EGAS41 - Swansea University

41 st EGAS CP 147 Gdańsk 2009
Corrections to transit time broadening
J. Ulmanis 1 , N.N. Bezuglov 2 , B. Mahrov 1 , C. Andreeva 3 , K. Miculis 1 , M. Bruvelis 1 ,
E. Saks 1 , A. Ekers 1
1 Laser Centre, University of Latvia, LV-1002 Riga, Latvia
2 Faculty of Physics, St. Petersburg State University, 198904 St. Petersburg, Russia
3 Institute of Electronics, Bulgarian Academy of Sciences, 1784 Sofia, Bulgaria
We reconsider the effect of transit time on line broadening in dilute gases in the weak
excitation limit, when the effects of reabsorption, saturation, and collisions are negligible.
Two-photon excitation of excited molecular levels by counter-propagating laser fields is
used in order to eliminate the Doppler broadening. Both laser fields have Gaussian profiles
and are focused onto the supersonic beam of Na 2 molecules such that transit time is
shorter than natural lifetime of the excited state. In experiment, the field 2 in the 2nd excitation
step is detuned by 1GHz off from one-photon resonance between the intermediate
rovibrational level in the A 1 Σ + u state and the final level 51 Σ + g (v=10,J=9). The excitation
spectrum of the final level is then recorded by scanning the field 1 across the two-photon
resonance.
Usually in such a situation one would describe the absorption profile by a Lorentz
function with the natural width Γ sp and transit time τ tr [1]:
P(∆) = π˜Γ/ ( ∆ 2 + ˜Γ 2) ; ∆ω =
˜Γ
2 = 1 2 (Γ sp + 1/τ tr ), (1)
where ∆ is the detuning of field 1 from the two-photon resonance, and ∆ω is the HWHM.
Transit time τ tr = L/v f is determined by the flow velocity of molecules in the beam v f
and the spot size of the laser beams L. In our case the lifetime is τ sp =42ns and the transit
time is τ tr =23ns.
Analytical solution for two-photon absorption shows that it is described by the Voight
profile, which is in good agreement with the measured line profiles (see Fig. 1). The
HWHM of this profile exceeds the value of ∆ω given by Eq. (1) by a factor of 2.5.
Fluorescence intensity (a.u.)
1,0 Calculated lineshape
Experimental data
0,8
0,6
0,4
0,2
0,0
-800 -700 -600 -500 -400 -300
(MHz)
Figure 1: Doppler-free two-photon excitation spectrum of the Na 2 (5 1 Σ + g ,v=10,J=9).
References
[1] B.W. Shore, The Theory of Coherent Atomic Excitation (Wiley, New York, 1990).
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