Doppler-free optoacoustic spectroscopy

The first observation of Doppler-free optoacoustic spectroscopy is reported. As a first example the P (193) line of the 11-0 band of the B←X transition of ¹²⁷I₂ is used. The output of cw single mode dye laser is split into two equal intensity beams chopped at frequencies ω₁ and ω₂. The nonlinear compoment of the optoacoustic signal at the frequency (ω₁ + ω₂) is detected and Doppler-free resolution is obtained. Comparing the Doppler-free optoacoustic and fluorescence spectra of iodine measured under similar conditions, good agreement is found. Since optoacoustic and fluorescence methods complement each other, this opens up new possibilities for weakly or nonfluorescing molecules.     

Doppler-free two-photon spectroscopy

We describe atomic transitions where two or more photons are simultaneously absorbed by an atom or a molecule (without any intermediate resonant level). We show theoretically and experimentally the possibilities of these transitions for numerous high-resolution spectroscopic studies. The principle of the method is discussed: the Doppler broadening is suppressed in two-photon transitions if the atoms are irradiated by two laser beams travelling in opposite directions, because the Doppler shift in one wave cancels that in the other. A generalization is made for multiphotonic transitions. Calculations of the multiphotonic transition probability are summarized and their hypotheses specified: one deals with line shape and orders of magnitude in practical conditions. The problem of light shifts is also discussed. A typical experimental set-up with a cw dye laser is described in detail. Typical recordings of two-photon transitions are given; they show line widths smaller than 4 MHz (on a Doppler width of 2000 MHz). Other experiments show the possibility of increasing the transition probability by using two lasers with slightly different wave lengths. One experiment with a three-photon Doppler-free transition is also described. In conclusion, we make a brief comparison with the saturated absorption technique.     

Doppler-free evanescent wave spectroscopy

Nonlinear Doppler-free spectroscopy at the interface between a gas and a dielectric medium is experimentally demonstrated on Na vapor. Both saturated absorption and dispersion of sodium atoms near the surface are monitored, depending on the light beam incidence relatively to the critical angle for total internal reflections. Resonance line widths as narrow as 20 MHz have been observed on the Na D₁ line. This technique makes possible Doppler-free diagnostics of optically dense media and should find applications in the analysis of atom-surface interactions.     

空心阴极灯中镧原子消多普勒光谱

我们在自制的La-Kr阴极灯中,用内调制、消多普勒荧光法研究了镧原子一些能级的超精细结构.据我们所知,其中20197.34cm~(-1)、21447.86cm~(-1)能级的超精细结构常数A、B尚无报道,这还是第一次.此外,还获得了线宽小于40MHz的线型和来自公共低能级的所有交岔线(Cross-Over line).对阴极灯中原子光谱特性进行了研究.     

Doppler-free spectroscopy in driven three-level systems

We demonstrate two techniques for studying the features of three-level systems driven by two lasers (called control and probe), when the transitions are Doppler broadened as in room-temperature vapor. For -type systems, the probe laser is split to produce a counter-propagating pump beam that saturates the transition for the zero-velocity atoms. Probe transmission then shows Doppler-free peaks which can even have sub-natural linewidth. For V-type systems, the transmission of the control beam is detected as the probe laser is scanned. The signal shows Doppler-free peaks when the probe laser is resonant with transitions for the zero-velocity group. Both techniques greatly simplify the study of three-level systems since theoretical predictions can be directly compared without complications from Doppler broadening and the presence of multiple hyperfine levels in the spectrum.Received: 10 September 2003, Published online: 6 January 2004PACS: 42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption - 42.50.-p Quantum optics     

Theory of multiple-pass two-photon Doppler-free spectroscopy

Electron stimulated desorption (ESD) of CO on Ru (001) leads to emission of CO⁺ and O⁺ ions from the same adsorption states. Following earlier work on this system, a correlated study of its ESD behaviour together with LEED, Δϕ, and TPD measurments has been carried out. The filling of the two states found earlier can be seen in ESD also. The dividing line between the high and low energy states is different in room temperature adsorption on the one hand, and desorption or equilibrium measurements at elevated temperatures on the other, which is explained by incomplete attainment of equilibrium in the layer under the first condition. The behaviour of the ESD ion currents with coverage showstthat part of the molecules occupying high energy sites in the ordered layer of intermediate coverage are shifted to less favourable sites at higher coverage, so that the full layer consists of a mixture of states due to the occupation of different sites in the compressed layer, possibly accompanied by lateral interactions. ESD also suggests that the high energy sites are the on-top sites rather than the threefold sites.     

Pulsed Doppler-free two-photon spectroscopy of polyatomic molecules

Doppler-free two-photon electronic spectra of a large polyatomic molecule are recorded for the first time with pulsed laser radiation of near Fourier-transform limited bandwidth (Δv～100 MHz). The resolution obtained is sufficient to resolve individual rotational lines. Due to the high density of these rotational transitions a strong Doppler-broadened background is observed, which is, however, subtantially reduced by suitable choice of photon polarizations. Different vibronic bands of benzene (C₆H₆) are investigated and very accurate rotational constants are found.     

