Ro-vibronic levels

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Figure 45. Schematic representation of the preparation and detection of rotational coherence in a molecule. The case depicted corresponds to the linearly polarized excitation (polarization vector ,) of a symmetric top molecule in ground-state ro-vibronic level S0v0 J0K0M0) to those rotational levels of the excited vibronic state 15,1 ,) allowed by the rotational selection rules germane to a parallel-type transition moment. The excitation process creates a superposition state of three rotational levels, the coherence properties of which can be probed by time resolving the polarized fluorescence (polarization it) to the manifold of ground-state ro-vibronic levels S0vf JfKfMfy, or by probing with a second, variably time-delayed laser pulse (polarization...

Fig. 2. Four types of light scattering processes used in molecular spectroscopy. Raman scattering takes place if the incident light corresponds to a transparent region of the molecular absorption spectrum the intermediate state is then virtual in that it does not closely resemble any particular molecular state. Preresonance Raman scattering takes over when the incident beam approaches an electronic absorption band, so that the corresponding electronic state dominates the intermediate state. It turns into resonance Raman scattering when the intermediate state is dominated by a few (ro-)vibronic levels in the vicinity of the incident light frequency. Ultimately the resonance fluorescence limit is reached when the incident beam coincides with a single sharp level of the electronic manifold.

The laser functions according to a four-level scheme similar in nature to that of a (molecular) dye laser. Excitation is from the lowest ro-vibronic level in the electronic ground state 2 into the E excited-state manifold. Fast relaxation to the lowest ro-vibronic level in this state occurs, where population inversion accumulates due to the long lifetime of that level of 3.5 ps. After radiative transitions into the high part of the ro-vibronic manifold of 2,... 

The ro-vibronic spectrum of molecules and the electronic transitions in atoms are only part of the whole story of transitions used in astronomy. Whenever there is a separation between energy levels within a particular target atom or molecule there is always a photon energy that corresponds to this energy separation and hence a probability of a transition. Astronomy has an additional advantage in that selection rules never completely forbid a transition, they just make it very unlikely. In the laboratory the transition has to occur during the timescale of the experiment, whereas in space the transition has to have occurred within the last 15 Gyr and as such can be almost forbidden. Astronomers have identified exotic transitions deep within molecules or atoms to assist in their identification and we are going to look at some of the important ones, the first of which is the maser. 

As previously discussed, if two or more excited eigenstates can combine in absorption with a common ground-state level, then these eigenstates can be excited so as to form a coherent superposition state. The superposition state, in turn, can give rise to quantum beat-modulated fluorescence decays. All this, of course, lies at the heart of the theory of vibrational coherence effects. However, it also implies that the same experimental conditions under which vibrational coherence effects are observed should allow for the observation of rotational coherence effects. That is, since more than one rotational level in the manifold of an excited vibronic state can combine in absorption with a single ground-state ro-vibrational level, then in a picosecond-resolved fluorescence experiment rotational quantum beats should obtain. 

These new algorithms made it possible to calculate the derivative couplings for general polyatomic molecules with much improved efficiency and accuracy. As discussed in Chapter 2 of this volume, the ACj j(X) produces a mass dependent modification to a Born ppenheimer potential energy surface that can be inferred from measurements of ro-vibronic energy levels of the isotopomers. Indeed, using the Bj j(X) determined at the MRCI level, we were able to resolve a discrepancy in the adiabatic correction for LiH(X E ) obtained from an analysis of experimental data, and from a theoretical prediction, based on highly specialized wave functions, see Sec. 8. 

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