Lamb-dip spectroscopy

38 2 ELECTROMAGNETIC RADIATION AND ITS INTERACTION WITH ATOMS AND MOLECULES 

Saturation is clearly achieved more readily if states m and n are close together, as is the case for microwave or millimetre wave transitions, but, even if they are far apart, a laser source may be sufficiently powerful to cause saturation. 

1 The number of collisions z that a molecule in the gas phase makes per unit time, when only one species is present, is given by 

2 Calculate in hertz the broadening Av of transitions in HCN at 25 °C due to the Doppler effect in regions of the spectrum typical of rotational transitions (10 cm ), vibrational transitions (1500 cm ) and electronic transitions (60 000 cm ). 

3 As a function of frequency, the spectral radiation density is given by 

Calculate typical values in the microwave (v = 50GHz) and near-ultraviolet (v = 30 000 cm ) regions. 

Ramsey, N. F. (1956) Molecular Beams, Oxford University Press, Oxford. 

---

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. 

Fig. 15. Lamb dip spectroscopy of Left side Stark spectrum (with...

The hyperfine structure of the R(127) rotational line in the 11-5 band of molecular iodine at X = 6328 A could be resolved by Lamb dip spectroscopy with a halfwidth of 4.5 MHz, and the influ-enie of isotopic substition and has been studied 38, 338a)k)... 

Lamb dip spectroscopy provides a very sensitive tool for studying small frequency shifts and broadening of spectral lines which normally would be undetectable because they may be small compared to the doppler width. These investigations yield information about collisions at low pressures, where the effect of far distant collisions is not suppressed by the more effective close collisions. This allows the potential between the collision partners at large intermolecular distances to be examined. 

The last two chapters discussed spectroscopic studies which used coincidences between laser lines and transitions in other atoms or molecules. These investigations have been performed either with lasers as external light sources, or inside the laser cavity. In the latter case coupling phenomena occur between the absorbing species and the laser emission, one example of which is the saturation effect employed in Lamb dip spectroscopy and laser frequency stabilization. This chapter will deal with spectroscopic investigations of the laser medium itself and some perceptions one may obtain from it. 

Suppose now that the two-photon transition is produced with a Laser beam reflected on itself with a mirror, as in Lamb-dip spectroscopy. 

Saturation spectroscopy is based on the velocity-selective saturation of Doppler-broadened molecular transitions, treated in Sect. 2.2. Here the spectral resolution is no longer limited by the Doppler width but only by the much narrower width of the Lamb dip. The gain in spectral resolution is illustrated by the example of two transitions from a common lower level c) to two closely spaced levels a) and b) (Fig. 2.9). Even when the Doppler profiles of the two transitions completely overlap, their narrow Lamb dips can clearly be separated, as long as Aco = cOca — < cb > 2/s Saturation spectroscopy is therefore often called Lamb-dip spectroscopy. 

For Lamb-dip spectroscopy with ultrahigh resolution, the output beam of the powerful laser is expanded before it is sent through the sample cell in order to minimize transit-time broadening (Vol. 1, Sect. 3.4). A retroreflector provides the coun-terpropagating probe wave for Lamb-dip spectroscopy. The real experimental setup is somewhat more complicated. A third laser is used to eliminate the troublesome region near the zero-offset frequency. Furthermore, optical decoupling elements have to be inserted to avoid optical feedback between the three lasers. A detailed description of the whole system can be found in [222]. 

G. Meijer, B. Janswen, J.J. ter Meulen, A. Dynamus, High resolution Lamb-dip spectroscopy on OD and SiCl in a molecular beam. Chem. Phys. Lett. 136, 519 (1987)... 

Saturation spectroscopy is therefore often called Lamb-dip spectroscopy. 

A retroreflector provides the counter propagating probe wave for Lamb dip spectroscopy. 

---