sub-Doppler Line Research Articles

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.

Frequency measurements and self-broadening of sub-Doppler transitions in the + band of C2H2.

Frequency comb-referenced measurements of sub-Doppler laser saturation dip absorption lines in the + band of acetylene near 1.5 μm are reported. These measurements include transitions involving higher rotational levels than previously frequency measured in this band. The accuracy of the measured frequencies is typically better than 10 kHz. Measurements of the observed sub-Doppler line widths as a function of pressure showed that the self-pressure-broadening coefficients are about 3.5 times larger than those derived from conventional pressure broadening of unsaturated Doppler-limited spectra. This is attributed to the contribution of velocity-changing collisions to the total dephasing rate in the low pressure sub-Doppler measurements. At higher pressures, when the homogeneous broadening becomes comparable to the typical Doppler shift per elastic collision, the velocity changing collisions cease to contribute significantly to the incremental pressure broadening. A time-dependent soft collision model is developed to illustrate the transition between low and high pressure regimes of sub-Doppler pressure-broadening.

EIT line shape in an open and partially closed multilevel V-type system

A detail theoretical analysis based on density matrix formulation is carried out to study the electromagnetically induced transparency in a multilevel open V-type system. The density matrix equations are solved numerically for both the Doppler-free and Doppler-broadened medium. We get two coherent electromagnetically induced transparency (EIT) peaks of sub-natural line width and five saturation peaks of sub-Doppler line width. The effect of decoherence on the EIT peak is studied using the theoretical model. A comparative study of the EIT line shape for open and closed V-type system has been done using the theoretical results. In our experiment, we have observed one EIT peak and four saturation peaks in an open V-type system corresponding to the D2 transition of 85Rb atoms and the EIT peak gets enhanced for partially closed V-type system compared to the open system. The dependence of height and width of the EIT line shape for different pump power values is also observed. The experimental observations are simulated numerically with a good agreement. We have also reported a noticeable change in the contrast of the EIT resonance due to the tuning of the pump laser frequency over the entire probe Doppler profile.

Spectroscopy and Dynamics of Jet-Cooled Polyynes in a Slit Supersonic Discharge: Sub-Doppler Infrared Studies of Diacetylene HCCCCH.

Fundamental, bending (ν6, ν7, ν8, ν9), and CC-stretch (ν2, ν3) hot band spectra in the antisymmetric CH stretch (ν4) region near 3330 cm(-1) have been observed and analyzed for jet cooled diacetylene (HC≡C-C≡CH) under sub-Doppler conditions. Diacetylene is generated in situ in the throat of a pulsed supersonic slit expansion by discharge dissociation of acetylene to form ethynyl (C≡CH) + H, followed by radical attack (HC≡CH + C≡C-H) to form HC≡C-C≡CH + H. The combination of (i) sub-Doppler line widths and (ii) absence of spectral congestion permits rotational structure and Coriolis interactions in the ν4 CH stretch fundamental to be observed and analyzed with improved precision. Of particular dynamical interest, the spectra reveal diacteylene formation in highly excited internal vibrational states. Specifically, multiple Π ← Π and Δ ← Δ hot bands built on the ν4 CH stretch fundamental are observed, due to doubly degenerate bending vibrations [cis C≡C-H bend (ν6), trans C-C≡C bend (ν7), trans C≡C-H bend (ν8) and cis C-C≡C bend (ν9)], as well as a heretofore unobserved Σ ← Σ band assigned to excitation of ν2 or 2ν3 CC stretch. Boltzmann analysis yields populations consistent with universally cold rotations (Trot ≈ 15 ± 5 K) and yet superthermal vibrations (Tvib ≈ 85-430 K), the latter of which is quite anomalous for the high collision densities in a slit jet expansion. In order to elucidate the physical mechanism for this excess vibrational excitation, high level ab initio CCSD(T) calculations have been pursued with explicitly correlated basis sets (VnZ-f12; n = 2,3) and extrapolated to the complete basis set (CBS) limit using MOLPRO quantum chemistry software. The results suggest that the extensive hot band structure observed arises from (i) highly exothermic CCH + HCCH addition to yield a strongly bent HCCHCCH radical intermediate (ΔH = -62.6 kcal/mol), followed by (ii) rapid fragmentation over a submerged transition state barrier (ΔH = -18.9 kcal/mol) to form vibrationally hot diacetylene + H products (ΔH = -25.6 kcal/mol), and consistent with crossed molecular beam studies by Kaiser et al. [Phys. Chem. Chem. Phys. 2002, 4, 2950.] Finally, RRKM fragmentation rates for this complex are calculated, which exceed collision frequencies in the slit jet expansion and suggest near unity quantum efficiency for diacetylene formation.

