Absolute Transition Frequencies Research Articles

Frequency-comb referenced collinear laser spectroscopy of Be+ for nuclear structure investigations and many-body QED tests

Absolute transition frequencies of the $2s\,^2{\rm{S}}_{1/2}$ $\rightarrow$ $2p\,^2{\rm{P}}_{1/2,3/2}$ transitions in Be$^+$ were measured with a frequency comb in stable and short-lived isotopes at ISOLDE (CERN) using collinear laser spectroscopy. Quasi-simultaneous measurements in copropagating and counterpropagating geometry were performed to become independent from acceleration voltage determinations for Doppler-shift corrections of the fast ion beam. Isotope shifts and fine structure splittings were obtained from the absolute transition frequencies with accuracies better than 1\,MHz and led to a precise determination of the nuclear charge radii of $^{7,10-12}$Be relative to the stable isotope $^9$Be. Moreover, an accurate determination of the $2p$ fine structure splitting allowed a test of high-precision bound-state QED calculations in the three-electron system. Here, we describe the laser spectroscopic method in detail, including several tests that were carried out to determine or estimate systematic uncertainties. Final values from two experimental runs at ISOLDE are presented and the results are discussed.

Ultracold metastable helium: Ramsey fringes and atom interferometry

We report on interference studies in the internal and external degrees of freedom of metastable triplet helium atoms trapped near quantum degeneracy in a \( 1.5\kern0.166667em \upmu \mathrm{m} \) optical dipole trap. Applying a single \( \pi /2 \) rf pulse we demonstrate that 50% of the atoms initially in the \( m=+1 \) state can be transferred to the magnetic field insensitive \( m=0 \) state. Two \( \pi /2 \) pulses with varying time delay allow a Ramsey-type measurement of the Zeeman shift for a high precision measurement of the \( {2}^3{S}_1 \)–\( {2}^1{S}_0 \) transition frequency. We show that this method also allows strong suppression of mean-field effects on the measurement of the Zeeman shift, which is necessary to reach the accuracy goal of 0.1 kHz on the absolute transition frequencies. Theoretically the feasibility of using metastable triplet helium atoms in the \( m=0 \) state for atom interferometry is studied demonstrating favorable conditions, compared to the alkali atoms that are used traditionally, for a non-QED determination of the fine structure constant.

Absolute molecular transition frequencies measured by three cavity-enhanced spectroscopy techniques.

Absolute frequencies of unperturbed (12)C(16)O transitions from the near-infrared (3-0) band were measured with uncertainties five-fold lower than previously available data. The frequency axis of spectra was linked to the primary frequency standard. Three different cavity enhanced absorption and dispersion spectroscopic methods and various approaches to data analysis were used to estimate potential systematic instrumental errors. Except for a well established frequency-stabilized cavity ring-down spectroscopy, we applied the cavity mode-width spectroscopy and the one-dimensional cavity mode-dispersion spectroscopy for measurement of absorption and dispersion spectra, respectively. We demonstrated the highest quality of the dispersion line shape measured in optical spectroscopy so far. We obtained line positions of the Doppler-broadened R24 and R28 transitions with relative uncertainties at the level of 10(-10). The pressure shifting coefficients were measured and the influence of the line asymmetry on unperturbed line positions was analyzed. Our dispersion spectra are the first demonstration of molecular spectroscopy with both axes of the spectra directly linked to the primary frequency standard, which is particularly desirable for the future reference-grade measurements of molecular spectra.

Polarization dependence of the direct two photon transitions of 87Rb atoms by erbium: Fiber laser frequency comb

The femtosecond fiber-based optical frequency combs have been proved to be powerful tools for investigating the energy levels of atoms and molecules. In this paper, an Er-doped fiber femtosecond optical frequency comb has been implemented for studying the polarization dependence of 5S–5D two-photon transitions in thermal gas of atomic rubidium 87 using an entirely symmetrical optical configuration. By changing the polarization states of the counter-propagating light beams, the polarization dependence of direct two photon transition spectrum is demonstrated, and a dramatic variation (up to 5.5 times) of the two-photon transitions strength has been observed. The theory for the polarization dependence of two photon transition based on the second-order perturbation was established, which is in good agreement with the experimental results. The measurement results indicate that the polarization state manipulation with the existing frequency comb is used for femtosecond optical frequency comb based two photon transition spectroscopic purposes, which will improve the precision measurement of the absolute transition frequency and related applications.

