Molecular Quantum-Beat Spectroscopy

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. 

One example is the measurement of hyperfine quantum beats in the polyatomic molecule propynal HC=CCHO by Huber and coworkers [877]. In order to simplify the absorption spectrum and to reduce the overlap of absorbing transitions from different lower levels, the molecules were cooled by a supersonic expansion (Sect. 4.2). The Fourier analysis of the complex beat pattern (Fig. 7.14) showed that several upper levels had been excited coherently. Excitation with linear and circular polarization with and without an external magnetic field, allowed the analysis of this complex pattern, which is due to singlet-triplet mixing of the excited levels [877, 878]. 

Many other molecules, such as SO2 [880], NO2 [881], or CS2 [882], have been investigated. A fine example of the capabilities of molecular quantum-beat spectroscopy is the determination of the magnitude and orientation of excited-state electric dipole moments in the vibrationless Si state of planar propynal [883]. 

More examples and experimental details as well as theoretical aspects of quantum-beat spectroscopy can be found in several reviews [868, 878, 884], papers [871-885, 887], and a book [886]. 

3 Excitation and Detection of Wave Packets in Atoms and Molecules 

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H. Bitto, J.R. Huber, Molecular quantum beat spectroscopy. Opt. Common. 80,184 (1990) R.T. Carter, R. Huber, Quantum beat spectroscopy in chemistry. Chem. Soc. Rev. 29, 305 (2000)... 

H. Bitto, J.R. Huber Molecular quantum beat spectroscopy. Opt. Commun. 80, 184 (1990)... 

Bitto, H., and Huber, J. R. (1992), Molecular Quantum Beats. High-Resolution Spectroscopy in the Time Domain, Acc. Chem. Res. 25, 65. 

Either two or more molecular levels of a molecule are excited coherently by a spectrally broad, short laser pulse (level-crossing and quantum-beat spectroscopy) or a whole ensemble of many atoms or molecules is coherently excited simultaneously into identical levels (photon-echo spectroscopy). This coherent excitation alters the spatial distribution or the time dependence of the total, emitted, or absorbed radiation amplitude, when compared with incoherent excitation. Whereas methods of incoherent spectroscopy measure only the total intensity, which is proportional to the population density and therefore to the square ir of the wave function iff, the coherent techniques, on the other hand, yield additional information on the amplitudes and phases of ir. 

Quantum-beat spectroscopy represents not only a beautiful demonstration of the fundamental principles of quantum mechanics, but this Doppler-free technique has also gained increasing importance in atomic and molecular spectroscopy. Whereas commonly used spectroscopy in the frequency domain yields information on the stationary states A ) of atoms and molecules, which are eigenstates of the total Hamiltonian... 

G. Herzberg, Molecular Spectra and Molecular Structure (van Nostrand, New York, 1950) N. Ochi, H. Watanabe, S. Tsuchiya, RotationaUy resolved laser-induced fluorescence and Zeeman quantum beat spectroscopy of the state of jet-cooled CS2. Chem. Phys. 113,... 

Fig. 12.1 la,b. Quantum-beat spectroscopy of atomic or molecular ground states measured by time-resolved polarization spectroscopy (a) experimental arrangement and (b) Zee-man quantum beat signal of the Na 3 Si/2 ground state recorded by a transient digitizer with a time resolution of 100 ns. (Single pump pulse, time scale 1 rs/div, magnetic field 1.63 X 10-4 T) [12.40]... 

Meth. LC QB OS ED MW MB EPR DR method of measurement for the values in this line of the table level crossing spectroscopy quantum beat spectroscopy optical spectroscopy electric deflection method microwave spectroscopy molecular beam resonance experiment-electric or magnetic resonance electron paramagnetic resonance double resonance experiments (MODR, RF/DR)... 

Meth DR LA LM MB MW QB RA method of measurement applied to obtain the reported values double resonance experiments (MODR and RFDR, compare 2.2.1) Doppler free laser spectroscopy Laser magnetic resonance molecular beam electric resonance or molecular beam resonance with laser detection microwave spectroscopy quantum beat spectroscopy radio astronomy... 

F1q.ll.25. Quantum beat spectroscopy in atomic or molecular ground states. The oscilloscope trace shows a Zeeman quantum beat signal for the Na 32 i/2 ground state, recorded with a transient digitizer (time resolution 100 ns) for a single pulse of the pump laser (time scale 1 ys/div, magnetic field 130 A/m) [11.35a]... 

A theory for the ultrafast pump-probe spectroscopy of large polyatomic molecules in condensed phases was developed in the work [15]. A multimode Brownian oscillator model was used to account for high-frequency molecular vibrations and local intermolecular modes as well as collective solvent motions. A semiclassical picture was provided using the density matrix in Liouville space. Conditions for the observation of quantum beats, spectral diffusion, and solvation dynamics (dynamic Stokes shift) are specified. 

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. 

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