Two-photon spectroscopy of trapped HD+ions in the Lamb-Dicke regime

We study the feasibility of nearly degenerate two-photon rovibrational spectroscopy in ensembles of trapped, sympathetically cooled hydrogen molecular ions using a resonance-enhanced multiphoton dissociation (REMPD) scheme. Taking advantage of quasicoincidences in the rovibrational spectrum, the excitation lasers are tuned close to an intermediate level to resonantly enhance two-photon absorption. Realistic simulations of the REMPD signal are obtained using a four-level model that takes into account saturation effects, ion trajectories, laser frequency noise, and redistribution of population by blackbody radiation. We show that the use of counterpropagating laser beams enables optical excitation in an effective Lamb-Dicke regime. Sub-Doppler lines having widths in the 100-Hz range can be observed with good signal-to-noise ratio for an optimal choice of laser detunings. Our results indicate the feasibility of molecular spectroscopy at the ${10}^{\ensuremath{-}14}$ accuracy level for improved tests of molecular QED, a new determination of the proton-to-electron mass ratio, and studies of the time (in)dependence of the latter.

Sub-Doppler Spectroscopy of the trans-HOCO Radical in the OH Stretching Mode

Rovibrational spectroscopy of the fundamental OH stretching mode of the trans-HOCO radical has been studied via sub-Doppler high-resolution infrared laser absorption in a discharge slit-jet expansion. The trans-HOCO radical is formed by discharge dissociation of H2O to form OH, which then combines with CO and cools in the Ne expansion to a rotational temperature of 13.0(6) K. Rigorous assignment of both a-type and b-type spectral transitions is made possible by two-line combination differences from microwave studies, with full rovibrational analysis of the spectrum based on a Watson asymmetric top Hamiltonian. Additionally, fine structure splittings of each line due to electron spin are completely resolved, thus permitting all three ε(aa), ε(bb), ε(cc) spin-rotation constants to be experimentally determined in the vibrationally excited state. Furthermore, as both a- and b-type transitions for trans-HOCO are observed for the first time, the ratio of transition dipole moment projections along the a and b principal axes is determined to be μ(a)/μ(b) = 1.78(5), which is in close agreement with density functional quantum theoretical predictions (B3LYP/6-311++g(3df,3pd), μ(a)/μ(b) = 1.85). Finally, we note the energetic possibility in the excited OH stretch state for predissociation dynamics (i.e., trans-HOCO → H + CO2), with the present sub-Doppler line widths providing a rigorous upper limit of >2.7 ns for the predissociation lifetime.

Highly-accurate line shape studies in the near-IR spectrum of H218O: Implications for the spectroscopic determination of the Boltzmann constant

The outcomes of a specific study on line shape deviations from the usual Voigt profile, in the near-IR spectrum of water, will be discussed. Particularly, a great attention is paid to the role of speed-dependence of collisional broadening. From the experimental point of view, high spectral fidelity has been reached by using the technique of offset-frequency locking between two extended-cavity diode lasers. In such a system, the master laser is frequency stabilized against a narrow sub-Doppler molecular line, while the slave laser is actively controlled so that its emission frequency maintains a given offset with respect to the frequency of the master. This method allows one to perform highly accurate and reproducible frequency scans of the slave laser around a given vibration-rotation line. I will present recent findings on the speed dependence of collision parameters, influencing the shape of H218O absorption lines. Finally, I will discuss the implications for the spectroscopic determination of the Boltzmann constant.

