Line Center Shift Research Articles

Development of a Stark shift measurement technique using excited-state oxygen atoms to determine electron number density in shock heated O2/Ar above 10 000 K

We present measurements of electron number density, n e, in partially ionized gases containing electronically excited atomic oxygen, at nominal translational temperatures between 10 100–11 200 K. The measurements were based on the Stark shift of the O() to O() transition at 926 nm. The current work scanned the laser across the transition and utilized the Stark shift of the absorbance for robust determination of n e. The measurements were conducted in a shock-heated 1% O2/Ar mixture, with a measurement time resolution of 20 μs. A line-center shift of −0.06 cm−1 was observed within around 500 μs of the test time after the mixture was instantaneously heated to 11 200 K by the reflected shock wave, which corresponded to an electron number density increase from 0 to 2 × 1021 m−3 due to collisional ionization. The measurement method and resulting data may be of interest in future studies of high-temperature air flows.

Fabrication and characterization of short acetylene-filled photonic microcells.

We have developed short (6-10 cm), connectorized acetylene-filled photonic microcells (PMCs) from photonic bandgap fibers that may replace near-IR frequency references for certain applications based on gas-filled glass cells. By using a tapering technique to seal the microcells, we were able to achieve a high transmission efficiency of 80% and moderate line center accuracy of 10 MHz (1σ). This approaches the National Institute of Standard Technology Standard Reference Material 2517a 10 MHz (2σ) accuracy. Using an earlier Q-tipping technique, 37% off-resonant transmission and 5 MHz accuracy were achieved in finding the line center, but a large 13% etalon-like effect appears on the wings of the optical depth. The etalon-like effect is reduced to less than 1% by using the tapering method. In both cases, the microcells could be connectorized, albeit with a reduction in off-resonant transmission efficiency, for integration into multimode fibers or free-space optical systems. Although contamination is introduced during both fabrication techniques, the P13 PMC line center shifts are small with respect to the sub-Doppler line center. This shows that the PMC can be used for moderate-accuracy frequency measurements. Finally, repeatable measurements show that PMCs are stable in terms of total pressure over approximately one year.

Study of anharmonicity in pure and doped InSe by Raman scattering

With the help of temperature dependence, Raman scattering anharmonic effect of various modes of layered semiconductor InSe over temperature range of 20–650 K has been studied. It was found that with an increase in temperature, anharmonicity will increase. Two and three phonons coupling with optical phonon, are used to describe temperature-induced anharmonicity in the linewidth of Raman modes. It was found that the temperature variation of the phonon parameter can be accounted for well by the cubic term in anharmonic model. To describe line-center shift of Raman modes, a model not considering independently cubic and quartic anharmonicity was used. A similar study has been done for InSe doped with different concentration of GaS dopant. The result of temperature study of InSe doped with GaS revealed that in this case anharmonicity increases with an increase in dopant and an increase in temperature.

Spectroscopic measurements of a CO2 absorption line in an open vertical path using an airborne lidar

We used an airborne pulsed integrated path differential absorption lidar to make spectroscopic measurements of the pressure-induced line broadening and line center shift of atmospheric carbon dioxide at the 1572.335 nm absorption line. We scanned the lidar wavelength over 13 GHz (110 pm) and measured the absorption lineshape at 30 discrete wavelengths in the vertical column between the aircraft and ground. A comparison of our measured absorption lineshape to calculations based on HIgh-resolution TRANsmission molecular absorption database shows excellent agreement with the peak optical depth accurate to within 0.3%. Additionally, we measure changes in the line center position to within 5.2 MHz of calculations and the absorption linewidth to within 0.6% of calculations. These measurements highlight the high precision of our technique, which can be applied to suitable absorption lines of any atmospheric gas.

Measurements and calculations of He-broadening and -shifting parameters of the water vapor transitions of the ν 1 + ν 2 + ν 3 band

The water vapor line broadening and shift in the ν 1 + ν 2 + ν 3 band induced by helium pressure were measured using a Bruker IFS 125HR FTIR spectrometer. The measurements were performed at room temperature at a spectral resolution of 0.01 cm−1 and in a wide He pressure range. The shifting coefficients δ are mainly positive, in contrast to the line center shift induced by other mono-atomic gases. Calculations of the coefficients γ and δ were performed in the framework of the semi-classical method. The influence of the rotational dependence of the isotropic intermolecular potential and accidental resonances in the H2O molecule on the shifting coefficients is discussed. Calculated values of line profile parameters are in a good agreement with observed parameters.

