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Sub-Doppler Spectroscopy of Collision Processes

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 [Pg.431]

Lamb peaks (inverse Lamb dips) at the line centers of the absorbing transitions (Sect. 2.3). The line profiles of these peaks are determined by the pressure in the absorption cell, by saturation broadening, and by transit-time broadening (Vol. 1, Sect. 3.4). Center frequency coq, linewidth Aco, and line profile Pl(co) are measured as a function of the pressure p (Fig. 8.2). The slope of the straight line Aco p) yields the line-broadening coefficient [977], while the measurement of coo p) gives the collision-induced line shift. [Pg.432]

A more detailed consideration of collisional broadening of Lamb dips or peaks must also take into account velocity-changing collisions. In Sect. 2.2 it was pointed out that only molecules with the velocity components = 0 di y/A can contribute to the simultaneous absorption of the two counterpropagating waves. The velocity vectors v of these molecules are confined to a small cone within the angles p 3ie around the plane v =0 (Fig. 8.3a), where [Pg.432]

V sinO y/k, the molecule after the collision is still in resonance with the standing light wave inside the laser resonator. Such soft collisions with deflection angles 0 therefore do not appreciably change the absorption probability of a molecule. Because of their statistical phase jumps (Vol. 1, Sect. 3.3) they do, however, contribute to the linewidth. The line profile of the Lamb dip broadened by soft collisions remains Lorentzian. [Pg.433]

Collisions with 0 e may shift the absorption frequency of the molecule out of resonance with the laser field. After a hard collision the molecule, therefore, can only contribute to the absorption in the line wings. [Pg.433]

If V sin y/k, the molecule after the collision is still in resonance with the standing light wave inside the laser resonator. Such soft collisions with deflection angles 0 therefore do not appreciably change the absorption [Pg.728]

The combined effect of both kinds of collisions gives a line profile with a kernel that can be described by a Lorentzian profile slightly broadened by soft collisions. The wings, however, form a broad background caused by velocity-changing collisions. The whole profile cannot be described by a single Lorentzian function. In Fig. 13.4 such a line profile is shown for the Lamb peak in the laser output at A = 3.39 p.m with a methane cell inside the [Pg.729]


The difference between optical nutation and free induction decay should be clear. While the optical nutation occurs at the Rabi frequency which depends on the product of laser field intensity and transition moment, the free induction decay is monitored as a heterodyne signal at the beat frequency 0) 2 which depends on the Stark shift. The importance of these coherent transient phenomena for time-resolved sub-Doppler spectroscopy is discussed in the next section. Its application to the study of collision processes is treated in Chap.12. For more detailed information the excellent reviews of BREWER [11.43,48] are recommended. [Pg.581]

While the previous chapter emphasized the high speotral resolution achievable with different sub-Doppler techniques, this chapter concentrates on some methods which allow high time resolution. The generation of extremely short and intense laser pulses has opened the way for the study of fast transient phenomena, such as molecular relaxation processes in gases or liquids due to spontaneous or collision-induced transitions. A new field of laser spectroscopy is the time-resolved detection of coherence and interference effects such as quantum beats or coherent transients monitored with pulse Fowoiev transform spectroscopy. [Pg.546]


See other pages where Sub-Doppler Spectroscopy of Collision Processes is mentioned: [Pg.431]    [Pg.727]    [Pg.696]    [Pg.431]    [Pg.727]    [Pg.696]    [Pg.201]   


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