Hyperfine structures in the v = 1–0 vibrational band of the [formula omitted] of N2

Hyperfine splittings were measured in selected rotational lines of the (1–0) vibrational band of the B∼3Πg-A∼3Σu+ transition of 14N2 in a low pressure discharge. Frequency modulated saturation dip spectroscopy produced hyperfine resolved, sub-Doppler lines with typical full-width at half-maximum line widths of 7MHz. Hyperfine transitions within 27 rotational lines in the R33, Q33, Q21, Q22, R11, and Q11 branches were recorded for J between 2 and 12, and fit with the same Hamiltonian used previously by Geisen, Neuschafer and Ottinger [Z. Phys. D 4, 263 (1987)]. The resulting parameters are (in MHz, with one standard deviation in parentheses): α=13.19(13), β=−12.36(18), e2q0Q=−1.19(25) for the (v″=0)A∼3Σu+ state; and K11=98.52(50), D11=13.09(59), D1−1=−44.11(16), G11=70.00(52), Q11=1.390(98) and Q1−1=1.65(11) for the (v′=1) B∼3Πg state.

Extra sub-Doppler lines in the vicinity of the third-resonance 6S-8Ptransition of atomic Cs attributed to optically induced Cs dimers

We report on the observation of additional sub-Doppler lines in a saturated absorption experiment when exploring the vicinity of the $6{S}_{1/2}\ensuremath{-}8{P}_{3/2}$ transition of Cs (\ensuremath{\lambda}$=$388 nm). These additional lines are observed only under a relatively strong irradiation of both the pump and the probe beams. Extra narrow lines are also observed in copropagating nonlinear spectroscopy, and around the lines of the V-type three-level system $8{P}_{3/2}\ensuremath{-}6{S}_{1/2}\ensuremath{-}8{P}_{1/2}$ (${\ensuremath{\lambda}}_{1}$$=$388 nm, ${\ensuremath{\lambda}}_{2}$$=$389 nm). We attribute theses additional lines to a probing of high-lying molecular cesium, produced as a result of the optical excitation of Cs atoms, as the low Cs atom density (\ensuremath{\le}10${}^{12}$cm${}^{\ensuremath{-}3}$) is unable to populate significantly the dimer states in the condition of thermal equilibrium.

Absolute frequency stabilization of an extended-cavity diode laser against Doppler-free H_2^17O absorption lines at 1384 μm

We report the frequency stabilization of a cw extended-cavity diode laser against saturated absorption lines of the H(2)O17 isotopologue of water vapor at around 1.384 microm. The saturation of rotovibrational transitions is achieved by filling a high-finesse optical resonator with H(2)O17 at low pressure and by locking the laser frequency to the resonator by using the Pound-Drever-Hall technique. Absolute frequency stabilization is obtained, locking the cavity resonance to the center of the sub-Doppler line by means of the wavelength modulation method. A relative frequency stability of sigma(y)(tau)=10(-13)(0.1tau(-2)+0.9)(1/2) is demonstrated for integration times in the range 4 ms<tau<30 s.

Jet-cooled infrared spectroscopy in slit supersonic discharges: Symmetric and antisymmetric CH2 stretching modes of fluoromethyl (CH2F) radical

The combination of shot noise-limited direct absorption spectroscopy with long-path-length slit supersonic discharges has been used to obtain first high-resolution infrared spectra for jet-cooled CH2F radicals in the symmetric (nu1) and antisymmetric (nu5) CH2 stretching modes. Spectral assignment has yielded refined lower- and upper-state rotational constants and fine-structure parameters from least-squares fits to the sub-Doppler line shapes for individual transitions. The rotational constants provide indications of large amplitude vibrational averaging over a low-barrier double minimum inversion-bending potential. This behavior is confirmed by high-level coupled cluster singles/doubles/triples calculations extrapolated to the complete basis set limit and adiabatically corrected for zero point energy. The calculations predict a nonplanar equilibrium structure (theta approximately 29 degrees, where theta is defined to be 180 degrees minus the angle between the C-F bond and the CH2 plane) with a 132 cm(-1) barrier to planarity and a vibrational bend frequency (nu(bend) approximately 276 cm(-1)), in good agreement with previous microwave estimates (nu(bend) = 300 (30) cm(-1)) by Hirota and co-workers [Y. Endo et al., J. Chem. Phys. 79, 1605 (1983)]. The nearly 2:1 ratio of absorption intensities for the symmetric versus antisymmetric bands is in good agreement with density functional theory calculations, but in sixfold contrast with simple local mode CH2 bond dipole predictions of 1:3. This discrepancy arises from a surprisingly strong dependence of the symmetric stretch intensity on the inversion bend angle and provides further experimental support for a nonplanar equilibrium structure.

