Lamb-dip technique

Line centers of a number of previously unresolved SF transitions in the fundamental Vj band have been observed by Rabinowitz et al. with this Lamb-dip technique. Measurements of the dip width at small saturation levels yielded a determination of the SFe-SFe cross-sections for phase interrupting collisions. 

With the sub-Doppler resolution of the Lamb-dip technique, about 500 Stark resonances have been identified for 44 lines of the V2 band and 31 lines of the V4 band [13]. Two Doppler-free lines with signs opposite to those of Lamb dips were observed in a study of the V2 °P(5,3) transition by laser Stark spectroscopy using the 10P(18) CO2 laser line. They were identified as infrared-infrared double resonance transitions, caused by accidental overlapping of the °P(5,3) line in the V2 band with the °P(4,3) line in the hot band 2V2-V2 [14 to 16]. 

Figure 6.2 provides a representative example for a Doppler-limited rotational spectrum, while Figure 6.3 shows such a comparison for resolved hyperfine structure (Lamb-dip technique in conjunction with frequency modulation has been employed for recording the spectrum [88]). 

The investigation of the hyperfine stmcture of the rotational spectrum of H CN by means of the Lamb-dip technique [136] provides a good example of how theory may also supply missing experimental data. In detail, it turned out to be essential to fix in... 

Natural line broadening is usually very small compared with other causes of broadening. However, not only is it of considerable theoretical importance but also, in the ingenious technique of Lamb dip spectroscopy (see Section 2.3.5.2), observations may be made of spectra in which all other sources of broadening are removed. 

This extremely high stability can be transferred to tunable lasers by a special frequency-offset locking technique [221]. Its basic principle is illustrated in Fig. 2.20. A reference laser is frequency stabilized onto the Lamb dip of a molecular transition at coq. The output from a second, more powerful laser at the frequency (D is mixed in detector D1 with the output from the reference laser at the frequency coq- An electronic device compares the difference frequency coq — co with the frequency co of a stable but tunable RF oscillator, and controls the piezo P2 such that a>o — CO = co dt all times. The frequency co of the powerful laser is therefore always locked to the offset frequency co = coo — co which can be controlled by tuning the RF frequency co. ... 

Fig. 4.25 (a) Lamb dips of hyperfine components of the rotational transition J = l J" = Q in a collimated NO2 beam. The residual Doppler width is 15 MHz. (b) The insert shows one component with suppression of the Doppler background by chopping the laser beams and using lock-in techniques [453]... 

With techniques of sub-Doppler spectroscopy, even small collisional broadening effects can be investigated with high accuracy. One example is the measurement of pressure broadening and shifts of narrow Lamb dips (Sect. 2.2) of atomic and molecular transitions, which is possible with an accuracy of a few kilohertz if stable lasers are used. The most accurate measurements have been performed with stabilized HeNe lasers on the transitions at 633 nm [975] and 3.39 um [976]. When the laser frequency co is tuned across the absorption profiles of the absorbing sample inside the laser resonator, the output power of the laser Pl(co) exhibits sharp... 

The transit-time broadening can greatly be reduced by the optical Ramsey method of separated fields. The best resolution of the recoil splittings has indeed been achieved with this technique (Sect. 14.4). The transit time can also be increased if only molecules with small transverse velocity components contribute to the Lamb dip. If the laser intensity is kept so small that saturation of the molecular transition is reached only for molecules that stay within the laser beam for a sufficiently long time, that is, molecules with small components Vx, Vy, the transit-time broadening is greatly reduced [14.11]. 

The Doppler-free techniques, which have been explained in Chap.10, eliminate the bothering Doppler width and therefore already small collision broadening can be sensitively monitored. One example is given by the investigation of broadening or shifts of Lamb dips in saturation spectroscopy (see Sect. 10.2), which can be measured with an accuracy of a few kilohertz if stabilized lasers are used. This high accuracy allows detection of even small interaction forces such as those experienced by atoms or molecules without permanent dipole moments. The interaction potential at large internuclear distances can be described in these cases by a van der Waals potential V(r) = -ar where the constant a depends on the polarizability of the collision partners. 

In lasers with inhomogeneously-broadened transitions it is found that oscillation usually occurs simultaneously on a ntunber of longitudinal cavity modes. The reasons for this behaviour are explained and we examine one method for stabilizing the intensities and inter-mode frequency differences in multi-mode operation. Next we consider several different techniques which have been used to obtain oscillation on a single longitudinal mode and this leads on naturally to a discussion of the output power versus oscillation frequency of single-frequency gas lasers. The experimental observation and theoretical interpretation of the Lamb dip is the main topic of section 13.8. 

The Lamb dip is an important manifestation of the saturation of the gain which occurs in inhomogeneously-broadened transitions when the oscillation frequency coincides with the centre of the laser line. A similar phenomenon occurs if the laser frequency is tuned close to the centre of an absorption line of a sample of atoms or molecules interacting with the standing wave field of the laser. This saturated absorption has become an important new technique in atomic and molecular spectroscopy since it removes the limit on the attainable resolution which was formerly imposed by the Doppler broadening of absorption lines. 

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