Manifestation of Atom-Surface Interactions

The sub-Doppler structure of the SR spectrum provides an opportunity to determine the frequency shift caused by the atom-surface interaction with high resolution. This singularity can be isolated if one applies a low-frequency modulation to the incident light. By detecting the amplitude-modulated reflected signal one obtains for the reflectivity at the modulation frequency, cOm, 

This technique was applied to the study of the reflection spectrum at the interface between a glass window and Cs vapor (Oria et al. 1991). The incident light was scaimed across the 6Si/2(F = 4) —s- 6P /2 R = 3,4,5) transitions of the caesium D2 line. The reflection spectrum was compared with the saturated absorption spectrum and a red shift of about 3 MHz, along with a remarkable asymmetry, were observed (Fig. 7.5). 

In order to fit the SR spectrum one has to take into accoimt the 2-dependence of the atomic transition frequency due to the atom-surface interaction (Section 2.2.1). It can be approximated as 

If an atom is in an excited state, the character of the atom-surface potential (attractive versus repulsive) is determined by the sum of the van der Waals interaction (Eq. (2.100)) and the classical shift (Eq. (2.101)). The first term may be either positive or negative. The second term also may have either sign if one of the transition frequencies is close to the frequency of a surface excitation determined by the equation n (cp) + 1 = 0. As a result, the interaction constant C may be either positive or negative. 

The latter situation was observed in SR from the interface between a sapphire surface and Cs vapor (Failache et al. 1999). The SR spectrum was registered at the 6Pi/2 — 6D3/2 transition while the transition 6S1/2 — 6P1/2 was pumped by another laser (Fig. 7.6). An important feature of this gas-solid system is that the wavelength of the transition 7Pi/2 6D3/2 (12.15 pm) is close to the wavelength of surface optical phonons ( 12 pm) which leads to a large positive contribution to the energy of the 6D3/2 state. As a consequence, the atom-surface interaction in this state is repulsive (Fichet et al. 1995) and the SR spectrum at the transition 6P1/2 - 6D3/2 is blue-shifted and can be fitted with C —160 kHz-pm.  

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