A new approach to spectral line shapes of the weak oxygen transitions for atmospheric applications

We propose to construct a new database of O2 molecular spectral lines for atmospheric application, consistent with recent IUPAC recommendation [Tennyson et al. Pure Appl Chem 2014;86:1931] going beyond Voigt profile by incorporation of the speed dependence of collisional broadening and shifting. For this purpose we collected the laboratory data for the self-perturbed oxygen B-band transitions. Line shapes were measured at low pressures by the Pound–Drever–Hall-locked frequency-stabilized cavity ring-down spectrometer linked to the optical frequency comb. Data were analyzed by means of the quadratic speed-dependent Voigt profile. The absolute transition frequencies are determined with accuracy even as good as 150kHz. Line intensities, pressure width and shift coefficients and the speed-dependent parameters are determined with subpercent accuracy.

Doppler-free two-photon absorption spectroscopy of rovibronic transition of naphthalene calibrated with an optical frequency comb

We performed Doppler-free two-photon absorption spectroscopy of naphthalene using an optical frequency comb as a frequency reference. Rotationally resolved rovibronic spectra were observed, and absolute frequencies of the rovibronic transitions were determined with an uncertainty of several tens of kHz. The resolution and precision of our system are finer than the natural width of naphthalene. We assigned 1466 lines of the Q(Ka)Q(J) transition and calculated molecular constants. We attribute systematic spectral line shifts to the Coriolis interaction, and discuss the origin of the spectral linewidths.

Precision Test of Many-Body QED in the Be+ 2p Fine Structure Doublet Using Short-Lived Isotopes.

Absolute transition frequencies of the 2s 2S{1/2}→2p2P{1/2,3/2} transitions in Be^{+} were measured for the isotopes ^{7,9-12}Be. The fine structure splitting of the 2p state and its isotope dependence are extracted and compared to results of ab initio calculations using explicitly correlated basis functions, including relativistic and quantum electrodynamics effects at the order of mα(6) and mα(7) ⁢ln α. Accuracy has been improved in both the theory and experiment by 2 orders of magnitude, and good agreement is observed. This represents one of the most accurate tests of quantum electrodynamics for many-electron systems, being insensitive to nuclear uncertainties.

Frequency-agile, rapid scanning cavity ring-down spectroscopy (FARS-CRDS) measurements of the (30012)←(00001) near-infrared carbon dioxide band

We present new high accuracy measurements of the (30012)←(00001) CO2 band near 1575nm recorded with a frequency-agile, rapid scanning cavity ring-down spectrometer. The resulting spectra were fit with the partially correlated, quadratic-speed-dependent Nelkin-Ghatak profile with line mixing. Significant differences were observed between the fitted line shape parameters and those found in existing databases, which are based upon more simplistic line profiles. Absolute transition frequencies, which were referenced to an optical frequency comb, are given, as well as the other line shape parameters needed to model this line profile. These high accuracy measurements should allow for improved atmospheric retrievals of greenhouse gas concentrations by current and future remote sensing missions.

High-precision laser spectroscopy of the CO A(1)Π - X(1)Σ(+) (2,0), (3,0), and (4,0) bands.

High-precision two-photon Doppler-free frequency measurements have been performed on the CO A(1)Π - X(1)Σ(+) fourth-positive system (2,0), (3,0), and (4,0) bands. Absolute frequencies of forty-three transitions, for rotational quantum numbers up to J = 5, have been determined at an accuracy of 1.6 × 10(-3) cm(-1), using advanced techniques of two-color 2 + 1' resonance-enhanced multi-photon ionization, Sagnac interferometry, frequency-chirp analysis on the laser pulses, and correction for AC-Stark shifts. The accurate transition frequencies of the CO A(1)Π - X(1)Σ(+) system are of relevance for comparison with astronomical data in the search for possible drifts of fundamental constants in the early universe. The present accuracies in laboratory wavelengths of Δλ/λ = 2 × 10(-8) may be considered exact for the purpose of such comparisons.

