Stark quantum-beat

Stark quantum-beat (SQB) spectroscopy was used by Brieger, et al., (1980) for LiH A1E+ and by Schweda, et al., (1980, 1985) for BaO A1E+ to measure electric dipole moments of several rotation-vibration levels. In a A = 0 state, the electric-field-induced shift of a J, M-level is proportional to 2/B because jjStark hyg only A J -= 1, AM = 0 matrix elements ... 

Figure 6.19 Stark quantum beats in BaO A1 +(u = 2, J = 1). The J = 1 level is excited via the R(0) line by radiation from an N2-laser-pumped dye laser. The pump radiation is linearly polarized at 45° to the 5-field direction in order to produce a coherent superposition of At = 0 with M = 1 components. The top trace shows the signal resulting when the polarization of the detected fluorescence is selected to be at 45° to and at 90° to the excitation polarization. The middle trace is for parallel excitation and detection polarizations. The bottom trace is the difference between the two detection geometries. [From Schweda, et ai.(1985).J...

The earliest pulsed laser quantum beat experiments were performed with nanosecond pulses (Haroche, et al., 1973 Wallenstein, et al., 1974 see review by Hack and Huber, 1991). Since the coherence width of a temporally smooth Gaussian 5 ns pulse is only 0.003 cm-1, (121/s <-> 121 cm"1 for a Gaussian pulse) nanosecond quantum beat experiments could only be used to measure very small level splittings [e.g. Stark (Vaccaro, et al., 1989) and Zeeman effects (Dupre, et al., 1991), hyperfine, and extremely weak perturbations between accidentally near degenerate levels (Abramson, et al., 1982 Wallenstein, et al., 1974)]. The advent of sub-picosecond lasers has expanded profoundly the scope of quantum beat spectroscopy. In fact, most pump/probe wavepacket dynamics experiments are actually quantum beat experiments cloaked in a different, more pictorial, interpretive framework,... 

Stark and Zeeman polarization quantum beats are discussed in Section 6.5.3. An external electric or magnetic field destroys the isotropy of space. As a result, the amplitudes for two transition sequences J", M" — J, M = M" 1 —> J ", M" interfere, and the intensity of X or Y (but not Z) polarized fluorescence is modulated at (Fj M =M"+i — Ejim =M"-i)/h. However, it is not necessary to destroy the isotropy of space in order to observe polarization quantum beats. 

Because quantum-beat spectroscopy offers Doppler-free spectral resolution, it has gained increasing importance in molecular physics for measurements of Zee-man and Stark splittings or of hyperfine structures and perturbations in excited molecules. The time-resolved measured signals yield not only information on the dynamics and the phase development in excited states but allow the determination of magnetic and electric dipole moments and of Lande g-factors. 

R. Schmidt, H. Bitto, J.R. Huber, Excited state dipole moments in a polyatomic molecule determined by Stark quantum beat spectroscopy. J. Chem. Rhys. 88,696 (1988)... 

An example of the Stark effect is given in Fig.9.39. Note, that while only the tensor Stark interaction constant 03 (Sect.2.5.2) can be determined in an LC experiment (Sect.7.1.5 Fig.9.18), the scalar interaction constant ag can be obtained in this type of experiment as well as 03. In the same way, isotope shifts can be measured by direct optical high-resolution methods while resonance methods and quantum-beat spectroscopy can only be used for measurements of splittings within the same atom. 

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