High-resolution infrared studies in slit supersonic discharges: CH2 stretch excitation of jet-cooled CH2Cl radical

First high-resolution infrared spectra are presented for jet-cooled CH2 35Cl and CH2 37Cl radicals in the symmetric (nu1) CH2 stretching mode. A detailed spectral assignment yields refined lower and upper state rotational constants, as well as fine structure spin-rotation parameters from least-squares fits to the sub-Doppler line shapes for individual transitions. The rotational constants are consistent with a nearly planar structure, but do not exclude substantial large amplitude bending motion over a small barrier to planarity accessible with zero-point excitation. High level coupled cluster (singles/doubles/triples) calculations, extrapolated to the complete basis set limit, predict a slightly nonplanar equilibrium structure (theta approximately 11 degrees), with a vibrationally adiabatic treatment of the bend coordinate yielding a v = 1<--0 anharmonic frequency (393 cm(-1)) in excellent agreement with matrix studies (nu(bend) approximately 400 cm(-1)). The antisymmetric CH2 stretch vibration is not observed despite high sensitivity detection (signal to noise ratio >20:1) in the symmetric stretch band. This is consistent with density functional theory intensity calculations indicating a >35-fold smaller antisymmetric stretch transition moment for CH2Cl, and yet contrasts dramatically with high-resolution infrared studies of CH2F radical, for which both symmetric and antisymmetric CH2 stretches are observed in a nearly 2:1 intensity ratio. A simple physical model is presented based on a competition between bond-dipole and "charge-sloshing" contributions to the transition moment, which nicely explains the trends in CH2X symmetric versus asymmetric stretch intensities as a function of electron withdrawing group (X = D,Br,Cl,F).

Frequency stabilization of a novel 1.5-μm Er-Yb bulk laser to a /sup 39/K sub-Doppler line at 770.1 nm

The second harmonic output at 770.1 nm of a novel and compact Er-Yb:glass laser was frequency stabilized against the sub-Doppler linewidth of a crossover line in the /sup 39/K 4S/sub 1/2/-4P/sub 1/2/ transition as obtained by saturation spectroscopy. Efficient frequency doubling, with a conversion efficiency of /spl sim/220% W/sup -1/, and with second harmonic power in excess of 15 /spl mu/W, was achieved in a waveguide made in a periodically poled lithium niobate crystal. As measured through the analysis of the closed-loop error signal, a laser frequency instability of /spl sim/200 Hz was obtained; the Allan standard deviation of the frequency samples was below 4/spl times/10/sup -12/ for integration times /spl tau/ between 100 ms and 100 s, and reached a lowest floor level of 8/spl times/10/sup -13/ for 20 s/spl les//spl tau//spl les/100 s. The measured frequency noise spectral density was in good agreement with the analysis performed in the time domain. Compared to previously published data for stabilized solid-state laser sources in this wavelength region, these results represent a significant improvement in the frequency stability.

Absolute frequencies of the [formula omitted] transitions near 1064 nm

We investigate the sub-Doppler lines of 133 Cs 2 using the FM spectroscopy and stabilize the 1064 nm Nd:YAG laser to 133 Cs 2 lines. We measure the absolute frequencies of thirty-one transitions of 133 Cs 2 with respect to a frequency-doubled Nd:YAG/I 2 reference. Our measurements have an accuracy of 120 kHz. Our results show a discrepancy of 2 MHz with respect to previous work. This indicates that the reproducibility of 133 Cs 2 -locked Nd:YAG laser is worse than 1 MHz although it shows good stability (better than 1×10 −10). These measurements provide a well-distributed reference line within the tuning range of a Nd:YAG laser near 1064 nm.

Frequency stability at the kilohertz level of a rubidium-locked diode laser at 192114 THz

The frequency stability of a 1560-nm diode laser, whose second harmonic was locked to (87)Rb sub-Doppler lines, was characterized by measuring the beat frequency relative to a 780-nm reference laser that was locked to sub-Doppler lines of another rubidium cell. The square root of the Allan variance reached a minimum value of 7.5 x 10(-12) in 1 s, which corresponded to frequency variations of 1.44 kHz for the 1560-nm laser. The frequency reproducibility of the system was approximately 1 x 10(-9). These values are better than those that can be achieved by locking to Doppler-broadened transitions at the 1550-nm wavelength band.