Intracavity spectroscopy of water vapor in the 1.06-micrometer range

Using an intracavity spectrometer with a neodymium glass laser with a spectral resolution of 0.035 cm−1 and threshold absorption sensitivity of 10−8 cm−1, water vapor absorption spectra have been studied. The H2O and HDO absorption spectra have been recorded in the temperature interval of 300–1000 K. The spectra have been identified, and many new energy levels of the H2O and HDO molecules have been determined. Coefficients of line center shifts induced by pressures of H2, O2, Ar, Xe, and Kr buffer gases and of the atmospheric air have been measured. For all buffer gases, a linear dependence of the shift magnitude on the pressure was observed. For a proper measurement of the coefficients of the line center shift, a technique employing a temperature-stabilized Fabry-Perot interferometer has been proposed.

Helium-induced halfwidths and line shifts of water vapor transitions of the ν 1 + ν 2 and ν 2 + ν 3 bands

The collisional broadening (γ) and shift (δ) coefficients of more than 100 absorption rovibrational lines of the ν 1 + ν 2 and ν 2 + ν 3 bands of the water vapor molecule in a H2O–He mixture were measured with the help of a Fourier-transform spectrometer over a large range of helium pressures. The shift coefficients δ are mainly positive and have an unusual rotational dependence, in contrast to the line center shift induced by other mono-atomic gases. The calculations of coefficients γ and δ are performed in the framework of the semi-classical method. The rotational dependence of these shift coefficients is explained by the rotational dependence of the isotropic intermolecular potential. The influence of accidental resonances in the H2O molecule is also discussed.

Broadening and shift of lines of the ν2 + ν3 band of water vapor induced by helium pressure

The broadening and shift coefficients of more than 100 absorption lines of the ν2 + ν3 band of water vapor that are induced by the pressure of helium are measured and calculated. The broadening and shift coefficients are obtained from analysis of the room-temperature absorption spectra of an H2O-He mixture measured with a resolution of 0.007 cm−1 on a Fourier spectrometer in a large range of helium pressures. The specific features in the rotational dependence of the line center shifts are determined, which, in contrast to the broadening induced by other gases, are mainly positive. The calculated coefficients of the line broadening and shift of line centers are determined by a semiclassical method. An unusual dependence of the shift coefficients is explained by the rotational dependence of the intermolecular isotropic interaction potential.

Resonantly enhanced three-photon excitation spectra of lithium

We present the first measurement on the resonantly enhanced three-photon excitation spectra of natural lithium using a Nd:YAG laser pumped dye laser in conjunction with a thermionic diode ion detector. Exploiting the linear and circular polarizations, the np2P3/2(8⩽n⩽11) and nf 2F7/2 (8⩽n⩽38) series have been observed via three-photon excitation from the ground state. The measured level energies reveal a dynamic shift from calculated values, which increases with an increase of the principal quantum number n. The ac stark shift and line broadening mechanisms are studied as a function of laser intensity. It is noted that the width increases and the line center shifts towards the higher energy side as the laser intensity is increased. The maximum observed shift for the 12f2F7/2 line is 0.33cm−1 corresponding to the laser intensity variation from 1.34×1012W/m2 to 1.03×1013W/m2, whereas its width increases from 0.36cm−1 to 0.82cm−1.

Asymmetric Deviation of the Scattering Cross Section around Lyα by Atomic Hydrogen

We investigate the asymmetry of the scattering cross section of radiation around Lyα by atomic hydrogen, which may be applied to analyses of scattering media with high neutral hydrogen column densities, including damped Lyα absorption systems of quasars. The exact scattering cross section is given by the Kramers-Heisenberg formula obtained from fully quantum mechanical second-order time-dependent theory, where, in the case of hydrogen, each matrix element is given in a closed analytical form. The asymmetric deviation of the scattering cross section from the Lorentzian near the line center is computed by expanding the Kramers-Heisenberg formula in terms of Δω/ωLyα, where ωLyα is the angular frequency of the Lyα transition and Δω is the deviation of incident radiation from ωLyα. To the first order of Δω/ωLyα, we obtain σ(ω) = σT(0.5f12ωLyα/Δω)2(1 - 1.79Δω/ωLyα), where σT is the Thomson scattering cross section and f12 = 0.4162 is the oscillator strength for the Lyα transition. With this deviation, the line center of the damped wing profile apparently shifts blueward of the true Lyα line center. In the case of a damped Lyα system with an H I column density 5 × 1021 cm-2, the apparent line-center shift relative to the true center amounts to 0.2 A, resulting in an underestimation of the redshift by Δz ~ 10-4. A measurable underestimation by an amount Δz ~ 10-3 is expected for absorbing systems with N ≥ 4 × 1022 cm-2.

