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Vibronic spectra involve simultaneous changes in the vibrational and electronic energy states of a molecule. In the gas phase vibronic transitions are accompanied by changes in rotational energy also. Vibronic spectra of diatomic molecules have been analysed in detail;〔Available for download at (community books )〕 emission spectra are more complicated than absorption spectra. The intensity of allowed vibronic transitions is governed by the Franck–Condon principle. Vibronic spectroscopy may provide information, such as bond-length, on electronic excited states of stable molecules. It has also been applied to the study of unstable molecules such as dicarbon, C2, in discharges, flames and astronomical objects.〔Hollas, p. 211.〕 == Principles == Electronic transitions are typically observed in the visible and ultraviolet regions, in the wavelength range approximately 200–700 nm (50,000–14,000 cm−1), whereas fundamental vibrations are observed below about 4000 cm−1.〔Energy is related to wavenumber by , where h=Planck's constant and c is the velocity of light〕 When the electronic and vibrational energy changes are so different, vibronic coupling (mixing of electronic and vibrational wave functions) can be neglected and the energy of a vibronic level can be taken as the sum of the electronic and vibrational (and rotational) energies; that is, the Born–Oppenheimer approximation applies.〔Banwell and McCash, p. 162.〕 The overall molecular energy depends not only on the electronic state but also on vibrational and rotational quantum numbers, denoted v and J respectively for diatomic molecules. It is conventional to add a double prime (v", J") for levels of the electronic ground state and a single prime (v', J') for electronically excited states. Each electronic transition may show vibrational coarse structure, and for molecules in the gas phase, rotational fine structure. This is true even when the molecule has a zero dipole moment and therefore has no vibration-rotation infrared spectrum or pure rotational microwave spectrum.〔Banwell and McCash, p. 163.〕 It is necessary to distinguish between absorption and emission spectra. With absorption the molecule starts in the ground electronic state, and usually also in the vibrational ground state because at ordinary temperatures the energy necessary for vibrational excitation is large compared to the average thermal energy. The molecule is excited to another electronic state and to many possible vibrational states . With emission, the molecule can start in various populated vibrational states, and finishes in the electronic ground state in one of many populated vibrational levels. The emission spectrum is more complicated than the absorption spectrum of the same molecule because there are more changes in vibrational energy level. For absorption spectra, the vibrational coarse structure for a given electronic transition forms a single ''progression'', or series of transitions with a common level, here the lower level .〔Hollas, p. 214〕 There are no selection rules for vibrational quantum numbers, which are zero in the ground vibrational level of the initial electronic ground state, but can take any integer values in the final electronic excited state. The term values for a harmonic oscillator are given by : where ''v'' is a vibrational quantum number, ωe is the harmonic wavenumber. In the next approximation the term values are given by : where χe is an anharmonicity constant. This is, in effect, a better approximation to the Morse potential near the potential minimum. The spacing between adjacent vibrational lines decreases with increasing quantum number because of anharmonicity in the vibration. Eventually the separation decreases to zero when the molecule photo-dissociates into a continuum of states. The second formula is adequate for small values of the vibrational quantum number. For higher values further anharmonicity terms are needed as the molecule approaches the dissociation limit, at the energy corresponding to the upper (final state) potential curve at infinite internuclear distance. The intensity of allowed vibronic transitions is governed by the Franck–Condon principle.〔Hollas, p. 215.〕 Since electronic transitions are very fast compared with nuclear motions, vibrational levels are favored when they correspond to a minimal change in the nuclear coordinates, that is, when the transition is "vertical" on the energy level diagram. Each line has a finite linewidth, dependent on a variety of factors.〔Hollas, pp. 30–33.〕 Vibronic spectra of diatomic molecules in the gas phase have been analyzed in detail.〔Hollas, pp. 210–228〕 Vibrational coarse structure can sometimes be observed in the spectra of molecules in liquid or solid phases and of molecules in solution. Related phenomena including photoelectron spectroscopy, resonance Raman spectroscopy, luminescence, and fluorescence are not discussed in this article, though they also involve vibronic transitions. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「vibronic spectroscopy」の詳細全文を読む スポンサード リンク
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