Frequency stabilization of a diode laser at 1540 nm by locking to sub-Doppler lines of potassium at 770 nm

An external cavity 1540-nm diode laser was frequency doubled in a 3-cm-long periodically poled LiNbO(3) waveguide doubler with 150% W(-1) conversion efficiency, thereby generating more than 3 muW at 770 nm. Second-harmonic light was used to detect and lock to sub-Doppler lines of the (39)K D(1) transition.

Quasi-phase-matched frequency doubling in a waveguide of a 1560-nm diode laser and locking to the rubidium D_2 absorption lines

An external-cavity 1560-nm diode laser was frequency doubled in a 3-cm-long periodically poled LiNbO(3) waveguide doubler with 120% W(-1) conversion efficiency. The 780-nm light was used to detect the D(2) transitions of Rb, and the laser frequency was locked to Doppler-broadened lines of Rb. Furthermore, the ~1 microW of second-harmonic power was sufficient for detecting the sub-Doppler lines of Rb, and the laser was locked to a (87)Rb crossover line.

Frequency stabilization of Nd:YAG lasers to ^133Cs_2 sub-Doppler lines near 1064 nm

Sub-Doppler lines of Cs2 were investigated with a Nd:YAG laser. Modulation transfer spectroscopy and FM spectroscopy were applied to yield error signals that were used for absolute stabilization of the laser frequency. The frequency stability was characterized with an I2-locked dual-wavelength Nd:YAG frequency reference. The root Allan variance of the beat frequency reached a minimum of 1.3 × 10−11 (beat frequency fluctuations of 3.65 kHz) for a measurement time of 20 s. Absolute frequencies of several Cs2 lines were determined with an accuracy of ∼1 MHz. Further improvement in sensitivity was demonstrated by insertion of the absorption cell into a Fabry–Perot cavity. While the laser was locked to the cavity, the cavity length was modulated and the transmitted probe beam was detected at the third harmonic of the modulation frequency.

10 4-fold isotope effect: vibrational predissociation in ArDF

High-resolution sub-Doppler line shapes for ArDF excited into the ν DF = 1 manifold are recorded using a tunable laser/slit supersonic expansion near-infrared spectrometer. Direct spectroscopic evidence is obtained for vibrational predissociation (2.6(14) MHz ⩽ Δν prediss. ⩽ 4.3 (8) MHz), in dramatic contrast with the highly metastable ArHF ( ν HF = 1) species observed in bolometry studies. To the best of our knowledge, this represents an unprecedentedly large (> 10 4-fold), inverse isotope effect. Furthermore, a clear dependence of the ArDF predissociation rates on intermolecular mode, excited in combination with the DF stretch, is observed (61(30) ⩾ τ νDF ⩾ 37(7) ns, τ νDF + II bend ⩾ 94(60) ns).

The H+OCS hot atom reaction: CO state distributions and translational energy from time-resolved infrared absorption spectroscopy

Time-resolved infrared diode laser spectroscopy has been used to probe CO internal and translational excitation from the reaction of hot H atoms with OCS. Product distributions should be strongly biased toward the maximum 1.4 eV collision energy obtained from 278 nm pulsed photolysis of HI. Rotations and vibrations are both colder than predicted by statistical density of states theory, as evidenced by large positive surprisal parameters. The bias against rotation is stronger than that against vibration, with measurable population as high as v=4. The average CO internal excitation is 1920 cm−1, accounting for only 13% of the available energy. Of the energy balance, time-resolved sub-Doppler line shape measurements show that more than 38% appears as relative translation of the separating CO and SH fragments. Studies of the relaxation kinetics indicate that some rotational energy transfer occurs on the time scale of our measurements, but the distributions do not relax sufficiently to alter our conclusions. Vibrational distributions are nascent, though vibrational relaxation of excited CO is unusually fast in the OCS bath, with rates approaching 3% of gas kinetic for v=1. At 9720 cm−1 above the activation barrier, energy partitioning is consistent with incomplete IVR in an HSCO intermediate, followed by repulsive release of a large fraction of the available energy into relative translation.