Lying Short-Lived Excited States

Double resonance (DR) spectroscopy in the sense defined above was introduced by Brossel and Bitter. The work on atoms carried out before 

Rotational constants could be calculated with the help of the measured transition frequencies. 

Extensive laser-microwave investigations have also been performed in the spectrum of NH2. Hills and Curl, Jr. observed strong electric dipole microwave transitions between a previously unobserved rovibronic level and the J = 1/2 and J = 3/2 spin-rotational levels of ho, A ir(0,10,0), respectively. The NH2 molecules passed through a resonant half-wave microwave cavity with a 60-mW single-mode cw dye laser beam along the axis. The microwave field was amplitude modulated, and the laser-excited fiuorescence signal was detected with a phase sensitive amplifier. Microwave transitions in that part of the NH2 spectrum were used by Hills to assign numerous optical transitions. 

Further work on NH2, concerning ground state microwave transitions and microwave modulated saturation spectroscopy can only be quoted by reference here. 

Remarkably, laser-microwave DR spectroscopy was also applied to large biomolecules. The investigations concentrated on the electronic structure and excited state dynamics of fluorescent organic systems via optical detection of magnetic resonance in the triplet state. Plenty of work was also performed utilizing classical light sources (see, e.g.. Ref. 167). 

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The factor kr is temperature-independenL The non-radiative rate constant contains contributions from a temperature-independent term, which accounts for the deactivation to the ground state, and a temperature-dependent term which can play a role when upper-lying short-lived excited states are thermally accessible (Thompson et al., 2002). Determination of quantum yields is not an easy task. The measurement of absolute quantum yields is critical and requires special equipment, because it is necessaiy to know the amount of excited light received by the sample. These measurements are done by the use of scattering agents and integrating spheres to calibrate the system. For routine woik, one is often satisfied with the determination of relative quantum yields. In this case the quantum yield of the unknown is compared with that of a reference sample ... 

The properties of the alkane triplet states are much less known than the properties of the singlet states. It seems there is a dissociative, and thereby short-living triplet state — 1 eV below the lowest singlet excited state [29]. The other triplet states are probably also dissociative. The low-lying and long-living triplet states, supposed in some works based on unusual energy-transfer results, still needs real experimental verification [7]. 

An alternative route to long-lived excited states in transition metal complexes is to populate an excited state localised on a pendant arm. For example, a switch from a very short lived MLCT state in [Pt(tpy)Cl] to a long-lived intra-pyrene excited state in [(4 -pyrene)-tpy-Pt-Cl] , has been demonstrated an electron-rich aryl substituent at the 4 position of the tpy ligand promotes the low-lying excited state with intra-ligand charge-transfer (ILCT) character, enhances the emission intensity, and extends the excited-state lifetime up to 64 ps in r.t. dichloromethane solution [35]. 

When C60 is irradiated with visible light, it is excited from the S0 ground state to a short-lived (-1.3 ns) S,-excited state (Es 46.1 keal/mol). The St state rapidly decays at a rate of 5.0 x 108 s and a triplet quantum yield (0T) of 1.0 to a lower lying triplet state Tj (/. ., . 37.5kcal/mol) with a long hfetime of 50-lOOps. The Sj—>Tt decay is... 

Polypyridine complexes of Fe(II) are strongly colored due to MLCT transitions. However, their MLCT excited states are very short-lived because of an efficient deactivation through lower-lying LF states. MLCT excited state lifetimes of [Fe(N,N)3] + range from 2.54 ns for tpy to 0.8 ns for bpy and phen [270]. Therefore, Fe complexes were deemed unsuitable for electron-transfer reactions. Recently, there is a renewed interest in excited state properties of Fe -polypyridines caused by the observation [304] of an ultrafast electron transfer from [Fe(4,4 -(COOH)2-bpy)2(CN)2] (tq = 330 ps) to Ti02, see Section 5.4.6. 

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