G.-Y. Yan

Doppler-free radiofrequency optogalvanic spectroscopy

Abstract Doppler-free optogalvanic spectra of two neon lines have been observed, with a radiofrequency oscillator used to both excite the discharge and detect the laser-induced change in ion density. Both intermodulated and polarization-intermodulated (POLINEX) methods have been used. The sensitivity is similar to that obtained with fluorescence detection.

High-contrast Doppler-free transmission spectroscopy

By applying Doppler-free saturated absorption spectroscopy in the regime of high integrated sample absorption, high-contrast Doppler-free laser transmission signals can be obtained as demonstrated in experiments on the sodium D lines. Natural linewidth background-free signals are observed.

Doppler-free ultraviolet excitation spectra of CΠ1 u −XΣ1g+ in Na 2 by modulated population spectroscopy

The C1Πu state of Na2 has been studied in detail by cw UV Doppler-free excitation spectra from its ground state, simplified by modulated population spectroscopy. 270 levels of e parity and 180 levels of f parity with 11 ≤ υ ≤ 35 and 8 ≤ J ≤ 70 were measured with high resolution. The observations reveal that many of the e levels with 15 ≤ υ ≤ 24 are perturbed. A likely candidate for the perturbing state is the (3)Σ1u+ state. A set of Dunham coefficients was deduced to fit the spectroscopic data after deperturbation and used to construct the Rydberg–Klein–Rees potential for the C1Πu state. In addition, Λ-type doubling of the state was observed directly.

Potential of the (5)Σ1g+ state of Na 2 by two-step excitation spectroscopy

The (5)Σ1g+ state has been studied by two-step excitation spectroscopy. 270 levels in the state were measured with high resolution. A set of Dunham coefficients was deduced to fit all levels with v < 19. The spectroscopic data were used to construct a Rydberg–Klein–Rees potential for the state. The potential shows that the state is perturbed by other Rydberg states and ionic states and seems likely to have a second well that is due to avoided crossings with these states.

Laser spectroscopy using beam-overlap modulation

A new Doppler-free laser spectroscopy method is demonstrated that employs modulation of the position of a laser beam rather than the commonly used intensity or polarization modulation. The technique is applicable in saturated absorption as well as fluorescence measurements, as is illustrated in experiments on sodium and iodine lines. A particular feature of the method is that Doppler- and background-free fluorescence spectra can be recorded without using intermodulation techniques.

Relaxation-oscillator detection of optogalvanic spectra

A neon bulb used in a relaxation oscillator is a sensitive detector for observing optogalvanic spectra of neon. A few percent change in the oscillation frequency was observed with resonant irradiation of tenths of microwatts for some neon lines. The percent change is greater than the corresponding current change in an ordinary optogalvanic detection by 2 orders of magnitude. The new technique, not requiring a lock-in amplifier and a special sample cell, probably provides the simplest and the cheapest method for calibrating wavelengths.

Two-photon line shapes with near-resonant enhancement☆

Abstract We have studied a number of Doppler-free two-photon lines in Na2 that are strongly enhanced by very nearly resonant intermediate states. Both experimentally and theoretically, it is found that the ratio of the height of the Doppler-free line to the Doppler-broadened background is a maximum when the offset of the enhancing level is about equal to the Doppler half-width.

Experimental observation of the (3)Σ1u+ state of Na 2 by deperturbation of the CΠ1 u −XΣ1g+ system

Doppler-free UV-excitation spectra, obtained by modulated population spectroscopy, reveal perturbations of a few hundred e levels of the C1Πu state in Na2. We ascribe the perturbations to the (3)Σ1u+ state. After deperturbation, information about 15 vibrational levels of the (3)Σ1u+ state was extracted. This information was used to deduce a set of Dunham coefficients and the Rydberg–Klein–Rees potential for the state. The experimental results show that the potential of the (3)Σ1u+ state is one with a double minimum or an inflection. In addition, it has a flattened bottom with Te = 31 247.82 cm−1. As a consequence, the vibrational spacing at first increases and then decreases as a function of υ. Comparison of the experimental results and theoretical calculations yielded fairly good agreement.

Constants of the 5s 1 Σg+ state of Na 2 from two-photon spectroscopy

Fluorescence spectroscopy is used as an aid in the identification of two-photon transitions in Na2. Several methods are introduced and used to identify the transitions and analyze the spectra. By using these methods, the upper-level rotational quantum number, energy, and electronic character may be determined without any assumptions about the state to which the upper level belongs. Twenty-nine two-photon transitions were observed and analyzed. A Dunham expansion for the energy levels allowed 18 of the 29 to be associated with the Σ1g+ (3s+5s) state. They were used to calculate new values of the Dunham coefficients for that state. These were in close agreement with values obtained in polarization-labeling experiments.

Polarization Intermodulated Excitation (POLINEX) Spectroscopy of Excited Atoms

We have demonstrated a sensitive and versatile new technique of Doppler-free saturation spectroscopy, which takes advantage of Polarization Intermodulated Excitation (POLINEX): the nonlinear interaction of two laser beams in an absorbing medium is studied by modulating the polarization of one or both beams. When the combined absorption depends on the relative polarization of the two beams, an intermodulation is observed in the total rate of excitation. In first exploratory experiments we have studied excited helium atoms in a positive column discharge [1], neon atoms in radiofrequency discharges [1,2], and copper atoms in a hollow cathode discharge [3]. Both optogalvanic detection and fluorescence detection have been used. Clean spectra free of Doppler-broadened background could be recorded despite velocity changing elastic collisions.