Laser spectroscopy with microwave

In Florence, we have chosen an approach that combines laser spectroscopy with the direct frequency measures of the microwave experiments [4]. We take advantage of the obvious consideration that to obtain the FS separations there s no need to precisely know the optical transitions frequencies but just their differences. Thus, if we have two laser frequencies whose difference can be accurately controlled, we may use one as a fixed reference and tune the second across the atomic resonances, as illustrated by Fig. 1. In fact, our approach reverts to an heterodyne technique, where all the transitions are measured with respect to the same reference frequency, that can take any arbitrary but stable value. In the experimental realisation we obtain the two frequencies by phase-locking two diode lasers (master and slave), i.e. phase-locking their beat note to a microwave oscillator [14]. We show in Fig 2 a full-view of the experimental set-up. 

CIRDLS = infrared diode laser spectroscopy ES = electronic spectroscopy, spectra with resolved vibrational structure MW = microwave spectroscopy force field calculations denote harmonic frequencies obtained on the basis of combined analysis of electron diffraction and vibrational spectroscopy data. 

Fig. 1. Muonium energy levels for states with principal quantum numbers n = 1 and n = 2. The indicated transitions could be induced to date using modern techniques of microwave or laser spectroscopy. High accuracy has been achieved for the indicated transitions which involve the ground state. The atoms can be produced very efficiently only in the Is state...

Both microwave and optical frequency standards have benefited greatly from the development of the laser and the methods of laser spectroscopy in atomic physics. In particular, the ability to determine both the internal and external (that is, motion) atomic states with laser light - by laser cooling for example - has opened up the prospect of frequency standards with relative uncertainties below lO, for example, the Cs atomic fountain clock. The best atomic theories in some cases at starting to match in accuracy that of measurement, providing thereby refined values of the fundamental, so-called atomic constants. Even quite practical measurements (such as used in GPS navigation and primary standards of length) have advanced in recent years. 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

The second scheme to be treated is based on a frequency modulation of the monochromatic incident wave. It was not designed specifically for laser spectroscopy, but was taken from microwave spectroscopy where it is a standard method. The laser frequency co] is modulated at the modulation frequency 2, which changes coi periodically from cul — Acol/ to cul + Acul/2. When the laser is tuned through the absorption spectrum, the difference APr = Py(col Al/2) is detected with a lock-in amplifier (phase-sensitive detector) tuned to the modulation frequency (Fig. 1.4). If the modulation sweep Acol is sufficiently small, the first term of the Taylor expansion... 

Optical-microwave double resonance (OMDR) can considerably improve the situation and extends the advantages of microwave spectroscopy to excited vibrational or electronic states, because selected levels in these states can be populated by optical pumping. Generally dye lasers or tunable diode lasers are used for optical pumping. However, even fixed frequency lasers can often be used. Many lines of intense infrared lasers (for example, CO2, N2O, CO, HF, and DF lasers) coincide with rotational-vibrational transitions of polyatomic molecules. Even for lines that are only close to molecular transitions the molecular lines may be tuned into resonance by external magnetic or electric fields (Sect. 1.6). The advantages of this OMDR may be summarized as follows ... 

Fig. 7.28 Heterodyne spectroscopy with two lasers, stabilized onto two molecular transitions. The difference frequency, generated in a nonlinear crystal, is either measured or a second downconversion is used by mixing with a microwave...

This chapter assesses the state of the art in laser microwave spectroscopy, i.e., investigations where both laser radiation and microwave radiation are involved. The many concepts and methods applied to numerous different atomic, molecular, and solid state systems are outlined, not always with emphasis on completeness of all the later references to the same method. This chapter hopefully stimulates further development of laser microwave spectroscopic methods and applications, even crossing the border of different disciplines. 

Microwave techniques were combined with fast ion beam laser spectroscopy (see Figure 11) in order to investigate the hyperfine structure of A 50-keV, isotopically pure ion beam with a current... 

Table 1. Hfs Splittings of the 2 S State for (a) and Li (b). Measured by Laser-Microwave Spectroscopy with an Ion Beam"...

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