Hydrogen, atomic Doppler-free spectroscopy

Multiphoton resonant processes with simplest fundamental quantum systems exposed to sufficiently strong laser fields attracted conspicuous attention over last years. Currently, this interest is being especially strongly stimulated by dramatic improvements in the precision of measurements presently attainable in spectroscopic experimental studies of hydrogenic and few-particle atoms. Using methods of ultra high precision Doppler-free spectroscopy, particularly impressive results have been recently obtained in studies of fundamental bounded systems such as hydrogen (H) and its natural isotopes deuterium (D) and tritium (T) [1,2,3,4,5,6,7], positronium [8,9], denoted Ps = (e+ — e ), muonium [10,11,9,12,13,14,15], denoted (M = — e ), and the helium atom (He) [16[... 

The advent of Doppler-free spectroscopy has reopened the discussion of atomic hydrogen at levels of precision and accuracy far beyond those traditionally available. The present-day laser spectroscopy concentrates on two issues First is the fully resolved multiple structure within Balmer a as pioneered by Hansch and collaborators [21] and subsequently exploited by others second is the hydrogen ls-2s transition which is available... 

The method we use is Doppler free two-photon laser spectroscopy, applied to the atomic hydrogen transitions from the metastable 2S state to the Rydberg nD states (n = 8, 10, 12) /8/. 

In 1976, reviewing the field of two-photon spectroscopy, BLOEMBERGEN and LEVENSON wrote " The IS - 2S transition in atomic hydrogen is among the most important in physics, and it can be resolved by the Doppler-free two-photon... 

Fig. 2. Recent measurements of the Rydberg constant by laser spectroscopy of hydrogen and deuterium. Data points A and B are derived from the wavelength of the Balmer-a [7] or Balmer-8 line [8], observed by quenching of a beam of metastable 2S atoms with crossed dye laser beams. Data C and D are obtained by Doppler-free two-photon spectroscopy of 2S-8..12S transitions, recorded by quenching of a metastable atomic beam [9,10]. The Rydberg values E and F have been measured by Doppler-free two-photon spectroscopy of the 1S-2S transition in a gas cell [11,12],...

In the following I shall discuss a number of recent laser experiments on two-body systems namely, hydrogen, positron urn and muonic atoms (u He). In describing these experiments I will be introducing several applications of Doppler-free laser spectroscopy and of frequency-doubled tunable radiation. I will spend first a little time on the theory of the hydrogen atom, contrasting it with that of positronium and muonium. I will then make one or two remarks about frequency calibration of Doppler-free spectra and then consider in some detail laser experiments performed in Oxford and Stanford on hydrogen. 

The possibilities of Doppler-free two-photon spectroscopy for metrology and fundamental physics has been impressively demonstrated by precision measurements of the 1S-2S transition in atomic hydrogen [260-263]. Precise measurements of this one-photon forbidden transition with a very narrow natural linewidth of 1.3 Hz yield accurate values of fundamental constants and can provide stringent tests of quantum electrodynamic theory (Sect. 9.7). A comparison of the 1S-2S transition frequency with the 2S-3P frequency allows the precise determination of the Lamb shift of the 15 ground state [261], whereas the 2S Lamb shift was already measured long ago by the famous Lamb-Rutherford experiments where the RF transition between 25 1/2 and 2P /2 were observed. Because of the isotope shift the 15-25 transitions of and differ by... 

Using the dispersion profiles of Doppler-free molecular lines in polarization spectroscopy (Sect. 7.4), it is possible to stabilize a laser to the line center without frequency modulation. An interesting alternative for stabilizing a dye laser on atomic or molecular transitions is based on Doppler-free two-photon transitions (Sect. 7.5) [5.77]. This method has the additional advantage that the lifetime of the upper state can be very long, and the natural linewidth may become extremely small. The narrow Is —2s two-photon transition in the hydrogen atom with a natural linewidth of 1.3 Hz provides the best known optical frequency reference to date [5.76]. 

A very interesting application of Doppler-free two-photon spectroscopy was demonstrated by HANSCH et al. [10.78]. The frequency w of a nitrogen laser pumped dye laser at A = 486 nm was frequency doubled in a lithium formiate monohydrate crystal. The fundamental dye laser output was used to perform saturation spectroscopy of the Balmer 3 line in a hydrogen discharge cell (see Fig.10.55), while the frequency-doubled output at X = 243 nm simultaneously excited a two-photon transition IS - 2S in a second flow cell where H atoms are produced in a gas discharge. The two-photon transition was observed through the collision-induced 2P-1S fluorescence at X = 121.5 nm. Without the Lamb shift the frequency of the 1S-2S transition should be four... 

Fig. 2 Top Balmer spectrum of atomic hydrogen. Center Doppler profile of the Balmer-a line at room temperature and theoretical fine structure components. Bottom Doppler-free spectrum of Balmer-a, recorded by saturated absorption spectroscopy with a pulsed dye laser.

DOPPLER-FREE TWO-PHOTON SPECTROSCOPY OF ATOMIC HYDROGEN 1S-2S... 

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