Light-force-induced fluorescence line-center shifts in high-precision optical spectroscopy: Simple model and experiment

Light-force-induced fluorescence line-center shifts in high-precision optical spectroscopy: Simple model and experiment

Frequency shift in saturation spectroscopy induced by mechanical effects of light

We report on a substantial light-force-induced line-center shift of the sub-Doppler dip in a saturation spectroscopy configuration where mechanical effects are expected to be negligible. We observe the shift on the $2{}^{3}{S}_{1}\ensuremath{\rightarrow}2{}^{3}{P}_{1}$ open transition of ${}^{4}\mathrm{He}$ that is relevant for a determination of the fine-structure constant $\ensuremath{\alpha}$ [Phys. Rev. Lett. 82, 1112 (1999)]. We discuss the physical origin of the shift and we provide a numerical analysis to support our interpretation. The shift can be quite important for various other high-precision spectroscopy measurements.

Measured pressure broadening and shift rates of the 1.73 μm (5d[3/2]1–6p[5/2]2) transition of xenon

Pressure broadening and line center shift rates as a function of helium, neon, and argon pressure have been measured for the 1.73 μm (5d[3/2]1–6p[5/2]2) transition in xenon. The pressure broadening rates are 20.3±6.4, 12.7± 3.5, and 19.7±2.9 MHz/Torr for helium, neon, and argon buffers, respectively.

High-resolution measurement of water-vapor overtone absorption in the visible by frequency-modulation spectroscopy

We demonstrate the use of near-shot-noise-limited frequency-modulation spectroscopy in the study of a weakly absorbing transition of water vapor at 16 940.27 cm−1. The high sensitivity of the technique allows a detailed study of the line shape to yield accurate air-induced pressure-broadening and line-center shift coefficients and the self-induced pressure-broadening coefficient of the transition.

Erratum: Fine structure of Rydberg states. IV. Completely resolved fine structure inD,F, andGstates ofHe4

The authors have made extensive new measurements in Rydberg (n=6 --11) D, F, and G states of helium. The new data represent over 1100 individual resonance scans, totalling some 2200 h of data-collection time. In order to perform such a large number of runs, data logging has been automated by interfacing a microcomputer to an apparatus previously used to make fine-structure measurements in one- and two-electron atoms. An exhaustive analysis of possible systematic line-center shifts, including black-body radiation effects, was carried out. The root-mean-square one-standard-deviation experimental uncertainty is 294 kHz for our 67 measurements. The large quantity of new measurements has allowed us to perform a global least-squares fit to all existing state-resolved data in D, F, and G states. The complete structure of the D, F, and G manifolds is now known for n> or =6 to a precision of a few megahertz or better. Current theoretical calculations for a typical (30-GHz) interval differ from the measurements by 6 x 10/sup -8/ a.u., or over 1000 experimental standard deviations.

Fine structure of Rydberg states. IV. Completely resolved fine structure inD,F, andGstates ofHe4

The authors have made extensive new measurements in Rydberg (n=6 --11) D, F, and G states of helium. The new data represent over 1100 individual resonance scans, totalling some 2200 h of data-collection time. In order to perform such a large number of runs, data logging has been automated by interfacing a microcomputer to an apparatus previously used to make fine-structure measurements in one- and two-electron atoms. An exhaustive analysis of possible systematic line-center shifts, including black-body radiation effects, was carried out. The root-mean-square one-standard-deviation experimental uncertainty is 294 kHz for our 67 measurements. The large quantity of new measurements has allowed us to perform a global least-squares fit to all existing state-resolved data in D, F, and G states. The complete structure of the D, F, and G manifolds is now known for n> or =6 to a precision of a few megahertz or better. Current theoretical calculations for a typical (30-GHz) interval differ from the measurements by 6 x 10/sup -8/ a.u., or over 1000 experimental standard deviations.

Infrared Spectrum of Hydrogen Fluoride: Line Positions and Line Shapes Part II Treatment of Data and Results*

The spectrometer and experimental methods described in a companion paper were used to measure the center frequencies of some absorption lines in the HF fundamental and first-overtone bands and to measure the pressure-broadened shapes of some lines in the fundamental. The centers of both bands were calculated, and strongly J-dependent, pressure-induced line center shifts were measured for the fundamental. A method for correcting the measured shape of a spectral line for the effects of instrumental broadening in order to obtain the “true shape” is explained. It was possible to obtain the true shape of lines whose pressure broadened half-width was as small (0.03 cm−1) as several times the Doppler half-width.It was found that the true line shape could be represented quite satisfactorily by a modified form of the Lorentz expression, i.e., α=α0(Δν)2[|ν−ν0|η+(Δν)2]−1. For spectral lines whose half-width at one-half the maximum percent absorptance was less than 2.25 cm−1, η=2 gave good agreement between the true shape and that predicted by the Lorentz expression. If this width was greater than 3 cm−1 then η=1.85 allowed the best fit to the true line shape. No pressure dependence of η could be measured in the range of pressures used (0–5 atm).