Frequency Ratio of Two Optical Clock Transitions inYb+171and Constraints on the Time Variation of Fundamental Constants

Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6×10(-16). Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, μ/μ=0.2(1.1)×10(-16) yr(-1), along with an improved constraint on time variation of the fine structure constant, α/α=-0.7(2.1)×10(-17) yr(-1).

Hyperfine-resolved transition frequency list of fundamental vibration bands of H35Cl and H37Cl

Sub-Doppler resolution spectroscopy of the fundamental vibration bands of H35Cl and H37Cl has been carried out from 87.1 to 89.9THz. We have determined the absolute transition frequencies of the hyperfine-resolved R(0) to R(4) transitions with a typical uncertainty of 10kHz. We have also yielded six molecular constants for each isotopomer in the vibrational excited state, which reproduce the determined frequencies with a standard deviation of about 10kHz.

Reservoir spectroscopy of5s5p3P2–5snd3D1,2,3transitions in strontium

We perform spectroscopy on the optical dipole transitions 5s5p $^3$P$_2$ - 5s$n$d $^3$D$_{1,2,3}$, $n \in (5,6)$, for all stable isotopes of atomic strontium. We develop a new spectroscopy scheme, in which atoms in the metastable $^3$P$_2$ state are stored in a reservoir before being probed. The method presented here increases the attained precision and accuracy by two orders of magnitude compared to similar experiments performed in a magneto-optical trap or discharge. We show how the state distribution and velocity spread of atoms in the reservoir can be tailored to increase the spectroscopy performance. The absolute transition frequencies are measured with an accuracy of 2 MHz. The isotope shifts are given to within 200 kHz. We calculate the $A$ and $Q$ parameters for the hyperfine structure of the fermionic isotope at the MHz-level. Furthermore, we investigate the branching ratios of the $^3$D$_{J}$ states into the $^3$P$_{J}$ states and discuss immediate implications on schemes of optical pumping and fluorescence detection.

Precision two-photon spectroscopy of alkali elements

In this paper, we have briefly reviewed the work on two-photon spectroscopy of alkali elements and its applications. The technique of Doppler-free two-photon spectroscopy is briefly summarized. A review of various techniques adopted for measuring absolute frequencies of the atomic transitions and precision measurements of isotope shifts and hyperfine structures (HFS) is presented. Some of the recent works on precision measurements of HFS constants of 6s 2 S 1/2 level of 39K and 41K, 9s 2 S 1/2level and 7d 2 D 3/2 level of 133Cs are also discussed.

Absolute 12C16O2 transition frequencies at the kHz-level from 1.6 to 7.8 µm

Absolute transition frequencies were measured for a series of transitions in the (30013)←(00001) near-infrared 12C16O2 band. These measurements were referenced to a cesium atomic clock through the use of an optical frequency comb. Combined standard uncertainties as low as 18kHz (6×10−7cm−1) were achieved. Importantly, deviations as large as 5MHz were observed relative to the HITRAN 2008 database. These measurements were then included in a global fit of 416 CO2 mid-infrared and near-infrared measurements each of which was absolute. The resulting spectroscopic parameters provide a series of secondary frequency standards with kHz-level uncertainties across a wide frequency range and should significantly improve spectroscopic retrieval algorithms for space-based measurements of atmospheric CO2.

Precision Frequency Metrology of Helium2S01→2P11Transition

We report a precision frequency measurement of the (4)He 2(1)S(0)→2(1)P(1) transition at 2058 nm. The saturated absorption spectroscopy is performed in a rf discharge sealed-off cell with a volume Bragg grating-based Tm:Ho:YLF laser. The absolute transition frequency measured using a fiber optical frequency comb is 145 622 892 822 (183) kHz with a relative uncertainty of 1.3×10(-9). Our result is ten times more precise than current best theoretical calculations and is in reasonable agreement with the calculated values. However, the ionization energy of the 2(1)P(1) state, derived from our result and other precisely measured transitions, shows a discrepancy of approximately 3.5σ with the most precise atomic theory. We have also determined the isotope shift between (3)He and (4)He to be 4248.7 (5.3) MHz, which is more precise than the previous measurement by one order of magnitude.

