Contains reports on one research project.Joint Services Electronics Program (Contract DAABO7-71-C-0300

Apt, Jerome, III

Ducas, Theodore W.

Pritchard, David E.

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III. ATOMIC RESONANCE AND SCATTERINGProf. Daniel KleppnerProf. David E. PritchardJerome Apt IIIGary M. CarterAcademic and Research StaffDr. Theodore W. DucasGraduate StudentsWilliam E. CookeMichael G. LittmanDr. Richard R. FreemanDr. Edward M. MattisonWilliam D. PhillipsFrancine WrightA. TWO-PHOTON HIGH-RESOLUTION SPECTROSCOPYJoint Services Electronics Program (Contract DAABO7-71-C-0300)Jerome Apt III, Theodore W. Ducas, David E. PritchardWe have excited both the 4D and 5S states of neutral atomic sodium by simultaneoustwo-photon absorption directly from the 3S ground state. This is the first high-resolutiontwo-photon spectroscopy performed in the visible region and the first to utilize continu-ous wave lasers. An atomic beam of ground-state sodium was excited by a Spectra-Physics System 580 single-mode tunable dye laser, and the resonance was detected bymonitoring the decay fluorescence from the excited state. The experimental arrange-ment is shown in Fig. III-1.SODIUMOVENHOT-WIRE BEAMMONITORFig. III-1. Experimental apparatus. A mirror that reflects the laserbeam back through the interaction region will be added toquadruple the signal and eliminate Doppler broadeningcaused by divergence of the atomic beam.Since the linewidth observed in the experiment was 5 X 10- 3 cm-1 , we were able toresolve fully the fine structure of the 4 2D state by electronically sweeping the laser(around 17, 274 cm-1, half of the 3S-4D transition energy). The fine-structure splitting(around 17, 274 cm , half of the 3S-4D transition energy). The fine-structure splittingQPR No. 113(III. ATOMIC RESONANCE AND SCATTERING)was measured by comparing the spacing between components of the observed spectrum(Fig. III-2) split by the 4D fine structure, A, with the spacing arising from thewell-known hyperfine splitting of the 3S ground state, Aw. We measured Af =. 0343 ±. 0002 cm . This error is roughly 1/10 that of any previous measurement offine -structure splitting.4D: j = 5/2 j 3/2 j 5/ 2  j 3/235: F=2 F=2 F=I F=. FREQUENCYAW I AA AW2 2Fig. I1-2. Detailed structure of the 3S - 4D two-photon transition (with fre-quency increasing to the right ~200 MHz/div). The levels involvedin each of the four components of the transition are shown abovethe corresponding peaks and the spacing is shown below.The transition rate for two-photon absorption is given by second-order time-dependent perturbation theory. If we take the perturbation to be H = i e A r, thenthe transition probability is proportional toAZ Af 2z (f *Er ik) (kl E l i)22 kRate oc (Laser Power)  k[(wki- fi/2) 2 k+y/4]where i and f refer to the initial and final states and the sum is taken over all inter-mediate states k, and wki (wfi) is the level spacing between the ground and intermediate(final) states.The interesting feature of this expression is the resonance denominator, which allowshigh transition rates when one of the intermediate states is nearly midway between the-1initial and final states. In the case of our 3S - 4D transition, the 3P state is 350 cm-1away from being halfway between the 3S and 4D levels (the sodium term diagram isshown in Fig. I1-3), thereby allowing saturation of the two-photon transition with a rela-tively low power (2 X 103 W/cm2). (No direct excitation of the 3P state is produced byQPR No. 113(III. ATOMIC RESONANCE AND SCATTERING)the laser when operating near the frequency wfi/2, since this is roughly 5 X 105 line-widths away from the resonance of the 3S - 3P transition.4P 3/2, 1/24S 1/23P 3/2, 1/2NEUTRAL SODIUMFig. 111-3. Energy-level diagram for neutral atomic sodium. The main diagramis roughly to scale, as are the enlargements of the 4D fine structureand the 3S hyperfine structure.Two-photon spectroscopy is applicable, with improved optics and possibly signalaveraging techniques, to systems in which the intermediate state is 1000-3000 cm1from being halfway between the initial and final states. Thus this technique is well-suited to the precise spectroscopy of many systems in which levels have previouslybeen inaccessible when using single-photon dipole transitions. Unlike stepwise excita-tion, this technique requires only a single laser, although a second cW laser (not tunable)at a different frequency can be utilized to extend the accessible frequency range consid-erably.QPR No. 113=3/24D32,5 5/24D3/2,5/23s1/ F =2F:=

Atomic Resonance and Scattering

http://dspace.mit.edu/bitstream/1721.1/56471/1/RLE_QPR_113_III.pdf

Atomic Resonance and Scattering

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