Doppler-free spectroscopy of rubidium atoms driven by a control laser

A scheme of Doppler-free spectroscopy is experimentally demonstrated with a co-propagating control laser locking to an atomic hyperfine transition, and the differential transmission of the probe and the reference laser is detected. Crossover resonances are eliminated by selecting the class of atoms with zero velocity in the direction of beam propagation. In addition, the sub-Doppler spectrum experiences optical gain compared to the conventional saturated-absorption spectrum as a result of optical pumping.     

Doppler-free frequency-modulation spectroscopy of atomic erbium in a hollow-cathode discharge cell

The erbium atomic system is a promising candidate for an atomic Bose–Einstein condensate of atoms with a non-vanishing orbital angular momentum (L≠0) of the electronic ground state. In this paper we report on the frequency stabilization of a blue external cavity diode laser system on the 400.91 nm laser cooling transition of atomic erbium. Doppler-free saturation spectroscopy is applied within a hollow-cathode discharge tube to the corresponding electronic transition of several of the erbium isotopes. Using the technique of frequency-modulation spectroscopy, a zero-crossing error signal is produced to lock the diode laser frequency on the atomic erbium resonance. The latter is taken as a reference laser to which a second main laser system, used for laser cooling of atomic erbium, is frequency stabilized.     

Doppler-free laser spectroscopy of atomic copper in a hollow cathode discharge

The isotope shift and hyperfine structure of the 5782 and 5105 Å ($\text{3d}{}^{10}\text{4p}{}^2\text{P}{}^0\text{? 3d}{}^9\text{4s}{}^2{}^2\text{D}$) laser transitions of Cu were examined using laser saturation spectroscopy. The atomic cooper density was generated in a hollow cathode discharge. This work demonstrates the suitability of the hollow cathode discharge to high-resolution spectroscopy of non-valatile materials.     

Doppler-free two-photon modulation transfer spectroscopy in sodium dimers

We performed an experiment to investigate modulation transfer spectroscopy in two-photon transition of Na₂ using a CW dye laser. Comparing with UV fluorescence detection, the signals obtained by modulation transfer spectroscopy have a very high S/N without the Doppler background problem and its line shape is in agreement with theoretical calculations.Project supported by National Natural Science Fund of ChinaPermanent address: Shanghai University of Technology, Shanghai 200072, People's Republic of China     

Doppler-free optoacoustic spectroscopy of O-18 methanol

The Doppler-free optoacoustic spectrum of the oxygen-18 species of methanol has been investigated with a CO₂ laser. Seventeen coincidences have been found between infrared absorptions of CH¹⁸₃OH and CO₂ laser lines. The strongest feature seen, pumped by the 10R(20) CO₂ laser line, has two plausible alternative identifications, determined from a parallel high-resolution spectroscopic study of the C-O stretching band. Further experimental tests are suggested to resolve the assignment, and predicted far-infrared laser frequencies are given for the two possibilities.     

Doppler-free two-photon spectroscopy using degenerate four-wave mixing

Using recently developed techniques of degenerate, four-wave mixing, Doppler-free two-photon spectra can be obtained. The signal level for such an experiment in xenon is calculated and found to be readily detectable as well as competing favorably with the method of observing fluoresence following two-photon absorption.     

Doppler-free infrared-acoustic spectroscopy using color center lasers

A single mode widely tunable color center laser has been applied to obtain Doppler-free infrared-acoustic spectra of hydrogencyanide gas. Saturated-absorption sub-Doppler spectroscopy of the R(13) and R(16) v₃ HCN transitions are reported using small-volume non-resonant photoacoustic detectors.     

Doppler-free polarization spectroscopy of diatomic molecules in flame reactions

Gas phase reactions are used to produce radicals or other molecules which can hardly be evaporated. The conditions of this production method require a sensitive spectroscopic technique if sub-Doppler spectra are to be recorded. Different techniques are discussed and Doppler-free polarization spectroscopy is shown to offer the highest sensitivity for the study of these species. First polarization spectra of the A ¹∑?X ¹∑ (1, 0) transition of BaO and the B ²∑?X ²∑ (0,0) transition of CaCl are reported.     

High-resolution Doppler-free molecular spectroscopy with a continuous-wave optical parametric oscillator

We present a reliable, narrow-linewidth (100-kHz) continous-wave optical parametric oscillator (OPO) suitable for high-resolution spectroscopy applications. The singly resonant OPO with a resonated pump is based on periodically poled lithium niobate crystal and features a specially designed intracavity etalon, which permits precise tuning to any desired wavelength in a wide range. We demonstrate Doppler-free spectroscopy of a rovibrational transition of methane at 3.39 mum.     

Two field nonlinear Faraday rotation in rubidium vapor in a Doppler-free geometry

Nonlinear Faraday rotation with counter-propagating light beams was investigated in a rubidium vapor cell. Observations across all hyperfine and crossover resonances of the D₂ line of ⁸⁷Rb atoms were made. Additionally, rotation spectra as a function of pump and probe intensity, pump polarization and external magnetic field strength were studied.The investigations were performed in a regime where the pump beam power was sufficiently low relative to the probe power to avoid polarization rotation due to pump-induced anisotropy. Our results are analyzed in the context of Bennett structures and coherent optical processes.