Widely tunable difference frequency generation source for high-precision mid-infrared spectroscopy

We have developed a widely tunable mid-infrared difference frequency generation (DFG) source by mixing ~ 1 W Ti:sapphire laser and 6 W Nd:YAG laser beams in a 50-mm MgO-doped long periodically poled lithium niobate (MgO:PPLN). The power of the DFG source is > 2 mW over the tuning range of 2.66-4.77 μm and its free-running linewidth is about 100 kHz. Combining various frequency stabilisation schemes for the Nd:YAG laser and the Ti:sapphire laser, the DFG frequency can be precisely controlled. Besides, its frequency can be determined better than 12 kHz by measuring the Ti:sapphire laser frequency using an optical frequency comb. Two high resolution spectroscopic studies on (12)C(16)O(2) molecule are demonstrated using this DFG source. The saturation spectra of R(18) and R(60) transitions of 00(0)1 ← 00(0)0 fundamental band at 4.2 μm and P(20) transition of [10(0)1, 02(0)1](I) ← 00(0)0 band at 2.7 μm have been observed and their absolute transition frequencies are measured with an accuracy better than 30 kHz.

Quantum interference and light polarization effects in unresolvable atomic lines: Application to a precise measurement of the6,7LiD2lines

We characterize the effect of quantum interference on the line shapes and measured line positions in atomic spectra. These effects, which occur when the excited-state splittings are of order of the natural line widths, represent an overlooked but significant systematic effect. We show that excited-state interference gives rise to non-Lorentzian line shapes that depend on excitation polarization, and we present expressions for the corrected line shapes. We present spectra of ${}^{6,7}$Li $D$ lines taken at multiple excitation laser polarizations and show that failure to account for interference changes the inferred line strengths and shifts the line centers by as much as 1 MHz. Using the correct line shape, we determine absolute optical transition frequencies with an uncertainty of $\ensuremath{\le}$25 kHz and provide an improved determination of the difference in mean-square nuclear charge radii between ${}^{6}$Li and ${}^{7}$Li. This analysis should be important for a number of high-resolution spectral measurements that include partially resolvable atomic lines.

Precise frequency measurements of iodine hyperfine transitions at 671 nm

We report absolute frequency measurements on the a(1), a(10), and a(15) hyperfine components of the R(78) 4-6 line of (127)I(2). An external-cavity diode laser system at 671 nm is frequency-stabilized to the saturated absorption center obtained by modulation transfer spectroscopy in an iodine vapor cell. Its absolute frequency is measured by an optical frequency comb. The effect of pressure shift is investigated to obtain the absolute transition frequency at zero pressure. Our determination of the line centers reaches a precision of better than 40 kHz and will provide useful input for theoretical calculations. This frequency-stabilized laser can be used as a reference laser for the spectroscopy of lithium D lines.

Vuv frequency-comb spectroscopy of atomic xenon

Direct frequency-comb spectroscopy of a single-photon transition ($5{p}^{6}\ensuremath{-}5{p}^{5}6s$) in ${}^{132}\mathrm{Xe}$ is performed with a vacuum ultraviolet (vuv) frequency comb at 147 nm. The vuv comb is generated from an Yb-doped fiber laser system at a repetition rate of $\ensuremath{\sim}$79 MHz via high harmonic generation in a Kr gas jet placed at the focus point of an external enhancement cavity. For Doppler-reduced spectroscopy, a collimated atomic beam of isotope-selected ${}^{132}\mathrm{Xe}$ is used as a target. Repeated measurements with different repetition frequencies and highly sensitive time-resolved detection allow us to determine the absolute transition frequency with an uncertainty of 140 MHz in the vuv region.

Absolute frequency measurement of the molecular iodine hyperfine transitions at 548 nm

We report absolute frequency measurement of the molecular iodine P(28) 24-0 a1, a10, and a15 hyperfine transitions at 548 nm. The light source is based on a frequency-doubled fiber amplifier system seeded by an external cavity diode laser. Saturated absorption is performed by modulation transfer spectroscopy, and the absolute transition frequency is measured by an optical frequency comb. The effects of pressure shift and pressure broadening are discussed. Our determination of the line centers reaches a precision of better than 20 kHz. This light source can be used as a reference laser for lithium ion spectroscopy.