ii) plasmon-assisted nonradiative energy transfer, the competition between them governing the structure of system eigenstates. Specifically, when emitters are removed by more than several nm from the metal surface, the emission is dominated by three superradiant states with the same quantum yield as a single emitter, resulting in a drastic reduction of ensemble radiated energy, while at smaller distances cooperative behavior is destroyed by nonradiative transitions. The crossover between two regimes can be observed in distance dependence of ensemble quantum efficiency. Our numerical calculations incorporating direct and plasmon-assisted interactions between the emitters indicate that they do not destroy the plasmonic Dicke effect.

Self-broadening coefficients and positions of acetylene around 1.533 μm studied by high-resolution diode laser absorption spectrometry

The self-broadening coefficients of acetylene at room temperature have been measured for 10 lines in the P branch of the ν 1 + ν 3 ( ∑ u + ) − 0 ( ∑ g + ) bands of 12C 2H 2 and 13C 12CH 2 near 1.533 μm, using a high resolution tunable diode laser spectrometer developed for the Martian space mission PHOBOS-Grunt. The collisional widths are obtained by fitting each recorded line with the Voigt profile as well as the Rautian profile accounting for the collisional Dicke narrowing effect. The standard Voigt model provides slightly smaller broadening coefficients than the Rautian model. Our data are thoroughly compared to the main atmospheric molecule database HITRAN and previous values in various bands of acetylene. Moreover, it is worth noting that a large number of new transitions not listed in the latest HITRAN08 were measured and identified for the first time.

Shot noise spectrum of artificial single-molecule magnets: Measuring spin relaxation times via the Dicke effect

We investigate theoretically shot noise in an artificial Single Molecule Magnet based upon a CdTe quantum dot doped with a single S=5/2 Mn spin and gated in the hole sector. Mn-hole exchange anisotropy is shown to lead to profound consequences on noise, like super-Poissonian behaviour. We report on a novel effect, similar to the Dicke effect in Quantum Optics, that allows to separately extract the hole and Mn spin relaxation times using frequency-resolved shot noise measurements. We expect that our findings may have further relevance to experiments in other S>1/2 systems including transport through Mn(12) molecules and STM spectroscopy of magnetic atoms.

Non-Voigt line-shape effects on retrievals of atmospheric ozone: Collisionally isolated lines

This paper addresses the question of errors in retrievals of vertical profiles of ozone from atmospheric spectra caused by assuming that the absorption lines have pure Voigt line shapes. The case of collisionally isolated transitions (no line mixing) is treated by considering only the effects of the speed dependence (SD) of the pressure broadening. The case of O 3 retrievals from a sequence of limb transmission spectra is first treated theoretically. The results show that the influence of SD is very small, leading to changes in the residuals and in the retrieved O 3 mixing ratios smaller than 1%. These findings are then confirmed by treating a series of spectra recorded by the ACE-FTS instrument. Similar exercises are also made for other observation techniques by treating simulated or measured limb and nadir emission spectra as well as ground-based solar and in-situ laser transmission data. All lead to the general conclusion that SD (and Dicke narrowing) can be neglected in retrievals of ozone amounts from recorded atmospheric spectra. Indeed, the biases caused in the ozone profiles by the use of pure Voigt line shapes still remain significantly smaller than the total error/uncertainty from other sources such as improper line intensities and widths, uncertainty in the instrument function, errors in the pressure and temperature profiles and so forth.